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Karen Petryna Shell Oil Products 880 E. Victor Rd., Stockton, CA Page 4 of 5 groundwater, which would be contrary to the State of California's antidegradation policy. As discussed above, the data from the groundwater extraction events demonstrate that the true MTBE concentration still exceeds 30,000 µg/I. The high remaining MTBE concentrations, the uncertainty of identifying all permeable pathways in the sand-rich subsurface of this site, and the importance of all groundwater resources leads EHD to conclude that mass reduction is still necessary and EHD directs that the interim monthly groundwater extraction be reinitiated immediately to reduce contaminant mass and that a work plan for a continuous contaminant mass reduction system be submitted by 03 January 2005 and put into operation by 21 February 2005. During startup of groundwater extraction, water samples should be collected daily and analyzed to demonstrate stabilization of MTBE and TBA concentrations, after which less frequent sampling should be conducted. During quarterly groundwater monitoring events and during extraction, samples should additionally be analyzed for dissolved oxygen (DO), oxidation/reduction potential (ORP), nitrates, sulfates and methane to demonstrate the presence of environmental conditions associated with biodegradation of MTBE to TBA. Cambria has offered to move proposed monitoring well MW-12 on-site to assess and monitor the gap between MW-6 and MW-7. EHD concurs with this proposal and suggests that proposed well MW-13 be moved to the best alternative position on the San Joaquin County Mosquito and Vector Control District site. If the data from the locations indicate a need for additional wells on the Teresi Trucking site, EHD will be in a better position to assist you in obtaining an access agreement with the property owner. EHD has also revisited the soil vapor extraction (SVE) test data to reevaluate it for potential vadose zone remediation. The achievable vacuum potential and flow rates appear to be adequate, but the contaminant concentrations recovered were low. The MTBE concentrations had a general increase toward the end of each test. Mapping the MTBE concentrations in soil between 25 and 33 feet bsg show the vapor extraction wells SVE-1, SVE-2 and SVE-3 to be just outside the impacted area. Contaminants in soil samples from the three extraction well borings were at less than detectable concentrations by both field screening with a PID and by laboratory analysis. The low concentrations detected in the soil gas samples collected during the field test could well have been due to the travel time required to move the contaminants and dilution induced by most of the extracted soil gas pulled from the surrounding clean area outside the plume. In addition to the above directives, EHD directs that the necessity for vadose zone remediation and the potential use of properly located extraction wells be