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THIRD QUARTER,1998 GROUNDWATER MONITORING REPORT • USA STATION#65-LODI AUGUST,1998 5.2 Results of Groundwater Laboratory Analyses Concentrations of TPH G from the July 29, 1998 sampling event ranged from 74 micrograms per liter(µg/1)in MW-2 to 18,000 µg/I in MW-4. TPH G was non-detect (ND) at the method detection limit in wells MWA, MW-5, MW-6, and MW-7. Benzene was detected in five wells sampled at concentrations ranging from 12 µg11 in MW-3 to 2,300 µg/1 in S-1. Benzene was ND at the method detection limit in wells MW-1, MW-2, MW-5, MW-6, and MW-7. MTBE was ND in all wells except S-1, S-2 and MW-8, where it was reported at 2,900, 2,500 and 40 4g/1, respectively. Groundwater analytical data are summarized in Table 2, and shown on Figure 6. Laboratory reports with COC documentation are included in Appendix B. Monitoring well MW-4 continues to have sporadic elevated concentrations of TPH G and benzene. The chromatographic signatures for MW-4 indicate an older weathered gasoline. The relationships between TPH G and benzene concentrations and groundwater elevations in MW-4 are shown on Figure 7. 6.0 CONCLUSIONS AND RECOMMENDATIONS TPH G concentrations remain low with the exception of sporadic seasonal concentrations in MW- .4. As stated in the second quarter QMR, GHH recommends the installation of a sock containing oxygen release compound (ORC) into MW-4. In addition to MW-4, the downgradient wells MW-5, MW-6 and MW-7 will also get ORC socks to take advantage of the elevated groundwater conditions. Given the high groundwater levels with the addition of oxygen may enhance aerobic bio-activity in the subsurface by an infusion of elemental oxygen to promote the natural degradation of petroleum hydrocarbons in the groundwater and capillary fringe. The ORC sock will be removed approximately two weeks prior to the next groundwater sampling event to allow the groundwater quality to equilibrate. The groundwater purging process prior to sample collection will ensure that a representative groundwater sample will be collected. With the replacement of the USTs and fuel distribution lines and the removal of most of the impacted soil during the site retrofit, potential risks associated with this service station have been significantly reduced. This site should be considered for no further action as a low risk site. , i 5