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Unocal Corporation 1 - 2000 Crow Canyon Place,,a 400 r San Ramon,California 34583 TeleFacs mile{hone 5 ) 10)277 2309 }`'''}' ; �,; 11 t F 11 fr S ' ? PN z: 37 UNOCALO September 9, 1994 Northern Region Corporate Environmental Remediation 8 Technology Mr. Harlin Knoll, Senior REHS Site Mitigation Unit Environmental Health Division San Joaquin County P. O. Box 388 Stockton, CA. 95201-0388 Unocal Service Station #6015 601 E. Kettleman Lane Lodi, CA Dear Harlin: This letter is in response your June 2, 1994 letter and your recent telephone phone call. I apologize for not responding sooner to your communications. In your letter, you indicated that t the Technical Review Committee had recommended that the original SESOIL program run by f Kaprealian Engineering, Inc. (KEI) should be modified to utilize a residual benzene concentration of 1.9 ppm at a depth of 32.5 and a groundwater level of 50 feet below grade (1986 data), for input data. It is not necessary to re-run the SESOIL program to determine the effect of a shallower groundwater. This can be seen by simply using a depth of approximately 530 cm. and would equate with a time of nearly 46 months. By extrapolation, there would be measurable benzene concentrations at groundwater at this time (in the 1 ppm range). Please note that this time and concentration is entirely dependent upon no other chemical or biological reactions taking place. As part of the model run, the original depth was set at 26 feet to simulate the effect of a point of a suspected leak. Please note that tank bottoms are normally at 12 - 14 feet. Deeper depths of initial contamination equate with the deeper downward movement of hydrocarbons. The -- effects of a different benzene concentration is slightly more complicated; however, it can be roughly approximated by doubling the initial concentration and end point (in actuality, there is a dispersion effect which reduces concentration over vertical distance). The most controversial condition is the use of a groundwater level of 50 feet for the depth to groundwater. Previously submitted data (November 2, 1993, KEI report) shows that groundwater throughout the region has dropped nearly 15 feet over the past 10 years, despite j the 1992 rain season. It is interesting that a property (ARCO service station) immediately southeast of our former property has water levels that are nearly 18 feet higher than those measured in our wells. I believe that is could be argued that a more reasonable depth to water on our former property should be 60 to 65 feet (for 1986). It should,berrioted that 1986 was the "100 year" rainfall and should not be used as an average for the valley. It is very unlikely that water levels could recover to the 1986 levels unless all pumping in the Central Valley (both agricultural and consumptive) is reduced. With the increased building and agricultural demands