Land contamination groundwater compliance points: quantitative risk assessments
How to select compliance points for the assessment of risks to groundwater from land contamination.
This guidance is for specialists who carry out groundwater risk assessments for historical pollution from land contamination. Use this guide to set targets to determine how much remediation (clean up) you need to do.
If you’re not a specialist you need to employ a specialist to do your groundwater risk assessment.
This guidance doesn’t apply where contamination is due to a breach of an environmental permit. If an incident has caused pollution, follow the Environment Agency’s position statement J1.
What your risk assessment should achieve
Where pollutants are in the soil and haven’t entered groundwater, you must take all necessary and reasonable measures to:
- prevent the input of hazardous substances into groundwater
- limit the entry of non-hazardous pollutants into groundwater to avoid pollution, deterioration in the status of groundwater bodies and to prevent sustained and upward trends in pollutant concentrations in groundwater
Where pollutants have already entered groundwater your priority is to take all necessary and reasonable measures to:
- minimise further entry of contaminants where there is a defined source
- limit the pollution of groundwater or any effect on the status of the groundwater body from the future expansion of the plume, if necessary, by actively reducing its extent
In groundwater risk assessments on sites affected by land contamination, “reasonable” means feasible without involving disproportionate costs. What costs are “disproportionate” depends on site specific circumstances. These circumstances may include:
- considerations of technical feasibility such as identified by the remedial options appraisal, this may be due to the distribution or nature of the contamination and the available remedial methods to treat the identified contamination
- sustainability considerations
You should follow a risk-based methodology such as the land contamination remedial targets methodology to remediate sites.
Remediation should be undertaken following sustainable remediation principles.
Passive discharges from contaminated soils aren’t normally regarded as discharges for the purposes of Environmental Permitting Regulations because there’s no surface activity to control. If your activity disturbs the contamination causing a new discharge of pollutants, you may need an environmental permit, unless an exclusion applies.
Develop your conceptual model
You need to develop a conceptual model. This will form the basis for your risk assessments and will help you successfully evaluate environmental risks.
Your model must look at the source, pathways, receptors, existing and potential future environmental effects.
You need to understand the possible pathways the pollutant may take and the factors that affect how the pollutant moves along these pathways. From this you can identify likely receptors.
You must identify all relevant receptors. Receptors are the actual, or potential (plausible) future uses of groundwater that may be affected or at risk from pollutant release from your site. Receptors receive a flow of groundwater from the identified source of contamination.
Identify the receptors (people, animals, property and anything else that could be affected by your hazard) at risk from the migration of pollutants, via groundwater, from your site.
Understand any existing environmental effects
Identify any existing effects on receptors, for example any existing contaminant plumes. The plume is the volume of aquifer down-gradient of a source within which groundwater is contaminated to a level that exceeds the relevant environmental standard. The relevant environmental standard is set to protect any likely or feasible future use of the groundwater.
Identify compliance points
Use compliance points within a risk assessment and remedial strategy to protect receptors from a source of contamination.
The compliance point is the point along the groundwater flow pathway where the defined target concentration (compliance limit or value) must not be exceeded. The compliance point may:
- be the receptor itself
- be a specified point along the source–pathway–receptor linkage, for example within the aquifer between the source and receptor
- represent pore water in the soil zone
The location of the compliance point will depend on the circumstances, the level of assessment and the sensitivity of the receptor.
The compliance point may be a virtual point for the purpose of predictive assessments (modelling) or it may be a physical monitoring point, for example a borehole or spring.
You need to understand how the receptors and the environmental standards apply at these receptors. You can then select an appropriate location for a compliance point and the relevant water quality criteria that should apply to it.
Decide which environmental standards to use
You must choose the environmental standard which best protects the use of the identified receptors at risk. Read section 4 of the land contamination remedial targets methodology to decide which environmental standard to use.
Set the target concentration
The target concentration is a concentration at the compliance point that should not be exceeded. The target concentration needs to be appropriate to ensure protection of the receptor for its current and potential future uses.
If you meet the target concentration, the relevant environmental standard for the receptor should also be met.
Where the compliance point is the receptor, you will set the target concentration as the relevant environmental standard or background groundwater quality.
Use the target concentration in the level 1 to 4 calculations to get a remedial target against which soil or groundwater concentrations are compared. The selected target concentration at a given compliance point will remain constant during the assessment process.
The remedial target is the soil or groundwater concentration got during the quantitative risk assessment. It’s the concentration that the soil or groundwater should be remediated to, to ensure the target concentration is met at the receptor.
Leaching of contaminants from soils into groundwater
You need to identify soil remedial targets that protect groundwater and other receptors fed by groundwater, from contamination. Mobile contaminants in the soil and unsaturated zone must be reduced to concentrations that will protect groundwater for its current, and future, uses. Although groundwater contamination might have already occurred from the site, the remedial targets set for the contaminated soil should:
- seek to prevent or limit the continued input of pollutants into groundwater
- not take any account of historical groundwater contamination unless this is overridden by sustainability considerations
Existing groundwater contamination
If there is existing groundwater contamination, whether or not related to a known source, you need to separately assess the need for remedial targets and any subsequent remediation associated with the existing plume. The aim of the remedial targets are to:
- avoid groundwater contamination causing harm to human or ecological users of the water, for example borehole abstractions, springs and rivers
- avoid further expansion of contaminant plumes to protect usable groundwater resources
- assess whether it’s necessary to carry out groundwater remediation to manage unacceptable risks to defined receptors and to groundwater resource
These aims assume there is no further input from the original source such as an area of land contamination or a spill. If pollutants are still leaching from overlying contaminated soils, you should also set compliance points for those contaminants leaching from soils.
See an illustration on where to set compliance points for a land contamination groundwater risk assessment (PDF, 92.1KB, 1 page) .
Where to put your compliance points
Where you set your compliance points depends on which of the 4 risk assessment levels you apply.
Level 1: soil leaching
The level 1 compliance point is within the soil zone.
For a level 1 assessment, you will need to compare the soil pore water quality data or leaching test results to either:
- the target concentration in the soil zone
- soil remedial target concentrations got from theoretical soil–water partitioning relationships
The target concentration is usually the same as the environmental standard that would apply to the most sensitive receptor at risk.
Level 2: unsaturated zone attenuation and dilution only
The level 2 compliance point is the groundwater immediately below the site, at or near the groundwater table.
For a level 2 assessment, you must meet the target concentration within the groundwater flow pathway at, or immediately downstream of, the source zone. The level 2 assessment takes account of dilution from infiltrating rainwater and groundwater flow. The source of contamination may be either leaching soils or an area of contaminated groundwater or both.
Where the source of contamination is leaching soils, the assessment may also consider attenuation in the unsaturated zone. Level 2 assessments don’t consider attenuation in the saturated zone.
The target concentration is usually the same as the environmental standard that would apply to the most sensitive receptor at risk.
Level 3: saturated zone attenuation
The level 3 compliance point is located at a point between the source and receptor where the target concentration will provide adequate protection to all identified receptors.
A level 3 assessment is similar to level 2 but also takes account of natural attenuation processes in the saturated zone.
Level 4: dilution in the receptor
The level 4 compliance point is at the receptor.
A level 4 assessment is similar to level 3, but also takes account of any additional dilution available at the receptor. For example from deeper within an abstraction borehole or from upstream flow in a surface water body.
A level 4 assessment must demonstrate either of the following:
- any effect on groundwater doesn’t jeopardise future use of the resource
- the cost of remediation is unreasonable in relation to the improvement of groundwater or surface water quality
Find out how to carry out each level of risk assessment in detail in the land contamination remedial targets methodology.
Hazardous substances compliance points
Set compliance points for hazardous substances at, or as close as practically possible to, the point which contaminants enter the saturated zone.
When to apply level 3 or level 4 compliance points
You should only apply level 3 or level 4 compliance points to hazardous substances in certain cases.
If the contaminant has already entered groundwater and you can show that returning affected groundwater to its natural background quality isn’t achievable or warranted. Base your decision by considering the technical feasibility or sustainability of returning affected groundwater.
If remediation to prevent entry of the contaminant at the water table is impractical for either of the following reasons:
- the distribution and nature of contamination
- it could be achieved only at unreasonable cost and that those costs can’t be reduced or recouped by other measures
In both cases, you need to justify and explain why the compliance point should not be set at, or as close as practically possible to, the point at which the contaminants are entering the saturated zone. You may need to provide this justification to the regulator or third party if they request it.
Your aim is to take all reasonable and practical measures to avoid hazardous substances entering groundwater.
When making an assessment, don’t rely on any of the following:
- dispersion of contaminants beyond the boundary of the discharge area
- higher dilution ratios, for example by including flow in the aquifer below the expected mixing zone or by including the outcrop area beyond the discharge area
- downstream attenuation in the saturation zone and consideration of effects to more distant receptors
Compliance points distances for resource protection
If there are no specifically identifiable groundwater receptors you should protect actual or potential groundwater resources. Do this by applying a level 3 compliance point.
To get a level 3 compliance point in this situation you must select a hypothetical surrogate receptor at which the appropriate environmental standard must be met.
The most appropriate environmental standard will protect the use of the local aquifer. To achieve this, you may identify areas of groundwater downstream from the source of contamination within which a degree of dilution and attenuation of contaminants within groundwater is allowable. You must ensure there is no nearer, feasible or likely future use of the groundwater identified.
Where there is no specifically identifiable receptor use the recommended default compliance point distance to protect groundwater resources. These are linked to the:
- type of contaminant (hazardous substance or non-hazardous pollutant)
- aquifer designation
When considering aquifers as receptors in their own right, they can be categorised as:
- having strategic groundwater resource potential – principal aquifers and some secondary A where these support significant local water supply
- having local groundwater resource potential – some secondary A and secondary B
- unproductive strata
Find out more about the aquifer categories.
The default compliance point distance for resource protection from the contaminant source is 50 metres for:
- all hazardous substances in all aquifers for contaminants already in the groundwater, or from inputs from soils and the unsaturated zone which can’t be prevented
- non-hazardous pollutants in groundwater with strategic resource potential
For non-hazardous pollutants in groundwater with local resource potential, a distance greater than 50 metres may be agreed with the Environment Agency but this should not normally exceed 250 metres.
Unproductive strata are low permeability deposits that are unable to support significant water abstraction or base flow to rivers. When contamination has already entered such strata, these strata are usually regarded as a potential pathway for contaminant migration to other receptors, rather than needing resource protection in their own right. You still need to prevent hazardous substances entering groundwater in these strata.
Resource protection where there is existing groundwater contamination
Where there is existing groundwater contamination, your aim is to avoid contaminant plumes extending beyond the 50 or 250 metre distances. This is measured from the boundary of the original pollutant source. If your hydrogeological risk assessment indicates this has or may occur, you must investigate, assess and if necessary take remedial action to:
- reduce the source term
- stabilise the migrating front of the plume
- reduce the expansion of the plume
If there is a specific receptor (for example a borehole, spring or river) located closer to the source than the 50 or 250 metre distance, use that specific receptor as your compliance point.
Alter the default compliance point distances
You may need to, or be able to, justify altering the 50 or 250 metre default compliance point distances for resource protection based on:
- Water Framework Directive objectives
- future use of groundwater
- sustainability assessment
- natural attenuation
Water Framework Directive objectives
You may need to decrease the compliance point distance to achieve or maintain water body objectives in line with river basin management plans. These objectives are for good status and to meet protected area requirements.
Future use of groundwater
You may be able to justify increasing the compliance point distance. This can only be done if there is credible information to demonstrate a significant physical constraint on the ability to use the groundwater resource.
Examples of physical constraints where development of the groundwater resource now or in the future may be constrained include:
- existing and future land use, such as an area designated for use as domestic housing with proposed mains supplies
- land ownership – there may be factors governing the long-term control of land or access to adjacent land, for example private estate, park land, major infrastructure development or extensive industrial complex
- topography – steep or inaccessible land or areas with unsuitable access
- natural limitations, for example potential yield, or the natural background groundwater quality
Attenuation processes in groundwater can have a significant effect on contaminant concentrations. You may consider selection of a compliance point distance over the default distance where strong evidence that natural attenuation processes are acting on the contaminant plume. In these cases you must provide several types of evidence to support your justification with an appropriate sustainability assessment. Your evidence must justify the increase in compliance distance.
You need to provide strong evidence that concentrations will reduce and that there is confidence that an acceptable environmental outcome will be achieved, within a reasonable time. You also need to include appropriate site-specific monitoring (monitored natural attenuation). The results must show that there is no effect on down-gradient compliance points.
You may be able to justify increasing the compliance point distance if it’s supported by a sustainability assessment. This may include a qualitative, semi-quantitative or quantitative sustainability appraisal such as described by SuRF-UK.
Manage the effect of non-aqueous phase liquids
If your site has mobile non-aqueous phase liquids (NAPLs) that are present on or below the water table, the Environment Agency considers the source of contamination to have already entered groundwater.
In these cases, you should follow the requirements for setting compliance points and:
- minimise further entry of hazardous substances to groundwater from the overlying unsaturated zone
- minimise expansion of the plume to prevent further pollution
NAPLs pose an indirect risk to receptors due to the dissolution of constituent compounds in groundwater and subsequent transport. Mobile NAPLs may also represent a direct risk to receptors via movement through the unsaturated or saturated zone providing a secondary source of contamination.
You need to manage the contamination to ensure you:
- remove or control mobile NAPL where its migration could present an unacceptable risk
- remove or control residual NAPL where its dissolution or volatilisation could present an unacceptable risk
- remediate dissolved phase or vapour phase hydrocarbons where they could create an unacceptable risk