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S in any significant impact to groundwater. Inasmuch as the groundwater levels since 1998 are abnormally high due to recent wet years and should drop below the contaminated soil in the future, it is not expected that any impact of significance will ever occur. Water level data are summarized in Table 4. 1 I 6.0 RECEPTOR SURVEY ' As part of the investigation of the adjacent property at 601 E. Miner Ave., Versar, Inc. conducted a survey of water supply wells within a 2,000 foot radius of the site (Environmental Assessment Report and Workplan Addendum, Versar, Inc. July 3, 1996). Five wells were located, the closest of ' which is approximately 1,000 northeast of the site. This well has not been used since approximately 1991. Information concerning the wells is contained in Attachment C. ' Currently the only potential receptors are individuals accessing the parking area at Stockton Clutch and Brake. 1 7.0 HEALTH RISK EVALUATION AND CONTAMINANT MASS ESTIMATE I ' Ground Zero completed an evaluation of the risk to human health and the environment resulting from the residual contamination beneath the former UST pit. The mass of residual contamination ' was also determined. The risk evaluation focused on potential human exposure to hydrocarbon vapors potentially emanating from the subsurface soil contamination. Quantification of risk was computed using the program RSCA TOOL KIT for Chemical Releases (Groundwater Services, Inc. 1998). This program uses the methodology of the ASTM document Standard Provisional Guide for Risk Based ' Corrective Action (ASTM PS-104, ASTM, 1998). Based on current and potential future property use, the exposure pathway of concern is outdoor vapor inhalation under a commercial or residential property use. It was determined that the lifetime excess cancer risk due to vapor inhalation under a ' commercial use is 7.1 E-8 (7.1 chances in one hundred million) and the hazard index for toxic effects is 1.5 E-3. With a residential property use the excess cancer risk is 1.0 E-7 (one chance in ten million) and the hazard index is 1.8 E-3. These values are orders of magnitude below the most ' stringent "levels of significance" prescribed by any governmental agency (1 E-6 excess cancer risk and hazard index of 1). The conclusion is that the residual hydrocarbons at the site do not pose an unacceptable health risk. Details of the risk evaluation process, parameters, calculations and ' conclusion are contained in Attachment D. The mass of residual contamination was also calculated. The approximately 385 cubic yards of residual contaminated soil at the site is estimated to contain approximately 4,400 pounds of gasoline and less than one pound of benzene. Detailed mass estimates are included in Attachment D. ' datalgroundzelvalleym\summu y.doe 6