SITE INFORMATION AND CORRESPONDENCE

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Comments _ a 1) The primary hydrocarbon source area at the site is believed1to have been immediately to the east of the former tank pit, adjacent f to the pump island. Definition of the probable source area is based on elevated concentrations of dissolved hydrocarbons measured lin MW-3 to -5 (prior to September 1989) , and pre-remediation soil samples. 2) Removal of hydrocarbon mass from the subsurface is indicatedby the recovery of the VES, analytical data from confirmation soil borings, and the reduction in dissolved hydrocarbon concentrations in downgradient well MW-11. It should be noted that groundwater has not been sampled from formerly impacted wells MW-3 , -4 and -5 since late 1989, due !to insufficient water. If the trend toward increasing water table elevation continues, it may be possible to sample these wells during a future monitoring event. Reduced hydrocarbon concentrations in groundwater samples collected from these wells will provide conclusive evidence for the effectiveness of former remediation activity. f f Indirect evidence of source removal is indicated by water quality data from MW-11. The observed reductions in the concentrationsi.of dissolved hydrocarbons in MW-11, located approximately 60 feet from the former USTs and dispensers, suggests an actual reduction in the hydrocarbon mass within the apparent source area. i 3) A stable dissolved hydrocarbon plume, with a defined finite migrational distance, is indicated by the history of nondetectto minimal hydrocarbon concentrations in downgradient wells 'MW-10,. - 12, -13 and -14 . I I 4 The apparent equilibrium quilibrium line of downgradient hydrocarbon migration which is noted at the site likely developed, at 'least �lin part, to the attenuation and destruction of dissolved hydrocarbons by a variety of processes. Destructive processes include biodegradation, abiotic oxidation and hydrolysis. The process of biodegradation results in the ultimate transformation of the hydrocarbon compound into the metabolic byproduct of bacterial activity; water and carbon dioxide. The limitation on the degree of biodegradation is often the absence "of a suitable electron acceptor compound available in the subsurface. By measuring groundwater for the presence of various electron acceptor compounds, the areal distribution of the dissolved phase may be explained. The most common electron acceptor preferred for aerobic bacteriIal biodegradation is dissolved oxygen. Alternative electron acceptor compounds which will facilitate anaerobic bacterial activity 3 1 i