SITE INFORMATION AND CORRESPONDENCE

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r • ,, MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VALLEY REGION 3443 Routier Road,Suite A Phone:(916)255-3000 Sacramento,CA 95827-3098 CALNET:8-494-3000 To: Wendy L. Cohen From: Joel M. Weiss Senior Engineer Engineering Geologist Date: 18 July 1996 Signature: Subject: STATUS OF INVESTIGATION AT CHEVRON PIP NE SITE, CORNER OF CORRAL HOLLOW AND GRANT LINE ROADS, TRACY, SAN JOA QUIN COUNTY I have reviewed the file and two latest reports entitled Results of Soil and Ground Water Investigation (15 December 1995) and April 1996 Ground Water Monitoring and Deep Ground Water Assessment (9 July 1996). The following is a summary of the work completed so far and the work needed to begin remediating the site. A pipeline leak in 1987 caused gasoline and diesel contamination of soil and ground water at the site. The immediate spill area was excavated (approximately 100 cubic yards) and backfilled with clean fill although it is unclear if any soil confirmation sampling was done in the excavation. The lateral extent of soil contamination has been fairly well-defined with borings, except for the area directly east of the leak. Chevron believes that the vertical extent of soil contamination is limited to about 12 feet below the ground surface (bgs), but there are no analytical data to back this up. It is unclear if soil sampling was conducted during the drilling of the ground water monitoring wells at the site. Ground water is approximately 10 feet bgs and flows to the east-northeast. There are 11 ground water monitoring wells at the site, all of them north of the leak area. Better lateral definition of ground water contamination is needed to the east and south of the leak area. The recent installation of MW14 included obtaining a hydropunch sample from a relatively deep portion of the aquifer. That sample contained 80 µg/1 of total petroleum hydrocarbons as gasoline (TPHg). Because the wellwas completed as a shallow well (screened from 6 to 26 feet bgs), there is still no means of monitoring the vertical extent of ground water contamination. Historical ground water data are sporadic, and sampling commonly occurred during the dry season. It would be appropriate to begin monitoring the wells at the site on a quarterly basis. Lead and methyl t-butyl ether should be included in the analytical protocol for at least the first quarter. Chevron believes that intrinsic bioremediation is occurring at the site based on certain indicator parameters. However, the spatial trends of these parameters are still unclear, and benzene and gasoline concentrations have not declined significantly in the last five years since ground water sampling began. Recent benzene concentrations are as high as 5,200 µg/l, and TPHg concentrations are up to 58,000 µg/1. In an August 1994 letter, Chevron proposed the use of oxygen-releasing compounds to help promote bioremediation. This technique should be acceptable as an interim remedial measure at this time as long as the plume is better defined and monitoring continues. JMW'mw/lsb