Land Contamination Risk Assessment

The evaluation of contaminated land begins at the Phase I Desk Study stage (often referred to as a Preliminary Risk Assessment), which determines whether any potentially significant pollutant linkages are likely to be present in the surrounding area, including considerations for groundwater risk assessments. The subsequent Phase II Ground Investigation tests the significance of these pollutant linkages, initially through a Generic Quantitative Risk Assessment (GQRA) based on standard published assessment criteria. Where a GQRA identifies a potential risk or is insufficient to determine the significance of any risks, a site-specific Detailed Quantitative Risk Assessment (DQRA) can be carried out to more accurately quantify risks and ensure an appropriate response.

A contaminated land risk assessment is necessary to comprehensively evaluate the risks from land contamination and its potential impact on the environment, and to focus remediation efforts on dealing with the risks that have been determined to be significant.

Contaminated Land Risk Assessment - Below ground fuel tanks

Contaminated Land Risk Assessment - Naphthalene distribution plan

Contaminated land risk assessment - A river edged by trees

What is Contaminated Land?

Contaminated land refers to any land that has been polluted with substances to a degree that can potentially harm human health or the environment. This can include industrial sites, other previously developed (brownfield) sites, and areas that have been subject to waste disposal, such as landfills. Whether land is considered to be contaminated is determined through a staged assessment process. This assessment is usually carried out by a geo-environmental consultant and involves an initial desk-based analysis of available environmental data, followed by intrusive investigations and subsequent qualitative and quantitative risk assessments.

Regulatory Framework

The regulatory framework for dealing with contaminated land is primarily covered by Part IIA of the Environmental Protection Act 1990, which details the mechanisms and responsibilities for addressing land that may present a risk to human health or the environment. However, land contamination is predominantly dealt with through the planning regime, particularly the requirements of the National Planning Policy Framework (NPPF). The general aim is to ensure that land does not present a 'significant possibility of significant harm', as defined by Part IIA. How to achieve this is detailed by a wealth of industry guidance, with key sources being the Part IIA Contaminated Land Statutory Guidance, and The Environment Agency's Land Contamination: Risk Management (LCRM) framework.

The Stages of Contaminated Land Risk Assessment

The first stage of contaminated land risk assessment is the Preliminary Risk Assessment, a qualitative assessment undertaken at the Phase I Desk Study stage. Quantitative contaminated land risk assessment is subsequently undertaken based on data acquired during the Phase II intrusive site investigation, and may take the form of a Generic Quantitative Risk Assessment (GQRA) or a Detailed Quantitative Risk Assessment (DQRA), depending on the site circumstances.

Contaminated Land Investigation

A contaminated land investigation is a thorough examination of a site to establish the properties and contamination characteristics of the land. This investigation is designed to obtain information sufficient to undertake a suitable assessment of the risks, and is undertaken in accordance with BS10175 “Code of Practice for the Investigation of Potentially Contaminated Sites” following the technical approach described by the “Land Contamination: Risk Management (LCRM)” framework (Environment Agency).

Conceptual Site Model

A Conceptual Site Model (CSM) is the foundation of a contaminated land risk assessment. The CSM is a visualisation or descriptive representation of the site setting that highlights the source-pathway-receptor pollutant linkages that may be present on a site in its current state, or that may be created based on future development proposals. The CSM is initially developed during the Preliminary Risk Assessment, the purpose being to determine whether any further works (a Phase II Site investigation) are required. If unacceptable risks are identified, further investigations may be necessary to ensure the site can be developed safely.

The CSM is commonly a written or diagrammatic representation of the the potential sources of contamination, the likely exposure pathways and receptors, which together form potential pollutant linkages. Initially developed based on the information gathered during the desk study and site walkover, the CSM is refined and updated as more information becomes available through later stages of investigation. The model is an essential tool for communicating the site’s risks to stakeholders and for informing further investigation and remediation efforts.

Generic Quantitative Risk Assessment for Contaminated Land

A Generic Quantitative Risk Assessment (GQRA) uses conservative criteria (often published by regulatory bodies or other organisations) as an initial screen to identify contaminants that are of potential concern. For soils, these are contaminant concentrations (known as Generic Assessment Criteria (GAC)) that, if exceeded, may warrant further consideration or remediation. They are produced using an appropriate exposure model (in the UK, this is usually the Contaminated Land Exposure Assessment (CLEA) model), which requires a wide range of toxicity and exposure parameters to be input to allow calculation of the GAC.

GACs are based on the ‘suitable for use’ principle and are available for several common land uses, such as residential, commercial, open space and allotments. The concentrations represented by the GAC are a function of the sensitivity of the site use, for example, the GAC for a residential land use are lower than for a commercial land use. The GACs used for contaminated land assessment in the UK are available from some sources, with the most commonly used being Category 4 Screening Levels (C4SLs) published by CL: AIRE on behalf of Defra in 2014, and Suitable 4 Use Levels (S4ULs) published by LQM/CIEH in 2015. Although developed using the same model, GACs published by different organisations can vary for the same substance due to the use of different input parameters. As such, the contaminated land assessor needs to be confident that the assumptions used in the development of the chosen GAC are appropriate for the subject site. However, all GACs are based on predominantly conservative assumptions (i.e. worst case) with regards to toxicity and exposure for each land use, and if simply used as thresholds for mitigation, can result in remediation works being undertaken that are not strictly necessary.

For waters, reference concentrations such as Drinking Water Standards (DWS) or freshwater Environmental Quality Standards (EQS) are often used as initial screening concentrations. However, these are not always directly applicable, especially when considering shallow groundwater quality and potential groundwater contamination.

Detailed Quantitative Risk Assessment for Contaminated Land

A DQRA seeks to refine the assessment by taking site-specific data, such as soil parameters, building types, exposure durations, groundwater flow direction, distance to the receptor, etc, to determine Site Specific Assessment Criteria (SSAC). The DQRA may be relatively straightforward, using data that is readily available as part of the ground investigation, as well as any further information needed. In other cases, it may be necessary to undertake further targeted site investigation works to assess specific sources, pathways and receptors and build a more comprehensive dataset to feed into the model. As with all contaminated land assessments, the amount of information and level of detail required for the DQRA will depend on the assessment aims and objectives. If initial screenings indicate potential risks, further assessment may be required to ensure accurate results.

For soil contamination and the risks to human health, the CLEA model is usually the basis for calculating the SSAC and determining whether any potentially significant risks exist. The SSAC will essentially be used as a threshold, above which remediation works are likely to be required. This may simply involve comparing individual measured concentrations with the SSAC, but it is often more appropriate to calculate averages or mean values that are representative of a particular material or location, as this approach may be more consistent with the assumptions behind the SSAC calculation. Ultimately, this is a decision for an experienced contaminated land assessor who understands the rationale behind the model.

For groundwater, models such as ConSim or the Environment Agency’s Remedial Targets Methodology can be used to determine if recorded concentrations are likely to impact a sensitive receptor, such as a watercourse or a groundwater abstraction, or to calculate target concentrations which are unlikely to present a risk.

Assessments of the risks from ground gases are often complex and require consideration of many factors, although there are criteria published by organisations such as CIRIA and the BSI that can be used as an indication of a potential problem. The monitoring and assessment of ground gases is covered in more detail here.

Human Health Considerations

Human health risk assessments are a critical part of the contaminated land risk assessment process. These assessments involve comparing the concentrations of contaminants in soil and other media (such as water or air) with benchmark concentrations derived through a GQRA or DQRA. The basis of these concentrations are Health Criteria Values, which are quantities (doses) of a particular substance that humans can be exposed to without significant health effects. These doses are determined through rigorous scientific studies, and have significant (orders of magnitude) factors of safety applied to ensure confidence that benchmark doses represent safe human exposure levels. Models such as CLEA then estimate the likely exposure of humans to media such as soil (via ingestion, inhalation and skin contact), and when combined the Health Criteria Values, can determine acceptable concentrations of contaminants in soil.

Risk Management and Remediation

Should a contaminated land risk assessment identify unacceptable risks, the next stage is consider the options for mitigation and management, and develop an appropriate Remediation Strategy. Remediation / mitigation measures may include the removal of contaminated soil, treatment of contaminated soil / groundwater on or off the site, or mitigation measures such as cover systems, material management or vapour protection measures. The most appropriate options will depend on the type and extent of contamination, but will also be a function of practical and economic considerations. The Local Planning Authority and Environment Agency play a role in regulating contaminated land remediation, ensuring that remediation strategies are appropriate and effective to protect human health and the environment.

G&J Contaminated Land Risk Assessment Services

G&J’s qualified and experienced contaminated land assessors will determine the appropriate level of assessment to identify and quantify potential contamination risks and establish where remediation works are necessary (and only where necessary) to ensure a safe development and compliance with legislation and planning conditions, through our comprehensive contaminated land services. We aim to avoid unnecessary remediation works where this can be demonstrated through a high-quality risk assessment.