3528 A New UK Approach: Development of vertical screening distances to support more sustainable assessment of the petroleum vapour intrusion pathway

Overview:

Petroleum hydrocarbons are known to be readily biodegraded under aerobic conditions as they migrate through the subsurface. However, under current UK regulatory guidance, biodegradation is not considered in the assessment of the risk to human health via the Petroleum Vapour Intrusion (PVI) pathway in the most frequently used Johnson and Ettinger sub-model. In countries where large-scale and robust empirical datasets assessing the impact of biodegradation on the PVI pathway exist, regulatory and good practice guidance has evolved to account for the significance of this process in developing risk-based assessment thresholds. The absence of a comparable empirical dataset in the UK has led to risk-based thresholds which may be overly conservative for the PVI pathway, in turn leading to land contamination management decision-making which is less sustainable.

This guidance includes the compilation of a UK-specific dataset to support robust decision-making for the PVI pathway in the UK, which required collaboration with multiple stakeholders across the UK, was undertaken using bespoke data collection forms for 144 sites with known petroleum hydrocarbons within soil or groundwater. Preliminary analysis of the dataset was undertaken to understand the significance of biodegradation as a factor affecting concentrations of petroleum hydrocarbons in vapours, versus the impact of other factors such as the presence of Non-Aqueous Phase Liquid (NAPL), the geology of the unsaturated and the concentration of petroleum hydrocarbons in groundwater.

Having completed the preliminary analysis and identified appropriate subsets within the data, the ‘Vertical Distance Method’ was used to derive vertical screening distances suitable for use in the UK. A comparison between the UK dataset, and resulting vertical screening distances, was undertaken against international datasets from the US and Australia. The datasets from all three countries were relatively consistent, with existing US and Australian vertical screening distances comparable to those proposed for the UK. The results of the research project have been used to develop a good practice guide for UK practitioners undertaking PVI pathway assessments. The guide includes appropriate application of vertical screening distances, including the need for a robust conceptual site model (CSM) and the absence of conditions which would preclude biodegradation.