SITE INFORMATION AND CORRESPONDENCE_FILE 1

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-- Me rr or and u m CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VALLEY REGION 3443 Routier Road Phone: (916) 361-5600 Sacramento, CA 95827-3098 - ATSS: 8-495-5600 TO: Antonia Vorster ' FROM: Dave Brent Senior Enginee �% y� Area Engineer CN F`ERWIIT/,6ERArETH S i DATE: 28 October 1987 SIGNATURE: SUBJECT: SITE INVESTIGATION REPORT FOR UNDERGROUND TANK LEAK AT EXXON STATION, 4444 PERSHING AVENUE I have reviewed the subject report dated 28 August 1987, prepared by EA Engi- neering. The Exxon Company claims that the ground water contamination below its site is from an off-site source and there is no threat to water supplies. The company has requested the San Joaquin Local Health District 's concurrence with this conclusion and its approval of the sale and development of the site. I do not agree with Exxon' s claim that the contamination is caning from off-site and find that further plume definition and clean up is necessary. My comments are as follows: 1. A soil vapor survey was conducted as part of this site investigation. Four- teen soil vapor probes ranging from 3.5 to 7.0 feet deep were collected for on-site analysis. The results of the survey indicate almost no soil contami- nation at the site. These results are not surprising since soil and ground water contamination was confirmed and the soil contamination was eliminated between June 1986 and January 1987 when monitoring wells 1, 2 and 3 were installed and the under- ground tanks , related pipes, and contaminated soils were removed. The contaminated soils were aerated and backfilled. As would be expected, the data contained in the subject report shows very little evidence of soil contamination. The data also shows that the contaminated ground water plume is now migrating off-site in the direction of the ground water gradient. 2. The report concludes that the ground water contamination increases to the north and northwest (downgradient from former tanks and piping) . Conclusions state further that the distribution of ground water contamination indicates it came from an off-site source, to the north or northwest, when the ground water gradient came from a different direction in the past. The contamination does increase to the north of the former tanks. This should be expected since the source has been removed and the "hot-spot" is no longer at the original source. There is no documentation or evidence of ground water gradients shifts in the area and the conclusion that a former source existed to the north of the site is unsubstantiated. The data collected has shown the highest level of contamination in the most downgradient well (Benzene at 3,100 ppb in MW-G) . The downgradient extent of the plume has not been defined and further monitoring wells are required.