Natural Source Zone Depletion Rates from Groundwater Temperature

Natural source zone depletion (NSZD) is the process whereby immiscible hydrocarbon mass in the subsurface is removed by natural degradation processes. Hydrocarbon degradation produces methane, which exothermically oxidizes to carbon dioxide and results in elevated subsurface temperatures. Temperature gradient measurements in the unsaturated zone above the methane oxidation zone are an established means to calculate NSZD rates, and the governing equations and calculation methods are readily available. In contrast, downward heat flux beneath the zone of methane oxidation is excluded from temperature gradient NSZD rate estimates when there is difficulty measuring the vertically compressed downward temperature gradient in the unsaturated zone. Available NSZD guidance documents acknowledge the value of temperature measurements for assessing downward heat flux below the methane oxidation zone; however, methods to quantitatively calculate this contribution to NSZD rate are not readily available in instances where the temperature profile data are insufficient. In this contribution, we provide a method to calculate the NSZD rate attributable to downward heat flux into the saturated zone using groundwater temperature measurements in the saturated zone and provide a freely available R and python script to perform the rate calculation.