28 Jul 2020

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.

For more information, please contact us