SITE INFORMATION AND CORRESPONDENCE

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I i r Ms. Mary F. Ways January.18, 1995 The Southland Corporation-9110 Thornton Rd., Stockton, CA 2 ,Free phase hydrocarbons Free-phase hydrocarbons were initially observed in the southern wells at the site, at a depth of about 30 feet below grade. These hydrocarbons were subsequently removed during an extensive groundwater extraction and product recovery program. Free-phase hydrocarbons have not been measured at the site since August, 1988. Dissolved hydrocarbons Dissolved hydrocarbons were initially measured around the site in an area extending off site, as reported In early assessment and monitoring reports. Of particular concerns were benzene levels reported at up to 12,000 ppb in well MW-1 in March, 1988. After an extensive program of groundwater extraction (wits, about 1.5 million gallons of groundwater treated), and source reduction by vadose remediation, and under a lowering groundwater table, the dissolved concentrations of hydrocarbons in groundwater samples have consistently been at or below state action levels. The monitoring well located in the center of the area of the suspected leak, and the area of highest reported past dissolved concentration (well MW-1), has shown consistently low levels of benzene of j less than 1 ppb since May 1990, and levels of total petroleum hydrocarbons of 51 ppb or less since July 1990. The most recent samples, collected in conjunction with the CRWQCB, indicate that well MW 1, located in the southern portion of the site, in the area of suspected highest hydrocarbon levels, contains total petroleum hydrocarbons, as TPH as gasoline, at 34 ppb, with 1 ppb benzene level, which corresponds to state action levels. As regular sampling of the wells was conducted prior to that sampling event (defining the migration direction of the plume) and as other wells provide a lateral boundary to the hydrocarbons in well MW-1, an estimate of the mass of hydrocarbons dissolved at the site can reasonably be calculated: assuming conservative lateral dimensions of the plume at 150 meters, with an aquifer thickness of 20 feet and a porosity of 30%, approximately 1 million gallons of groundwater at the site may conservatively contain hydrocarbons. The estimated mass of dissolved benzene and gasoline hydrocarbons, based on the reported concentrations, is less than 0.01 lbs and 0.3 lbs respectively. The risks associated with dissolved hydrocarbons must also be estimated by evaluating the risks of further groundwater degradation from sources.in the vadose zone, as adsorbed or vapor hydrocarbons potentially migrating.to the groundwater. These additional potential sources are discussed in the following.sections. STHLSTO.TX1'(STHL�) ❑❑� GROUNDWATER ❑❑❑ TECHNOLOGY i