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CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VA <br /> Phone: (916) 255-3000 <br /> 3443 Routier Road, Suite A � �' CALNET: 8-494-3000 <br /> Sacramento, 6A 95827-3098 <br /> Ott <br /> From: Elizabeth A. Thayer <br /> To: RJM, CMH <br /> l 1 Associate Engineer <br /> DATE: 12 November 1996 SIGNATURE: <br /> Subject: EXXONSTATION, 4444PE,RSHING,STOC%TON,SAN JOAQUIN COUNTY <br /> I reviewed the"Groundwater Remediation System Technical Plan",received 31 October 1996,two case <br /> studies submitted 11 October 1996, and the file for 4444 Pershing (Exxon) in order to determine whether <br /> Waste Discharge Requirements were necessary for the proposed remediation. I also reviewed files for <br /> the nearby sites at 4445 (Shell) and 4501 (7-Eleven)Pershing. <br /> My review can be divided into two separate categories;the investigation and the remediation proposal as <br /> follows: <br /> THE INVESTIGATION <br /> In addition to numerous wells on- and off-site,the investigation at the Exxon station involved <br /> installation of a monitoring well near the Shell and one near the 7-Eleven. Contamination was detected <br /> in both wells. For some time there was speculation that the Exxon station was the source of <br /> contamination in both wells even though the Shell is upgradient and the 7-Eleven is cross gradient from <br /> the Exxon. Both the 7-Eleven and Shell stations had documented releases. <br /> During the course of my review,I drew a ground water contamination concentration map using data <br /> from the three sites. The map is attached. Based on the shapes of the plumes, it appears that there are <br /> three plumes; a large one emanating from the Exxon,which once had free product, a smaller one <br /> emanating from the Shell and commingling downgradient with Exxon's plume, and a third plume <br /> emanating from the 7-Eleven, which appears to be unassociated with the other two plumes. <br /> The Shell station has installed 4 monitoring wells,two have shown sporadic low levels of contamination <br /> and two have been consistently ND. However, it appears that the wells installed by Shell have <br /> completely missed the ground water plume Of the four wells, one is cross gradient and three are <br /> upgradient.. Several wells on-site at the Exxon,appear to be down gradient of and monitoring Shell's <br /> contaminant plume and/or the commingled plumes, including;MW1,MW4,MW7,IW1, and P1 and P2. <br /> According to our file,the 7-Eleven has not yet started a ground water investigation. <br /> Also,based on the attached map, it appears that Exxon has not completely defined the lateral extent of <br /> ground water contamination to the northwest of the source area. <br /> < �' <br />