A RISK BASED METHODOLOGY TO ASSIST IN THE REGULATION OF DOMESTIC
WASTE WATER TREATMENT SYSTEMS
NOVEMBER 2012
i
ii
EXECUTIVE SUMMARY
Aim of Report
Under the
Water Services (Amendment) Act, 2012 (S.I. No. 2 of 2012) the EPA is responsible
for making a National Inspection Plan having regard to relevant risks to human health and
the environment. The aim of this report is to set out a methodology to enable the EPA to
adopt a risk-based approach to organising inspections of DWWTSs, whereby the level of
inspection will be proportionate to the risk posed to human health and the environment.
Risk-based Methodology
The methodology is based on the source-pathway-receptor (S-P-R) model for environmental
management. The development of the methodology was influenced by:
the data and map information available as GIS datasets;
the current understanding of the hydrological and hydrogeological settings present
in Ireland;
results of research on DWWTSs and hydrogeology undertaken in Ireland.
Discharges from DWWTSs
DWWTSs located, constructed, installed and maintained in accordance with the best
practice guidance generally provide adequate treatment and disposal of domestic waste
water. However, where the location, construction, operation and/or maintenance are
inadequate, impacts may occur. This report focuses on the issues that may arise in the areas
that are problematical with regard to inadequate percolation and/or attenuation. Three
pollutants were taken as representative of the threat posed by discharges from DWWTSs to
water quality and human health – molybdate reactive phosphate (MRP), nitrate and
microbial pathogens.
Receptors of Concern
The receptors of concern are human health from direct contact with microbial pathogens,
surface water from eutrophication and/or polluted groundwater being used as a private
water supply (e.g. untreated well water).
Evaluation of Pathways Linking DWWTSs with Receptors
The pathway is the link between the source of pollution and the receptor, and can either be
at or close to the surface or underground, or a combination of both. Natural vertical and
horizontal pathways for effluent migration are determined by the on-site subsurface
geology, particularly the nature of the soils, subsoils and underlying aquifers. Artificial
pathways may include drainage ditches, land drainage pipes and stream culverts.
The characteristics of both the surface and subsurface pathways are defined by the
‘
pathway susceptibility’, which is a measure of the degree of attenuation between source
and receptor.
The factors that influence attenuation along the
surface pathway (which is present
where percolation is inadequate) include: whether the effluent is piped directly to
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ditches/streams or ponds at the surface; uptake of nutrient by plants in the ponded
areas; die-off or predation of microbial pathogens; attenuation in the topsoil; and
percolation during dry weather conditions.
The factors that influence attenuation along the
subsurface pathway include: the
thickness and permeability of the subsoil; the type of aquifer (whether bedrock or
sand/gravel); and whether the bedrock facilitates de-nitrification or not.
The presence of areas with ‘inadequate percolation’ due to low permeability soils and
subsoils, high water tables and/or low permeability bedrock presents the greatest challenge
in Ireland to dealing with effluent from DWWTSs, as engineering measures to alleviate the
situation are not usually readily available. It is estimated that overall proportion of the
country with inadequate percolation, which can arise all year round or be intermittent
during wet weather conditions, is approximately 39%.
Risk Characterisation
The risk is determined by a combination of the following elements:
Estimated pollutant load from each individual DWWTS, derived from typical
discharge concentrations and quantities.
Pathway susceptibility indicating the ease with which pollutants can enter surface
water or groundwater.
Cumulative load entering the surface water or groundwater environment derived
from DWWTS density and estimations of attenuation.
Dilution of load at the water receptor; calculations are based on the effective rainfall
and groundwater recharge estimations in 1 km2 grids country-wide.
Risk ranking using estimates of predicted pollutant – MRP and nitrate –
concentrations at the receptor in comparison to appropriate standards for MRP and
nitrate. Microbial pathogens are considered to be influenced by pathway factors in a
similar manner to MRP.
Four categories of relative risk are used: low, moderate, high and very high.
The percentage areas of the country in the different relative risk categories are given in the
table below.
Relative risk
MRP & Pathogens
Nitrate
category
Streams via
Streams and wells via
Streams via surface
Streams and wells via
surface pathway
subsurface pathway
pathway
subsurface pathway
Low
63.1
89.0
97.3
97.6
Moderate
10.5
4.1
0.2
<0.1
High
6.4
1.9
<0.1
<0.1
Very High
17.8
2.8
0.1
<0.1
Area Sewered
2.3
2.3
2.3
2.3
*Percentages may not add to 100% due to rounding
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The results indicate that:
The risk to human health from DWWTS waste water is significantly higher in areas
with a high density of DWWTSs and inadequate percolation; and in vulnerable areas
with private wells.
MRP is the main pollutant posing a threat to the environment, particularly to surface
water, either where there is inadequate percolation or where there is inadequate
attenuation prior to entry of waste water into bedrock aquifers, particularly
karstified (cavernous limestone) aquifers. While the cumulative pollutant load arising
from DWWTSs will be insignificant compared to urban waste water treatment
systems and agriculture at river basin scale, it can be significant in certain physical
settings at small catchment scale.
The threat posed by nitrogen from DWWTSs is low at catchment scale and at the
scale of this assessment – 1km2– due to dilution; however, in exceptional
circumstances, at site-scale (a few hectares), a high density of DWWTSs can cause
localised plumes with elevated nitrate concentrations in groundwater.
Next Steps
The output from the Risk Based Methodology indicates the relative risk of impacts from
DWWTSs. Detailed criteria for site selection, which take account of sensitive receptors,
have been being established for use in conjunction with the Risk Based Methodology in
developing the National Inspection Plan and in proposing the level of inspection, based on
risk. This will assist Water Service Authorities in identifying areas to focus inspections and
achieve the maximum outcome for the environment.
The results of the inspections undertaken will be used to verify and calibrate the S-P-R
model as appropriate.
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Table of Contents
1
Background ........................................................................................................................ 1
1.1
Introduction................................................................................................................. 1
1.2
Report Scope ............................................................................................................... 1
1.3
Domestic Waste Water Treatment Systems ............................................................... 2
1.4
Historical and current requirements for siting, design and installation of DWWTS .. 3
1.5
Risk Based Enforcement .............................................................................................. 4
1.6
National Inspection Plan ............................................................................................. 4
2
A risk-based approach to assessing the impact of existing Domestic Waste Water
Treatment Systems .................................................................................................................... 5
2.1
Risk assessment ........................................................................................................... 5
2.2
Source-Pathway-Receptor Framework ....................................................................... 5
3
Source Characteristics ........................................................................................................ 9
3.1
Domestic waste water quality ..................................................................................... 9
3.1.1
Microbial pathogens ............................................................................................ 9
3.1.2
Phosphorus ........................................................................................................ 10
3.1.3
Nitrogen ............................................................................................................. 10
3.2
Volumes of waste water generated by DWWTSs ..................................................... 10
3.3
Pollutant Load ........................................................................................................... 10
4
Surface and subsurface pathways ................................................................................... 11
4.1
Understanding and using the ‘Pathway’ concept ..................................................... 11
4.2
Characteristics of surface and subsurface pathways ................................................ 11
4.2.1
Data Availability ................................................................................................. 12
4.2.2
Attenuation ........................................................................................................ 13
4.3
Pathway Susceptibility .............................................................................................. 14
4.4
Factors influencing surface water susceptibility to contamination .......................... 14
4.5
Factors influencing groundwater susceptibility to contamination ........................... 15
5
Classifying pathway susceptibility ................................................................................... 17
5.1
Introduction............................................................................................................... 17
5.2
Inadequate percolation ............................................................................................. 17
5.3
Inadequate attenuation ............................................................................................ 19
5.3.1
Pathogen and molybdate reactive phosphorus contamination ........................ 19
5.3.2
Nitrate contamination ....................................................................................... 19
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5.4
Use of pathway susceptibility ranking maps ............................................................. 20
5.5
Map confidence and forthcoming county maps ....................................................... 21
6
Risk characterisation ........................................................................................................ 25
6.1
Introduction............................................................................................................... 25
6.2
Pollutant Load ........................................................................................................... 26
6.3
Pathway Attenuation ................................................................................................ 26
6.3.1
Surface pathway ................................................................................................ 27
6.3.2
Subsurface pathway ........................................................................................... 27
6.4
Estimating Cumulative Load and Resultant Concentration ...................................... 28
6.4.1
Cumulative Load ................................................................................................ 28
6.4.2
Dilution ............................................................................................................... 29
6.5
Risk Ranking ............................................................................................................... 30
6.6
Conclusions from Risk Ranking Process .................................................................... 31
7
Summary and Conclusions ............................................................................................... 39
8
Glossary ............................................................................................................................ 41
9
References ....................................................................................................................... 45
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Authors and Acknowledgements
This guidance document, as well as the maps which accompany it, are the result of work by
EPA personnel, Geological Survey of Ireland Groundwater Section staff, and an independent
geological consultant. A peer review process was undertaken.
Working Group Members
Donal Daly, Environmental Protection Agency
Leo Sweeney, Environmental Protection Agency
Claire Byrne, Environmental Protection Agency
Matthew Craig, Environmental Protection Agency
Natalya Hunter-Williams, Groundwater Section, Geological Survey of Ireland
Margaret Keegan, Environmental Protection Agency
Monica Lee, Groundwater Section, Geological Survey of Ireland
Anthony Mannix, Environmental Protection Agency
Robert Meehan, Consultant Geologist
Peer Reviewers
Phil Jordan, Professor of Catchment Science, School of Environmental Sciences
University of Ulster
Tony Marsland, formerly Groundwater Policy Manager, Environment Agency, England and
Wales.
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1 BACKGROUND
1.1 Introduction
A domestic waste water treatment system (DWWTS) is the primary method used for the
treatment and disposal of sewage from houses in rural and suburban areas that are not
serviced by a public sewer system. Properly located, constructed, installed and operated
domestic waste water treatment systems generally provide adequate treatment of domestic
waste water and will ensure that the waste water is discharged with minimal risk to human
health or the environment.
The
Water Services (Amendment) Act, 2012 (S.I. No. 2 of 2012), requires home owners
connected to a DWWTS to register and ensure that the system does not constitute a risk to
human health or the environment through the compliance with standards for the
performance and operation of DWWTSs. Water Services Authorities (Local Authorities) are
required to maintain a register of DWWTS and undertake inspections to regulate the
discharges from these systems. The Environmental Protection Agency (EPA) is responsible
for the development of the National Inspection Plan (NIP); for the appointment of
inspectors; the establishment and maintenance of a register of inspectors and is the
supervisory authority over the WSA in the performance of their functions under the Act.
The new legislation will also assist Ireland in meeting the objectives of the
Water Framework
Directive (2000/60/EC). Ireland must maintain current ‘high’ and ‘good’ status water bodies
and it is of critical importance that this is achieved. All water bodies at less than good status
must show no deterioration and must be restored to good status by 2015 (there are
extended deadlines to 2021 and 2027 for some water bodies). A major programme, through
the implementation of River Basin Management Plans, is under way to achieve this
ambitious target. The NIP specifically addresses one of the measures in the River Basin
Management Plans, which deals with inspection and remediation of DWWTSs.
A review of enforcement procedures internationally revealed that environmental
considerations are key to developing enforcement strategies, in prioritising enforcement
activities and in allocating resources where they are needed most. The Environmental
Protection Agency (EPA) has developed this environmental risk assessment methodology to
allow it to prioritise the inspection of domestic waste water treatment systems through the
implementation of the NIP.
1.2 Report Scope
The aim of this report is to provide a scientific basis for enabling consideration of the risk to
human health and the environment when deciding on the inspection regime. The report
includes descriptions of the following:
The risk-based approach.
Characteristics (quality and quantity) of discharges from DWWTSs.
Characteristics of the surface and subsurface pathways for the discharge after it
leaves the wastewater treatment tank.
The process followed in ranking pathway susceptibility.
The basis for ranking areas based on the risk of impact to human health and the
environment.
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The maps produced to accompany this methodology are intended to give a guide as to
where issues are most likely to occur with respect to inadequate percolation or susceptibility
of groundwater to the occurrence of pathogens/molybdate reactive phosphorus or nitrate
across Ireland, with respect to existing DWWTSs. The maps have been produced at a
1:40,000 scale. Further enlargement would potentially be misleading as the spatial
resolution of the underlying data is insufficient to show detail beyond this scale. The
depicted boundaries and interpretations derived from the maps do not eliminate the need
for on-site sampling, testing, and detailed study of specific sites. The maps should be used
appropriately, and users are responsible for this process.
1.3 Domestic Waste Water Treatment Systems
Domestic waste water disposal accounts for approximately one-third of residences in
Ireland. There are an estimated 497,000 (CSO, 2012) domestic waste water treatment
systems (DWWTSs) in Ireland treating waste water from single houses not connected to a
public sewer system.
In most cases the DWWTS utilised is a conventional septic tank system. The conventional
septic tank system consists of two main parts:
The septic tank itself, and
The percolation area, which comprises the effluent distribution system and the
adsorption and treatment beneath it, which occurs in the biomat and in the soil and
subsoil layers and where the main treatment of the effluent takes place.
Secondary treatment systems (often called ‘Advanced’ systems) are also employed. These
systems offer secondary treatment of discharged effluent and include those constructed on-
site and packaged treatment systems. Secondary waste water treatment systems may
include a package treatment system and a polishing filter, which may comprise soil, sand or
peat.
Both conventional septic tank systems and secondary treatment systems must include the
following two elements in their make-up, namely the tank or mechanical treatment unit, and
the percolation or polishing filter set in the ground. Septic tank systems require greater
depths of subsoil and a larger area for distribution of discharged effluent than secondary
treatment systems.
All new DWWTSs, whether of the conventional septic tank type or of the secondary type
should meet the requirements of the appropriate European Standards (EN 12566 series of
standards). Such systems are designed to:
Treat the wastewater to minimise contamination of soils and water bodies;
Prevent direct discharge of untreated wastewater to the groundwater or surface
water;
Protect humans from contact with wastewater;
Keep animals, insects, and vermin from contact with waste water; and
Minimise the generation of foul odours.
It should be noted that there are many localities across Ireland where neither a conventional
septic tank nor a secondary treatment system discharging to ground will operate adequately,
due to unsuitable site conditions. For existing systems, in such cases, some additional
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remedial measures or non-conventional systems may be necessary to provide adequate
protection (e.g. importation of suitable soil for the construction of mound systems or
tertiary treated effluent discharging to surface waters with a licence).
1.4 Historical and current requirements for siting, design and installation of DWWTS
In 1975, the Institute for Industrial Research and Standards (IIRS) published the
‘
Recommendation for Septic Tank Drainage Systems suitable for single houses (SR6:1975)’,
which was the first document outlining best practice with respect to septic tank systems in
Ireland. The National Standard Authority of Ireland (NSAI) followed this in 1991 by
publishing ‘
Septic tank systems: recommendations for domestic effluent treatment and
disposal from a single house (SR6:1991)’. This document required that a site suitability
assessment be carried out before the installation of a DWWTS, and outlined how this
assessment should be carried out and how percolation areas should be constructed. Where
sites were deemed unsuitable such as due to poor percolation, remedial measures were
suggested which included mounding percolation areas using imported suitable material.
However, while remedial measures such as this are satisfactory in some circumstances, they
may not address sufficiently the hydraulic issue, where there is inadequate percolation.
The EPA published the guidance manual ‘
Wastewater Treatment Manuals; Treatment
Systems for Single Houses’ in 2000, which further defined the site assessment process and
gave detailed accounts of the types of secondary treatment systems available in Ireland at
that time. Acceptable limits to percolation values were set out in this document, and the site
assessment process from this time on was a more involved procedure.
Currently, where new houses are being constructed in un-sewered areas and waste water
from a single house needs to be treated on-site the ‘
EPA Code of Practice: Wastewater
Treatment and Disposal Systems Serving Single Houses (p.e.< 10)’ provides guidance from
the assessment stage through to the design, installation and maintenance stages of a
domestic wastewater treatment system, in such a way as to prevent water pollution and
protect public health. This document, published in late 2009, applies to new systems only
and does not account for the many older houses with existing DWWTSs that pre-date the
Code.
The importance of proper design, installation, operation and maintenance of DWWTSs
cannot be under-estimated. In particular, a well designed and constructed distribution
device and percolation area/polishing filter is critically important in terms of maintaining an
even flow of waste water and promoting biomat development that will adequately distribute
and allow treatment of the effluent. A clogged percolation area/polishing filter due to poor
construction and maintenance can result in backing up and surface ponding in some
locations otherwise suitable for subsurface percolation. However, a good percolation
area/polishing filter can occasionally assist where marginal subsurface percolation
conditions exist but it will never overcome the problem of fundamentally unsuitable
subsurface percolation conditions, hence the focus on soil/geological conditions in this
report.
Correct operation and maintenance of DWWTSs is essential to ensure on-going treatment of
waste water. A ‘duty of care’ is placed on homeowners by Section 70 of the
Water Services
Act, 2007 and the
Water Services Acts 2007 and 2012 (Domestic Waste Water Treatment
Systems) Regulations 2012 (S.I. 223 of 2012), which details the requirements for operation
and maintenance of systems including the requirements for de-sludging of DWWTSs. If
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systems are not de-sludged properly solids can be carried over and cause blockages in the
percolation area/polishing filter thus preventing adequate treatment and/or ponding of
effluent on-site.
It is expected that all new DWWTSs will be designed, installed, operated and maintained to
accepted standards, however, the risk assessment methodology does not make particular
assumptions about adherence of existing systems to these standards as its purpose is to
underpin a risk based inspection scheme relying only on underlying environmental
conditions.
1.5 Risk Based Enforcement
The EPA is committed to delivering effective risk-based regulation (Lynott and O’Leary,
2011), recognizing that the most effective and efficient way to protect human health and the
environment is to target resources towards activities which pose the greatest risk and those
areas at greatest risk of impact. The EPA considers that, while the overall risk from DWWTSs
to the environment at a national scale is lower than agricultural activities and urban
wastewater treatment systems, there are, however, areas of the country where the
potential risk from DWWTSs may be important at a local level due to the density of systems
and the prevailing ground conditions.
In line with European and International best practice, the EPA has developed this risk-based
methodology to provide for the prioritising of the enforcement of DWWTS management.
The methodology has built on previous research work carried out by the Western River Basin
District in the development of the River Basin Management Plans (WRBD, 2008), which
estimated that a significant proportion of existing septic tanks have the potential to impact
on groundwater and/or surface waters.
1.6 National Inspection Plan
The
Water Services (Amendment) Act 2012 requires the EPA to prepare a National Inspection
Plan for DWWTSs. It sets out the issues to be considered by the EPA to include relevant and
potential risks to human health and the environment when drawing up the plan and makes
provision for the revision of the plan. The Water Service Authorities (WSAs) will be required
to give effect to the plan. The development of this Risk Based Methodology provides a
decision making framework to prioritise and target sensitive areas, allocate and deploy
resources with a view to facilitating compliance with the
Water Services (Amendment) Act.
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2 A RISK-BASED APPROACH TO ASSESSING THE IMPACT OF EXISTING
DOMESTIC WASTE WATER TREATMENT SYSTEMS
2.1 Risk assessment
Risk assessment is a fundamental step in the protection of human health and the
environment, and in effective water management planning. Risk assessment allows
environmental problems to be identified, likely problem areas to be located, monitoring
programmes to be designed, and appropriate cost effective inspection, protection and
improvement measures to be formulated and implemented.
The assessment of the performance of DWWTSs, as outlined in this report, is receptor-
focussed and risk-based. The receptors of concern are human health, aquatic ecosystems
and groundwater resources.
2.2 Source-Pathway-Receptor Framework
The basis for risk assessment is the source-pathway-receptor (S-P-R) model, which underpins
all groundwater protection schemes in Ireland, as well as the EU Water Framework Directive
on which both surface water and groundwater regulations are based.
The S-P-R model is depicted schematically in Figure 1
below, whereby a
source is linked to
one or more receptors
via pathways. In the example, the source is represented by a
DWWTS, which disposes of discharged effluent through a percolation area situated in an
area where bedrock is at a shallow depth. The discharge infiltrates through subsoils into
groundwater in the bedrock, from where it migrates through fractures and fissures to a
down-gradient abstraction well and towards a river. There are in fact three potential
receptors in the diagram: the abstraction well, the river, and the bedrock aquifer (the latter
as a groundwater resource). Figure 2 represents a situation where a properly installed and
maintained DWWTS will not impact on the water receptors
via underground flow due to the
protection provided by the subsoils underlying the percolation area. However, if the
permeability of the subsoil is sufficiently low that adequate percolation cannot occur,
ponding of DWWTS discharge at the surface is likely with a consequent threat to human
health. In such a situation, effluent can also flow ‘downhill’ in the more permeable topsoil,
could enter wells down the outside of well casings where well protection may be
inadequate, and typically will enter nearby ditches or streams, maintaining a water impact
especially during low flows.
Every DWWTS carries a degree of risk of impacting on water quality and receptors. In many
cases, the risk may be low or manageable through well sited, designed and managed
systems. In other cases, the discharge activity can pose a significant threat to human health,
groundwater or surface water quality and related receptors.
It is relatively easy to develop a conceptual model of a surface water flow system around a
DWWTS from a topographic map and a site walkover survey. It is more difficult to obtain a
similar, site-specific image for groundwater flow because groundwater is not visible.
Changes in the geology in three dimensions will influence the volume and velocity of
groundwater flows, as well as groundwater levels, directions and chemistry. The S-P-R model
for environmental management presents a basis for a conceptual model of both the
groundwater and surface water flow systems.
5
Water table
Figure 1: S-P-R Model for domestic waste water treatment system with subsurface pathways (permeable subsoil)l
(graphic sourced from the WFD Visual website, SNIFFER, 2007)1
In order to fully assess the impact of a DWWTS at a site it is necessary to have a credible
conceptual model to determine the potential pathways underground, and to assess the risk
to down-gradient receptors. In particular, it is important to realise that a two dimensional
‘plan view’ is not adequate for assessing the risk posed by an existing DWWTS, as
groundwater flow systems consist of flows in three dimensions underground. Changing
conditions through time also need to be taken into account.
When examining S-P-R relationships, the main questions to be considered are:
Source characterisation – how significant is the potential discharge (input) from the
DWWTS, and what volume of wastewater is involved? What are the pollutants of
concern? What is the density of systems in the area? What is the nature and
condition of these systems?
Pathways analysis – how and where would the pollutants flow, and to what extent
would the pollutants be expected to attenuate? Is there a hydrogeological or
hydrological link that can deliver a pollutant source to a nearby receptor?
Receptor identification – who or what would potentially be affected? Receptors may
be of different types, and may be linked to a source
via different pathways. Are any
wells present nearby and down-gradient of the DWWTS? Are there any particularly
sensitive ecosystems nearby?
These three elements are dealt with in more detail in the following sections.
1 OSWTS is the acronym for ‘on-site wastewater treatment system’ and can be used interchangeably with
DWWTS.
6
Water table
Figure 2: S-P-R Model for domestic waste water treatment system with surface pathways (impermeable subsoil)l (graphic
sourced from the WFD Visual website, SNIFFER, 2007)
7
8
3 SOURCE CHARACTERISTICS
The pollution source is characterised by its location, size, quantity, and type. Key source
descriptors for domestic waste water include information on its composition, discharge rate
and resulting load to both surface water and groundwater.
3.1 Domestic waste water quality
The quality of domestic waste water is summarised in Table 1. The discharge from the tank
component of conventional septic tank systems and secondary treatment systems poses a
hazard to human health and the water environment, particularly if the waste water ponds
on the surface or enters groundwater without adequate treatment. It is therefore essential
that further treatment of this effluent occurs to facilitate safe disposal.
Table 1: Typical pollutant concentrations that arise from DWWTSs, per household (after
Ó’Súilleabháin, 2004 and Gill et al., 2005)
Pollutant
Conventional Septic
Secondary Treatment
Tank
Tank
Faecal Coliforms
> 1 million/100ml
> 5-10,000/100ml
Nitrogen (mg/l N)
30-80
20-35
Phosphorus (mg/l P)
5-20
1-5
BOD (mg/l)
150-500
20-50
Where DWWTSs are not properly located, designed, installed, operated and managed they
pose a risk to the homeowner’s health through possible contamination of their own or their
neighbour’s well or by resulting in effluent ponding in their gardens, thus restricting the play
activities of their children or pets. In addition to the risk posed to human health,
malfunctioning DWWTS also pose a risk to our watercourses and therefore may result in
impact on fishing, bathing waters and other amenities. It is essential that adequate
treatment of this effluent occurs before safe disposal by percolation.
Typically there are more than 1 million coliform bacteria (includes faecal coliforms) in 100
mls of effluent from a septic tank serving a normal household while the drinking water
standard is zero. Also, in general a domestic wastewater treatment system emits 0.5 kg
phosphorous/person/year and that is enough to pollute 14.5 million litres of water.
The main pollutants in wastewater dealt with in this report are microbial pathogens,
phosphorus and nitrogen.
3.1.1 Microbial pathogens
Microbial pathogens are bacteria, viruses and protozoa which can cause gastro-enteritis,
polio, hepatitis, meningitis and eye infections, among others. The occurrence of faecal
indicator organisms (FIO) such as
E. coli, enterococci,
streptococci and faecal coliforms, with
the same enteric origin as other microbial pathogens, indicate whether these pathogens may
9
be present in waste water. The drinking water standard for
E.coli and coliform bacteria is
zero.
3.1.2 Phosphorus
Phosphorus is the major limiting factor for plant growth in many freshwater ecosystems. The
addition of phosphorus encourages algal growth, depletes dissolved oxygen, causes algal
blooms in lakes and fish kills in rivers. Phosphorus is the main cause of eutrophication in
rivers and lakes in Ireland.
For the purposes of this report, molybdate reactive phosphorus, or MRP, which is often
taken as a measure of the soluble reactive inorganic phosphorus in water, is taken as the
primary phosphorus pollutant arising from DWWTSs.
3.1.3 Nitrogen
The percolation process converts nitrogen and ammonia from organic matter almost
entirely into nitrite and then to nitrate. Nitrate, unlike ammonium, is mobile in the ground
and therefore is a good indicator of contamination. Reduction of nitrate concentrations in
groundwater occurs primarily through dilution, both by recharge from rainfall and, where
background nitrate concentrations are low, by groundwater. In certain hydrogeological
settings in Ireland, de-nitrification can occur (see Appendix 2).
In this report, nitrate is taken as the main nitrogen pollutant, although in some
circumstances ammonium from DWWTSs also causes water pollution.
3.2 Volumes of waste water generated by DWWTSs
DWWTSs accept waste water from toilets, showers, sinks, wash hand basins, washing
machines and dishwashers. The greater the population of the dwelling, the greater the
volume of waste water produced. In order to calculate the waste water capacity for any
DWWTS, it is assumed that a typical daily hydraulic loading for each person is 150 litres, as
stated in the CoP. (2009).
3.3 Pollutant Load
The pollutant load is derived from multiplying the hydraulic loading from the number of
people by the average pollutant concentrations. Further details are given in Section 6.2.
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4 SURFACE AND SUBSURFACE PATHWAYS
The pathway is the link between the source of pollution and the receptor, and can either be
at or close to the surface or underground, or a combination of both. Natural vertical and
horizontal pathways for effluent migration are determined by the on-site subsurface
geology, particularly the nature of the soils, subsoils and underlying aquifers. Artificial
pathways may include drainage ditches, land drainage pipes and stream culverts.
4.1 Understanding and using the ‘Pathway’ concept
Understanding and taking account of the pathways through which pollutants from a site
move towards a receptor creates a 3-D conceptual understanding of water presence and
movement at a site and is critical to:
Assessing the link between domestic waste water treatment systems (DWWTS)
and impacts;
Locating critical source areas that contribute contaminant load;
Predicting the likelihood of an impact;
Describing ‘why’ there could be or has been an impact;
Locating any monitoring that may be required;
Enabling monitoring data to be understood and assessed;
Enabling ‘responses’ to the risk or appropriate ‘measures’ to be derived and
implemented.
There is often a danger that the critical role of the characteristics of the ‘environmental
pathway’ may be forgotten about, as emphases may tend to be put on 1) the system itself,
its type and workings, and/or 2) monitoring/impact data. Encouraging greater consideration
of the ‘pathway’ elements can prevent important factors from being missed, such as:
the possible role of the subsurface pathway in both attenuating pollutants and in
transmitting pollutants to receptors, for instance, rivers and wells; and/or
the role of hydrogeological information/maps in helping understand ponding,
runoff, percolation rates and in predicting impacts.
4.2 Characteristics of surface and subsurface pathways
The effluent that leaves a DWWTS may receive a degree of attenuation in the environment
depending on the soil/subsoil/bedrock properties along the pathway to the water
environment.
Pathway characteristics are determined from hydrological and hydrogeological information
accessed from various data sources, as well as site walkover surveys and site investigations.
Key pathway descriptors include subsoil type and permeability; wet or dry soil type; and
aquifer type and hydraulic properties. While slope can be a factor in certain circumstances,
it has not been included in the methodology outlined in this report.
11
4.2.1 Data Availability
Table 2 outlines the main regional-scale pathway components – all of which have been
mapped and are available in national GIS data layers – and summarises their characteristics
and relevance.
Table 2: Examples of relevant characteristics of pathways interacting with wastewater
following initial treatment, and their implications (adapted from Table 2 of WGGW, 2005a)
Component
Factor
Relevant ‘intrinsic’ Implication
Water Receptor at
Characteristic
Risk
Soil
‘Wet’ (gley) soils
Low permeability
Rapid runoff
Surface water
‘Dry’ (brown
Moderate/high
Leaching of
Groundwater and
earth,
etc.)
permeability
pollutants
e.g. NO3 surface water
and P
‘organic’
Low permeability
High % of runoff
Surface water
Subsoil
2
SAND and
High permeability
Leaching of
Groundwater and
GRAVEL
pollutants
e.g. NO3 surface water
CLAY
Low permeability
Rapid runoff
Surface water
Depth to bedrock
Bedrock at or near
a) No protection of Groundwater and
(<0.6m) of the
groundwater
surface water
surface
b) rapid runoff if
low permeability
Groundwater
‘Extreme’ and
Rapid transit time
High leaching
Groundwater and
Vulnerability
‘High’
potential
surface water (
via
groundwater)
Slow vertical
‘Low’
transit time and
Minimal leaching
Surface water
groundwater
potential and
recharge
often rapid runoff
High attenuation
Aquifer flow
Poor aquifers
Short underground
High surface
Surface water
regime
flowpaths
drainage density
Denitrification
possible
Regionally
Long underground
Low surface
Groundwater and
important
flowpaths
drainage density
surface water (
via
aquifers
groundwater)
Karst aquifers
Point recharge
Pollutants can
Groundwater and
reach receptor
surface water (
via
quickly
groundwater)
Sand/gravel
Rapid infiltration
Mobility of NO3
Groundwater and
aquifers
Attenuation
Phosphate
surface water (
via
groundwater)
Karstification
Point recharge
Presence of
No retardation of
Groundwater and
swallow holes and
contaminants
surface water (
via
bare rock
groundwater)
Topography
Slope
Gradient
Rate of runoff
Surface water
2 See Table 3 and British Standards: BS5930 (1999) methodology.
12
The EPA Code of Practice (EPA, 2009) requires that site suitability assessments are
undertaken prior to applying for permission for a DWWTS. This consists of a desk study,
visual assessment, trial hole assessment (giving subsoil type, depth to bedrock and depth to
water table) and percolation tests (giving ‘T’ values3).
The GSI have undertaken a national programme of mapping groundwater vulnerability. As
part of this programme, three categories of subsoil permeability are mapped, as noted in
Table 3.
Table 3: BS5930 descriptions and permeability rates of subsoil permeability categories.
Subsoil Permeability
BS5930 Descriptions
Permeability (m/s)
Category
High
GRAVEL; sandy GRAVEL; SAND
>10-4
Moderate
SAND; clayey SAND; SILT; sandy SILT;
some SILT/CLAY; some sandy
10-4- 10-8
SILT/CLAY*
Low
SILT/CLAY; sandy SILT/CLAY; sandy
<~10-8
CLAY; CLAY*
4.2.2 Attenuation
Physical removal of some faecal indicator organisms (FIOs) and associated microbial
pathogens occurs through filtration. Microbial pathogens are also removed by
sedimentation, where they settle out on soil and subsoil particles; by predation, where
they are consumed or broken down by other micro-organisms in the soil and subsoil; and
by die-off, where they reach the end of their life-cycle naturally.
Phosphorus is removed in the soil and subsoil by precipitation to mineral phosphorus and
adsorption to soil particles. Phosphorus will also be removed by plant uptake but this
only happens when the discharged effluent is close to the ground surface or has ponded
at the surface.
Organic nitrogen removal in the soil and subsoil occurs through a number of processes.
Initially, the organic nitrogen is mineralised to ammonium nitrogen. Nitrification occurs
as ammonium is further changed by microorganisms to the nitrate form, and is then
available to leach to groundwater. The biological reduction of nitrate to nitrous oxide or
nitrogen gas that escapes into the atmosphere, also known as denitrification, may occur
in the soil, subsoil and bedrock.
Nitrate will also be removed by biological oxidation and plant uptake, again only where
the discharged effluent is close to the surface. When nitrate reaches the groundwater, it
moves freely. Reduction of nitrate concentrations in groundwater may occur, however,
primarily through dispersion and dilution, although this depends on the background
nitrate concentrations.
3 The percolation rates in minutes expressed as the time for water to fall 25mm in a 300mm x 300mm square
hole (T= time mins/25mm)– see EPA (2009) for further details.
13
4.3 Pathway Susceptibility
‘Pathway susceptibility’ is a measure of the degree of attenuation between source and
receptor. It is a measure of the ability of the pathway factors to reduce the impact of a
pressure, in terms of: time to reach the receptor; proportion of pollutant load reaching the
receptor; pollutant concentration level in the receptor; and duration of the pollution event.
The pathway susceptibility concept has been used previously in the Irish context in the
derivation of risk matrices for groundwater in Ireland by the Working Group on Groundwater
(2005a, b).
In this report, the ‘pathway susceptibility’ concept has been applied specifically to discharges
from DWWTSs. Keeping in mind the purposes of this document, only three pollutants arising
from DWWTSs were considered – microbial pathogens, MRP and nitrate.
Pathway susceptibility is based on combinations of the following maps that capture the
relevant hydrogeological properties of an area:
Soil type;
Subsoil permeability;
Groundwater vulnerability4; and
Aquifer category5.
Four categories of susceptibility are used: ‘very high’, ‘high’, ‘moderate’ and ‘low’. Generally,
the categories of most concern are ‘very high’ and ‘high’.
4.4 Factors influencing surface water susceptibility to contamination
Surface water receptors are at risk where there is inadequate percolation in the ground for
the waste water arising from DWWTSs. This can arise from both poorly constructed
drainage fields and (more likely) from low permeability subsoil and bedrock.
In areas where there is inadequate percolation, the combination of rainwater and DWWTS
waste water can result in saturation of the soil and subsoil and, at times, in ponding and
breakout of untreated or partially treated effluent at surface, backup of sewage in pipes,
odour issues, and the potential for insects and vermin. In short, the effluent cannot drain
away and this situation poses a general risk to human health and surface water, both on the
site itself and in drains and streams around the site.
Where wet (gleyed) soils occur and where inadequate percolation exists in subsoil layers, the
vast majority of pathway attenuation processes are limited, as effluent cannot enter the
subsurface environment. Some attenuation of contaminants does occur where there is
ponding of discharge, but only
via small amounts of nitrate and phosphorus being taken up
by plant roots. Also, predation and die-off of pathogens can occur. In many circumstances,
effluent enters directly into ditches and streams with no attenuation other than dilution in
4 Groundwater vulnerability is the term used to represent the intrinsic geological and hydrogeological
characteristics that determine the ease with which groundwater may be contaminated by human activities.
Maps at 1:50,000 scale have been produced based on mapping i) subsoil permeability, ii) subsoil thickness and
iii) karst features. Further information can be obtained in the Groundwater Protection Schemes Report
(DELG/EPA/GSI (1999) at the following lin
k: www.gsi.ie .
5 For more information on aquifer categories, see the Groundwater Protection Schemes document of
DELG/GSI/EPA (1999).
14
the surface water receptor. In other circumstances, ponded effluent may become flushed
during subsequent storm flows and be mixed with other diffuse signals of pollution runoff.
The main factors influencing inadequate percolation in the subsurface, and therefore the
susceptibility of surface water to contaminants, are:
The subsoil type on the site: if an area has subsoil with a high proportion of CLAY,
this material does not have sufficiently large pore spaces within to allow water to
flow through it. CLAY is, by its very nature, a low permeability material. Examples of
areas where CLAY subsoil dominates includes the north central and northeastern
portions of the country.
The type of bedrock under the site: if soil and subsoil depth is relatively shallow and
the bedrock is of low permeability, the rock has few significant fractures within and
therefore water ‘backs-up’ in the subsurface over time, resulting in a waterlogged
landscape with a dense network of streams. Examples include the uplands of the
west and northwest of the country.
The landscape setting: if a portion of land is in a low-lying area within the landscape,
where the water table is close to the surface for all or part of the year, there may not
be enough depth of ‘dry’ soil and subsoil to allow percolation to occur. Examples
throughout Ireland occur along the floodplains of rivers, areas reclaimed from bogs,
and other flat, low-lying portions of the landscape.
4.5 Factors influencing groundwater susceptibility to contamination
The factors influencing the susceptibility of groundwater to contamination by pollutants
arising from DWWTSs are:
the thickness and permeability of the subsoil;
the type of soil (whether wet or dry);
the type of aquifer (whether bedrock or sand and gravel) and
whether the bedrock enables denitrification or not.
Where only a shallow cover of soil/subsoil over bedrock exists on a site with an existing
DWWTS, elevated levels of nitrate, MRP and FIOs/pathogens in the underlying groundwater
may result. Such cases occur where bedrock is within 1-2 m of the surface and preferential
flowpaths in soil and subsoil take the contaminants rapidly towards groundwater below. In
these areas, the attenuation processes of filtration, sedimentation, cation exchange,
adsorption, precipitation and biological oxidation, which remove contaminants where soil
and subsoil treat wastewater effectively, are limited, as there is an insufficient depth of soil
and subsoil on sites to allow them to occur effectively.
In cases such as these, discharge to ground may be acceptable, but only through site
remediation works such as the importation of suitable soil/subsoil to enable construction of
an adequate percolation area or polishing filter. Historically, this was not often completed
as the percolation on-site was adequate and the processes which treat the wastewater in
the subsoil before it reaches groundwater were not recognized or not taken into account.
In some situations around Ireland where DWWTSs have been installed without an adequate
depth of suitable subsoil to remove pathogens and molybdate reactive phosphorous by the
processes outlined above there is little protection of groundwater. In circumstances such as
15
this, a high density of DWWTSs may also pose a threat to surface water receptors as
groundwater provides a high proportion of surface water flow in dry weather. In addition,
pollution of nearby wells by FIOs and associated microbial pathogens can occur.
Some rock types have high levels of pyrite and other minerals which can lead to de-nitrifying
conditions within the bedrock itself and thus natural removal of nitrate (see Appendix 2).
Therefore, even where there is a high density of DWWTSs, nitrate concentrations are not
likely to be increased significantly in these areas.
In most areas of Ireland, however, the bedrock does not have a natural capacity to reduce
nitrate concentration in groundwater. Consequently, in areas where there is adequate
percolation, there may be small, localized plumes of relatively high nitrate, particularly
where the density of DWWTSs is high. This scenario is especially of concern where nearby
private wells are sources of drinking water, and where there are sensitive groundwater
terrestrial ecosystems nearby.
16
5 CLASSIFYING PATHWAY SUSCEPTIBILITY
5.1 Introduction
As all effluent from DWWTSs poses a threat to human health and the environment, whether
the DWWTSs are properly constructed or not, the key factor in assessing the degree of
potential impact is ‘pathway susceptibility’. This Section describes the considerations that
are taken into account when combining the soils, subsoil permeability, groundwater
vulnerability and aquifer national data layers in order to derive the ‘pathway susceptibility’
category (i.e. ‘very high’, ‘high’, ‘moderate’ and ‘low’) for effluent from DWWTSs.
The way in which these data layers are combined is illustrated in the ‘susceptibility matrices’
(Appendix 1). By combining these national data layers in a GIS, the resulting pathway
susceptibility category can be displayed as simple, colour coded risk assessment maps.
The susceptibility matrices were developed in the context of two over-arching
environmental scenarios:
Inadequate percolation, which may result in surface ponding of effluent, bypass
directly to surface water and the associated threats to human health and surface
water quality, and
Insufficient attenuation (subsurface treatment of the effluent), which may result in
directly polluting groundwater/drinking water supplies (wells and springs), and/or
indirectly impacting on surface water.
This report only considers pathway susceptibility with respect to microbial pathogens, MRP
and nitrate.
5.2 Inadequate percolation
The presence of inadequate subsurface percolation at any point in the landscape is
determined by the soil, subsoil and bedrock permeability (as indicated by aquifer category).
The different scenarios that are likely to result in inadequate percolation are outlined in
Table A1, Appendix 1. The main considerations are summarised below:
In areas of ‘extreme’ groundwater vulnerability, (
i.e. soil/subsoil <3 m thick), the
subsoil permeability can be very heterogeneous and therefore is not classified by the
Geological Survey of Ireland. In these areas, the likelihood of inadequate percolation
is determined by evaluating the aquifer type (an indication of permeability) and the
drainage class of the topsoil, as shown in Table A1, Appendix 1.
Where soil/suboil is greater than 3m thick, subsoil permeability (‘high’, ‘moderate’ or
‘low’) (see Table 3 for details) and soil types (‘wet’ or ‘dry’) determine the likelihood
of inadequate percolation.
Where the percolation ‘T’ test results on a site are found to be greater than 90, the
site is deemed to be unsuitable for discharge of treated effluent to ground owing to
inadequate percolation. The value of 90 means that it takes greater than 5 hours for
water to drop 100 mm (or 4 inches) in a percolation test hole. In this situation, the
principal subsoil types recorded in trial holes are usually CLAY or SILT/CLAY. T>90
corresponds to a ‘low’ subsoil permeability.
17
The likelihood of inadequate percolation arising at a site is subdivided into four categories –
low, moderate, high and very high (Table A1, Appendix 1). Based on an evaluation of the
hydrogeological settings outlined in Table A1 and practical site assessment experience, the
probability of finding inadequate percolation or inadequate depth to water table within
these categories is given in Table 4.
Table 4: The probability of finding inadequate percolation for each susceptibility category
Susceptibility Category
Probability of finding inadequate
percolation within a category
Low
<5% of sites.
Moderate
Approximately 25% of sites
High
Approximately 50% of sites
Very High
>80% of sites
In all cases, these figures represent the average within a range. Also, the mapping scale
(approximately 1:40,000) will not have enabled local variations to be captured.
Groundwater discharge zones, low lying areas and areas with a low slope gradient may have
groundwater levels close to surface in winter and may have water table constraint issues.
Many of the wet soils, and thus areas with inadequate percolation, in high or moderate
permeability subsoil areas occur in such localities and will therefore indicate such zones,
however, shallow groundwater table is not mapped and therefore not directly included in
this risk assessment.
By combining the available data layers as outlined in Table A1, Appendix 1, a national map of
the likelihood of inadequate percolation has been derived. This is shown in Figure 3 for
illustration purposes. The proportion of the country in each category is given in Table 5. The
overall proportion of the country with inadequate percolation is estimated to be 39% – this
proportion is derived by applying the probabilities given in Table 4. A corresponding
summary for County Meath is also presented in Table 6 below as an example at a county
scale.
Table 5: National summary of areas within each susceptibility category and the overall
likelihood of finding inadequate percolation
Susceptibility Category
Percentage (%)
Overall national likelihood of
Land Area
Inadequate Percolation (%)
Low
25.8
Moderate
25.7
High
22.0
39
Very High
25.2
Made Ground
1.3
18
Table 6: Summary of areas within each susceptibility category and the overall likelihood of
finding inadequate percolation for County Meath
Susceptibility
Percentage Area (%)
Overall likelihood of
Category
Inadequate Percolation
for Co. Meath (%)
Low
43.7
Moderate
12.4
High
11.2
36
Very High
31.4
Made Ground
1.3
5.3 Inadequate attenuation
5.3.1 Pathogen and molybdate reactive phosphorus contamination
Within the scope of the data layers used for this assessment, the likelihood of pathogens or
molybdate reactive phosphorus (MRP) reaching a groundwater or surface water receptor is
determined by the same factors: type of aquifer (bedrock or sand and gravel); depth of soil/
subsoil (as derived from vulnerability maps). Therefore the pathway susceptibility is the
same for both pollutants (Table A2, Appendix 1).
While there are four general susceptibility categories, only three relative categories apply to
pathogen and MRP susceptibility. No locations were found to have a ‘moderate’
susceptibility, given the mobility of pathogens and MRP, and the type of pathways that exist.
Consequently, susceptibility is either ‘very high’ or ‘high’ where groundwater vulnerability is
classed as ‘extreme’, and ‘low’ in all other cases, as the subsoil cover overlying the bedrock
receptor is considered to provide sufficient protection. The map illustrating the susceptibility
of groundwater to percolation of pathogens and MRP is shown in Figure 4. The proportion of
the country in each category is given in Table 7. A corresponding summary for County Meath
is also presented in Table 8 below as an example at a county scale.
5.3.2 Nitrate contamination
The likelihood of nitrate percolation to groundwater (Table A3, Appendix 1) is determined by
the bedrock type (whether the rock will de-nitrify groundwater or not), the subsoil
permeability (allowing nitrate leaching or not), the soil type (wet or dry) and the
groundwater vulnerability (extreme or other). ‘De-nitrifying bedrock’ includes all bedrock
units which are rich in pyrite, other metal sulphides and organic carbon and will hence
reduce nitrate levels through microbially-assisted oxidation of the electron donor minerals.
The bedrock units listed in Appendix 2 are considered to have the potential for
denitrification.
Where wet soil occurs, it is assumed there will be reducing conditions in the underlying
soil/subsoil, and hence groundwater is relatively well protected from nitrate percolation.
Three of the four susceptibility categories are considered as sufficient to apply to nitrate
susceptibility: no areas are classed as ‘high’ susceptibility due to the mobile nature of
19
nitrate. Susceptibility was considered to be ‘very high’ where dry soil and infiltration occurs
readily, ‘moderate’ where de-nitrifying bedrock and high permeability subsoils are in
evidence, or ‘low’ where wet soils and all other situations overlying de-nitrifying bedrock are
found. The map illustrating the susceptibility of groundwater to percolation of nitrate is
given in Figure 5. The proportion of the country in each category is given in Table 7. A
corresponding summary for County Meath is also presented in Table 8 below as an example
at a county scale.
Table 7: National summary of areas within each category of susceptibility of groundwater
to percolation of pathogens and MRP and to the percolation of nitrate.*
Percentage Area for each Susceptibility Category (%)
Percolation of
Percolation of Nitrate
Susceptibility Category
Pathogens/MRP to GW (%)
to GW (%)
Low
61.0
67.8
Moderate
n/a
0.7
High
23.1
n/a
Very High
14.6
30.3
Made Ground
1.3
1.3
*Percentages may not add to 100% due to rounding
Table 8: Summary of areas within each category of susceptibility of groundwater to
percolation of pathogens and MRP and to the percolation of nitrate for County Meath.*
Percentage Area for each Susceptibility Category (%)
Percolation of
Percolation of Nitrate
Susceptibility Category
Pathogens/MRP to GW (%)
to GW (%)
Low
87.1
72.6
Moderate
n/a
5.1
High
6.5
n/a
Very High
5.2
21.1
Made Ground
1.3
1.3
*Percentages may not add to 100% due to rounding
5.4 Use of pathway susceptibility ranking maps
The susceptibility maps are designed for general information and strategic planning usage;
modelled evidence and local details have been generalized to fit the map scale, which is
approximately 1:40,000. As these geological and hydrogeological settings are complex in
some areas, exceptions can be expected.
The matrices and maps derived apply to the discharge of treated wastewater to ground from
DWWTSs only. They do not reflect risks associated with any other potential environmental
issues and thus should not be used for assessments other than that intended.
20
5.5 Map confidence and forthcoming county maps
The subsoil permeability map is one of the most critical datasets used for the generation of
the map showing the likelihood of inadequate percolation, as well as the risk of high nitrates,
and pathogens/MRP occurring in groundwater. This map was obtained by the EPA from the
Groundwater Section of the Geological Survey of Ireland (GSI).
Three counties have preliminary work completed on the subsoil permeability across their
extents, and therefore have a slightly lower confidence level than all other counties depicted
on the maps. These are Wicklow, Laois and Kilkenny, which will have updated and revised
subsoil permeability maps produced by the GSI in early 2013.
21
Figure 3: Map illustrating the distribution of susceptibility categories for inadequate
percolation. Data captured at 1:40,000 scale. [This map summarises the relevant
hydro(geo)logical parameters that characterise the surface ‘pathway’ for water in the source-pathway-
receptor framework.]
22
Figure 4: Map illustrating the susceptibility of groundwater to percolation of
pathogens and MRP from DWWTSS. Data captured at 1:40,000 scale. [This map
summarises the relevant hydro(geo)logical parameters that characterise the subsurface ‘pathway’ for
pathogens and MRP in the source-pathway-receptor framework. It does not provide any indication of
likely impacts]
23
Figure 5: Map illustrating the susceptibility of groundwater to percolation of nitrate
from DWWTSs. Data captured at 1:40,000 scale. [This map summarises the relevant
hydro(geo)logical parameters that characterise the subsurface ‘pathway’ for nitrate in the source-
pathway-receptor framework. It does not provide any indication of likely impacts]
24
6 RISK CHARACTERISATION
6.1 Introduction
The purpose of the approach outlined in this section is to rank the risk to human health and
surface water and groundwater quality from domestic waste water treatment systems
(DWWTSs), as a means of apportioning the inspections relative to the risk presented. The
general concept is represented graphically in Figure 6.
Load to Surface
Cumulative Load to
RISK via
Surface Water
water from
Surface Water from
Load from individual DWWTS
Surface
Susceptibility
individual
multiple DWWTS
Pathway
DWWTS
(based on density)
Groundwater Susceptibility
Susceptibility
Load to Groundwater from
-
Very High
High
Moderate
Low
individual DWWTS
Very High
yt
Cumulative Load to
is
Risk to Receptor via Surface
OR
Groundwater from multiple
n
High
e
DWWTS (based on density)
D
Underground Pathway
Moderate
for individual pollutant
RISK via Subsurface
Low
Pathway
Figure 6: General concept for determining risk from DWWTSs contamination via the
surface or subsurface pathways
The risk characterisation is based on the combination of the following elements:
Pollutant load from each DWWTS, derived from typical discharge concentrations and
quantities.
Pathway susceptibility, which includes consideration of attenuation by physical
process, such as dilution, and biological and chemical processes. Two pathways are
considered: surface (i.e. surface water) and subsurface (i.e. groundwater).
Cumulative load entering the surface water or groundwater environment derived
from DWWTS density and estimation of attenuation.
Dilution of load at the water receptor.
Risk ranking using estimates of predicted pollutant concentrations at the receptor.
A more detailed graphical summary of the method proposed to estimate the risk is provided
in Figure 7. Worked examples of the calculations are presented in Appendix 3 and Appendix
4; the examples are simplified to some degree to demonstrate the methodology used (each
1km2 grid is assumed to have uniform pathway susceptibility – this is not the approach taken
in GIS processing where the pathway susceptibility at each individual DWWTS was used).
While the approach used predictions of pollutant concentrations as the basis for the risk
ranking, the maps are not intended to be used for predicting precise impacts; they are
intended to show relative risk on which an inspection regime can be based.
25
Figure 7: Outline of methodology for risk ranking
6.2 Pollutant Load
The pollutant load is derived by combining typical effluent quantity and quality from each
DWWTS.
The average inputs of pathogens, MRP and nitrate from an individual septic tank, prior to
treatment in the subsoil or polishing filter, are given in Table 10. These data can enable an
estimate of the pollutant load produced in an area to be calculated by multiplication of the
values by the number of systems present there.
6.3 Pathway Attenuation
The pollutant load to water may be reduced by amounts that depend on the various factors
that have been described in Section 5.3 on pathway susceptibility to attenuation. In general
terms, the higher the category of pathway susceptibility, the lower the degree of
attenuation and the greater the likelihood that contaminants will enter water.
It should be noted that the risk ranking for MRP also reflects the risk ranking for microbial
pathogens, and therefore the risk to human health.
26
Table 10: Data sources for the calculation of overall load
Input Parameter
Input Value
Data Source
Pathogen Load (
E. Coli)
5,000 -1 million per 100 ml
Gill
et al. TCD Research
Phosphate Load in kg per Person/year 0.56
Gill
et al. TCD Research
(in liquid discharge leaving Septic Tank)
Nitrate Load in kg per Person/year (in 2.77
Gill
et al. TCD Research
liquid discharge leaving Septic Tank)
Persons Per House
2.8
CSO data
GIS layer created for DWWTS
Density of Systems
Variable
locations
based
on
use
of
Geodirectory and unsewered areas
6.3.1 Surface pathway
Where discharges cannot infiltrate underground in areas of inadequate percolation, many
systems are either piped directly to ditches and/or streams, or ponding and overflow into
ditches and/or streams occurs. The inadequate percolation map is used as the basis for
calculating the pollutant quantity that may either pond or is piped to surface water. Using
Table 4, 80% of the initial pollutant load is estimated to be present at the surface or piped
away directly to drains and/or streams where the likelihood of inadequate percolation is
‘very high’, whereas this estimate is only 5% where the likelihood of inadequate percolation
is ‘low’. All effluent not directed to surface water is assumed to move via the subsurface
pathway noted below and attenuated accordingly.
Where effluent ponds or is piped to ditches/streams, it is likely that there will be some
removal of MRP and nitrogen, for instance uptake by plants growing in the ponded areas,
attenuation as a proportion of the effluent moves through the topsoil or some percolation in
dry weather. An arbitrary pollutant reduction factor of 25% is taken to account for a best
estimate attenuation in the immediate vicinity of the percolation area. Evidence from future
research will be used to refine the risk methodology.
Table 11 summarises the factors used in estimating the pollutant load entering surface water
via the surface pathway from each DWWTS in any area.
6.3.2 Subsurface pathway
The factors applied to enable an estimation of attenuation in the geological materials as
discharged effluent percolates underground are given in Table 12. Therefore, it is assumed,
for instance, that where the susceptibility of groundwater to percolation of microbial
pathogens and MRP is ‘low’, no pathogens or MRP will reach groundwater. Where the
susceptibility is high (see Table A2 for physical setting), it is assumed that the effluent will
percolate through at least 1 m of subsoil, with a consequent significant reduction of MRP
6 Compares with SRP loads cited by Withers et al. (2011) of 0.38 kg/yr in and 0.44 kg/yr in Britain and Northern
Ireland, respectively.
7 Compares with total dissolved nitrogen loads cited by Withers et al. (2011) of 2-4 kg/yr in the USA, UK and
Netherlands.
27
concentration (this is based on research by Gill
et al (2009)). Where the susceptibility is very
high, little attenuation of MRP is considered to occur.
With regard to nitrate, significant attenuation in the biomat is assumed to occur; this
reduces the loading proportions given in Table 12.
Table 11: Factors applied to estimate contaminant load from individual DWWTS reaching
surface water by the surface pathway
Surface Water Pathway
Input Value
Data
Comment
Source
% of Load leaving
Septic Tank that will
reach receptor8
LOW Susceptibility
5
These figures relate to the
likelihood of finding
MODERATE Susceptibility
25
inadequate depth to water
Risk
HIGH Susceptibility
50
table or inadequate
Matrices
percolation as noted in
Table 4
VERY HIGH Susceptibility
80
Second factor applied for reduction in overland flow
This is a best estimate –
75
some DWWTSs will be
piped directly to streams
Overland Flow (Pathogens/MRP and
with 100% of load reaching
Estimate
Nitrate)
(% of Load that will
surface water; in other
NOT be removed in
scenarios attenuation may
overland flow)
occur during ponding, re-
infiltration etc.
6.4 Estimating Cumulative Load and Resultant Concentration
For each 1 km grid square, the load of pollutants calculated to reach the surface water or
groundwater receptor via the surface or underground pathways from individual DWWTSs
were summed to estimate the cumulative load. Dilution of the cumulative load in the
environment was then applied to derive an estimated concentration. The method is
described below.
6.4.1 Cumulative Load
The density of DWWTSs used to calculate cumulative load was derived by combining
information derived from the An Post’s Geodirectory and the sewered areas as recorded by
WRBD (2008). In the unsewered areas, it is assumed that all houses use DWWTSs. The
accuracy of the un-sewered areas used in the methodology outlined in this report will need
to be verified at a local level by the Water Services Authorities when implementing the
National Inspection Plan.
8 Percentages will be a range, with the values averaged here.
28
A map illustrating the density of systems in unsewered areas in County Meath is shown in
Figure 8 on page 31.
Table 12: Factors applied to estimate contaminant load from individual DWWTS reaching
groundwater
Groundwater Pathway
Input Value
Data Source
Comment
% of Load leaving
Septic Tank that will
reach receptor
LOW Susceptibility for
0
MRP/Pathogens
Guidance on the
No MRP or pathogens
Authorisation of
HIGH Susceptibility for
to groundwater at
10
Discharges to
MRP/Pathogens
LOW Susceptibility
Groundwater 9
VERY HIGH Susceptibility for
90
MRP/Pathogens
Note - these
figures apply
For Nitrate it is
only to
assumed that at least
LOW Susceptibility for Nitrate
10
conventional
70% reduction in the
MODERATE Susceptibility for
15
septic tanks.
biomat giving
Nitrate
maximum input value
of 30%
VERY HIGH Susceptibility for Nitrate
30
6.4.2 Dilution
Once contaminants reach a water receptor, further attenuation will occur due to dilution.
The main factor determining the degree of dilution of the contaminant load in groundwater
and surface water is the estimated effective rainfall at each locality. The approach taken to
estimate the degree of dilution is outlined in Table 13.
With regard to groundwater, the quantity of water in an area providing dilution is derived
from the GSI recharge map. The average annual recharge for each locality is regarded as
adequate for the purposes of this risk ranking procedure.
DWWTSs are likely to have a greater impact on surface water during low flow conditions
than at other flow conditions (Macintosh
et al, 2011). This is due to the lower dilution
capacity (and thus higher resultant concentration) at these times. A low flow reduction
factor can be applied to the average flow volume in an attempt to account for low flow
conditions. Q90/Q50 is a ratio which is often used as an index of baseflow contribution. In
this case an estimation of Q90/Q50 of 0.22 was applied nationally to average flow to give an
estimation of low flow conditions. The value of 0.22 10 was taken as an approximate mid-
range value of the Q90/Q50 that might be expected in Irish conditions.
9 This document can be downloaded from the EPA Webs
ite, http://www.epa.ie/whatwedo/advice/wat/
guidegw/dischgw/
10 Median of Q90/Q50 from EPA’s hydrotool for estimation of flow for ungauged catchments
29
Table 13: Model values used to calculate the volume of water to dilute the nutrient load
Receptor
Field
Input Value
Data Source
Comment
Effective
Rainfall
Effective Rainfall
Variable
(mm/yr)
GIS Layer
Median of Q90/Q50 from
Surface water
EPA’s tool for estimation of
Low Flow factor
Variable
Hydrotool Output
flow for ungauged
catchments - 0.22 was used
in these calculations
Bedrock recharge
Recharge GIS
Groundwater Recharge (mm/yr)
Variable
acceptance capacity limit
Layer11
was not used
6.5 Risk Ranking
The final step in deriving relative risk maps involves calculating the resulting concentrations
of MRP and nitrate entering water from DWWTSs for each 1 km2 area.
It should be noted that the MRP map also reflects the likely presence of microbial pathogens
in both surface water and groundwater, and therefore the risk to human health.
In the case of MRP, where the predicted resulting concentration in either surface water
(during arbitrary baseflow conditions) or groundwater is greater than 0.035 mg/l P – the
environmental quality standard (EQS) that forms the boundary between good and moderate
status river water bodies (DEHLG, 2009) – a ranking of ‘very high’ is given. The ‘high’
category for MRP is based on the EQS that forms the boundary between high and good
status river water bodies (0.025 mg/l P) (see Table 14).
In the case of nitrate, the categories are based on boundaries used by the European
Environment Agency for cross European comparison (EPA, 2010) (see Table 14).
Table 14: Molybdate Reactive Phosphorus and nitrate concentrations used in deriving
surface pathway and subsurface pathway risk ranking
Risk Ranking
Low
Medium
High
Very High
Likely
MRP
Impact <0.015
0.015-0.025
0.025-0.035
>0.035
(concentration mg/l P)
Likely Nitrate Impact
<2
2-3.6
3.6-5.6
>5.6
(concentration mg/l N)
The MRP and nitrate risk ranking for groundwater as a receptor is the same as for the
surface water receptor as it is based on the concentration of nutrients in groundwater that is
delivered to surface water.
11 The GSI provided both the effective rainfall and recharge maps.
30
While the approach outlined here uses results from the prediction of pollutant
concentrations, the maps are not intended to be used for predicting precise impacts; they
are intended to show relative risk on which an inspection regime can be based.
6.6 Results of Risk Ranking Process
The results are available in GIS data layers produced by the Informatics and GIS Section of
EPA and will be offered to each local authority12.
The following maps for County Meath have been produced to illustrate the results that arise
from this process:
Figure
Receptor
Pollutant
Pathway
9
Surface water
Microbial pathogens and MRP
Surface
10
Groundwater
Microbial pathogens and MRP
Underground
11
Surface water
Nitrate
Surface
12
Groundwater
Nitrate
Underground
The percentage areas in each relative risk category for County Meath and nationally for the
following are given in Tables 15 and 16, respectively:
Relative risk of ponding and pollution of streams by MRP and pathogens via the
surface pathway due to inadequate percolation.
Relative risk of pollution of streams and wells by MRP and pathogens via the
subsurface pathway due to inadequate attenuation.
Relative risk of pollution of streams by nitrogen via the surface pathway due to
inadequate percolation.
Relative risk of pollution of streams and wells by nitrogen via the subsurface pathway
due to inadequate attenuation.
These four categories are given for the following reasons:
1. The subdivision into surface and subsurface pathways may influence the approach
taken to site inspections, particularly the visual assessments.
2. The subdivision based on pollutant type enables a better understanding of the threat
posed to human health and the environment.
12 The maps have been produced at a 1:40,000 scale. Further enlargement would potentially be misleading as
the spatial resolution of the underlying data is insufficient to show detail beyond this scale. The maps are
intended as a guide as to where issues are most likely to occur with respect to inadequate percolation or
susceptibility of groundwater to the occurrence of pathogens/MRP or nitrate across the Irish landscape, with
respect to existing DWWTSs. The depicted boundaries and interpretations derived from the maps do not
eliminate the need for on-site sampling, testing, and detailed study of specific sites.
31
Table 15: Percentage areas in the different relative risk categories for County Meath*
Relative risk
MRP & Pathogens
Nitrate
category
Streams via
Streams and wells via
Streams via surface
Streams and wells via
surface pathway
subsurface pathway
pathway
subsurface pathway
Low
50.7
86.8
95.3
95.9
Moderate
10.0
3.5
0.4
0.1
High
6.8
2.0
0.1
0.2
Very High
28.9
4.0
0.5
0.1
Area Sewered
3.7
3.7
3.7
3.7
*Percentages may not add to 100% due to rounding
Table 16: Percentage areas in the different relative risk categories nationally*
Relative risk
MRP & Pathogens
Nitrate
category
Streams via
Streams and wells via
Streams via surface
Streams and wells via
surface pathway
subsurface pathway
pathway
subsurface pathway
Low
63.1
89.0
97.3
97.6
Moderate
10.5
4.1
0.2
<0.1
High
6.4
1.9
<0.1
<0.1
Very High
17.8
2.8
0.1
<0.1
Area Sewered
2.3
2.3
2.3
2.3
*Percentages may not add to 100% due to rounding
Two national maps – Figures 13 and 14 – illustrate the relative risk of ponding and pollution
of water by MRP and pathogens via surface and subsurface pathways, respectively. In
general, the ‘very high’ risk ranking category coincides with areas where there is a relatively
high density of DWWTSs and either a high likelihood of inadequate percolation or extreme
groundwater vulnerability.
6.7 Next Steps
The output from the Risk Based Methodology will be used in developing the National
Inspection Plan and in proposing the level of inspection, based on risk. Detailed criteria for
site selection, which take account of sensitive receptors, have been being developed for use
in conjunction with the Risk Based Methodology. This will assist Water Service Authorities in
identifying areas to focus inspections and achieve the maximum outcome for the
environment.
32
Figure 8: Map of housing density across County Meath
Figure 9: Relative risk of water pollution (streams) from MRP and pathogens in DWWTS
waste water via the surface pathway in County Meath
33
Figure 10: Relative risk of water pollution (streams and wells) from MRP and pathogens in
DWWTS waste water via the subsurface pathway in County Meath
Figure 11: Relative risk of water pollution (streams) from nitrogen in DWWTS waste water
via the surface pathway in County Meath
34
Figure 12: Relative risk of water pollution (streams and wells) from nitrogen in DWWTS
waste water via the subsurface pathway
35
Figure 13: Relative risk of water pollution (streams) from MRP and pathogens in DWWTS
waste water via the surface pathway
36
Figure 14: Relative risk of water pollution (streams and wells) from MRP and pathogens in
DWWTS waste water via the subsurface pathway
37
38
7 SUMMARY AND CONCLUSIONS
The aim of this report is to set out a methodology to enable the EPA to adopt a risk-
based approach to organising inspections of DWWTSs, whereby the level of inspection
will be proportionate to the risk posed to human health and the environment.
The development of the methodology was influenced by:
the data and map information available as GIS datasets;
the current understanding of the hydrological and hydrogeological settings
present in Ireland;
results of research undertaken in Ireland.
The methodology is based on the source-pathway-receptor (S-P-R) model for
environmental management.
DWWTSs located, constructed and installed in accordance with the best practice
guidance generally provide adequate treatment and disposal of domestic waste water.
However there are areas where the percolation characteristics are problematical due
to the hydrogeology – the properties of the soils, subsoils and bedrock – resulting in
inadequate percolation or over rapid percolation. The methodology uses the available
information to locate these areas.
The risk ranking outcome is based on calculating the concentration of two pollutants –
MRP and nitrate – in both surface water and groundwater arising from existing
DWWTSs.
The results for MRP are considered to reflect the likely presence of microbial
pathogens and therefore the risk to human health from waste water that has not
percolated underground.
The approach uses a 1 km2 area as appropriate to evaluating likely impacts from
DWWTSs.
The calculations take account of housing density, attenuation in both surface and
subsurface pathways, dilution at the water receptor and predicted concentrations of
pollutants in the water receptor.
Four categories of risk are used: low, medium, high and very high.
The results indicate that:
A substantial proportion of the country is problematical with regard to
percolation characteristics.
The risk to human health from DWWTS waste water is significantly higher in
areas with a high housing density and inadequate percolation; and/or where
there are private wells in vulnerable areas.
MRP is the main pollutant posing a threat to the environment, particularly to
surface water, either where there is inadequate percolation or where there is
inadequate attenuation prior to entry of waste water into bedrock aquifers,
particularly karstified (cavernous limestone) aquifers. While the cumulative
pollutant load arising from DWWTSs will be insignificant compared to urban
waste water treatment systems and agriculture at river basin scale, it can be
significant in certain physical settings at small catchment scale.
The next stage in the National Inspection Plan will be to apportion the level of
inspections based on the risk ranking outcome. While this will be the main basis,
39
account will be taken of sensitive receptors and the possibility of inadequate design
and maintenance of systems in areas generally suitable for DWWTSs, and random
inspections will be undertaken.
40
8 GLOSSARY
Aquifer
A subsurface layer or layers of rock, or other geological strata, of sufficient porosity and
permeability to allow either a significant flow of groundwater or the abstraction of
significant quantities of groundwater (Groundwater Regulations, 2010).
Attenuation
A decrease in pollutant concentrations, flux, or toxicity as a function of physical, chemical
and/or biological processes, individually or in combination, in the subsurface environment.
Attenuation processes include dilution, dispersion, filtration, sorption, decay, and
retardation.
Biomat
A biologically active layer that covers the bottom and sides of percolation trenches and
penetrates a short distance in the percolation soil. It includes complex bacterial saccharides
and accumulates organic substances as well as micro-organisms.
Conceptual Hydrogeological Model
A simplified representation or working description of how a real hydrogeological system is
believed to behave on the basis of qualitative analysis of desk study information, field
observations and field data. A quantitative conceptual model includes preliminary
calculations of water balances, including groundwater flow.
Diffuse Sources
Diffuse sources of pollution are spread over wider geographical areas rather than at
individual point locations. Diffuse sources include general land use activities and
landspreading of industrial, municipal wastes and agricultural organic and inorganic
fertilisers.
Domestic Waste Water
Waste water of a composition and concentration (biological and chemical) normally
discharged by a household, and which originates predominantly from the human
metabolism or from day to day domestic type human activities, including washing and
sanitation, but does not include fats, oils, grease or food particles discharged from a
premises in the course of, or in preparation for, providing a related service or carrying on a
related trade. (Water Services Act, 2007).
Domestic Waste Water Treatment Systems (DWWTS)
Domestic waste water treatment system means a system involving physical, chemical,
biological or thermal processes, or a combination of such processes, utilised for the
treatment or disposal of domestic waste water, or the sludge derived from domestic waste
water, and includes:
(a) all septic tanks and waste water tanks and systems receiving, storing, treating or
disposing of domestic waste water and all drains associated with such tanks or systems, and
(b) all drains associated with the discharge of domestic waste water, whether or not they
discharge to a septic tank or waste water tank (i.e. including percolation area or polishing
filter)
41
Down-gradient
The direction of decreasing groundwater levels,
i.e. flow direction. Opposite of up-gradient.
Groundwater
All water which is below the surface of the ground in the saturation zone and in direct
contact with the ground or subsoil (Groundwater Regulations, 2010).
Groundwater Dependent Terrestrial Ecosystems (GWDTEs)
These are groundwater dependent wetlands, whereby the dependency is either on
groundwater flow, level or chemistry as the controlling factors or qualifying interests of
associated habitats. Examples are raised bogs, alkaline fens and turloughs. Groundwater
dependent terrestrial ecosystems are listed on the EPA’s register of protected areas in
accordance with Regulation 8 of the Water Policy Regulations, 2003.
Groundwater Protection Scheme (GWPS)
A
scheme comprising two principal components: a land surface zoning map which
encompasses the hydrogeological elements of risk (of pollution); and a groundwater
protection response matrix for different potentially polluting activities (DELG/EPA/GSI,
1999).
Groundwater Recharge
Two definitions: a) the process of rainwater or surface water infiltrating to the groundwater
table; b) the volume (amount) of water added to a groundwater system.
Groundwater Resource
An aquifer capable of providing a groundwater supply of more than 10 m3 a day as an
average or serving more than 50 persons.
Hydraulic Gradient
The change in total head of water with distance; the slope of the groundwater table or the
piezometric surface.
Karst
A distinctive landform characterised by features such as surface collapses, sinking streams,
swallow holes, caves, turloughs and dry valleys, and a distinctive groundwater flow regime
where drainage is largely underground in solutionally enlarged fissures and conduits.
On-site Waste Water Treatment Systems (OSWTSs)
A generic term for small-scale waste water treatment systems associated with single houses
and small communities or facilities, and mostly associated with septic tanks and intermittent
filter systems offering secondary treatment of raw waste water effluent.
Pathway
The route which a particle of water and/or chemical or biological substance takes through
the environment from a source to a receptor location. Pathways are determined by natural
hydrogeological characteristics and the nature of the contaminant, but can also be
influenced by the presence of features resulting from human activities (e.g., abandoned
42
ungrouted boreholes which can direct surface water and associated pollutants preferentially
to groundwater).
Percolation
The movement and filtering of fluids through porous materials; with regard to DWWTSs this
refers to the movement and filtering through soil and/or subsoil.
Permeability
A measure of a soil or rock’s ability or capacity to transmit water under a potential hydraulic
gradient (synonymous with hydraulic conductivity).
Point Source
Any discernible, confined or discrete conveyance from which pollutants are or may be
discharged. These may exist in the form of pipes, ditches, channels, tunnels, conduits,
containers, and sheds, or may exist as distinct percolation areas, integrated constructed
wetlands, or other surface application of pollutants at individual locations. Examples are
discharges from waste water works and effluent discharges from industry.
Pollution
The direct or indirect introduction, as a result of human activity, of substances or heat into
the air, water or land which may be harmful to human health or the quality of aquatic
ecosystems or terrestrial ecosystems directly depending on aquatic ecosystems which result
in damage to material property, or which impair or interfere with amenities and other
legitimate uses of the environment (Groundwater Regulations, 2010).
Poorly Productive Aquifers (PPAs)
Low-yielding bedrock aquifers that are generally not regarded as important sources of water
for public water supply but that nonetheless may be important in terms of providing
domestic and small community water supplies and of delivering water and associated
pollutants to rivers and lakes via shallow groundwater pathways.
Population Equivalent (p.e.)
A conversion value which aims at evaluating non-domestic pollution in reference to
domestic pollution fixed by EEC directive (Urban Waste Water Treatment Directive
91/271/EEC) at 60 g/day BOD.
Preferential Flow
A generic term used to describe water movement along favoured pathways through a
geological medium, bypassing other parts of the medium. Examples include pores formed by
soil fauna, plant root channels, weathering cracks, fissures and/or fractures.
Receptors
Receptors are existing and potential future groundwater resources, drinking water supplies
(
e.g. springs and abstraction wells), surface water bodies into which groundwater discharges
(
e.g. streams) and groundwater dependent terrestrial ecosystems (GWDTEs).
River Basin
43
The area of land from which all surface water run-off flows, through a sequence of streams,
rivers and lakes, into the sea at a single river mouth, estuary or delta.
River Basin District (RBD)
A group of river basins formally defined by Water Policy (2003) for the purposes of reporting
Water Framework Directive requirements to the European Commission.
Saturated Zone
The zone below the water table in an aquifer in which all pores and fissures and fractures are
filled with water at a pressure that is greater than atmospheric.
Soil (topsoil)
The uppermost layer of the earth in which plants grow and which is capable of supporting
life.
Source Pathway Receptor (SPR) Model
A SPR model involves identifying whether and how pollution sources are connected to a
receptor via a pathway. A conceptual model provides an understanding of all the
relationships between SPR factors in a particular hydrogeological setting.
Subsoil
Unlithified (uncemented) geological strata or materials beneath the topsoil and above
bedrock.
Surface Water
An element of water on the land’s surface such as a lake, reservoir, stream, river or canal.
Can also be part of transitional or coastal waters. (Surface Waters Regulations, 2009.).
UK TAG
The United Kingdom Technical Advisory Group, a partnership of UK environment and
conservation agencies set up to interpret and support the implementation of the Water
Framework Directive. The EPA is an invited member of the UK TAG.
Unsaturated Zone
The zone between the land surface and the water table, in which pores, fractures and
fissures are only partially filled with water. Also known as the vadose zone.
Vulnerability
The intrinsic geological and hydrogeological characteristics that determine the ease with
which groundwater may be contaminated by human activities (Fitzsimmons et al, 2003).
Water Table
The uppermost level of saturation in an aquifer at which the pressure is atmospheric.
44
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47
Appendix 1
Susceptibility Matrices
48
Table A1(a): Susceptibility of inadequate percolation for a Single House Treatment System in various settings across Ireland – Extreme groundwater vulnerability
Aquifer/Soil type:
Karst (Rk, Lk)
Karst (Rk, Lk)
Sand and Gravel
Sand and Gravel
Productive
Productive bedrock
Poorly
Poorly Productive
Poorly
Poorly Productive
(Rg, Lg)
(Rg, Lg)
bedrock (Rf,
(Rf, Lm)
Productive
Bedrock (Ll)
Productive
Bedrock (Pl, Pu)
Vulnerability
Lm)
Bedrock (Ll)
Bedrock (Pl, Pu)
/Subsoil Permeability:
Dry soil
Wet soil
Dry soil
Dry soil
Dry soil
Wet soil
Wet soil
Wet soil
Dry soil
Wet soil
Extreme Vulnerability Percolation
rate Percolation
rate Percolation rate Percolation
rate Percolation
Percolation
rate Percolation rate Percolation
rate Percolation rate Percolation
rate
(Subsoil thickness 0-
depends on depth moderate
to
low low and generally high and generally rate
depends depends on depth variable and variable
and variable and variable but often
to bedrock, subsoil depending on time of <10; well drained <10 depending on on depth to to
bedrock
and depends
on depends
on depends
on problematical; lateral
3m and in vicinity of
type
and
the year of test; water soils dominate.
time of year of test; bedrock
and subsoil type; but permeability of permeability
of permeability of groundwater
karst features)
potential presence table
<3m
from
water table <3m subsoil
type; generally
upper bedrock upper
bedrock upper bedrock movement
limited;
of
preferential surface,
generally
from
surface, but
generally satisfactory; water layers;
winter layers; winter water layers;
winter rainfall
runoff
flowpaths owing to owing to low-lying
generally owing to satisfactory;
table potentially a water table may table may be high in water table may predominates;
(Subsoil permeability shallow depths to topography, and may
low-lying
water table not constraint;
be high in low low lying or flat be high in low shallow water table
variable and not
bedrock
but be a constraint.
topography,
and a
constraint; saturated
soils lying
or
flat areas;
lateral lying
or
flat especially in winter;
generally
may
be
a well
drained dominate.
areas;
lateral groundwater
areas;
lateral poorly drained soils,
considered in
satisfactory; water
constraint.
soils dominate
groundwater
movement may be groundwater
peats or bare rock
assessment; this
table
not
a
as
bedrock
movement may limited
in
some movement may dominate.
should be considered constraint;
well
permeable.
be limited in circumstances;
be limited in
in the site
drained
soils
some
variable
soils
or some
assessment)
dominate
as
circumstances;
bare rock dominate. circumstances;
bedrock
highly
variable soils or
variable soils or
permeable.
bare
rock
bare
rock
dominate.
dominate.
Likelihood of
Low
Moderate
Low
Moderate
Low
Moderate
Moderate
Moderate
Moderate
High
inadequate
percolation
Single House domestic
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation often
waste water
generally
be variable and in
generally
be variable and
generally
be variable and
may be
be variable and
may be
inadequate and
treatment system
adequate and
low lying or flat
adequate and
in low lying or
adequate
in low lying or
variable and
in low lying or
variable and
lateral
percolation issues
no hydraulic
areas hydraulic
no hydraulic
flat areas
and no
flat areas
in low lying
flat areas
in low lying
groundwater
issue
issues may arise;
issue.
hydraulic issues
hydraulic
hydraulic issues
or flat areas
hydraulic issues
or flat areas
movement often
careful site
may arise;
issue
may arise;
hydraulic
may arise;
hydraulic
restricted,
assessments
careful site
careful site
issues may
careful site
issues may
thereby giving
focussing on
assessments
assessments
arise; careful
assessments
arise; careful
rise to hydraulic
potential
focussing on
focussing on
site
focussing on
site
issues
hydraulic
potential
potential
assessments
potential
assessments
problems
hydraulic
hydraulic
focussing on
hydraulic
focussing on
required.
problems
problems
potential
problems
potential
required.
required.
hydraulic
required.
hydraulic
problems
problems
required.
required.
49
Table A1(b): Susceptibility of inadequate percolation for a Single House Treatment System in various settings across Ireland – High permeability subsoil
Aquifer/Soil type:
Karst (Rk, Lk)
Karst (Rk, Lk)
Sand and Gravel
Sand and Gravel
Productive
Productive bedrock
Poorly
Poorly Productive
Poorly
Poorly Productive
(Rg, Lg)
(Rg, Lg)
bedrock (Rf,
(Rf, Lm)
Productive
Bedrock (Ll)
Productive
Bedrock (Pl, Pu)
Vulnerability
Lm)
Bedrock (Ll)
Bedrock (Pl, Pu)
/Subsoil Permeability:
Dry soil
Wet soil
Dry soil
Dry soil
Dry soil
Wet soil
Wet soil
Wet soil
Dry soil
Wet soil
High Permeability
Percolation
rate Percolation rate high Percolation rate Percolation
rate Percolation
Percolation
rate Percolation rate Percolation
rate Percolation rate Percolation rate high
Subsoil ~ (> 3m thick
high and generally < and generally <10 high
and high and generally rate high and high and generally high
and high and generally < high
and and generally < 10
10; > 3m of subsoil; depending on time of generally < 10; > <10 depending on generally < 10; <10 depending on generally <10; > 10 depending on generally < 10; > depending on time of
and with permeability water table not a year of test; water 3m of subsoil; time of year of test; > 3m of subsoil; time of year of test; 3m of subsoil; time of year of test; 3m of subsoil; year of test; > 3m of
>10-4 m/s); Broadly
constraint;
well table
<3m
from water table not a water table <3m water table not water table <3m water table not > 3m of subsoil; water table not subsoil; water table
equate to BS5930
drained
soils surface,
generally constraint;
well from
surface, a
constraint; from
surface, a
constraint; water table may be a
constraint; may be near-surface
GRAVEL, sandy
dominate.
owing to low-lying drained
soils generally owing to well
drained generally owing to well
drained near-surface owing well
drained owing to low-lying
GRAVEL and SAND
topography, and may dominate.
low-lying
soils dominate.
low-lying
topo-
soils dominate.
to low-lying topo-
soils dominate.
topography;
be a constraint.
topography,
and
graphy, and may be
graphy;
saturated
saturated
soils
may
be
a
a constraint.
soils dominate.
dominate.
constraint.
Likelihood of
Low
Moderate
Low
Moderate
Low
Moderate
Low
Moderate
Low
Moderate
inadequate
percolation
Single House domestic
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
waste water
generally
be variable and in
generally
be variable and
generally
be variable and
generally
be variable and
generally
be variable and in
treatment system
adequate and
low lying or flat
adequate and
in low lying or
adequate
in low lying or
adequate
in low lying or
adequate
low lying or flat
percolation issues
no hydraulic
areas hydraulic
no hydraulic
flat areas
and no
flat areas
and no
flat areas
and no
areas hydraulic
issue
issues may arise;
issue
hydraulic issues
hydraulic
hydraulic issues
hydraulic
hydraulic issues
hydraulic
issues may arise;
careful site
may arise;
issue
may arise;
issue
may arise;
issue
careful site
assessments
careful site
careful site
careful site
assessments
focussing on
assessments
assessments
assessments
focussing on
potential
focussing on
focussing on
focussing on
potential
hydraulic
potential
potential
potential
hydraulic
problems
hydraulic
hydraulic
hydraulic
problems
required.
problems
problems
problems
required.
required.
required.
required.
50
Table A1(c): Susceptibility of inadequate percolation for a Single House Treatment System in various settings across Ireland – Moderate permeability subsoil
Aquifer/Soil type:
Karst (Rk, Lk)
Karst (Rk, Lk)
Sand and Gravel
Sand and Gravel
Productive
Productive bedrock
Poorly
Poorly Productive
Poorly
Poorly Productive
(Rg, Lg)
(Rg, Lg)
bedrock (Rf,
(Rf, Lm)
Productive
Bedrock (Ll)
Productive
Bedrock (Pl, Pu)
Vulnerability
Lm)
Bedrock (Ll)
Bedrock (Pl, Pu)
/Subsoil Permeability:
Dry soil
Wet soil
Dry soil
Dry soil
Dry soil
Wet soil
Wet soil
Wet soil
Dry soil
Wet soil
Moderate
Percolation
rate Percolation
rate
Limited to
Limited to small Percolation
Percolation
rate Percolation rate Percolation rate low Percolation rate Percolation rate low
permeability subsoil ~ moderate,
water moderate to
low;
small areas of areas of country rate moderate; moderate to low; moderate; water depending on time moderate; water depending on time of
table
not
a water
table
water table not water
table table
not
a of year of test; table
not
a year of test; water
(Subsoil >3m thick
country only
only
constraint;
well potentially
a
a
constraint; potentially
a constraint; well water
table constraint; well table
probably
a
and with permeability drained
soils constraint owing to
well
drained constraint owing to drained
soils probably
a drained
soils constraint owing to
in range 10-4 - 10-8
dominate.
low-lying topography;
soils
usually low-lying
usually
constraint owing to usually
low-lying topography;
m/s.) Broadly equates
saturated
soils
dominate.
topography;
dominate.
low-lying
dominate.
saturated
soils
to BS5930; silty SAND,
dominate.
saturated
soils
topography;
dominate.
dominate.
saturated
soils
clayey SAND, SILT,
dominate.
sandy SILT, some
SILT/CLAY and some
sandy SILT/CLAY (as
well as gravelly
equivalents of each)
Likelihood of
Low
Moderate
Moderate
High where this
Low
Moderate
Low
High
Low
High
inadequate
where this
setting occurs
percolation
setting occurs
Single House domestic
Percolation
Percolation may
Limited to
Limited to small
Percolation
Percolation may
Percolation
Percolation may
Percolation
Percolation may
waste water
generally
be variable and in small areas of areas of country
generally
be variable and
generally
be variable and
generally
be variable and in
treatment system
adequate and
low lying or flat
country
adequate
in low lying or
adequate
in low lying or
adequate
low lying or flat
percolation issues
no hydraulic
areas hydraulic
and no
flat areas
and no
flat areas
and no
areas hydraulic
issue
issues may arise;
hydraulic
hydraulic issues
hydraulic
hydraulic issues
hydraulic
issues may arise;
careful site
issue.
may arise;
issue.
may arise;
issue.
careful site
assessments
careful site
careful site
assessments
focussing on
assessments
assessments
focussing on
potential
focussing on
focussing on
potential
hydraulic
potential
potential
hydraulic
problems
hydraulic
hydraulic
problems
required.
problems
problems
required.
required.
required.
51
Table A1(d): Susceptibility of inadequate percolation for a Single House Treatment System in various settings across Ireland – Low permeability subsoil
Aquifer/Soil type:
Karst (Rk, Lk)
Karst (Rk, Lk)
Sand and Gravel
Sand and Gravel
Productive
Productive bedrock
Poorly
Poorly Productive
Poorly
Poorly Productive
(Rg, Lg)
(Rg, Lg)
bedrock (Rf,
(Rf, Lm)
Productive
Bedrock (Ll)
Productive
Bedrock (Pl, Pu)
Vulnerability
Lm)
Bedrock (Ll)
Bedrock (Pl, Pu)
/Subsoil Permeability:
Dry soil
Wet soil
Dry soil
Dry soil
Dry soil
Wet soil
Wet soil
Wet soil
Dry soil
Wet soil
Low Permeability
Percolation
rate Percolation rate very
Limited to
Limited to small Percolation
Percolation
rate Percolation rate Percolation
rate Percolation rate Percolation rate very
Subsoil ~ (>3m thick
very low except low except where
small areas of areas of country rate very low; very low; rainfall very low; rainfall very low; rainfall very low; rainfall low; rainfall runoff
where bypass flow bypass flow at karst
rainfall runoff runoff
runoff
runoff
runoff
predominates;
and with permeability
country
at karst features features (
e.g. swallow
predominates;
predominates;
predominates;
predominates;
predominates;
generally
shallow
<~10-8 m/s.); Broadly (
e.g. swallow holes, holes,
dolines);
generally
generally
shallow generally
generally
shallow generally
‘perched’ water table;
equates to BS 5930;
dolines); rainfall rainfall
runoff
shallow
‘perched’
water shallow
‘perched’
water shallow
poorly drained soils
some SILT/CLAY,
runoff
predominates;
‘perched’ water table;
poorly ‘perched’ water table;
poorly ‘perched’ water dominate.
some sandy
predominates;
generally
shallow
table;
poorly drained
soils table;
poorly drained
soils table;
poorly
generally
shallow ‘perched’ water table
drained
soils dominate.
drained
soils dominate.
drained
soils
SILT/CLAY, CLAY,
‘perched’
water in
winter;
poorly
dominate.
dominate.
dominate.
sandy CLAY, CLAY,
table
in
winter; drained
soils
and the gravelly
poorly drained soils dominate.
equivalents of each of dominate.
these
Likelihood of
Very High
Very High
Very High
Very High
Very High
Very High
Very High
Very High
Very High
Very High
inadequate
where this
where this
percolation
setting occurs
setting occurs
Single House domestic
Percolation
Percolation often
Limited to
Limited to small
Percolation
Percolation
Percolation
Percolation
Percolation
Percolation often
waste water
often
inadequate and
small areas of
areas of the
often
often
often
often
often
inadequate and
treatment system
inadequate and
therefore
the country
country
inadequate
inadequate and
inadequate
inadequate and
inadequate
therefore
percolation issues
therefore
saturated subsoil
and
therefore
and therefore
therefore
and therefore saturated subsoil
saturated
a constraint in
therefore
saturated
saturated
saturated
saturated
a constraint in
subsoil a
winter. Hydraulic
saturated
subsoil a
subsoil a
subsoil a
subsoil a
winter. Hydraulic
constraint in
issues likely.
subsoil a
constraint in
constraint in
constraint in
constraint in
issues likely.
winter.
constraint in
winter.
winter.
winter.
winter.
Hydraulic issues
winter.
Hydraulic issues
Hydraulic
Hydraulic issues
Hydraulic
likely.
Hydraulic
likely.
issues likely.
likely.
issues likely.
issues likely.
52
Table A2: Susceptibility of MRP and pathogens entering groundwater via subsurface pathways from a Single House Treatment System
Vulnerability/Subsoil Permeability
Bedrock aquifers
Sand and gravel aquifers
Extreme Vulnerability (X - subsoil thickness 0-1m and in vicinity of karst features)
Very high
n/a
Extreme Vulnerability (E – subsoil thickness 1m-3m)
High
High
High, Moderate, Low Vulnerability
High Subsoil Permeability
Low
Low
Moderate Subsoil Permeability
Low
Low
Low Subsoil Permeability
Low
n/a
Table A3: Susceptibility of nitrate entering groundwater via subsurface pathways from a Single House Treatment System
Bedrock/Soil
‘Denitrifying’ bedrock
type
Sand and gravel aquifers
Sand and gravel aquifers
Bedrock aquifers – dry soil
Bedrock aquifers – wet
aquifers
soil
– dry soil
– wet soil
includes rocks with
includes most bedrock
remove nitrate through
types
includes most bedrock
Vulnerability/Subsoil Permeability
chemical reactions13
types
Extreme Vulnerability (X and E - subsoil thickness 0-
Very high
Low
Very high
Low
Low
3m and in vicinity of karst features)
High, Moderate,
High Subsoil Permeability
Very high
Low
Very high
Low
Moderate
Low Vulnerability Moderate Subsoil Permeability
Very high
Low
Very high
Low
Low
Low Subsoil Permeability
n/a
n/a
Low
Low
Low
13 See appendix 3 for list of bedrock types which remove nitrate naturally
53
Appendix 2
Bedrock types which are rich in pyrite and hence denitrify
(‘Denitrifying’ bedrock aquifers)
Impure Limestones
Westphalian Shales
Ballina Limestone Formation
Coolbaun Formation
Ballymartin Formation
Moyadd Coal Formation
Ballysteen Formation
Westphalian (undifferentiated)
Calp
Finlough Formation
Ordovician Volcanics
Loughshinny Formation
Aghamore Formation
Lucan Formation
Avoca Formation
Parsonage and Corgrig Lodge Formation
Ballyhoge Formation
Ballymalone Formation
Dinantian (early) Sandstones, Shales
and Limestones
Dunabrattin Formation
Lower Limestone Shale
Tawnyinagh Formation
Namurian Sandstones
Ordovician Metasediments
Ballynahown Sandstone Formation
Carrickatee Formation
Cloone Flagstone Formation
Carrighalia Formation
Feale Sandstone Formation
Hornfels in Finnlaghhta Formation
Kehernaghkilly Formation
Namurian Shales
Laragh Formation
Ardagh Shale Formation
One Brook Formation
Bencroy Shale Formation
Toberelatan Formation
Clare Shale Formation
Craggagh Shale Formation
Silurian Metasediments and Volcanics
Dergvone Shale Formation
Aghaward Formation
East Point Formation
Giants Grave Formation
Basalts and other volcanic rocks
Glenoween Shale Formation
Carrigcleenamore Volcanics
Gowlaun Shale Formation
Killeshin Siltstone Formation
Precambrian Quartzites, Gneisses and Schists
Lackantedane Formation
Cornamona Marble Formation
Longstone Shale Member
Luggacurren Shale Formation
Precambrian Marbles
Moanour Formation
Lakes Marble Formation
Granites and other igneous intrusive rocks
Crossdoney Granite
54
Appendix 3
Worked Example of Calculations of Load Input, Attenuation,
Dilution and Impact Risk for an area
Step 1: Calculation of Load Input for an area of 1 km2.*
Load = (
load per person (kg/yr)) x (
number of persons per house) x (
density of houses (per km2)) = kg/km2/yr
Housing density is 20 houses per km2.
N Load = 2.7 kg/yr x 2.8 persons per house x 20 houses/km2 = 151.2 kg/km2/yr
MRP Load = 0.5 kg/yr x 2.8 persons per house x 20 houses/km2 = 28 kg/km2/yr
*
A 1 km2 grid was taken as the most appropriate for the analysis
Step 2: Calculation of Load following Attenuation (Surface Pathway).
Load following Attenuation (Groundwater Pathway) = Load Input (kg/km2/yr) x (% of Load leaving Septic
Tank that will reach receptor based on Susceptibility Risk) x (% of Load that will NOT be removed in overland
flow
(i)
N Load
N Load = 151.2 kg/km2/yr and there is LOW Susceptibility for N
N Load following attenuation = 151.2 kg/km2/yr x 5% x 75% = 5.67 kg/km2/yr
(ii)
MRP Load
MRP Load = 28 kg/km2/yr and there is MODERATE Susceptibility for MRP
MRP Load following attenuation = 28 kg/km2/yr x 25% x 75% = 5.25 kg/km2/yr
The example is simplified to a certain degree to demonstrate the methodology used. – The calculation of the
Load following Attenuation will be calculated for each DWWTS individually and aggregated over the given
grid square. This would allow for taking into account variation in susceptibility ranking within a given area.
55
Step 3: Calculation of Load following Attenuation (Subsurface Pathway).
Load following Attenuation (Subsurface Pathway) = Load Input (kg/km2/yr) x (% of Load leaving Septic Tank
that will reach receptor based on Susceptibility Risk)
(iii)
N Load
N Load = 151.2 kg/km2/yr and there is VERY HIGH Susceptibility for N
N Load following attenuation = 151.2 kg/km2/yr x 30% = 45.36 kg/km2/yr
(iv)
MRP Load
MRP Load = 28 kg/km2/yr and there is VERY HIGH Susceptibility for MRP
MRP Load following attenuation = 28 kg/km2/yr x 90% = 25.2 kg/km2/yr
The example is simplified to a certain degree to demonstrate the methodology used. The calculation of the
load following attenuation will be calculated for each domestic waste water treatment system individually
and aggregated over the given grid square. This would allow for taking into account variation in susceptibility
ranking within a given area.
56
Step 4: Calculation of Dilution of Load and resultant concentration
Surface Pathway
𝑚3
𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑢𝑚𝑒 𝑘𝑚2
𝑦𝑟
𝑚𝑚
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑦𝑟
=
𝑚𝑚
× 𝐴𝑟𝑒𝑎 𝑘𝑚2 × 1,000,000 𝑚2 × 𝐿𝑜𝑤 𝐹𝑙𝑜𝑤 𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
1,000 𝑚
Effective Rainfall = 500 mm/yr; Low Flow reduction Factor =0.22
Dilution Volume (at low flow) per 1km2 area = (500/1,000)*(1*1,000,000)*0.22= 110,000 m3
𝑘𝑔
𝑚𝑔
𝑚𝑔
𝐿𝑜𝑎𝑑 𝑓𝑜𝑙𝑙𝑜𝑤𝑖𝑛𝑔 𝑎𝑡𝑡𝑒𝑛𝑡𝑢𝑎𝑡𝑖𝑜𝑛 𝑦𝑟 × 1,000,000 𝑘𝑔
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
=
𝑙
𝑚3
𝑙
𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑢𝑚𝑒 𝑘𝑚2
𝑦𝑟 × 1,000 𝑚3
N Load following attenuation = 5.67 kg/km2/yr
N Concentration (Surface water)= (5.67*1,000,000)/(110,000*1,000) = 0.052 mg/l N
MRP Load following attenuation = 5.25 kg/km2/yr
MRP Concentration (Surface water)= (5.25*1,000,000)/(110,000*1,000) = 0.048 mg/l P
Subsurface Pathway
𝑚3
𝑚𝑚
𝑅𝑒𝑐ℎ𝑎𝑟𝑔𝑒 𝑦𝑟
𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑢𝑚𝑒 𝑘𝑚2 =
× 𝐴𝑟𝑒𝑎 𝑘𝑚2 × 1,000,000 𝑚2
𝑦𝑟
𝑚𝑚
1,000 𝑚
Recharge = 350 mm/yr
Annual Dilution Volume per 1km2 area = (350/1,000)*(1*1,000,000)= 350,000 m3
𝑘𝑔
𝑚𝑔
𝑚𝑔
𝐿𝑜𝑎𝑑 𝑓𝑜𝑙𝑙𝑜𝑤𝑖𝑛𝑔 𝑎𝑡𝑡𝑒𝑛𝑡𝑢𝑎𝑡𝑖𝑜𝑛 𝑦𝑟 × 1,000,000 𝑘𝑔
𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
=
𝑙
𝑚3
𝑙
𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑢𝑚𝑒 𝑘𝑚2
𝑦𝑟 × 1,000 𝑚3
N Load following attenuation = 45.36 kg/km2/yr
N Concentration (Groundwater) = (45.36*1,000,000)/(350,000*1,000) = 0.130 mg/l N
MRP Load following attenuation = 25.2 kg/km2/yr
MRP Concentration (Groundwater)= (25.2*1,000,000)/(350,000*1,000) = 0.072 mg/l P
57
Step 5: Summary and Risk derived from proposed Risk Ranking
Surface Pathway
From earlier steps: Density = 20 houses per km2 ; Effective Rainfall = 500 mm/yr; LOW Susceptibility for N;
MODERATE Susceptibility for MRP
N Concentration (Surface water) = 0.052 mg/l N -
Impact Risk = LOW
MRP Concentration (Surface water)= 0.048 mg/l P -
Impact Risk = VERY HIGH
Subsurface Pathway
From earlier steps: Density = 20 houses per km2 ; Recharge = 350 mm/yr; VERY HIGH Susceptibility for N;
VERY HIGH Susceptibility for MRP
N Concentration (Groundwater)
= 0.130 mg/l N -
Impact Risk = LOW
MRP Concentration (Groundwater)= 0.072 mg/l P -
Impact Risk = VERY HIGH
58
Appendix 4
Examples of the Output of the Excel Workbook for
Surface and Subsurface Pathways.
SURFACE Pathway
Pollutant
MRP
Load kg per Person/year
(in liquid effluent leaving
Septic Tank)
0.5
Persons Per House
2.8
% of Load that will
% of Load
NOT be removed by % of Load that will
Leaving Septic
travel through
NOT be removed
tank that will
PATHWAY
subsurface pathway
in overland flow
reach receptor
LOW Susceptibility
5
4
MODERATE
Susceptibility
25
19
75
HIGH Susceptibility
50
38
P Impact Risk Ranking
VERY HIGH
Susceptibility
80
60
<0.015
0.015-0.025
0.025-0.035
>0.035
Effective Rainfall mm/yr
500
Low Flow reduction factor
(Q90/Q50)
0.22
LOAD to Surfacewater kg P per year/km2
Low Flow Concentration in Surfacewater mg/l P
Housing density (per
LOAD kg P per
MODERATE
HIGH
VERY HIGH
MODERATE
VERY HIGH
km2)
year/km2
LOW Susceptibility
Susceptibility
Susceptibility
Susceptibility
LOW Susceptibility
Susceptibility
HIGH Susceptibility
Susceptibility
2
3
0.11
0.53
1.05
1.68
0.001
0.005
0.010
0.015
4
6
0.21
1.05
2.10
3.36
0.002
0.010
0.019
0.031
6
8
0.32
1.58
3.15
5.04
0.003
0.014
0.029
0.046
8
11
0.42
2.10
4.20
6.72
0.004
0.019
0.038
0.061
10
14
0.53
2.63
5.25
8.40
0.005
0.024
0.048
0.076
12
17
0.63
3.15
6.30
10.08
0.006
0.029
0.057
0.092
14
20
0.74
3.68
7.35
11.76
0.007
0.033
0.067
0.107
16
22
0.84
4.20
8.40
13.44
0.008
0.038
0.076
0.122
18
25
0.95
4.73
9.45
15.12
0.009
0.043
0.086
0.137
20
28
1.05
5.25
10.50
16.80
0.010
0.048
0.095
0.153
22
31
1.16
5.78
11.55
18.48
0.011
0.053
0.105
0.168
24
34
1.26
6.30
12.60
20.16
0.011
0.057
0.115
0.183
26
36
1.37
6.83
13.65
21.84
0.012
0.062
0.124
0.199
28
39
1.47
7.35
14.70
23.52
0.013
0.067
0.134
0.214
30
42
1.58
7.88
15.75
25.20
0.014
0.072
0.143
0.229
32
45
1.68
8.40
16.80
26.88
0.015
0.076
0.153
0.244
34
48
1.79
8.93
17.85
28.56
0.016
0.081
0.162
0.260
36
50
1.89
9.45
18.90
30.24
0.017
0.086
0.172
0.275
38
53
2.00
9.98
19.95
31.92
0.018
0.091
0.181
0.290
40
56
2.10
10.50
21.00
33.60
0.019
0.095
0.191
0.305
42
59
2.21
11.03
22.05
35.28
0.020
0.100
0.200
0.321
44
62
2.31
11.55
23.10
36.96
0.021
0.105
0.210
0.336
46
64
2.42
12.08
24.15
38.64
0.022
0.110
0.220
0.351
48
67
2.52
12.60
25.20
40.32
0.023
0.115
0.229
0.367
50
70
3
13
26
42
0.024
0.119
0.239
0.382
Pollutant
N
Load kg per Person/year
(in liquid effluent leaving
Septic Tank)
2.7
Persons Per House
2.8
% of Load that will
% of Load
NOT be removed by % of Load that will
Leaving Septic
travel through
NOT be removed
tank that will
PATHWAY
subsurface pathway
in overland flow
reach receptor
LOW Susceptibility
5
4
MODERATE
Susceptibility
25
19
75
HIGH Susceptibility
50
38
N Impact Risk ranking
VERY HIGH
Susceptibility
80
60
<2
2-3.6
3.6-5.6
>5.6
Effective Rainfall mm/yr
500
Low Flow reduction factor
(Q90/Q50)
0.22
LOAD to Surfacewater kg N per year/km2
Low Flow Concentration in Surfacewater mg/l N
Housing density (per
LOAD kg N per
MODERATE
HIGH
VERY HIGH
MODERATE
VERY HIGH
km2)
year/km2
LOW Susceptibility
Susceptibility
Susceptibility
Susceptibility
LOW Susceptibility
Susceptibility
HIGH Susceptibility
Susceptibility
2
15
0.57
2.84
5.67
9.07
0.005
0.026
0.052
0.082
4
30
1.13
5.67
11.34
18.14
0.010
0.052
0.103
0.165
6
45
1.70
8.51
17.01
27.22
0.015
0.077
0.155
0.247
8
60
2.27
11.34
22.68
36.29
0.021
0.103
0.206
0.330
10
76
2.84
14.18
28.35
45.36
0.026
0.129
0.258
0.412
12
91
3.40
17.01
34.02
54.43
0.031
0.155
0.309
0.495
14
106
3.97
19.85
39.69
63.50
0.036
0.180
0.361
0.577
16
121
4.54
22.68
45.36
72.58
0.041
0.206
0.412
0.660
18
136
5.10
25.52
51.03
81.65
0.046
0.232
0.464
0.742
20
151
5.67
28.35
56.70
90.72
0.052
0.258
0.515
0.825
22
166
6.24
31.19
62.37
99.79
0.057
0.284
0.567
0.907
24
181
6.80
34.02
68.04
108.86
0.062
0.309
0.619
0.990
26
197
7.37
36.86
73.71
117.94
0.067
0.335
0.670
1.072
28
212
7.94
39.69
79.38
127.01
0.072
0.361
0.722
1.155
30
227
8.51
42.53
85.05
136.08
0.077
0.387
0.773
1.237
32
242
9.07
45.36
90.72
145.15
0.082
0.412
0.825
1.320
34
257
9.64
48.20
96.39
154.22
0.088
0.438
0.876
1.402
36
272
10.21
51.03
102.06
163.30
0.093
0.464
0.928
1.485
38
287
10.77
53.87
107.73
172.37
0.098
0.490
0.979
1.567
40
302
11.34
56.70
113.40
181.44
0.103
0.515
1.031
1.649
42
318
11.91
59.54
119.07
190.51
0.108
0.541
1.082
1.732
44
333
12.47
62.37
124.74
199.58
0.113
0.567
1.134
1.814
46
348
13.04
65.21
130.41
208.66
0.119
0.593
1.186
1.897
48
363
13.61
68.04
136.08
217.73
0.124
0.619
1.237
1.979
50
378
14
71
142
227
0.129
0.644
1.289
2.062
59
SUBSURFACE Pathway
Pollutant
MRP
Load kg per Person/year
(in liquid effluent leaving
Septic Tank)
0.5
Persons Per House
2.8
% of Load that will NOT be removed
by travel through subsurface
pathway (ie % of Load leaving
PATHWAY
Septic Tank that will reach receptor)
LOW Susceptibility
0
HIGH Susceptibility
10
P Impact Risk Ranking
VERY HIGH
Susceptibility
90
<0.015
0.015-0.025
0.025-0.035
>0.035
Recharge mm/yr
350
LOAD in Groundwater kg P per year/km2
Concentration in Groundwater mg/l P
VERY HIGH
Housing density (per
LOW MRP
HIGH MRP
MRP
LOW MRP
HIGH MRP
VERY HIGH MRP
km2)
LOAD kg P per year/km2
Susceptibility
Susceptibility
Susceptibility
Susceptibility
Susceptibility
Susceptibility
2
3
0
0
3
0.000
0.001
0.007
4
6
0
1
5
0.000
0.002
0.014
6
8
0
1
8
0.000
0.002
0.022
8
11
0
1
10
0.000
0.003
0.029
10
14
0
1
13
0.000
0.004
0.036
12
17
0
2
15
0.000
0.005
0.043
14
20
0
2
18
0.000
0.006
0.050
16
22
0
2
20
0.000
0.006
0.058
18
25
0
3
23
0.000
0.007
0.065
20
28
0
3
25
0.000
0.008
0.072
22
31
0
3
28
0.000
0.009
0.079
24
34
0
3
30
0.000
0.010
0.086
26
36
0
4
33
0.000
0.010
0.094
28
39
0
4
35
0.000
0.011
0.101
30
42
0
4
38
0.000
0.012
0.108
32
45
0
4
40
0.000
0.013
0.115
34
48
0
5
43
0.000
0.014
0.122
36
50
0
5
45
0.000
0.014
0.130
38
53
0
5
48
0.000
0.015
0.137
40
56
0
6
50
0.000
0.016
0.144
42
59
0
6
53
0.000
0.017
0.151
44
62
0
6
55
0.000
0.018
0.158
46
64
0
6
58
0.000
0.018
0.166
48
67
0
7
60
0.000
0.019
0.173
50
70
0
7
63
0.000
0.020
0.180
Pollutant
N
Load kg per Person/year
(in liquid effluent leaving
Septic Tank)
2.7
Persons Per House
2.8
% of Load that will NOT be removed
by travel through subsurface
pathway (ie % of Load leaving
PATHWAY
Septic Tank that will reach receptor)
LOW Susceptibility
10
MODERATE
Susceptibility
15
N Impact Risk Ranking
VERY HIGH
Susceptibility
30
<2
2-3.6
3.6-5.6
>5.6
Recharge mm/yr
350
LOAD in Groundwater kg N per year/km2
Concentration in Groundwater mg/l N
Housing density (per
LOW N
HIGH N
VERY HIGH N
VERY HIGH N
km2)
LOAD kg N per year/km2
Susceptibility
Susceptibility
Susceptibility
LOW N Susceptibility
HIGH N Susceptibility
Susceptibility
2
15
2
2
5
0.004
0.006
0.013
4
30
3
5
9
0.009
0.013
0.026
6
45
5
7
14
0.013
0.019
0.039
8
60
6
9
18
0.017
0.026
0.052
10
76
8
11
23
0.022
0.032
0.065
12
91
9
14
27
0.026
0.039
0.078
14
106
11
16
32
0.030
0.045
0.091
16
121
12
18
36
0.035
0.052
0.104
18
136
14
20
41
0.039
0.058
0.117
20
151
15
23
45
0.043
0.065
0.130
22
166
17
25
50
0.048
0.071
0.143
24
181
18
27
54
0.052
0.078
0.156
26
197
20
29
59
0.056
0.084
0.168
28
212
21
32
64
0.060
0.091
0.181
30
227
23
34
68
0.065
0.097
0.194
32
242
24
36
73
0.069
0.104
0.207
34
257
26
39
77
0.073
0.110
0.220
36
272
27
41
82
0.078
0.117
0.233
38
287
29
43
86
0.082
0.123
0.246
40
302
30
45
91
0.086
0.130
0.259
42
318
32
48
95
0.091
0.136
0.272
44
333
33
50
100
0.095
0.143
0.285
46
348
35
52
104
0.099
0.149
0.298
48
363
36
54
109
0.104
0.156
0.311
50
378
38
57
113
0.108
0.162
0.324
60