A pilot test of passive oxygen release for enhancement of in situ bioremediation of BTEX-contaminated ground water

A pilot-scale field demonstration of the use of Oxygen Release Compound(TM) (ORC) was conducted at the site of a former gasoline service station. ORC was installed into a barrier consisting of a tight pattern of ''treatment wells'' located relatively near the apparent source of hydrocarbon contamination (20 to 30 m downgradient). The purpose of the barrier was to enhance in situ biodegradation of BTEX in ground water by the passive release of oxygen from the unpumped treatment wells placed across the migration path of the plume. Detailed monitoring was carried out using fencelines of multilevel monitoring wells located up- and downgradient of the barrier. Total BTEX concentrations influent to the barrier were found to be highly variable in space (nondetect to greater than 60 mg/L) and time. Total influent BTEX concentrations averaged on a cross section transverse to flow were less variable over time, ranging from 10 to 16 mg/L. Significant decreases in BTEX mass flux through the zone impacted by the treatment wells were observed. For the entire portion of the plume impacted by the treatment wells, estimated BTEX treatment efficiency was approximately 70 percent on Day 51 of the test and declined thereafter. The decrease in efficiency was likely due to the observed increase in the influent BTEX load as the trial progressed along viith an apparent decline of the oxygen release rate from the ORC. However, along flow-paths in which influent total BTEX concentrations were below about 5 mg/L, nearly complete removal of BTEX occurred for the entire trial (132 days). Mass flux estimates of BTEX and dissolved oxygen indicated that less than 10 percent of the oxygen estimated to have been released from the treatment wells contributed to BTEX degradation or was observed as ''excess'' oxygen. NonBTEX components of the organic contamination appeared to exert a significant oxygen demand and account for at least part of this discrepancy.