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i I22 December 1998 AGE-NC Project No 95-0128 Page 6 of 8 I TBA was detected in the ground water samples collected from MW-3 at 5 9 'cg/1 and from MW-7 at 22 ,ug/l I Laboratory results of water samples analyzed for petroleum hydrocarbons are summarized in Tables 3, 4, 5 and 6 The laboratory reports for the quarterly monitoring event (MAI Laboratory ID No 88799 to 88806) QA/QC reports and chain-of-custody form are included in Appendix D 3 5 GROUND WATER ELEVATION AND GRADIENT ' The casing elevation of each well was surveyed relative to a City of Stockton bench mark The elevation of ground water to each well was calculated by subtracting the ground water depth from ' the surveyed casing elevation The depth of ground water, casing elevations and ground water elevations for the past year are summarized on Table 4 The hydraulic gradient was determined by ' contouring the ground water elevation in each well Ground water gradient on 16 June 1996 was calculated to be 0 0016 ft/ft over the site, directed toward the north The ground water elevation map for the site is illustrated in Figure 3 4.0. SUMMARY AND CONCLUSIONS ' Based on results of this investigation AGE concludes ' • Hydrocarbon-impacted soil was not present below the static water level in the areas north of MW-5, west and northwest of MW-7, or east to northeast of wells MW-2 and MW-4 ' • Overall, concentrations of petroleum hydrocarbons detected in water samples collected from VW-1 decreased However, TPH-g and BTEX concentrations increased in ground water samples from well MW-6, which included the highest concentrations of benzene detected ' this quarter • Data from analysis of ground water samples collected from the on-site wells strongly suggests that the dissolved TPH-g and BTEX plumes extend off-site toward the north and nonhedst Data derived from the grab ground water samples indic.dws th it the uissulvcd TPH-g plume does not extend to the off-site soil probe boring sampling points Minor ' concentrations of BTEX were detected in some off-site grab ground water samples • The plume of dissolved TPH-d extends off-site and was detected at all sampling locations except B19 The total extent of dissolved TPH-d is not well defined, although the concentrations detected in samples from most off-site sampling points contained relatively low concentrations suggestive of proximity to the plume margin Advanced GeoEm tronmental Inc t