Rapid Field Approach to Evaluating Natural Source Zone Depletion for a Range of Light Non‐Aqueous Phase Liquid Sites

Abstract

AbstractMore 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.