SITE INFORMATION AND CORRESPONDENCE

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• Regardless of the final outcome concerning the status of well MW-4, review of the historical groundwater data for the site suggests that the groundwater contaminant plume is likely expanding., There are indications that remaining soil contamination (which was apparently inaccessible to excavation due to site constraints) may be acting as an additional contaminant source. Given that groundwater levels have risen considerably in the last several years due to above-average rainfall, it is likely that the rise in the water table has mobilized contamination in the previously unsaturated vadose zone. • An additional problem created by the rising water table is the effectiveness of the monitoring wells. The wells are currently screened approximately B to 7 feet below the water table surface. Since gasoline has a density less than water, the concentrations will typically decrease substantially with increasing depth. Based on this, the submerged screens make it difficult to collect "representative" groundwater samples. Consequently, it is likely that actual groundwater contaminant concentrations are higher than those currently being reported. If the actual concentrations exceed the solubility of the various gasoline components, then free product will begin to accumulate atop the water table. Given that the submerged well screens are 6 to 7 feet below the water table, it is possible that free product may already be present in the vicinity of well MW-2_ This can be evaluated during the pilot testing, as described later in this letter. While some regulatory agencies accept Risk-Based Corrective- Action (RBCA) assessments as a means to evaluate the potential health threat of a contaminated site, Mr. Sasson has indicated that San Joaquin County is reluctant to accept this approach. Consequently, anticipating the site to be F closed based solely on a R13CA assessment is likely fruitless. Further !! corroborating this point of view is the fact that the groundwater contaminant S plume appears to be expanding, which suggests that the site is even lees likely to be a candidate for closure based solely on a RBCA assessment. Based on the above conclusions, JJW Geosciences recommends the following further actions: 1. Continue quarterly monitoring and sampling of the four groundwater -- monitoring wells. As suggested above, JJW Geosciences recommends that split samples be collected and analyzed for wells MW-3 and MW-4 for the next sampling event. Task 1 on the enclosed Cost Estimate Worksheet details JJW Geosciences costs for quarterly groundwater sampling and reporting. 2. Prepare a workplan to install an extraction well and conduct emedi��pflot tests. 3. install one 44nch extraction well. JJW Geosciences proposes install the At well in the vicinity of the area of highest contaminant concentrations, between and approximately equidistant from existing wells MW-1, MW-2 and MW-3. The extraction well is proposed to be installed to a depth of approximately 25 feet, and screened between the depths of 5 and 25 feet.