This study evaluates the theory, and some practical aspects of using temperature measurements to assess aerobic biodegradation in hydrocarbon contaminated soil. The method provides an easily applicable alternative for quantifying the rate of biodegradation and/or evaluating the performance of in situ remediation systems. The method involves two nonintrusive procedures for measuring vertical temperature profiles down existing monitoring wells; one using a thermistor on a cable for one‐time measurements and the other using compact temperature data loggers deployed for 3‐month to 1‐year period. These vertical temperature profile measurements are used to identify the depth and lateral extent of biodegradation as well as to monitor seasonal temperature changes throughout the year. The basic theory for using temperature measurements to estimate the minimum rate of biodegradation will be developed, and used to evaluate field measurements from sites in California where biodegradation of spilled petroleum hydrocarbons is due to natural processes. Following, temperature data will be used to evaluate the relative rates of biodegradation due to natural processes and soil vapor extraction (SVE) at a former refinery site in the North‐Central United States. The results from this study show that the temperature method can be a simple, cost effective tool for assessing biodegradation in the soil, and optimizing remediation systems at a wide variety of hydrocarbon spill sites.

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More rapid cost‐effective approaches are required to enable field evaluation of natural source zone depletion (NSZD) at petroleum LNAPL sites. Long and costly cleanups have continued as NSZD has not become widely acceptable as a remedial option. We have developed a low cost rapid field approach (RFA) and expanded the site conceptual models (SCMs) to include NSZD. Results from two sites are presented that illustrate the benefit of the RFA in supplementing existing data and updating SCMs. At present, most NSZD guidelines follow a single generic SCM (aerobic oxidation of methane) as the primary contributor to NSZD. Aerobic oxidation of LNAPL is also possible, and can be documented by subsurface measurements of soil‐gas composition and/or heat generated from biodegradation. The RFA was developed so that sites could be easily separated into those with and without a methane plume overlying LNAPL. This distinction is important as it defines the relative rates of oxygen transport into the soil, and methane generation and release from saturated zone anaerobic biodegradation. For the RFA, evidence for direct LNAPL oxidation is absence of methane (<0.1%‐v) in the unsaturated zone above LNAPL. This measurement can be readily made in the field using existing monitoring wells. The absence of a methane plume above LNAPL would indicate that NSZD is a result of direct biodegradation of LNAPL. This alternative SCM needs to be considered and evaluated prior to implementing more costly data collection or other remediation or site management options.