Guidance

Landfill developments: groundwater risk assessment for leachate

You must carry out a risk assessment when applying for a new landfill permit or extending the area of your current landfill.

This guide explains what information you need to submit to the Environment Agency as part of your environmental permit application.

Risk assessment

You must carry out a hydrogeological risk assessment (HRA) showing that leachate from the planned landfill won’t pose an unacceptable risk to groundwater at any stage of its lifecycle.

Your HRA and related assessments will let you propose groundwater control levels and compliance limits for the landfill. They’ll help you determine the engineering standards and other operational controls necessary to protect groundwater.

Landfills that will accept hazardous or non-hazardous wastes are likely to produce leachate that contains many potentially polluting substances. If you’re applying to accept such wastes you’ll need a complex risk assessment using site-specific data, including what hazardous substances and non-hazardous pollutants may be present in the leachate.

Lifecycle

Leachate generation will occur throughout the landfill’s operational phase and continue into the aftercare phase, long after waste disposal at the site stops. Discharge rates and leachate quality may change over time so you must take into account the uncertainties of liner and capping systems’ durability.

Your assessment must consider:

  • degradation of artificial lining systems (and other management systems such as leachate collection, transfer and treatment)
  • the capacity of the geological barrier to attenuate the discharge of leachate for the landfill’s whole lifecycle
  • the changing pollution potential of the leachate over time

Site conceptual model (SCM)

Your risk assessment should start with a site conceptual model.

You should use your SCM as the basis for any pre-application discussions with the Environment Agency and other interested parties such as the local council.

Your SCM must show any proposed technical precautions including:

  • limitations on both the rates of input and concentrations of contaminants in proposed waste types, loading rates and methods of disposal
  • the engineering systems of the site associated with leachate containment and drainage
  • leachate management and monitoring

Carry out a site investigation

You may need to carry out a site investigation to:

  • increase your understanding of the hydrogeological conditions at the site
  • establish that the site is suitable for its intended purpose
  • develop the engineering design of the site
  • reduce uncertainty in the conceptual model
  • design a monitoring programme to identify any impacts from the site

You must employ a qualified geologist, hydrogeologist or geophysicist to do your site investigation. Their investigations should include:

  • field observations of the proposed landfill development, geological exposures and hydrogeological features such as springs
  • installation and logging of geological boreholes and groundwater wells
  • in-situ testing of the soil beneath the site to determine geotechnical properties
  • laboratory testing of soil and rock materials to determine both the geotechnical and attenuating properties
  • tracer tests and groundwater quality monitoring over time
  • non-intrusive surface and down borehole geophysics

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

Risk screening

Risk screening will help you identify whether the landfill development potentially represents a hazard to groundwater and surface water.

Follow the qualitative risk screening process in the groundwater risk assessment guide.

Geological barrier

Your site investigation and the desk study part of developing your SCM should show if a natural geological barrier is present. The geological barrier must provide sufficient attenuation between the landfill source and any potential groundwater receptor to protect groundwater quality.

Where you intend to rely on the naturally occurring mineral material as a geological barrier you must provide an assessment, including evidence to support that its attenuation properties are adequate, in your permit application.

If the assessment shows that the natural mineral material won’t provide sufficient attenuation, you may need to install an artificial geological barrier (eg compacted clay or bentonite enriched sand) to ensure that your landfill development doesn’t impact groundwater quality for the whole life of the site.

Leachate characterisation

You need to adequately characterise leachate quality for all levels of risk assessment. Where possible, you should use test data from representative samples of leachate from either the landfill or representative analogue sites that take similar wastes.

You should compare leachate chemistry to assess its potential hazard against one or both of the following:

  • water quality standards (The Water Supply (Water Quality) Regulations 2000)
  • background groundwater quality

You can use this table on the potential sources of information on leachate quality to predict likely leachate chemistry.

Development scenarios Information sources
No existing leachate information is available. Consequently, leachate quality has to be determined from:
A literature review
of similar landfills that the operator may own.
Information on landfills that take similar waste streams that are operated by a third party. This information is available from the Environment Agency’s public register.
This scenario demands complete reliance on information gathered from other sources. Consequently the comparability of the information must be assured. To do this, you should:
Obtain information about waste stream and potential leachate quality.
Review data usability (completeness, comparability, representative, precision, accuracy).
Review the data and identify chemicals of concern.
Calculate the source term.
You should derive leachate quality information from landfills that have been permitted under the Landfill Directive.
For waste types such as soils and inert materials you may carry out leaching tests. Leaching tests should be undertaken using an appropriate test method as detailed in the EC Council Decision (2003/ 33/ EC), annex, section 3.
You must take extreme care when interpreting leaching test results owing to the potential heterogeneous nature of most waste materials and their potential inability to fully replicate the leaching process under landfill conditions.
You should identify the determinands to be tested (that will depend on the properties of the wastes being analysed) in the site conceptual model.

Screening landfill leachates

Use the landfill leachate analytical framework (MS Word Document, 73.5KB) to assess what hazardous substances are likely to be present in the leachate.

The framework explains what information you need to specify to laboratories so they know what tests to apply.

Inert waste landfills

Landfills for inert waste are unlikely to produce leachate that presents a risk to groundwater quality, as inert waste doesn’t undergo any significant physical, chemical or biological transformations.

If your risk screening shows that the inert waste is unlikely to present a risk to groundwater, then you won’t need to carry out a more detailed groundwater risk assessment. However, if your inert waste landfill is in a sensitive area, such as in an aquifer, source protection zone or below the water table, then you will need to carry out a more detailed risk assessment.

Where a landfill for inert waste in not in a sensitive area you can rely on the specification in the Landfill Directive Annex I for the geological barrier. It must be a minimum of 1m thick with a permeability of less than 1 x 10-7 metres per second.

Quantitative risk assessments

Your SCM, site investigation and risk screening will help you decide on the complexity level of any further risk assessment:

Risk assessment level guide

The level of risk assessment you need should provide confidence in the predicted impacts. You should consider the level of uncertainty and the likelihood of a risk happening.

Your risk assessment should:

  • determine that the input of hazardous substances into groundwater will be prevented and there will be no pollution of groundwater (or associated receptors) by non-hazardous pollutants over the whole lifecycle of the landfill
  • provide the basis for deciding whether the engineering measures and other proposed technical precautions fulfil the requirements of the Landfill Directive (LfD) and the Groundwater Directive (GWD)
  • ensure that the development complies with both the GWD and the LfD

You must do the following to meet the risk assessment aims:

  • confirm the hydrogeological and hydrological settings in which the site is located
  • investigate the sensitivity of water receptors
  • investigate and quantify the likely magnitude of environmental impacts arising from leachate generation and migration
  • investigate the likelihood of environmental impacts over the whole lifecycle of the landfill
  • quantify the source, pathway and receptor linkages over the whole lifecycle of the landfill
  • investigate the likely impact of accidents
  • investigate means of limiting the transport of pollutants along the source, pathway and receptor linkages over the short and long-term
  • develop indicative completion criteria with respect to groundwater

Use this indicative risk assessment levels for a range of scenarios (MS Word Document, 39.5KB) table to find out if you need a DQRA or GQRA.

Find out what tools you should use to complete and submit your risk assessment.

When you’ll need to do a detailed quantitative risk assessment

You should carry out a DQRA for landfill developments unless you’re on a site of low permeability, eg unproductive strata remote from surface water bodies where risk screening or GQRA may be adequate.

Compliance points

At a compliance point an Environmental Assessment Level (EAL) is set to ensure relevant environmental standards will be met at all receptors at risk.

For predictive modelling of hazardous substances, your compliance point will normally be set immediately downgradient of the discharge, at a point just below the water table adjacent to the edge of the discharge area and within the expected vertical mixing depth.

Practically, compliance with control levels and compliance limits for hazardous substances are assessed at monitoring points which are normally one or more boreholes directly adjacent to the landfill. This reflects the practical problems in collecting samples from beneath a landfill.

For non-hazardous pollutants the compliance point will also normally be the monitoring boreholes adjacent to the landfill. Where groundwater has no current or potential future resource value, boreholes for monitoring non-hazardous pollutants further from the site may be appropriate.

If you choose a compliance point other than at the perimeter of the site you have to justify that in your application with reference to the sensitivity of the landfill’s location.

Surface water features as a compliance point

If you don’t consider that groundwater is a resource for future human use, you may assign the compliance point for non-hazardous pollutants as a local surface water feature downgradient of the landfill.

The Environment Agency is only likely to accept a surface water feature as a compliance point where you’ve considered all the source, pathway and receptor linkages and:

  • identified the surface water as the highest priority risk
  • the Environment Agency agrees that it represents the most significant (water) receptor for any contamination from the landfill (that is where groundwater is not a useable resource and is for example in unproductive strata)

Groundwater control levels

You must use groundwater control levels as site-specific assessment criteria to:

  • determine whether a landfill is performing as designed
  • draw the attention of site management to the development of adverse trends in the monitoring data

If groundwater control levels are breached, they indicate that the landfill may not be performing as predicted. You should use them as an early warning system to implement appropriate investigation or corrective measures.

Control levels should:

  • highlight variations between the conceptual model (that is assumed behaviour) and observed conditions
  • identify adverse trends which are indicative of leachate impacts
  • allow for variation in natural water quality from baseline conditions
  • give sufficient time to take corrective or remedial action before compliance limits are breached

Pollutant calculations

Your assessment must show that you’ve taken all reasonable measures to prevent the discharge of hazardous substances and to limit the discharge of non-hazardous pollutants into groundwater.

You need to support your risk assessment with calculations dependent on the environmental setting of the site and the development proposals. Examples of calculations include the:

  • travel time for the leachate to migrate through any lining systems or geological barriers to a potential receptor (normally groundwater but possibly a surface water receptor) or both
  • potential retardation and decay of contaminants as they migrate through the lining systems or geological barriers (or both), provided there’s evidence that these processes are likely to occur
  • predicted concentrations of contaminants at appropriate assessment points in the subsurface (to derive relevant control levels)
  • potential attenuation of contaminants within the geological barrier, eg the retardation of ammonium due to cation exchange, or sorption of organic compounds
  • predicted decline in the leachate strength over time
  • predicted degradation of any artificial components of the liner and engineering systems
  • proposed completion criteria for leachate quality given the long-term attenuation capacity of any mineral liner and geological barrier
  • predicted time at which active management of the landfill will cease (eg extraction of leachate and maintenance of leachate collection systems)

Read the groundwater risk assessment guide for more information on probabilistic calculations.

Accidents and possible failures

You should include the eventual degradation of engineered systems in your risk assessment to predict the risk of pollution throughout the landfill’s life cycle. For example, you should consider the degradation of synthetic landfill liners in assessing the long-term flux of pollutants discharged from the landfill.

Examples of accidents and possible failures

Accident Direct consequence of an accident
Fire, vehicle accident, machine driver error Damage to geomembrane side or basal liner
Fire, structural failure, machine driver error, subsidence, flooding Destruction or degradation of leachate management system
Drilling, penetration of waste Perforation of artificial sealing liner or artificially established geological barrier (or both)
Stability failure, unforeseeable pore water pressure, subsidence, landslides Failure of side wall lining system
Drilling, stability failure, subsidence, void migration, landslides, sub-grade failure Failure of artificial sealing liner or artificially established geological barrier (or both)
Waste slippage, vehicle accident Waste or leachate outside contained area (or both)

Site closure and aftercare plan

You must include a site closure and aftercare plan as part of your permit application and maintain it. You must consider the landfill site’s risks for its whole life up until the point where the site no longer poses an unacceptable risk to the environment.

Your plan must include:

  • proposed completion criteria based on predictions of leachate quality and quantity
  • calculated time period for achieving the predicted hydrogeological surrender conditions
  • a series of performance criteria throughout the life of the landfill that can be used to validate issues such as the declining source term

What to do if your risk assessment fails

If your risk assessment fails to provide confidence that the landfill site will not cause pollution you can:

  • collect additional site-specific data (such as attenuation properties or groundwater levels, etc) to reduce uncertainty and allow the use of less conservative assumptions in the model
  • carry out more detailed risk assessment work at a higher level of complexity (only applicable if the risk assessment has been carried out at a simple level)
  • alter the nature of the development so that it presents a reduced hazard or risk to the groundwater environment (this could include altering the proposed waste types to be deposited, relocating the facility to a less sensitive environment, or upgrading the engineering, etc)
  • identify alternative waste management options not involving landfill.

Monitoring

You must monitor groundwater around and leachate quality within your landfill to confirm your risk assessment assumptions. You should start to monitor groundwater 12 months before your disposal activity starts to establish background quality. You must continue to monitor around your site to confirm that it’s having no impact on groundwater throughout the life of the site.

Once you start accepting waste you must monitor leachate quality and compare the results of that monitoring with your original risk assessment predictions. You must review your original risk assessment against actual leachate and groundwater quality data annually.

Reviews

You must carry out a formal review of your hydrogeological risk assessment every 6 years.

Published 1 February 2016