Assessment and delineation of a chlorinated solvent plume in Falkirk, UK

In Falkirk, a technical college on land used as an engineering works in the 1960s was going to be redeveloped to have a modern campus. The owner contracted RSK Geosciences to undertake a geoenvironmental assessment before finalising the redevelopment plans.

Several previous investigations had indicated elevated concentrations of chlorinated solvents in the shallow groundwater underlying the site. The contamination posed a potential risk to humans and water environment receptors. Our work involved investigating and documenting the extent and significance of the contamination, which helped the client to apply for and get planning permission and begin constructing the teaching units in a timely manner.

We designed a supplementary risk assessment investigation to document the chlorinated solvent groundwater plume and enable a site-specific, detailed quantitative risk assessment. A key part of the project was developing a detailed hydrogeological conceptual site model. We recorded chlorinated solvents at elevated concentrations in the underlying shallow groundwater. The source of the trichloroethene solvent was considered to be historical metal degreasing associated with the site’s use as an engineering works. The primary source area was immediately north and downgradient of the former engineering work building. A secondary source was in a former tank area to the east of the engineering building. We did not identify an ongoing soil source or any free-phase product, which suggested that all the contamination had leached into the groundwater.

The contaminated groundwater was within the glaciomarine deposits, which did not meet the criteria for a groundwater body with resource potential, as set out in Scottish Environment Protection Agency guidance, WAT-PS-10-01 (2014). Groundwater in the underlying bedrock was confined, and there was no viable pathway for migration of the contaminants to deeper groundwater bodies. Therefore, the detailed quantitative risk assessment investigated the risks to the nearest surface watercourse down the hydraulic gradient from the dissolved-phase chlorinated solvent plume.

To reduce the uncertainty in the results, the model was calibrated using site data to provide the best simulation of site conditions. As part of the sensitivity analysis, consideration was given to the potential for trichloroethene daughter products to increase in concentration within the aquifer over time. The results of the assessment indicated that the plume would attenuate within the aquifer before migrating to the surface watercourse. Consequently, it was unlikely that there was a risk of pollution to the water environment from the site.

Engagement with the regulators throughout the project enabled resolution of the planning conditions without protracted discussions. This meant there were no significant delays to the construction programme.