Guidance

Preparing a hydrogeological impact assessment

Published 13 March 2025

Applies to England

• Publication for Northern Ireland
• Publication for Scotland
• Publication for Wales

You must prepare a hydrogeological impact assessment (HIA) before you apply for a groundwater abstraction licence to:

• work out the risks

• design mitigation measures which manage the impact to other abstractions and the environment

You must prepare a HIA using a tiered approach to work out risks and impacts in context of the location and size of the groundwater abstraction. It must consider:

• how complex the hydrogeological setting is

• how sensitive the environment is

If you need a full description of the tiered approach, check the guidance on hydrogeological impact appraisal for dewatering and groundwater abstractions.

1. Establish the water resource status

You must check your local abstraction licensing strategy to find out what the water availability status is in your:

• catchment
• groundwater body

You must also check the Water Framework Directive Regulations status of the groundwater body and any impacted surface water bodies. Use the Catchment Data Explorer.

1.1 Who should carry out a HIA

A HIA is a detailed process which needs specialist skills to complete. The person who carries it out must be someone who either:

• holds an appropriate accreditation
• is working towards an appropriate accreditation under the supervision of an accredited industry professional

Appropriate accreditations include: 

• Chartered geologist
• Chartered scientist (specialising in hydrogeology)
• Chartered Institution of Water and Environmental Management (CIWEM) (specialising in hydrogeology)
• Chartered engineer (specialising in hydrogeology)

1.2 Why a HIA is important

The Environment Agency uses the HIA to make a decision on (determine) an abstraction licence. They may refuse your application or grant a licence with restrictive conditions because of:

• lack of water availability
• potential for derogation of protected rights and lawful users
• environmental impact (wetlands, site of special scientific interest – SSSIs, Habitats Regulations sites)
• risk of deterioration in status, or failure to meet other Water Framework Directive (WFD) objectives
• damage to buildings of historic interest or archaeology
• impact on water quality
• potential damage to fisheries or migratory routes
• other reasons, including those raised by representations

A good HIA with mitigation measures can avoid delays in the application process and also in making a decision on the licence application.

2. Purpose of the proposed groundwater abstraction

It is important to explain what the groundwater abstraction is for and the rate of abstraction needed to achieve the requirements. The groundwater abstraction application and the HIA requires you to meet the following 3 tests:

• environmental sustainability
• justification of need
• efficient use of water

Within a groundwater abstraction licence application, you must confirm:

• why you need the water

• when you need the water

• how much water you need to abstract

These requirements are important to refer back to when you develop and complete the HIA.

3. Develop a conceptual model

You should use a conceptual model in your HIA to show your understanding of the proposed groundwater abstraction within the local and regional hydrogeological setting. A conceptual model is a simplified representation of a groundwater system within the area of the abstraction.

Your model should bring together the key features of the hydrogeological setting, such as:

• geology
• topography
• water features
• groundwater recharge areas
• groundwater discharge areas
• flow directions
• other water users
• groundwater properties
• flow

You should do this using:

• maps
• sketches
• diagrams
• graphs
• cross-sections

You must take care when assessing the hydrogeological impacts from groundwater abstraction as they can be complex. You must develop good and appropriate conceptual models which takes into account the uncertainty, the size and magnitude of the abstraction and environmental risks. The conceptual model must use the appropriate tools and techniques suitable for the abstraction location.

A conceptual model should change throughout project development. It should start as an initial model at project inception with refinement during design testing, field investigations and finally to one acceptable for an abstraction licence application. For abstractions which are low risk and reasonable certainty, it may be possible to develop the model to support an application from a good desk study, incorporating environmental data and evidence supported by an appropriately conservative analysis. It is important that the model is appropriate for the scale of abstraction and local hydrogeological risks.

A conceptual model is necessary to describe the potential environmental impacts associated with the proposed abstraction, and any uncertainties in how the abstraction will interact with the hydrogeological setting. The nature and scale of these uncertainties may require additional site investigations and guide the development of any monitoring programmes.

The conceptual model will help you understand the level of risk that the groundwater abstraction may have on the environment. You may need to produce a more detailed conceptual model if there is a high risk of the groundwater abstraction impacting the water environment, such as if wetlands or rivers are nearby.

A conceptual model will also help you to realise if:

• the design and location of the groundwater abstraction is satisfactory

• you need to make changes, such as moving the abstraction location further away from a particular feature

3.1 Define the scope of a conceptual model

The size and extent of your conceptual model will depend on the level of risk associated with the groundwater abstraction. You could use a default radius of 1km for the conceptual model. But if your abstraction will be greater than 1,000 cubic metres per day, you may need to use a radius of 5km.

Your conceptual model area may be irregular and larger in shape and extent if the hydrogeological setting is complex. For example, in:

• karst and highly fractured groundwater systems
• mined areas

You may need to refine the conceptual model if the assumptions about the design, construction, location and rate of groundwater abstraction change. While the original purpose and need for abstraction may not have changed, it is important to review the proposed activity against the conceptual model when you make any changes or get more information.

3.2 Data for the conceptual model

To get information on existing abstraction licences, email enquiries@environment-agency.gov.uk and include the groundwater abstraction location and a search radius.

The data you collect must cover a wide range of factors for your conceptual model, including:

• geology

• hydrology

• topography

• interaction with dependent features (such as rivers and wetlands)

• land use

Data sources include:

• Defra Magic Map
• British Geological Survey – GeoIndex (onshore)
• local county council or unitary authority for private wells data
• public register for environmental information

3.3 Rate of abstraction

The rate of abstraction for the purpose of use must be valid for the risk assessment and reflect the rates and quantities (daily and annual) specified within the abstraction licence application. The analysis of impact will use the rate of abstraction. Abstraction rates may change from the initial design after you have created the conceptual model. You must base the hydrogeological impact assessment on a reasonable worst-case scenario of abstraction rates and amounts.

For some abstraction types, such as quarry dewatering, it is important to consider the maximum pumping rate that may occur during the construction, operational and post operational phases. The maximum pumping rate is likely to occur when the depth to the groundwater level is highest. For activities such as quarrying, the abstraction rate may also need to take into account the contribution from rainfall and runoff.

4. Water features survey

You must identify all water features within the potential area of abstraction impact. Some features are easy to identify but others (such as private wells or springs) may require local knowledge. You should complete a comprehensive water features survey initially at a desk study level.

You should complete a walk-over survey, as well as review published maps of the area, to confirm you have identified all water features. Walk-over surveys are important when the risk assessment identifies a possible impact on protected rights or environmental features. You may need to discuss mitigation or make agreements with a third party.

4.1 Features you should identify

Features which you could include in a water features survey include, but are not limited to: 

• boreholes, wells, springs and seepages
• watercourses, including rivers, ditches, burns, becks, leats and streams
• impoundments, including weirs, locks, sluices
• lakes, ponds, and reservoirs (including fish farms and water sports areas), both natural and artificial
• wetlands and groundwater dependent habitats
• coastlines and coastal lagoons
• surface water and groundwater abstractions (licensed, unlicensed, and private)
• areas of historic mining
• large discharges to surface waters (for example, water company water treatment works)
• landfills and areas of known contamination

4.2 Updating your conceptual model

When you complete the water features survey you should review and, where appropriate, update your conceptual model and mapping. It is important to review your conceptual model at this point as the data collected may change your assumptions.

4.3 Acquiring field data

You may need to approach neighbours and local landowners to get permission to access land or get information. When you carry out a water features survey and risk assessment, it does not give you rights to enter others property without permission. If you try to carry out the survey, but someone refuses you access, you should record this.

You must take care when investigating water features and take appropriate health and safety precautions. Domestic boreholes supplying drinking water may not have treatment of their private water supply and you must have measures in place to prevent contamination of the water source. 

5. Assess impacts to the water features

You must assess how the groundwater level and flow will change at each water feature as a direct consequence of your proposed abstraction. Each water feature will have unique characteristics and will experience different impacts.

Impacts of groundwater abstractions must include any changes in water quality. This is particularly important if the abstraction could lead to the movement of pollutants from landfills and known areas of contamination.

5.1 How to obtain aquifer parameters

You must select the appropriate aquifer parameters for the conceptual model when you use either analytical tools or groundwater models. You should also be aware of the assumption made when using tools and parameters.

You can get information and parameters for the aquifer by conducting a pumping test from the proposed groundwater abstraction point. A pumping test will also provide assurances that the aquifer can provide the amount and quality of water needed for your purposes.

You must include monitoring during the pump test to provide data that allows you to assess the risk appropriately. If existing monitoring of the water features is not possible, then additional monitoring that allows you to quantify the risk may have to be installed

If you want to do a pumping test, check the guidance on how to apply for consent to investigate a groundwater source.

Where a pumping test is not feasible, or appropriate for the type of abstraction activity, then you could obtain parameters from published sources, including:

• The physical properties of major aquifers in England and Wales
• The physical properties of minor aquifers in England and Wales

Aquifer parameters may also be available from expert judgement and experience or from previous testing at the abstraction location or a similar abstraction site.

Where there is uncertainty in the aquifer parameters then it may be suitable to carry out a sensitivity analysis.

5.2 How to calculate groundwater level impact

The method of assessing impact will depend on the complexity of the conceptual model and level of risk from groundwater abstraction impacts on the water features. Appropriate analytical tools are likely to be suitable in most cases. But a more complex situation may require a numerical groundwater model.

It is important to consider the cumulative impact of your abstraction in combination with existing abstractions in the area of each water feature. While you may assess that your abstraction may have a small impact on a water feature, the feature could already be at risk of damage and have mitigation measures in place from other impacts.

Analytical tools

Analytical tools can provide a simplified estimate for groundwater level change at a water feature at a given distance from the proposed abstraction location. You should use the best analytical tool for your conceptual model as a standard approach may not be suitable for your location and situation.

Different analytical tools are available for an HIA, you can find a selection within Tier 1 analytic tools which calculate groundwater level impacts (or drawdown) within the aquifer. You can request a copy of the tool by emailing national.groundwater@environment-agency.gov.uk.

For water features at surface level such as rivers and wetlands, you should use another method to estimate the impact of groundwater abstraction on river flows.

Generally, analytical tools are simple to use and are low cost. However, you must not use analytical tools which are not capable or suitable for the abstraction type, location, or sensitivity of water features (including protected wetlands). You must refer to the conceptual model to make sure the assumptions of the analytical tools are suitable.

Groundwater modelling

A groundwater model is a numerical representation of the conceptual model. These models are complex and only experienced hydrogeologists and groundwater modellers should build them. Groundwater models are most useful in complex hydrogeological settings, where several different processes are operating and interacting, that analytical models cannot take into account. You should consider using groundwater models early in the project design as they can take a significant amount of time and cost to build and use.

The Environment Agency has numerical groundwater models in some areas of the country that may be useful for some groundwater abstractions. If you would like to find out more, email enquries@environment-agency.gov.uk.

An example of the protocol for groundwater modelling is from USGS Guidelines for Evaluating Ground-Water Flow Models.

6. Risk assessment of groundwater abstraction

The risk assessment brings together the conceptual model, identification of water features and the magnitude of changes to the groundwater level and flows to assess the impact at protected features and the wider environment. The risk assessment must explicitly identify whether there is potential for adverse impacts on the water environment and other water users as a result of the proposed abstraction.

You should assess and report the following considerations for each water feature: 

• identify the consequences
• estimate the magnitude of impact
• estimate the probability of impact
• consider the timing of the impact
• evaluate the significance of the risk

6.1 Baseline assessment

You must complete the risk assessment using the reasonable worst-case condition for the aquifer to assess the maximum predicted effect. Using the reasonable worst-case condition as a baseline, then you may need to consider the following in the risk assessment.

Will the drawdown impact the performance of an existing abstractions or wells?

This could include:

• drying up boreholes or wells
• pumped water level falling below the freeboard or suction level of an existing pump
• increased pumping head and reduce abstraction rate

Will any wetland see a reduction in groundwater level or contributing flow?

If it is possible, then you should consult an ecologist as further investigations, including groundwater modelling, may be needed.

Will the reduction in river flow (or flow in other types of watercourse) have an impact?

This could include:

• impact on the ecology
• risk to surface water abstractions with ‘hands off’ flow level restrictions

Will the abstraction impact on water features?

This could include springs, lakes, ponds or other amenity features.

Will the abstraction cause deterioration in the quality of groundwater or at any of the water features?

For example, will the abstraction cause saline intrusion or expand an existing pollution plume?

In some situations, you may need to complete the risk assessment considering the maximum predicted impact during the lowest groundwater level. Where the abstraction location does not have historic groundwater level data, groundwater levels within the local area may be available from the Environment Agency. To find out more see Hydrology Data Explorer.

7. Mitigation measures

You may need mitigation if the proposed groundwater abstraction will cause an unacceptable impact on existing abstractions or the environment. The Environment Agency may:

• refuse a formal application if there is an unacceptable impact on water features

• place restrictions on the abstraction licence to mitigate the risk of damage

Designing effective mitigation measures may allow the approval of an an abstraction licence application.

If there is the potential for an unacceptable impact, then you must investigate mitigation measures which will offset or reduce the risk to an acceptable level.

Mitigation measures must be:

• actions to offset the risk of damage to a water feature

• in place or agreed before abstraction starts

Examples of mitigation include:

• recharge of the groundwater between the abstraction and the water feature

• restricting the daily abstraction limit

• minimum flow levels (or hands off flow) to protect the environment when flows and levels are low

• monitoring for a cessation threshold

You must not have measures which seek to investigate if damage has occurred.

It is your responsibility to propose appropriate mitigation measures.

8. Presenting your HIA

The HIA should be a structured and evidenced report, summarising:

• resource availability for the catchment and groundwater body
• the conceptual model, complete with maps and cross-sections
• size and scale of groundwater abstraction
• water features survey
• assessment of impact
• identification of risk
• mitigation measures

9. Enhanced (paid-for) advice

If you need more in-depth advice on preparing a hydrogeological impact assessment, the Environment Agency offers an enhanced pre-application advice service.

Check how to get advice before you apply for a water abstraction or impounding licence.