Assessing the risk on polluted land

Before any site regeneration can be initiated by a private developer or a local authority an essential preliminary task is to carry out a risk assessment of the contaminated area. In this special contribution, Steve Wilson, Associate Director of Card Geotechnics Ltd, outlines how risk assessments are undertaken.

Risk assessment has become an essential part of brownfield redevelopment as the Environmental Protection Act (EPA) Part IIA requires a risk-based approach to identifying contaminated sites. When applying for planning permission on land used previously most local authorities now require a risk assessment to back up remediation proposals..

One of the most difficult aspects, yet very important part of this process, is to agree the framework for risk assessment and its parameters with the regulators at the Environment Agency and Environmental Health Office so that, although subjective, it is accepted by all parties as part of the planning application.

The simplest form of risk assessment is to look at levels of contamination (source), how it can reach its target (pathway) and what may be affected (the target). This is a descriptive assessment, known as a “source – pathway – target model”, which is a fundamental part of the risk assessment requirements defined in the EPA Pt. IIA.

The first stage of any risk assessment is to carry out a desk top study. This is partly intuitive, based on information gained from local reference libraries, old ordnance survey maps, etc about the history and past uses of the site. From this information, a preliminary risk assessment and a conceptual ground model is produced. Site investigation is then designed and carried out to gather more site specific information, such as actual type and level of contaminants present, which is used to confirm and refine the ground model.

These levels can then be compared with the generic guidelines set down in ICRCL 59/83 and the Netherland (Dutch) intervention levels. In most cases, this effectively screens out many of the contaminants found as they are below the intervention levels. This leaves a limited list of “contaminants of concern” to be assessed in more detail.

Potential pathways for contaminants to reach targets are identified and where areas of contamination have been shown to exceed the guidelines, most can be removed at source by excavation, treated by bioremediation and other techniques or barriers installed to stop migration by encapsulation. Thus, either the source or pathway is removed and the risks minimised.

Quantitative analysis

The preliminary form of risk assessment is largely qualitative being based on many assumptions or uncertainties. Depending on what has been discovered, a decision must then be taken, on a cost/benefit basis, if further detailed investigation and quantitative assessment is justified to quantify the preliminary findings and reduce the scope of the remediation works that will be required. The most common sites that need this additional research are those that contain hydrocarbons where there is a potential risk of migration into groundwater and rivers.

As the requirements for information increase, the scope of the site investigations become greater. It may start with trial pits and laboratory testing and increase to drilling boreholes. For very detailed quantitative risk assessment some specialist techniques may be required such as membrane interface probes or laser induced florescence testing.

Case study

Risk assessments can also be used to reduce costs by preventing an unnecessary amount of remediation or prove that the work proposed will not harm the surrounding environment. One recent example of the latter was when Card Geotechnics was commissioned to demonstrate that the proposed construction of an office/residential development in Dublin’s docklands would not adversely effect the local groundwater resources or the nearby River Liffey.

The foundation design called for the basement walls that would pass through the Made Ground and underlying gravel for stability reasons. The risk assessment demonstrated that this could also be used to remediate the contamination present as it formed a cut off preventing the possibility of any contaminants migrating into the river.

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