SITE INFORMATION AND CORRESPONDENCE

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Mr. John Welty %WOI April 10, 1990 Page 12 Downgradient well MW-10 indicated detectable concentrations of TPH as recently as November 30, 1988. The highest TPH level found in Well 10 was 630 ppb in November of 1986. Downgradient well PW-2 has indicated TPH levels greater than 100 ppb in one of 45 sampling events. Detectable levels, less than 100 ppb, have been found in 4 of the 45 sampling events. The most recent detection was in January of 1989 at a concentration of 13 ppb. The February 1990 Project Update Report prepared by GTI summarized that dissolved hydrocarbons beneath the site continued to be concentrated near the former underground storage tanks and pump islands. The surrounding domestic wells did not contain detectable levels of dissolved hydrocarbons. GTI also stated that the initial high dissolved hydrocarbon concentration recovered in RW-1 dropped to below Method Detection Limit when pumping created a cone of depression and the groundwater elevation in MW-1 fell below the permeable gravelly coarse sand unit. GTI proposes that the subsurface soil and groundwater impacted by the highest concentration of dissolved hydrocarbons may be restricted to the upper stratigraphic sequence, particularly in the gravelly coarse sand unit. Therefore, their expansion of the vapor extraction system to include the four dry on-site wells would work to clean up this area if the groundwater stays lower than the wells. Regulatory Response and Southland's Implementation The following information was obtained from letters in the RWQCB files for the site. A letter to Southland from San Joaquin County Environmental Health Division (County), dated June 6, 1989, reported results of the County's review of GTI's quarterly reports and an April 1989 Work Plan for the installation of additional monitoring wells. The County found the work plan to be inadequate, and included the following considerations: 1. Data is insufficient to adequately illustrate a true gradient or recovery capture. 2. Additional work is needed to adequately define the area,or areas of influence and to provide for "zero-line" definition. The installation of clustered monitoring wells may provide needed data. However, the southern occurrence (Well 10 and 8968, located southeast or downgradient of the TGA property) of some sampling detections of hydrocarbons must be addressed. 3. Hydraulic control of the contaminant plume is doubtful. Pumping wells PW-1 and RW-1 do not appear to be creating large enough cones of depression to capture the entire plume. in addition, a leaking water pipe behind the store has caused the Brown and Caldwell C