SITE INFORMATION AND CORRESPONDENCE

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o» in ao x0viz 'p•51u 64Z 5Z5L CHEVRON PRODUCTS 14004/005 Comments \\\ 1) The primary hydrocarbon source area at the site is believed to have been immediately to the east of the former tank pit, adjacent to the pump island. Definition of the probable source area is based on elevated concentrations of dissolved hydrocarbons measured in MW-3 to -5 (prior to September 1989) , and pre-remediation soil samples. 2) Removal of hydrocarbon mass from the subsurface is indicated by 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. 3) The apparent equilibrium line of dowagradient hydrocarbon migration which is noted at the site likely developed, at least in 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 bacterial biodegradation is dissolved oxygen. Alternative electron acceptor compounds which will facilitate anaerobic bacterial activity include nitrates, ferric iron and sulfates. The following electron acceptor data was analyzed at the site (see attached Laboratory Analysis Data Table) : (a) Dissolved Sulfate - concentrations are uniformly elevated at all sampled wells (see attached Site Map/Dissolved Sulfate Concentrations) . The presence of relatively abundant dissolved sulfate indicates sufficient concentrations of the anaerobic electron acceptor to facilitate the biodegradation of dissolved hydrocarbons both on-site and off-site. 3