SITE INFORMATION AND CORRESPONDENCE_FILE 2

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KEY ISSUES FORMER MOBIL OIL BULK PLANT 500 East Grantline Road Tracy, California January, 1994 ■ No liquid- or separate-phase hydrocarbons have been observed or encountered in the groundwater at any time during soil excavation/remediation activities and throughout the groundwater monitoring program to this date. Soil was excavated to below the groundwater and no separate-phase product was observed in the groundwater at any time, even as sheen. ■ Petroleum hydrocarbons detected in the capillary fringe or zone are addressed as part of the dissolved-phase hydrocarbon plume in the groundwater. ■ Grab water sample from Boring B-7 is not considered an accurate representative of the water-bearing zone. This sampling technique is used for screening purposes only since the procedure is not consistent with standard sampling protocol for properly constructed monitoring wells. In some instances, the grab water sample can be spiked with the overlying contaminated soil, if present, such that the analytical result is higher than for a similar sample collected from a properly constructed and developed well. Analysis of groundwater samples collected from the confirmation monitoring well AW-9 installed adjacent to Boring B-7 did indeed reveal lower hydrocarbon concentration and in some instances, no detectable levels of hydrocarbon constituents. While several regulatory agencies have allowed the use of this qualitative survey method as a cost-effective measure to assist in the investigation of hydrocarbon plumes, it still does not provide a quantitative representation of actual field conditions. ■ Residual petroleum hydrocarbons remaining beneath the site,if any, are primarily in the capillary zone of the upper water-bearing unit and in the groundwater. ■ Adsorbed-phase petroleum hydrocarbons in the vadose (unsaturated) zone beneath the site have been adequately defined and effectively removed and remediated. ■ The extent of dissolved-phase hydrocarbon plume in the groundwater has been adequately defined both onsite and offsite. Historical monitoring and sampling data indicate a generally decreasing trend of the hydrocarbon concentration in the groundwater over time. ■ Benzene was detected only in one of the samples collected from the S onsite wells in October 1993 at a concentration of 0.76 parts per billion. During this sampling event, total petroleum hydrocarbons (TPH) were not detected in any of the water samples, except for TPH as diesel in two of the onsite wells at a concentration of up to 190 ppb. ■ The linear velocity of groundwater represents advective flow and the mass movement of groundwater through the media. Contaminant transport/migration, however, is influenced by other factors,such as sorption,biodegradation,and volatilization. The rate of contaminant transport is lower than the advective movement of groundwater.