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NV C 4 o A S S O C I A T E S I N C During the survey, a domestic well was observed at 2541 Holly Drive According to the tenant who has been living at the residence for two years, the house is supplied with municipal drinking water and the property owner stated that the well is not operational Approximately 900 to 1,000 feet west of the site on Grant Line Road, the logs identify a site called "Jordan Real Estates" where there are: three monitoring wells identified as MW-1, MW-2, and MW-3 Each well is 24 feet deep and screened from a depth of 4 to 24 feet These wells are located hydraulically crossgradient from the site Approximately 1,800 feet to the west of the site, well logs identify an irrigation well with a completion depth of 212 feet and a screened interval ranging from 159 to 194 feet below ground surface This well is located hydraulically crossgradient from the site and is screened in an aquifer much deeper than the impacted aquifer Based on this information, it is ATC's judgement that it is unlikely that these wells have been impacted or are threatened by petroleum hydrocarbons from the site 5.0 MASS CONTAMINANTS • Using groundwater analytical results presented in Table 3, the most recent data available, ATC estimated the lateral extent of dissolved TPHg concentrations beneath the site (Attachment 5) and was then used by ATC to perform mass balance calculations to estimate the mass of residual contamination in the site's saturated zone The total mass of dissolved TPHg present in the saturated zone is estimated to be approximately 1 7 pounds Mass balance calculations are included in Attachment 5 The combined mass balance of benzene, toluene, ethylbenzene, and xylenes (BTEX compounds) are estimated to be far less than one pound since all of these constituents and MTBE were reported at concentrations below method detection limits 6.0 EVALUATION OF LOW RISK CRITERIA Using the criteria listed in California Regional Water Quality Control Board's No Further Action (NFA) requirements, as outlined by CRWQCB (Appendix B of the Tri-Regional Recommendations), conditions at the subject site were evaluated to determine if NFA as a "low risk, Groundwater Case" is appropriate for this site Criterion 1: Contaminants remaining in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants No contaminants have ever been detected in the vadose zone at the site with the exception of trace levels of ethyl benzene and xylenes in one sample collected below the water table Soil samples were not collected from the vicinity of associated piping removed in 1988 If soil beneath the piping had been impacted with petroleum hydrocarbons it has likely degraded and does not pose a threat to groundwater as W 192054 00021closure-addendum doc 4