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Remedial Action Plan Addendum January 14, 1993 Southland Store No. 20304, 455 Grant Line Rd., Tracy, CA 4 MW the location of the building, the closest monitoring well (MW-5) in that direction is approximately 50 V feet north and has not exhibited detectable hydrocarbon concentrations. Groundwater samples collected from wells south and west of the potential source area have also Indicated an absence of hydrocarbons above detectable levels. Concentrations within individual wells have fluctuated significantly, although the variances appear to coincide among the wells as a group. The variances are probably caused by seasonal groundwater elevation fluctuations. The distribution of hydrocarbons along the trough may also be related to the depth of the wells in this area. Generally, groundwater samples collected from wells installed to less than 10 feet below grade, such as MW-9 and MW-10, have contained hydrocarbon concentrations an order of magnitude greater than those installed to 15 or 25 feet below grade. Therefore, the observed hydrocarbon distribution may be V affected by the depth and source from which the water samples were collected: The shallower wells are recharging solely from the upper few feet of the aquifer, where higher dissolved concentrations of hydrocarbons are expected to occur. 2.0 PROPOSED REMEDIAL ACTION The primary purpose of proposed remedial action at the site is to reduce hydrocarbon 64+%.1.- concentrations in soil and groundwater to obtain site closure. Groundwater Technology proposes to remediate the site In two phases. Phase I will consist of gaining hydraulic control (containment) at the site through groundwater extraction and treatment. Phase II will consist of remediating impacted soils that continue to contaminate the groundwater, using an appropriate remedial alternative. The alternative will be determined after the effectiveness of groundwater extraction and treatment system has been evaluated. Figure 4 illustrates the decision process that will be used in choosing Phase II remedial strategy. 2.1 Groundwater Containment and Treatment (Phase 1) Data review suggests the presence of highly variable hydraulic conductivities in the saturated zone. `A These conditions have led to channeling dissolved hydrocarbons in high conductivity areas, Increasing the accumulation of petroleum hydrocarbons in finer materials or lower hydraulic 4 conductivity areas. Based on this Information, containing and remediating the groundwater should be the first priority to reduce potential adverse environmental impact. Containing the affected R3263A1.SLi �ED�' GROUNDWATER nE TECHNOLOGY,INC.