SITE INFORMATION AND CORRESPONDENCE

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James Munch Page 2 documentation to support this statement. The first reference to this amount of soil was made in a work plan dated August 6, 1993 prepared by PEG and they were not the consultant at the time of the excavation activities. There is a substantial difference between 350 and 1100 cubic yards of soil being removed. UNOCAL has also stated that the vapor extraction system was shut down because the level of contaminants being removed had become asymptotic. As with most vapor extraction systems, the influent levels were higher when the system was started. The levels then evened out as shown by the fact that the influent level in February 1996 for TPH was 3800 ug/L and the influent level in November 1996 for TPH was 3000 ug/L. The vapor extraction equipment had problems during the period of operation and in October and November 1996 vapor samples indicated the system was not meeting the San Joaquin Valley Unified Air Pollution Control District permit requirements. The system was therefore shut down (documented in PEG letter dated December 20, 1996 enclosed). Vapor extraction appears to be a viable active remediation alternative for this site. The vertical extent of soil and groundwater contamination at this site has not been defined and, therefore. the lateral extent of contamination has not been defined. When monitoring well MW2 was installed, there were readings on the organic vapor analyzer meter to 50 feet bgs. When vapor extraction well VW2 was drilled, it was noted by PHSIEHD staff that there were high levels of TPH at 51 feet bgs. in 1992, groundwater level was about 54 feet bgs and TPH levels in monitoring well MW6 were 53,000 ug/L. Thus, we know that contamination exists more than 54 feet bgs. This site was originally leased in 1947 and was remodeled in 1967. San Joaquin County Flood Control maps indicate that depth to groundwater was 65-70 feet bgs in 1978. To define the vertical extent of contamination, a continuous core boring should be drilled near MW2 with collection of soil and groundwater samples to a depth of at least 70 feet bgs and further contamination is not found. When the contamination has been defined vertically, a decision can be made regarding whether it has been defined laterally. Soil sampling at the shallower depths will also indicate whether the vapor extraction successfully removed the contamination from the piping leak or if it still exists. UNOCAL has stated that the contamination will remediate by natural attenuation. Data to support that statement should also be submitted. We know that contamination exists below the current groundwater level which slows remediation. Installation of a air sparge or biosparge well to increase its effectiveness should be considered. As discussed at the December 1997 meeting, our agency cannot prevent the City of Stockton or other water purveyors from placing future wells in this area. UNOCAL must demonstrate that the groundwater contamination will be remediated before there is a need to use the groundwater in this area.