Detailed Evaluation of Natural Source Zone Depletion at a Paved Former Petrol Station

Report no. 13/20: This report presents the results of an investigation that was carried out to demonstrate Natural Source Zone Depletion (NSZD) occurred under a paved site and compare various monitoring measurement methods in that context. The site is considered typical of many services stations in Europe, as a paved surface is present and extends beyond the boundaries of the site. Below the pavement and surface fill, native unconsolidated material comprising pebbles and gravels in a fine sand to clayey matrix extends down to a perched water table at 8 - 12 m depth. Two plumes exist at the site; a dissolved phase gasoline plume and a diesel light non-aqueous phase liquid (LNAPL) plume, both extending in a south-easterly direction. Three well-documented monitoring methods were utilized to assess NSZD at the site:

• CO2 Traps, which involve measurement of CO2 efflux from the soil at ground surface;
• The soil gas concentration gradient method, based on measurement of subsurface O2 and CO₂ concentration profiles; and
• The biogenic heat method based on subsurface temperature measurements.

The use of multiple monitoring measurement methods provided insights into the conceptual site model and allowed for identification of site-specific interferences between some of the measurements. Complex soil gas concentration profiles and near-surface CO2 contributions in some areas of the site presented data interpretation challenges. However, all data indicate ongoing biodegradation near the interface of LNAPL and water table.