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' Groundwater Inve t's tt,a t<pn,Monitor Well Destruction and Installation,and interim Remediation Pump Test Report Boyett Petroleum,419 S.Main S1.,Manteca,CA January 30,2004 Page 16 The pump test data indicate the aquifer is slightly more transmissive in the area of EW-1 than r ' 4 g y t o In the area of MW-I I and MW-9. Transmissivities indicated by the pump test data are likely on the order of 11,000 to 15,000 gallons per day per foot (gpd/ft). The 8.7 gpm pumping rate does not appear to have stressed the aquifer very much. Drawdown data after three days of pumping were plotted on the distance drawdown graph in Appendix C. The graph indicates the cone of depression after three days of pumping may have extended to a distance of 1,000 feet from the pumped well. A cone of depression with a 1,000-foot radius should extend beyond the leading edge of the contaminant plume at the site. ' 6.0 FINDINGS AND RECOMMENDATIONS ' Laboratory analytical results of the groundwater samples collected during the groundwater investigation conducted on October 20 through 23, 2003, indicate that the MTBE plume extends off site to the • northwest along Stockton Street. Although the historic gradient for the site has been estimated in a westward direction, laboratory analytical results from the direct push borings completed along Wetmore Street (Figure 4, Appendix A) do not indicate that the MTBE plume has migrated to the west of the site in shallow groundwater. _ A review of the CPT logs and existing well logs indicates that MW-I likely provided a conduit for ' increased vertical contaminant migration. MTBE is present at elevated concentrations in groundwater at the locations of CPT-3, CPT-4, and CPT-5. TBA, TAME, and 1,2-DCA were also detected in tgroundwater samples collected during the investigation. Groundwater elevations during the months of September through December 2003 were at the lowest since groundwater monitoring began in 1999. Historic MTBE laboratory analytical results from groundwater samples collected from MW-7 and the shallow groundwater sample collected from CPT-4 (12,000 µg/L MTBE from 17-22 feet bgs) indicate that the highest concentrations of MTBE have been detected in groundwater above the clay horizon located at 22-28 feet bgs (Figures 7 and 8, Appendix A). Laboratory analytical results from groundwater samples collected directly below the clay horizon also identified elevated MTBE concentrations; however, the concentrations were much lower than those detected from above the clay horizon. The aquifer pump test at monitor well MW-11 indicated a potential radius of influence of 1,000 feet (Distance Drawdown Chart, Appendix C) at a pumping rate of 8.7 gpm Approximately 0.2 pounds of MTBE were removed from groundwater during the pump test. The lateral extent of the MTBE plume has not been adequately investigated in the area northwest of the ' site along Stockton Street. Additional groundwater investigation may be needed to determine the lateral and vertical extent of the MTBE plume in that area. Condor recommends installing and operating a groundwater pump and treat system to remediate contaminated groundwater at the site. Groundwater should be extracted from MW-11, which is screened in the lower groundwater zone. If groundwater elevations in the upper groundwater zone recover sufficiently, groundwater extraction should also be conducted at MW-10. It is possible that the upper groundwater zone will be dewatered during the ' pumping of MW-1 I. _ 7.0 LIMITATIONS w ('nndnr fit-m-InT1Pli the intarnratolinrc n a nl„n; « «� 1 i___._ i_ y .'...,. Cvrlr..i.a.iva YrC3C1"tCU 111r1�;111 iii c1t.UUiUd1LCC W1111 gC11CI-ally . accepted principles and practice. Conclusions presented in this report are professional opinions based on limited information obtained at the time of our site visits. Condor has endeavored to determine as much as ' practical about the site using conventional practices given our scope of services. Condor makes no ' �'� CONDOR