SITE INFORMATION AND CORRESPONDENCE_1994-1996

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MEMORANDUM 0 CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD •CENTRAL VALLEY REGION 3443 Routier Road, Suite A Pone. �-6 '455-3000 Sacramento, CA 95827-3098 CAL : 8-494-3000 -9 PH 2: 32 TO: Wendy L. Cohen FROM: Philip S. Isorena Senior Engineer Associate Engineer ian DATE: 5 May 1994 SIGNATURE: i��2 • SUBJECT: COMMENTS ON THE REMEDIAL ACTION PLAN, SHELL OIL COMPANY, STOCKTON TERMINAL, SAN JOAQUIN COUNTY On behalf of Shell Oil Company, Pacific Environmental Group, Inc. (Pacific) submitted the Remedial Action Plan (RAP) for Shell's facility on Rough and Ready Island in Stockton. I have reviewed the RAP along with Shell's case file and previously submitted reports. My comments on these documents are presented below. Remedial Action Plan 1. The vertical extent of contamination needs to be defined. DW-1, which is the deepest screened monitoring well (MW) with a screen interval at 44-54 feet below ground surface, contains total petroleum hydrocarbons as diesel and gasoline (TPHD/G) and benzene. 2. At least two additional deep monitoring wells are needed to determine the vertical gradient and flow direction. 3. In August 1992, MW 23 had the following concentrations in ppb: TPHD - 2,700; TPHG - 16,000; benzene - 4,100; toluene - 3,300; ethylbenzene - 520; and total xylenes - 1,900. The data show that MW 23 is highly contaminated. The attached Figure 1, which is MW 23's drilling log, shows there is a confining clay layer above the aquifer comprised of the black silty sand. The log also shows that MW 23's screen completely penetrates the aquifer and the confining layer. The soil above the confining layer is highly contaminated with petroleum hydrocarbon compounds. If the water level rises into the soil above the confining layer, the contaminants may dissolve in the ground water and migrate through the well into the aquifer. Due to the well construction, it is not known if the ground water contamination is coming from the saturated clay or from the aquifer. Therefore, a well which is screened exclusively in the aquifer below the clay should be installed to determine the source of ground water contamination. 4. MWs 24-27 and DW-1 need to be surveyed. 5. The attached Figure 2, which is reduced copy of Figure 3 of the RAP, shows secondary source area (SSA) 2 does not extend beyond DW 1 to the east and MW 26 to the south. Since both DW 1 and MW 26 are contaminated, SSA 2 is underestimated. SSA 3 should extend beyond MW 27 which also is contaminated. 6. Cross-sections should be included in the RAP. Graphical representation of the site model also should be included to help identify if additional investigation and/or remediation is needed.