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GcologicaI hclurics kc. Page 4 - Groundwater Monitoring Report Project No. 750.2 j October 5,2001 j 1.3 Laboratory Analyses Theroundwater samples collected on Au g p gust 15, 2001, were delivered to California Laboratory Services of Rancho Cordova, California (Department of Health Services Certification.No. 1400), for analysis. k The groundwater samples were analyzed for: • Benzene, Toluene, Ethyl Benzene, and Xylene (BTEX) by EPA method 602/5030 • Total. Petroleum Hydrocarbons as Gasoline (TPH-G) by EPA method 5030/8015(M) • Fuel oxygenated compounds (MTBE, DIPE, ETBE, TAME & TBA) by EPA method 8260 The detection limits for the above analyses are listed in Table 2 of Appendix A and the lab data is presented in Appendix B. 2.0 FINDINGS AND DISCUSSION 1 The results of the groundwater sample analyses show the following: • Since monitoring was initiated in April 2000, well MW-1 has consistently contained gasoline range petroleum hydrocarbons (TPH-G, BTEX, & MTBE). These constituents were not detected in the well for the May 15, 2001 event. These contaminants returned for the August event but at reduced levels that indicate a shrinking or stable plume. • Figure 6 illustrates groundwater elevation and TPH-G concentrations in MW-1 versus time. After an initial high of 516 lig/1 the concentrations of TPH-G have fallen to stable levels as described above. In addition., the graph suggests a fairly direct relationship between groundwater elevation changes and contaminant concentrations. • Figure 7 illustrates groundwater elevation and MTBE concentrations in MW-1 versus g time. After an initial high of 220 µg11 the concentrations of MTBE have fallen to fluctuating but stable levels as described above. • The discretely screened deep well MW-101 did not contain contaminant concentrations above the laboratory reporting limits. This suggests that the previously recorded contamination was a drilling artifact. k { i • The remaining wells were found to be free of contaminants, which is consistent with other events. • Vertical groundwater gradient was calculated and deten-nined to be downward and j greater than the horizontal gradient of the water table, suggesting that the contaminants in the water table may be transported to deeper depths in the subsurface. /�` • Oxidation-Reduction Potential measurements (Table 4) are positive for all the wells, I; suggesting that the volume of contaminants leaching into groundwater is minimal and { I �