ARCHIVED REPORTS_XR0006228 - Laserfiche WebLink

Home Browse Search

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