Real-world uncertainties during a site assessment of vapour migration into a residential house from soil and groundwater

Abstract

Assessing the relationship between hydrocarbon groundwater concentrations and overlying vapour concentrations is fraught with uncertainties resulting from effects of moisture and biodegradation in the capillary fringe above the water table, the combined effects of solubility and Raoult's law associated with free phase hydrocarbon and, not least, the subsequent potential vapour transport mechanisms. This paper reports on a UK site containing an uninhabited house adjacent to soil and groundwater source zones. Hydrocarbon concentrations within shallow groundwater, soil, soil vapour and indoor air have been measured over several sampling events. Potential for biodegradation has been assessed using biotraps. Recent publications and this site's data suggest that models are conservative and may overestimate potential indoor air concentrations. Attenuation from the subsurface into the house may be less than predicted if preferential pathways exist, but preferential pathways limit modelling applicability. As always, a robust and full understanding of the conceptual model is critical. At this site, and most real-world sites, this understanding is often prohibitively expensive to obtain. Notwithstanding this caveat, the results at this site suggest that both modelling algorithms and source-separation distance approaches are valid screening tools in the UK provided that a robust understanding of the conceptual model is developed.