Groundwater risk assessment for your environmental permit

How to carry out a groundwater risk assessment as part of an application for an environmental permit.

You may need to carry out a groundwater risk assessment to carry out activities that could directly or indirectly pollute groundwater. This is all water underground in the saturation zone (below the water table) and in direct contact with the ground or subsoil.

Read the risk assessment overview to find out if you need to carry out a groundwater risk assessment as part of your permit application.

If you’re carrying out groundwater risk assessments, you should be an industry professional with an appropriate accreditation, or working under the supervision of one, towards an accreditation such as a:

  • chartered geologist
  • Chartered Institution of Water and Environmental Management (CIWEM)
  • chartered engineer

You should read this guide along with:

When the Environment Agency will do your risk assessment for you

The Environment Agency will usually carry out a risk assessment for you if you’re applying for a permit to discharge to ground:

  • waste sheep dip, pesticide, or pesticide washings (liquid waste left over from washing or cleaning equipment used to apply pesticides)
  • less than 15 cubic metres of treated domestic sewage, for example from a septic tank or small sewage treatment plant in non-sensitive areas – discharges within sensitive areas, such as a groundwater Source Protection Zone 1 (SPZ1) require you to undertake the risk assessment and submit to the Environment Agency for approval

Contact the Environment Agency before you apply for a permit.

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.

You’ll need to develop and refine your model iteratively within each level of risk assessment you carry out.

Conceptual models for groundwater protection describe important hydraulic, hydro-chemical and biological processes that are at work in the soil, the unsaturated zone and the groundwater itself.

Your model should describe potential environmental impacts associated with the site, and any uncertainties in how the activity will interact with the hydrogeological setting. The nature and scale of these uncertainties will determine the need for any subsequent site investigations and guide the development of any monitoring programmes.

What your model should show

Your model should aim to demonstrate the:

  • physical and chemical nature of the discharge or source of contamination (installation or contaminated part of the subsurface)
  • physical and chemical characteristics of the aquifer
  • subsurface processes (for example dilution and degradation) that act on the pollutant as it moves down towards the water table or moves within the groundwater flow
  • location of all the receptors and their relationships to groundwater flow
  • environmental standards (for water quality) that apply to the receptors and by which harm can be measured, as well as criteria to protect groundwater ecosystems

As pollutants often travel through the unsaturated zone to reach groundwater, you should include the processes acting on pollutants in the unsaturated zone where appropriate.

The conceptual model must explicitly identify whether there is potential for a direct or indirect input of any hazardous substances or non-hazardous pollutants to groundwater. All hazardous substances and non-hazardous pollutants (with the exception of ammoniacal nitrogen, ammonium and suspended solids) are known as specific substances. If your discharge includes specific substances, a specific substances assessment will be needed as part of your risk assessment.

If you identify the potential for a direct discharge in the conceptual model and risk screening stage, then you must carry out a risk assessment that is correspondingly more detailed.

The main stages to developing an effective conceptual model include:

How to carry out a desk study

The desk study examines the environmental setting and any potential contamination from past activities on or next to the site where the activity is proposed.

You can find more guidance on the development of desk studies and ground investigations in the British Standards Institution’s code of practice for ground investigations (BS 5930:2015).

Collect together all available and relevant information to characterise the site and its surroundings from literature, public registers and site reconnaissance.

Sources of information include:

See the guidance for an understanding of the hierarchy of groundwater protection.

Water features survey

You should undertake a water features survey to include the details of any private and licensed groundwater abstractions in the vicinity of the proposed activity including their:

  • location on a map
  • use (for example potable water supply or water for food manufacturing)
  • rate at which they are licensed to abstract

The search radius for these will depend on the activity, but typically should be about 1 km radius from your discharge area.

Where the geology and hydrogeology of the area is layered, you need to ascertain the construction details of wells and springs to check whether the abstraction is from a shallow vulnerable layer or deeper, confined and protected layer.

Your water features survey should also include information on surface water receptors.

The desk study will identify any uncertainties related to both the activity and the site’s hydrogeological setting, and how these might interact. The nature and scale of these uncertainties will determine your need for site investigations and guide the development of any site investigation programme.

Use this information to form an initial site conceptual model. This is an iterative process and as you get further site-specific information, you’ll need to review and refine your model.

You should get preliminary views from the Environment Agency and other interested parties (such as local authorities) through a pre-application meeting using the initial site conceptual model as a basis for discussion. You may need to update your conceptual understanding of the activity and its potential impact on the environment accordingly.

You can then use this information to produce a more detailed conceptual model.

Sources, pathways and receptors

As part of your desk study, you need to research how your activity may be a ‘source’ of pollution to groundwater, the ‘pathways’ that the pollution could take to reach groundwater from your site, and the potential groundwater ‘receptors’ that could be affected by that pollution.

For example:

  • sources – such as discharging treated sewage effluent to ground, landfill or other permanent deposits of waste on land (for example, for recovery)
  • pathways – such as through engineered measures (a landfill lining system or infiltration system), or via contaminants passing through the unsaturated zone and saturated zone
  • receptors – such as abstraction boreholes used for drinking water, the ecosystem dependent on the groundwater, the groundwater itself, or any other conservation site, like a Site of Special Scientific Interest (SSSI)

Research the source

You need to research and model both hazardous substances and non-hazardous pollutants that are likely to be present depending on the type of activity being proposed. For example, a landfill source will need to include a range of hazardous substances and non-hazardous pollutants. The risk assessment for land spreading of domestic treated sewage effluent may only need to include a limited range of non-hazardous pollutants.

You can read guidance on:

For some activities there may only be a single substance on which any subsequent risk assessment should focus, for example land spreading waste sheep dip.

Activities involving discharges (such as landfill) with many potentially polluting substances need a more complex risk assessment which usually needs site-specific data.

You’ll need to include a section in your risk assessment stating why you decided which substances to model.

Understand the pathways

You’ll need a basic understanding of the possible pathways the pollutant may take (and the factors that affect flow along these pathways) before you can identify likely receptors.

Effective rainfall and recharge

Effective rainfall is the proportion of rainfall that does not run-off directly to surface waters, evaporate or get taken up by vegetation, but which percolates into the ground and ‘recharges’ underlying aquifers. A change in the amount of effective rainfall will alter the rate of recharge and the flow of any contaminants. Different types of aquifer also have different recharge characteristics.

You need to investigate whether effective rainfall is an important consideration for your activity and its proposed location.

If it is, you need to estimate how much becomes recharge (expressed as mm of rain per year) as part of your risk assessment.

Drainage, flood risk and surface water features

You need to research surface water features (wetlands, ditches, streams, rivers, estuaries or coastal waters) that may:

  • influence or interact with groundwater flowing from beneath the site
  • receive drainage (or are a receptor) from your discharge area

Geology and hydrogeology

The unsaturated zone is where (ideally) the concentration of all hazardous substances in any discharge is attenuated to below detectable levels before it enters groundwater (the saturated zone). In addition, the concentration of all non-hazardous pollutants should have been sufficiently reduced through attenuation, degradation and dilution to prevent pollution after mixing with groundwater.

You can find more guidance on the hierarchy of groundwater protection.

Your conceptual model will normally include a description (geological nature and thickness) of the soils (including fill material), strata and rocks separating the activity from the groundwater. You can get this information from regional maps (normally 1:50 000 scale from the British Geological Survey) and using logs from any boreholes or excavations in the vicinity of the activity. It’s important that you describe the spatial variability in the location, nature or thickness of such strata. You should do this by including a geological map and cross section(s) in your groundwater risk assessment.

You must provide as part of your conceptual model:

  • a description of any engineered barriers in terms of their nature (including clay, sealed concrete surface) and thickness separating the discharge from underlying natural ground or historically placed fill (such as landfill liners and sand filters beneath infiltration systems)
  • a description of the groundwater vulnerability
  • logs of boreholes (or wells) or excavations (such as trial pits) that have been constructed in the vicinity of your discharge, and that provide details on what lies beneath (include the logs and a plan of the site showing the locations of these boreholes)
  • a description of any man-made (for example unsealed boreholes and wells, foundations and piles from buildings and structures, and mine shafts) and karst or karstic features (for example sinkholes, pipes, large fissures and cave systems) that could provide rapid pathways between the activity and groundwater
  • information on the thickness of the unsaturated zone and, for some activities, measurements of water levels in wet and dry periods from boreholes or wells – seasonal variations are often important for specific types of aquifer, such as chalk

You may need to monitor the site to find out the typical maximum upper level of the saturated layer of the unconfined aquifer, if the records show uncertainty about what the maximum groundwater levels is likely to be.

You may also need to examine the effect of non-typical maximum groundwater levels in areas sensitive to groundwater flooding, or those with a high water table, to see if the likely effect on groundwater levels during periods of prolonged rainfall affects the overall risk that the activity presents to water quality.

If you need to carry out a quantitative assessment, you’ll also need to provide information on the moisture content of the unsaturated zone and saturated effective porosity of the different soils and rocks.

Character and importance of the aquifer

You need to investigate the character and importance of the aquifer at your site as part of your conceptual model. Aquifers differ in the way they transmit water to springs, wells, boreholes and rivers. Locally their importance in providing water to receptors also varies as does their vulnerability to pollution.

You can read guidance on aquifers.

You need to define the character and importance of the aquifer by:

  • using a hydrogeological assessment to investigate the aquifer’s general capacity to transmit water
  • carrying out a water features survey
  • describing whether the aquifer transmits water by intergranular flow, fracture flow or by dual porosity flow
  • describing the background quality of the water – you’ll need to provide monitoring data from boreholes around the proposed activity for some assessments
  • including the presence of any geological layering or preferential pathways (for example, faults) within the strata that could affect flow and mixing

Direction and rate of groundwater flow

The direction and rate of groundwater flow are important for indicating:

  • in which direction receptors may be at risk
  • the ability of the underlying groundwater to dilute the concentrations of any non-hazardous pollutants
  • the rate at which any residual pollutants could move towards the receptors

You need to provide the direction of groundwater flow in your conceptual model. This should be based where possible on gradients derived from water level measurements in boreholes, but in the absence of these you can use published maps. As a last resort, you can infer the direction if you cannot find either of these – groundwater usually flows naturally from hills to rivers.

You need to consider whether your activity will affect the natural flow gradient and whether the direction of groundwater flow may change seasonally.

Other potential future influences on the direction of groundwater flow could be from:

  • mine water rebound (the flooding back of deep mines)
  • stopping quarry dewatering
  • changes in borehole abstraction regimes

Identify the receptors

You must identify all relevant receptors.

The receptors are the actual, or potential (plausible) future, uses of groundwater that receive their flow, at least in part, from the vicinity of the discharge.

Main receptors include:

  • groundwater as a resource in aquifers, including all current abstractions from groundwater and all feasible future uses
  • discharges from groundwater, such as springs and base flow to rivers
  • surface watercourses, lakes and ponds
  • wetlands and groundwater dependent ecosystems
  • estuaries and foreshore environments

If you cannot identify a specific groundwater receptor in the vicinity of the groundwater resource, then you should aim to protect the resource itself. In these cases, you should create a surrogate receptor (such as a hypothetical abstraction borehole) and assign it the relevant environmental standard which must be met.

You need to take into account the sensitivity of the receptor, the timescale necessary for the assessment and the consequences of any impact within the risk assessment.

If surface water is a possible receptor you need to research:

  • the probability that there is hydraulic continuity between groundwater and surface water
  • stream and river water quality information (for example chemical and biological status)
  • flood risk and presence of indicative flood plains (flooding could lead to contamination of surface waters or more rapid movement of pollutants to groundwater)

If there is a potential risk to groundwater and specific receptors, you need to assess how to protect receptors through the use of compliance points.

The risk assessment also needs to consider the potential future use of groundwater. This should include a discussion with the Environment Agency about:

  • resource potential (yield and quality)
  • planned exploitation
  • likelihood and feasibility of water resource development

Carry out a site investigation

You’ll need to carry out a site investigation for some activities.

You should always carry one out for:

  • landfill activities and other permanent deposits of waste on land
  • higher risk activities such as a complex sewage effluent discharge

This will be confirmed in a pre-application discussion with the Environment Agency.

Site investigation can include soil and water sampling and the excavation of trial pits including drilling, construction, testing and sampling of boreholes. You can find more guidance in the British Standards Institution’s code of practice for ground investigations (BS 5930:2015).

How to present your conceptual model

Use your knowledge from carrying out your desk study and establishing the source-pathway-receptor relationship to produce your conceptual hydrogeological model.

You can present your model in a number of different ways. The aim is to:

  • present the characteristics of the site
  • provide a systematic indication of the risks that your activity presents
  • identify any uncertainties, further assessment needs or other actions

The main approaches – which you can combine – are:

  • a written description of the site
  • tabular or matrix description
  • a drawing or other diagrammatic illustration

One of the clearest ways of representing a conceptual model is on an annotated hydrogeological conceptual model plan (a map and cross section). The cross section should be oriented in the direction of groundwater flow – see an example of a hydrogeological conceptual model plan (PDF, 68.9KB, 1 page) .

Refine your model

You need to update and refine your model throughout the assessment as you get more data or your parameters change. You should aim to include as much data and remove as much uncertainty from the processes of the model as you can.

Use a tiered approach to your risk assessment

You should carry out your risk assessment after you’ve developed your conceptual hydrogeological model.

You should follow a ‘tiered approach’ to your risk assessment. This means that the greater your risk of groundwater pollution is, the more detailed assessment you undertake. You can stop your risk assessment at any stage if you obtain enough information to demonstrate that your activity does not pose a pollution risk to groundwater.

By adopting this method, it should ensure that the cost, time and effort you put into your risk assessment are proportional to the effort or measures needed to make the risks from an activity acceptable.

The 3 tiers are:

  • Tier 1 – qualitative risk screening – investigate what the risks are, whether more detailed assessment is needed and what that would need to focus on (risk prioritisation)
  • Tier 2 – generic quantitative risk assessment – to collect more information so you can make an informed decision on the risk posed by the site – you’ll also need to identify your compliance points
  • Tier 3 – detailed quantitative risk assessment – to collect more information and formulate a plan if there are clear source-pathway-receptor relationships

For each tier of assessment, you should assess and report the following considerations:

  • identify the consequences
  • estimate the magnitude of the consequences (‘impacts’)
  • estimate the probability of the consequences (‘impacts’)
  • evaluate the significance of the risk

Identify compliance points

As part of the overall risk assessment process you’ll need to identify compliance points.

The compliance point is the point along the groundwater flow pathway where the defined target concentration (compliance limit or value) must not be exceeded, as this would represent an unacceptable risk of harm to the receptor. The compliance point may be the receptor itself or a specified point along the source–pathway–receptor linkage (for example, within an aquifer nearer to the contamination source). Alternatively, it may 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 (such as a borehole or spring).

A compliance point lets you:

  • calculate an acceptable concentration of pollutant and volume of discharge that complies with environmental standards at a receptor
  • decide where to put a monitoring point (for example at an observation borehole or natural spring) to check that you’re complying with your permitted discharge (under the Environmental Permitting Regulations 2016 this is known as requisite surveillance)

You can set a limit on your compliance points which can be used as a value to trigger action (at a physical monitoring point). If the limit is exceeded you must take action because there is evidence of a polluting discharge that could result in a breach of a compliance limit.

Where to put your compliance points

You can set a compliance point at the receptor itself but this may not be possible or desirable. You may want to set the compliance point between the point of discharge and the receptor. If so, you should assess it using criteria that predict the effects of dilution, attenuation and degradation, to protect the downstream receptors.

If your compliance point is also your physical monitoring point, you may need to site it closer to the discharge than the receptor to:

  • be sure the monitoring takes place near enough to the discharge’s zone of influence (the area of aquifer that has the potential to be impacted by the discharge)
  • get advance warning of the development of any contaminant plume – data can then be gathered on the contaminant flux to protect the receptor before any environmental threshold is breached
  • overcome constraints you have on accessing any sampling points

If the receptor is not an abstraction point, but could be one in future, you should set a compliance point that protects the nearest point where you reasonably expect abstraction could take place. This may be subject to practical constraints.

Where there is no plausible use of groundwater closer to the point of discharge use the following to form your pollution assessment:

  • existing abstractions
  • natural discharges
  • other passive uses of groundwater

Setting the compliance limit

The target concentration (also known as a compliance limit) is a concentration at the compliance point that must not be exceeded. Provided the target concentration is met, the relevant environmental standard for the receptors should also be met or complied with.

Where the compliance point is the receptor, the target concentration will be set as the relevant environmental standard or natural background groundwater quality.

A compliance value can be:

  • theoretical if used during predictive modelling
  • a limit set in a permit for physical monitoring

Decide which environmental standards to use

Tailor the environmental standard to protect the use of the identified aquifer at risk. The Water Framework Directive (standards and classification) Directions (England and Wales) 2015 sets out values for assessing the status of groundwater bodies. These standards should not be used as part of site specific (local) investigation. They may, however, prove useful as overall indicators of groundwater quality when protecting groundwater dependent wetlands.

The following standards are often used as surrogates to represent a protection of groundwater relevant to the current or intended use of the aquifer (for example for strategic potable water resources, or base flow support to river flows):

  • Drinking Water Standards (DWS) which are maximum acceptable concentrations in consumer supplies after treatment
  • Surface Water Environmental Quality standards (EQS) which are set to protect the ecology in rivers, lakes, estuaries and coastal waters

However, the inappropriate use of standards intended for other purposes can lead to over or under-protection of the resource. You should only use values from other regimes (such as DWS or EQS) after you’ve carefully considered whether or not their inclusion is relevant to the local circumstances

Include other factors such as the natural background quality, or flow regimes. You may need to use a safety factor at the compliance point, suitable to the level of risk and accuracy in the assessment, to ensure that the receptor is protected.

Environmental standards for some contaminants (such as pesticides) can be very low, and their use ensures that there is unlikely to be any deterioration in background quality. For other contaminants, the standard may be significantly above background quality.

For example, the standard for chloride would be the drinking water maximum acceptable concentration of 250mg per litre which is much higher than the background concentration which is often less than 50mg per litre. In these circumstances it’s possible that some local deterioration in groundwater quality may occur but not to the extent of allowing deterioration up to the DWS.

The acceptability of this should be assessed in relation to the:

  • sensitivity of the receptor at risk
  • current or potential use of the water resource
  • degree to which the down gradient quality will deteriorate as a result

You may need to set the target concentrations at a level between an appropriate environmental standard and natural background level dependent on sensitivity of the site to sufficiently protect water quality for good quality aquifers which are either:

  • extensively developed for potable supplies
  • providing a significant flow component to surface water

You should take into account the compliance regime that the site will be operating under when setting a compliance limit.

Compliance regimes

A compliance regime is what you must do to meet compliance in your particular circumstance. The complexity of your compliance regime should depend on the overall risk, regulatory effort and necessary sampling frequency.

As a minimum, the compliance regime should set out the:

  • compliance limit
  • compliance location
  • sampling frequency
  • compliance statistic associated with the value (for example, a mean, percentile or absolute limit)
  • time period for monitoring
  • area the criteria apply to (for example at a point or as a spatial average over a groundwater body)

The regime should also include information on:

  • detection limits
  • precision of measurement

When identifying parameters to use in a compliance regime, you need to consider what pollutants are in your discharge. It’s often impractical to derive standards for all the discharge constituents, so select representative parameters, including some of the mobile constituents and likely key indicators of pollution for the type of discharge. You should take into account the likely fate and transport characteristics of the key components in your discharge.

Qualitative risk screening

Your qualitative risk screening should assess whether the potential discharge from your activity is acceptable and so will not require further assessment.

This could be because:

  • the discharge has acceptably low concentrations of hazardous substances, or in concentrations that are the same as the natural background levels in the groundwater (whichever is the higher concentration)
  • the discharge has concentrations of non-hazardous pollutants that are within the relevant environmental standards, or in concentrations that are the same as the natural background levels in the groundwater
  • there’s a very low risk to groundwater-fed receptors due to the presence of unproductive drift or unproductive bedrock strata (and there are no aquifers present or near your activity) and remoteness from surface waters
  • the volume or hydraulic loading rate of the discharge is so small such that only minimal dilution in underlying groundwater will be needed to avoid pollution by non-hazardous pollutants

Read the groundwater guidance for the definition of hazardous substances and non-hazardous pollutants.

You should aim to get from your risk screening assessment:

  • justification for the level (tier) of risk assessment you’re going to use next
  • prioritisation of the most important source-pathway-receptor relationships for further evaluation

You must carry out a generic quantitative risk assessment if your risk screening suggests there’s an unacceptable risk.

How to carry out a qualitative risk screening

You can support your qualitative risk screening by carrying out some basic calculations in the following order.

  1. Calculate the dilution factor for the discharge diluted by groundwater flowing in the mixing zone beneath the site.
  2. Calculate the attenuation factor for each of your selected substances for downwards movement from your discharge to the point of arrival in groundwater beneath the site. You should calculate separate attenuation factors for movement through different layers (for example, soil, drift, unsaturated bedrock) where these have different properties and where confidence in their properties varies due to data availability.

If these calculations suggest the pollution risk to groundwater is low you should submit them as evidence as part of the risk assessment document in your permit application.

The Environment Agency will use this evidence together with consideration of other issues such as the risk your discharge represents to nature conservation and Habitat Directive sites, to decide if your permit application is successful.

You must use conservative or well-supported data.

Generic quantitative risk assessment

A generic quantitative risk assessment involves a relatively simple assessment of the impact your activity may have on water quality, including groundwater.

You may need to carry out a generic quantitative risk assessment if any of the following apply:

  • your qualitative risk screening was not detailed enough to allow you to make an informed decision on the risk posed by the site
  • you’ve identified a link from source to path to receptor that you think is feasible
  • you’re preparing to carry out a more complex assessment
  • hazards are relatively low and the location is insensitive enough to mean your activity will have no significant impact
  • you can define source, pathway and receptor with enough certainty that you believe using worst case scenario figures represents them well enough

How to carry out a generic quantitative risk assessment

Generic quantitative risk assessments use hydrogeological calculations which are typically analytical solutions solved in a deterministic fashion.

You must use conservative (worst case) assumptions in your generic assessment.

There must be sufficient attenuation between the source of contamination and any potential groundwater receptor to demonstrate that the environmental protection afforded by any attenuating layer is sufficient, or if it will need to be artificially enhanced, for example by engineered solutions.

The assessment will need to demonstrate that the proposal poses little likelihood of unacceptable inputs to groundwater.

A detailed quantitative risk assessment should be carried out where a well-informed decision cannot be made using conservative inputs, methods and assumptions.

For example:

  • there’s uncertainty regarding any of the source, pathway and receptor terms
  • there are undefined groundwater patterns including the potential for fissure or conduit flow
  • where long-term liner integrity in a landfill or other permanent waste deposits needs further consideration or other engineering concerns

Detailed quantitative risk assessment

You should carry out a detailed quantitative assessment when:

  • the site setting is sensitive – for example on permeable strata such as Principal or Secondary A Aquifers, within an SPZ or close to sensitive surface water bodies
  • the uncertainty in aspects of the source, pathway and receptor terms cannot be overcome using conservative assumptions, because those assumptions lead to an unsatisfactory outcome in terms of risks to groundwater

Where cause for concern is clearly demonstrated at an early stage it may be necessary to introduce risk management action directly rather than spend more time on higher tiers of risk assessment.

A detailed quantitative risk assessment will normally use a ‘stochastic’ risk assessment technique, such as a ‘probabilistic approach’, to assess the impact of uncertainties in input data.

Assess groundwater properties

For a detailed quantitative risk assessment you’ll need to provide the:

  • hydraulic gradient in the direction of groundwater flow and whether this changes seasonally or is affected by your discharge
  • cross flow width of your discharge - the width of the area over which your discharge occurs measured perpendicular to the direction of groundwater flow
  • saturated thickness over which groundwater flows laterally
  • mixing zone depth over which any pollutants from your discharge are diluted
  • hydraulic conductivity (permeability) and effective porosity of the aquifer
  • groundwater flow rate in the mixing zone, calculated as the product of the hydraulic gradient, cross flow width, mixing zone depth and hydraulic conductivity
  • groundwater flow velocity for water moving from beneath your site, calculated as the product of the hydraulic gradient and hydraulic conductivity divided by the effective porosity

Examine attenuation processes and dilution

You need to examine the natural processes that could reduce the concentration of hazardous substances and non-hazardous pollutants as the discharge migrates down and laterally beneath the discharge area.

Many of these processes affect different substances to different extents and also vary between different soils and substrata and different settings, for example in the hyporheic zone around streams and rivers.

You need to consider:

  • physical processes, such as volatilisation, filtration, dispersion, dilution
  • chemical processes, such as precipitation, sorption, retardation, oxidation, reduction, hydrolysis
  • biological processes, such as nitrification, biodegradation

Although physical and chemical processes may occur throughout the movement of the discharged water, biological activity and processes become much less significant below the soil zone and away from the hyporheic zone.

Probabilistic calculations

The simplest form of probabilistic calculation of risks is to use a range of pessimistic, likely and optimistic input values in scoping calculations to illustrate what the outcome might be under the combination of these.

If you get:

  • an acceptable result with all pessimistic values combined – it suggests there’s a low risk
  • an unacceptable result with all optimistic values – it suggests a very high risk that will not be reduced without risk management
  • a mixture of answers – suggests you need to do probabilistic calculations

Probabilistic calculations use ranges of realistic input values, with an informed assumption of the distribution (for example normal, log normal) of values within the range, to produce a distribution of output values.

You can carry out probabilistic calculations using:

  • the Crystal Ball add-in to Microsoft Excel
  • probabilistic tools developed by the Environment Agency (ConSim and LandSim)
  • risk assessment models from commercial organisations

The Environment Agency usually expects the 95th percentile concentration to meet acceptable concentrations when judging the acceptability of the range of output values produced by this approach. This means that the concentration predicted in 95% of all calculations or model runs, which use sensible and justified ranges of input data and have a sound underpinning hydrogeological conceptual model, should be lower than an agreed target concentration. In other words, there’s a 1 in 20 chance of this concentration being exceeded. In sensitive settings, the Environment Agency may be more precautionary and require a higher percentile value (for example, the 99th percentile) to be acceptable.

Lifecycle phases and scenarios to assess

Your risk assessment needs to consider the potential effects on groundwater throughout the ‘life’ of the activity.

For many activities, the discharge will only occur during operation, and this usually means that a single risk assessment scenario is appropriate. There may, however, be occasions when the activity’s setting changes, such as groundwater levels rise because dewatering activities stop in a nearby mine or excavation. If such a change is predictable, then the second scenario should be assessed to check if the discharge will also be safe under these changed circumstances.

For other activities (particularly landfill, where the discharge occurs as a result of leaching of solid wastes), the discharge will continue long after the operational period. After this period the discharge rate may change because of deterioration of barrier and collection systems. However, the quality of the discharge may improve with time.

You need to model scenarios reflecting different lifecycle phases where either of the following apply:

  • activities have planned or anticipated phases of operation and aftercare
  • the discharge may change as a result of deterioration in an irreplaceable barrier or control system

The risk assessment needs to include a section on how you have decided on the scenarios to assess if they are applicable.

Other things you need to consider

You need to provide as part of your risk assessment:

  • an appraisal of the main uncertainties that may affect your predicted outcome
  • a sensitivity analysis demonstrating how the predicted effect on groundwater and associated receptors may change if you use more conservative data
  • any validation of the model used from field results or monitoring
  • an assessment of the risk of accidents, their consequences and what you will do to reduce their likelihood


Uncertainty is a measure of how far the result of an assessment is likely to be from the actual situation.

You need to consider, document and report uncertainties such as:

  • whether the activity can be controlled as well as predicted
  • the quality of the data that has been generated
  • whether conservative assumptions for data are valid
  • uncertainties in the hydrogeological conceptual model
  • uncertainties in the ability of science to simulate natural processes through the use of mathematical expressions

The approach used to take account of uncertainty should be clearly documented in any modelling report.

Analysing uncertainties: Best Estimate (BE) Prediction

The calculation (or model) is performed using the most likely value for each parameter. This should always be carried out to get an understanding of what the model is doing. The value arising from this calculation gives a starting point for uncertainty analysis. This version of the calculation is the one that should be checked thoroughly because this model is the foundation of the prediction – if it’s flawed then anything that follows from it has no value. However, on its own, this version of the model gives no understanding of the magnitude of uncertainty.

Analysing uncertainties: Worst Case (WC) Prediction

This is the same model as used for Best Estimate but with the parameters set at their most conservative possible values. This is a very useful calculation and will usually be overly conservative.

The difference between the Best Estimate prediction and the Worst Case prediction may indicate the magnitude of the uncertainty involved.

Sensitivity analysis

Sensitivity analysis is an important part of your risk assessment.

You can identify the most important factors affecting the outcome using sensitivity analysis. This will allow any variability in these factors to be better controlled through permit conditions.

A sensitivity analysis should be undertaken as part of the quantitative model to determine which parameters have the greatest influence on the model results. Some parameters and their input values have a much bigger influence on the predicted effect of the discharge than others.

A sensitivity analysis is usually undertaken by:

  • varying each parameter in turn by a given percentage, for example by +20%
  • calculating how this changes the model result

This analysis lets the most sensitive parameters be identified so a reasoned judgement can be made on whether further data is needed to better constrain the parameter that is being tested. This will provide greater confidence in the model results.

You need to consider:

  • the sensitivity analysis provides information on the sensitivity of the model (the equations used to represent the site) and does not necessarily reflect the sensitivity of the real environment
  • the range in the parameter values should reflect the range as determined from the field and laboratory testing – there’s no benefit in demonstrating the model using parameter values which exceed the known range of feasible values


If the model correctly predicts observed contamination at the receptor or at any intervening point, this:

  • significantly improves the confidence that can be attached to the model
  • provides assurance that the model provides a credible and acceptable representation of system behaviour

In reviewing the results of a model validation exercise you should consider:

  • whether sufficient field data are available to validate the model
  • if the model fits the observed both spatially and with time – a model may be able to match field data at one particular time, but may fail to represent changes in contaminant concentrations with time
  • the acceptability of the model fit to the observed data and whether any inconsistencies have been adequately explained – in some cases differences between the model and observed data can point to a flaw in the modelling approach
  • to provide assurance that the model provides a credible and acceptable representation of system behaviour

Consider the risk of accidents

You must provide a section in the risk assessment for your activity which describes any possible accidents that could affect risks to groundwater and their consequences in terms of groundwater pollution.

You must say how you will reduce the likelihood of these accidents happening.

Where the consequences of plausible accidents are serious, you should say how you’ll reduce the likelihood of accidents happening and their consequences should they happen.

How to submit your risk assessment

You can submit your risk assessment as a:

  • chapter in a groundwater risk assessment report
  • separate report that is to be used in combination with an underpinning activity and site setting report (such as a Hydrogeological Risk Assessment report template designed for use in permit applications for landfill)

Model output should be clearly and succinctly presented and in graphical format where this is appropriate. Validation data (such as comparison of modelled and observed contaminant concentrations) should be given for key model runs or checked where models are supplied on disk. Decisions made on the basis of model results should be justified with the appropriate model output.

For spreadsheet models and those using common codes such as LandSim or ConSim, the input files or spreadsheets should be provided in digital format. This lets the simulations be re-run and checked (for example to check consistency between the model and the reported results).

Review your risk management options

After you’ve completed your risk assessment, you’ll need to identify and evaluate your options for risk management. For example:

  • pre-treating the source material
  • enhancing the engineering measures, such as a different landfill liner or upgrading effluent treatment schemes prior to discharge
  • tightening the operational and aftercare controls

Groundwater monitoring

If your activities could affect groundwater it’s very likely that you’ll need to carry out groundwater monitoring. When assessing your permit application, the Environment Agency will decide whether monitoring is needed.

Your permit will tell you whether you need to carry out groundwater monitoring and if so:

  • how often you need to monitor
  • what type of data to collect
  • how to store it

You must carry out your monitoring in accordance with the design and frequency stipulated in your specific risk assessment and as reflected in your permit.

Regular monitoring allows you to check that you’re successfully preventing hazardous substances or non-hazardous pollutants from entering groundwater.

If your risk assessment shows that the risk to groundwater from your operation is non-existent, or very low, the Environment Agency may decide that groundwater monitoring is not necessary. Depending on the size and type of discharge from your activity you may still have to monitor any potential impact on groundwater by checking your discharge against limit values. These requirements will be specified in your permit.

The Environment Agency will ask to see your monitoring data either as part of an inspection or they may ask you to send them your data on a regular basis so they can review it. In some cases they may also carry out monitoring of effluent and groundwater (where boreholes are available) to check you’re complying with your permit.

How to monitor

The level of monitoring you need to do will depend on the size and type of the discharge and the sensitivity of the environmental setting.

Your risk assessment supporting your application should set out proposals for:

  • which compliance points you will monitor
  • which substances you’re measuring
  • how often you will monitor
  • how you will record and report monitoring information

Examples of types of monitoring you may need to carry out include measuring the quality of water in:

  • a borehole or spring after the attenuated discharge has mixed with the underlying groundwater beneath your site
  • a stream or other water course that is groundwater baseflow fed (following mixing with your discharge) – if that stream or water course is the main receptor, or supplies water to the main receptor, for example wetlands

It may not always be feasible to install boreholes for monitoring, due to cost or practicality, so compliance point monitoring may need to be done in other ways. For example, by testing soils to check that waste sheep dip chemicals have not travelled beyond the base of the soil layer.

For infiltration systems, monitoring could include the measurement of:

  • the discharge rate and effluent quality
  • groundwater levels and groundwater quality in boreholes located around the infiltration system
  • water quality in related receptors, for example a down gradient spring source

Where groundwater levels affect the outcome of your risk assessment, you should include proposals of how you will monitor these levels. Groundwater levels change seasonally affecting the thickness of the unsaturated zone, and the slope and direction of the hydraulic gradient.

A minimum of 3 monitoring points in triangulation formation is needed to define a gradient. Routine monitoring of each of these points will allow you to check on whether the gradient is different from the assumptions in your risk assessment.

Where the geology and hydrogeology is complex, with groundwater present in 2 or more separated layers, then use this system of monitoring for each layer. Additional monitoring points are likely to be needed if the site is complex.

Designing groundwater monitoring

Groundwater monitoring needs to be designed on a case-by-case basis in order to work out the right:

  • parameters to measure, sample or analyse
  • frequency for sampling
  • location of monitoring points

Refer to British Standard BS ISO 5667-11:2009 (Guidance on sampling of groundwater) for more detail on groundwater monitoring.

You should design your groundwater monitoring programme in line with CIS17 (European Commission, 2007).

Up-gradient or background monitoring

You may need to report on the unaffected or background quality of the groundwater either before a new activity is set up or up-gradient of an existing source of contamination.

Your report lets the Environment Agency know the baseline quality of the groundwater before you discharge waste so they can:

  • monitor any deterioration in the groundwater quality down-gradient of your discharge
  • compare and contrast to what the quality was before you started your activity

If the groundwater is contaminated from a historic source of pollution the Environment Agency will know:

  • how the baseline has changed before your works starts
  • what additional impacts they may have down gradient when you start work

Monitoring intervals

Your monitoring frequency should take into account:

  • the behaviour (such as travel time) of known pollutants and their degradation products
  • the hydrogeological conditions underlying your site (this links to your conceptual model)


The technical characteristics of the monitoring wells and the depth of monitoring in each observation well should be designed according to the type of pollutant present and seasonal water level fluctuations.

Sampling methods

Sample preservation and analysis methods will be dependent on the nature of the input and its expected pollutant concentration. Commercial analytical laboratories can advise on sample preservation and analysis.

Parameters to monitor

The parameters monitored at each well should be indicative of the type of pollutant(s) and their expected impact. Possible indicator parameters (redox, pH, electrical conductivity, temperature, salts) could be used to reduce the monitoring effort.

Cost and benefits

You should look at the cost benefit ratio between the number of wells and the levels of information they will provide.

If you have more wells, you’ll have more data for collection and analysis. This will give a better understanding of your site’s hydrogeology and its impact.


Contact the Environment Agency if you need help with your risk assessment.

General enquiries

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PO Box 544
S60 1BY

Monday to Friday, 8am to 6pm

Published 1 February 2016
Last updated 3 April 2018 + show all updates
  1. All hazardous substances and non-hazardous pollutants (except ammoniacal nitrogen, ammonium and suspended solids) are known as specific substances. If your discharge includes specific substances your risk assessment will need to include a specific substances assessment.
  2. Definitions and guidance on hazardous substances and non-hazardous pollutants have been moved to the groundwater content. Added links to the groundwater content.
  3. New link for the updated list of hazardous substances on JAGDAG's website.
  4. First published.