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i 40 Impacted groundwater is calculated to be approximately 76 kilograms The TPH-G mass calculation is shown in Appendix 5 5.2 Associated Health Risks The public health risks associated with the Impacted groundwater at Earthg rains is limited due to a lack of potential receptors The two known public water wells within a 2,000 feet radius of the site are both situated in cross-gradient locations relative to groundwater flow The groundwater plume has not migrated In the direction of these public water wells Monitoring wells MW-2, MW-5, and MW-11, positioned between the plume and these public water wells have not been impacted As an added safeguard, the closest water well is an irrigation well that is surface-cased down to a depth of 180 feet, providing protection against shallower contamination, such as that ' at Earthgrains The next closest well Is surface-cased down to 140 feet ' 5.3 Natural Attenuation It is evident that petroleum hydrocarbon concentrations are degrading naturally over time Analysis of degradation parameters indicate that microbiological activity may be the cause of the petroleum hydrocarbon decreases observed at this site If current reduction trends continue, regression analyses of TPH-G and benzene concentrations (see Table 8 and Table 9) suggest that they may be reduced to non-detectable values by the years 2001 and 2006, respectively i i May 1999 Subsurface Investigation and Groundwater Monitoring Report ort The Earthgrams Company Faculty 2651 South Airport Way Stockton,Callforrlla Prepared for THE EARTHGRAINS COMPANY Prepared by Philip Services Corp E 11213821121382EG1125997t0599REP DOC June 22, 1999 ' 12