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SECOND QUARTER, 1997 GROUNDWATER MONITORING REPORT USA STATION#65-LODI JUNE, 1997 Concentrations of TPH G for this sampling event ranged from 98 micrograms per liter(µg/1) in ' MW-2 to 18,000 µg/1 in S-2 for those wells containing TPH G TPH G was non-detect at the method detection limit (ND) in wells MW-1, MW-5, NfW-6 and N W-7 Benzene was detected in seven monitoring wells (MW-1, MW-3, MW-4, MW-5, MW-8, S-1 and S-2) at concentrations of 0 85 µg/1 in MW-1 to 1,700 µg11 in S-1 Benzene was ND in wells MW-2, MW-6 and MW-7 MTBE was ND in all wells Analytical results are summarized to Table 2, and shown on Figure 5 Copies of laboratory reports and COC documentation are included in Appendix B The groundwater elevations have continued to rise since the January 23, 1997 sampling event Groundwater elevations are up an average of 0 63-feet and remain at a historical high This high groundwater may have come in contact with some impacted soil in the vicinity of the UST's and MW-3 and MW-4, which may explain the increase of petroleum hydrocarbons present in the ' monitoring wells MW-3 and MW-4 have TPH G and benzene concentrations of 490 and 7500 µg11 and 3 0 and 70 pg/l, respectively Histoncal analytical results for groundwater in these wells have been ND to very low values The chromatogram signature in Apprendix B for groundwater in these wells indicates a weathered gasoline The high groundwater levels appear to have resolubilized degraded petroleum hydrocarbon compounds, wluch had been in the capillary fringe or vadose zone The impacted soil and groundwater can be shown to have a vertical extent at MW-3 and MW-4 of hydrocarbon migration from historical chemistry and aquifer gauging data to appoximately 41-feet bgs The lateral extent of impacted groundwater is limited as indicated by the lack of TPH in the downgradsent wells There does not appear to be any indication of plume migration At this time, GHH recommends that monitoring be continued r , 5