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This publication is available at https://www.gov.uk/government/publications/groundwater-protection-technical-guidance/groundwater-protection-technical-guidance
If you’re carrying out an activity that could lead to the input of substances to ground which could affect the quality or quantity of groundwater you need to understand:
- what type of input you can make
- how to assess the discernibility of hazardous substances
- when geological formations can be determined as permanently unsuitable for other purposes
The Environment Agency will consider the geological characteristics of your location when assessing your permit or licence application. You should consider your location when you plan your activities.
This guidance will help you with your permit or licence application.
1. Groundwater discharges: inputs
You need to know if your activity will result in a direct or indirect input of substances to groundwater. You must comply with the prevent and limit legal requirements so you prevent pollution. Inputs of pollutants to groundwater can be direct or indirect.
A ‘direct input’ into groundwater is the introduction of a pollutant to groundwater without percolation through soil or subsoil.
An input is direct if:
- the discharge goes into an open, artificial structure like a shaft, borehole or well that extends down to or into the water table
- the discharge uses a natural feature like a swallow hole with rapid flow to the water table – meaning a travel time of minutes
- any leachate from waste deposited below the water table moves into surrounding groundwater without the presence of a geological barrier
Indirect input of pollutants to groundwater happens when there is percolation through the subsurface and if:
- the discharge is made into a natural feature, even if it may involve rapid conduit flow, but connection between the surface and the saturated zone is gradual and there is potential for attenuation
- an unsaturated zone is maintained following discharge through an infiltration system, drainage field or other similar feature
- any leachate from waste deposited below the water table migrates into surrounding ground across a natural or built geological barrier, fulfilling the requirements of the Landfill Directive
You can make the input indirect if you backfill the structure with a suitable material, for example gravel or uncontaminated sand, to create an artificial unsaturated zone. You can contact the Environment Agency if you need further advice on this.
If you’re uncertain about rapid flow, you need a site-specific assessment showing the expected discharge volume and stability of any natural infill.
1.1 Discharges to periodically saturated ground
You may consider an input to periodically saturated ground indirect when:
- there are minor fluctuations in the water table, either naturally or because of the discharge itself, for example due to effluent mounding
- the transition from indirect to direct does not alter the technical acceptability of the discharge
Where saturation conditions predominate, you should regard the input as direct.
Use site-specific judgement of the groundwater, and consider whether you can alter the design of the infiltration system or drainage field to minimise repeated events. These considerations don’t apply to sub-water table waste deposits.
2. Discernibility of hazardous substances
You must consider the potential discernibility of hazardous substances in groundwater from your activity. This applies to any activity that inputs hazardous substances to ground.
You must take all necessary measures to prevent inputs of hazardous substances to groundwater. You can consider the input of hazardous substances to be prevented if there are no attributable, discernible concentrations of hazardous substances in the groundwater immediately down-gradient of the discharge zone.
If there are, or are likely to be, discernible concentrations in the groundwater, the input may still be regarded as being prevented if all ‘necessary and reasonable measures’ have been taken to avoid it. Read about necessary and reasonable measures in section 4.19 to 4.22 of Environmental permitting guidance: Groundwater activities.
2.1 Discernible concentrations
A substance is discernible if its concentration at a defined point is greater than either:
- that found naturally in groundwater (known as the natural background quality)
- a minimum reporting value (MRV), usually the limit of quantification or other value set out in legislation
Whichever of the two has the highest concentration will be the discernible concentration. You need to ensure where an input is unavoidable from your activity that the concentration is environmentally trivial immediately downstream in the groundwater flow system.
The most common activities with hazardous substances for which you need to assess ‘discernibility’ are:
- discharging liquid effluents into infiltration systems
- spreading waste sheep dip and agricultural pesticides on to land
2.2 Assessing discernibility
Assess discernibility by considering the hydrogeology of your site and making a site- specific assessment of how you’ll detect substances. You may need help from a groundwater specialist to make this assessment.
You should measure discernibility at a point just below the water table, next to the edge of the discharge area. For example, the furthest point of a drainage field or the boundary of a landfill site. The discharge must not be discernible after immediate dilution after the discharge enters groundwater.
When assessing discernibility, don’t rely on:
- 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 outside the discharge area
- downstream attenuation in the saturation zone and more distant receptor impacts
Predicting discernible inputs, especially from effluent discharges, can be difficult because of the range of uncertainties associated with attenuation processes in the unsaturated zone.
With established effluent discharges, you could collect borehole monitoring data and track a record of impacts over time. All measurements must be as near as reasonably possible to the point of discharge entry.
2.3 Trivial exceedances
If you plan to assess discernibility from measured concentrations in monitoring boreholes, you need to know when a measured concentration is:
- significant, meaning the input is unlawful as inputs have not been prevented
- trivial, with no environmental significance
Trivial exceedance may happen when:
- detections of hazardous substances are not representative – examples include random spikes, sampling errors or contamination
- results are very close to the MRV, for example for an MRV of 0.1 micrograms per litre, 7 readings comply but 2 to 3 readings are between 0.12 and 0.15 micrograms per litre
No exceedance is trivial when it:
- results in harm to groundwater, people or the environment, now or in future – this includes near to the discharge area
- causes a consistent, statistically significant increasing trend, including increased frequency of pollutant spikes
You should use a professional with specialist knowledge of the local hydrogeology and who can expertly interpret the data to decide what trivial exceedances are.
2.4 Discernibility in historically polluted groundwater
If groundwater has previously been polluted by the same hazardous substance you’re monitoring, it may not be possible to measure discernibility.
Instead you must base discernibility on predictive assessments using the MRV or surrounding clean groundwater as your guide on the pre-existing background quality.
You must not allow further inputs of a hazardous substance even though the input quality may be better than the existing polluted groundwater. The only exceptions are if:
- reinjection of hazardous substances occurs after abstraction and treatment as part of a remedial scheme to improve groundwater quality – you’ll need an environmental permit to reinject the abstracted water
- a disproportionate cost for land contamination exclusion applies to the input
3. Geological formations permanently unsuitable for other purposes
The Environment Agency may grant a permit, in limited situations, to discharge pollutants directly into geological formations which for natural reasons can be determined permanently unsuitable for other purposes.
Your submission to the Environment Agency for a geological formation to be assessed as permanently unsuitable should be made as part of your groundwater activity permit application.
You must demonstrate the geological formations affected by the discharge, for natural reasons, are permanently unsuitable for all other purposes. Contamination from human activity isn’t a valid reason for a permanently unsuitable determination.
The formation won’t be determined as permanently unsuitable for other purposes if there are any current or potential future uses.
The circumstances when the Environment Agency may grant a permit to input substances into a formation that has been determined permanently unsuitable are:
- injecting water containing substances, resulting from the operations for exploration and extraction of hydrocarbons or mining activities (produced water)
- injection of natural gas or liquid petroleum gas for storage
For oil and gas, discharges can either be to the same formation or to a permanently unsuitable formation.
These inputs may be allowed given the particular nature of the substance, and the condition of the formation involved.
You can apply for an environmental permit to inject in permanently unsuitable aquifers. Contact your local Environment Agency Groundwater and Contaminated Land team to talk about your plans first.
Any concerns about groundwater or surface water impact will prevent the Environment Agency from determining permanently unsuitable and issuing a permit for the proposed discharge.
You and the Environment Agency will have to consider:
- the impact on existing or potential uses of ground resources
- hydraulic properties of the rock strata
- the quality of any receiving groundwater
- any likely changes in circumstances over the time in which the injection will have an impact
An initial assessment, using published, conservative data may be enough to identify whether the proposal for permanently unsuitable is valid.
If the initial assessment doesn’t identify this, you’ll need to prepare a detailed risk assessment with site-specific data. You may also need a groundwater investigation consent and an abstraction licence.
The Environment Agency will use this information to decide if the strata is permanently unsuitable for other purposes and if so it may grant you a permit.
3.1 Discharge permit conditions
A permit’s conditions will depend on certain information and characteristics of the groundwater.
3.2 Ground resources and other environment systems
Any change caused by the injection must not obstruct exploiting mineral and agricultural resources now or in future. You should include mineral planning documents and records of past uses in your assessments to identify any potential future exploitation.
You must isolate any discharged pollutants from the soil zone or vegetation. This includes the:
- depth of the soil zone
- maximum depth of future roots
3.3 Groundwater quantity
You must consider the hydraulic properties of the geological formation. This includes whether:
- the yield of a rock type is minimal
- groundwater is isolated or inaccessible
The groundwater must not discharge to another formation. This means the only applicable situations for permanently unsuitable are likely to be:
- very deep, isolated permeable strata, such as former oil-bearing strata kilometres below the surface
- very low permeability strata
- isolated lenses with minimal resource value
3.4 Managed aquifer recharge (MAR)
Site-specific conditions will show if managed aquifer recharge (MAR) is possible. Even if an aquifer with poor water quality isn’t naturally usable, it may be possible to use it to store good quality water by:
- injecting water from a cleaner source
- re-injecting after abstracting and treating the groundwater at the surface
It’s important not to lose the potential to develop an aquifer in this way.
A low-yielding aquifer is not suitable for MAR. To determine if MAR is possible you will need to make an assessment of the site-specific conditions as part of your proposal.
For more information on MAR read section Q of the groundwater protection position statements.
3.5 Groundwater quality
A permanently unsuitable designation is only for naturally occurring reasons. Poor groundwater quality resulting from human activity is not acceptable grounds.
The basis for evaluating naturally poor groundwater quality is the effort needed to bring it back to a quality suitable for human consumption. In any assessment include all possible measures, regardless of cost. However, to be permanently unsuitable there must also not be other potential uses, like cooling.
If even after the most intensive treatment the groundwater will still prove unsuitable for human consumption, then the geological formation may be permanently unsuitable.
If the groundwater is treatable to drinking water standards, the geological formation can only be determined as permanently unsuitable when the quantity of water is either:
- extremely low