Thermal estimation of natural source zone depletion rates without background correction

Abstract

Real-time monitoring of subsurface temperature profiles is a promising approach to resolving natural source zone depletion (NSZD) rates for shallow petroleum liquids. Herein, a new “single stick” computational method for transforming temperature data into NSZD rates is advanced. The method is predicated on subsurface temperatures being a function of surface heating and cooling, and the heat associated with NSZD. Given subsurface temperature at two points, a system of two-equation two-unknown is used to resolve NSZD rates. Mathematical formulations and computational algorithms are validated through computational tests showing near perfect agreement between prescribed and predicted NSZD heating, and observed and predicted subsurface temperatures. The method is applied to temperature data from five field sites. Results include lower NSZD rates in areas where petroleum liquids are absent by a factor of 0.5–7.5 as compared to background correction methods. While the single stick method provides the lowest rate in unimpacted areas, it also provides reasonable rates as compared to two of three background correction methods. In addition, the single stick method yields a coefficient of variation equal to 6% in a triplicate analysis and reasonable estimates of NSZD in LNAPL-impacted areas without background corrected data.