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KEI-P91-0202 .QR5 October 27 , 1993 Page 4 depth to ground water in the four monitoring wells varied from 49 . 83 to 52 . 86 feet below grade, and the direction of ground water flow was towards the northwest at a gradient of 0 . 02 . Analyses of the ground water samples collected from these wells yielded non- detectable results for TPH as gasoline and BTEX. Analysis of the soil samples from the borings for the wells indicated concentra- tions of TPH as gasoline and benzene ranging from non-detectable to 310 ppm and 0 . 29 ppm, respectively. The elevation of ground water at the Arco station is approximately 12 to 15 feet higher than at the Unocal site. The ground water elevations at the Arco and Unocal sites appear to be controlled by a low permeability clay unit, which most of the Arco and Unocal wells terminate within. This clay unit is present at the Arco site between 50 and 53 feet below grade, and at the Unocal site between 74 and 80 feet below grade. The elevation of ground water in the Arco and Unocal wells is generally within several feet of the upper contact of the clay unit. Based on the historical analytical and monitoring data for the Arco and Unocal sites, it does not appear that contamination from these sites has commingled. KEI has previously recommended that a meeting be held between both the San Joaquin County Department of Public Health and the RWQCB, to discuss the subsurface investigation that has been conducted at the Unocal site to date. As of the date of this report, Unocal has been unable to arrange a meeting. DISTRIBUTION A copy of this report should be sent to the San Joaquin County Public Health Services, and to the RWQCB, 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. The results of this study are based on the data obtained from the field and laboratory analyses obtained from a state-certified . laboratory. We have analyzed these data using what we believe to be currently applicable engineering techniques and principles in