SITE INFORMATION AND CORRESPONDENCE FILE 2

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Former Chevron Service Station 9-0557 Page 4 139 South Center Street May 6, 2010 Stockton, California The EHD has several concerns with the FS and the conclusions and recommendations for MNA. For the reasons given below, the EHD recommends that the pilot designs, data and interpretations be reevaluated by HFA. An estimate of the sorbed and dissolved contaminant masses was not.provided in either the SCM or the FS. Although the groundwater monitored by the outboard monitoring wells MW-1, MW-2, MW-5, MW-6 and MW-8 apparently cleaned up between February 1996 and May 2000, the decline trend in the inboard monitoring well data (MW-15D, MW-15, MW-18D2) is not as clear. A degradation rate for the residual contaminants and an estimate of the time required to achieve cleanup goals utilizing MNA was not provide. The lack of this information makes evaluation of the recommendation problematical. As the site and the contaminant distribution were characterized in the SCM, much of the residual contamination resides in fine-grained soil in the saturated zone, rendering remediation a challenge. The reported results of the pilot tests lead HFA to conclude that none of the tested technologies would be cost-effective. The EHD has the following concerns with the field tests and interpretations: Pumping Test— Groundwater Extraction The radius of influence inferred from extracting groundwater from MW-9A (screened fifty-two to fifty-seven feet bsg) at a rate of 0.75 gallons per minute was fifty-six feet, which seems to be a reasonable distance. HFA concludes that groundwater extraction was not likely to be cost-effective, but with no contaminant mass estimate to compare to the extracted mass estimate or the mass extraction rate estimate, one cannot easily evaluate the cost- effectiveness of the method. Two Phase Extraction This test was conducted by extracting air and groundwater from monitoring well MW-9A and MW-9S, primarily utilizing the latter well. The test utilizing MW-9A was not likely to be very effective as the well was screened from fifty-two to fifty-seven feet bsg, approximately thirty feet below the water table. While the extraction in MW-9A may drop the water level to the screened interval of the well, it seems unlikely that the water table outside the well would also decline to the screened interval and thereby provide a path for soil gas to enter the well. If this is true, one could not expect much vapor recovery from MW-9A. Extraction from MW-9S, screened seventeen to thirty-two feet bsg, would have an initial seven feet of screen open to the vadose zone, as the depth to groundwater was approximately twenty-four feet at the time of the test; this should have improved the vapor recovery portion of the test compared to extraction from MW-9A. The observation wells (MW-11 and MW-12) with screen intervals matching the extraction well, were approximately thirty feet and seventy feet away from MW-9S, with MW-11 and MW-12 on the opposite side of the former UST excavation; most other wells were screened entirely in the saturated zone, and would not be likely to be subjected to much pressure drop. Although no pressure decline was observed, the great distances of wells MW-11 and MW-12 from the extraction well may have dampened the pressure drop; the intervening backfilled UST over-excavation may have allowed a 'short- circuiting' of atmospheric air that relieved the pressure drop.