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KEI-P91-0202 .R7 March 18 , 1993 Page 6 Lastly, KEI has reviewed the City of Lodi Public Works Department ' s historical water table data . Annual water depth data was analyzed for a total of five city wells for the period of 1972 through 1991 . During the study period, the ground water table data showed a decreasing trend (deepening of the water table) in each of the five wells . These results support the use of a constant depth to water of 75 feet below grade in the three simulations as a conservative approach. DISCUSSION AND RECOMMENDATIONS As recommended in the CWRCB LUFT Manual , the SESOIL application of the EPA' s Graphical Exposure Modeling System was utilized to model the migration of hydrocarbon contamination in the subsurface soil at the subject site. Results of the three SESOIL computer simulations indicate that no significant impact to ground water can be expected from the source of contamination at the subject Unocal site. Of the three simulations , benzene reached the soil./ground water interface only in Simulation #3 . Upon contacting the ground water in the lith year, however, the concentration of dissolved benzene was less than the California Department of Health MCL of 1 ppb. Additionally, in the ten months subsequent to initial contact with the ground water, the benzene concentration consistently attenuated to a non-detectable level (<0 . 5 ppb) . Based on the results of the SESOIL models, and based on the analytical results of all soil and ground water samples collected at the subject site to date, KEI recommends that no further contamination delineation or remediation work is warranted at the subject site. DISTRIBUTION A copy of this report should be sent to the San Joaquin County Local Health District, and to the Regional Water Quality Control Board, Central Valley Region. LIMITATIONS Environmental changes, either naturally-occurring or artificially- induced, may cause changes in ground water levels and flow paths, thereby changing the extent and concentration of any contaminants. Our studies assume that the field and laboratory data are reason- ably representative of the site as a whole, and assume that subsur- face conditions are reasonably conducive to interpolation and extrapolation.