Assessing the hydrocarbon natural source zone depletion rate of non-aqueous phase hydrocarbons by measuring soil gas flux

Cardno Lane Piper (Cardno) undertook an assessment of the Natural Source Zone Depletion (NSZD) rate of Non-Aqueous Phase Liquid (NAPL) in the unsaturated zone at two former fuel depot sites in Victoria. NSZD is the biodegradation of NAPL in the groundwater and vadose zone. The study assessed the NSZD rate of NAPL by measuring soil gas flux using E-Flux carbon dioxide (CO₂) traps (provided by E-Flux, LLC). CO₂ is produced by microorganisms as a by-product of the biodegradation of NAPL which diffuses to the surface and is collected in the E-flux traps by conversion of calcium hydroxide to calcium carbonate (McCoy, et. al., 2014). Background CO₂ flux measurements allows the subtraction of background flux, and carbon isotope analysis allows the subtraction of CO₂ generated by natural soil respiration (such as the breakdown of organic matter), in order to determine the fossil fuel carbon fractions of the sorbed CO₂ (Zimbron et. Al., 2011). Finally, the quantity of sorbed CO₂ is converted to the NSZD rate of NAPL (in mass/area/time) using an appropriate stoichiometric ratio between CO₂ and NAPL. (author)