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1 Quarterly Groundwater Report • Frontier Transportation 1199-184 December 2, 1999 Page 2 through MW-7) to assess the extent of subsurface contamination beneath the site (See Figure 2, Site Layout Map). Soil samples were collected every five feet. Laboratory analysis of soil samples identified no detectable concentrations of TPH-g, BTEX, or MTBE. Groundwater samples were subsequently collected. Results are tabulated in Table 1. 3.0) SITE GEOLOGY/HYDROGEOLOGY The site is located on the boundary of the Sacramento Valley and the San Joaquin Valley within the west-central portion of the Great Valley Geomorphic province of California. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlay sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlay a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Reference; Site Characterization Report, Twining Laboratories, October 16, 1996). BEII encountered predominantly clays with traces of interbedded sands from 0' to 25' bg during well advancement in January of 1998. lie First subsurface water was encountered at depths from 11' to 12' bg during tank removal and monitoring well installation at this site in 1995 and 1996. Since then, shallow subsurface water has been encountered between 6' and 10' bg. Data collected in November of 1999 identified the flow of groundwater in a generally western direction beneath the site (See Figure 3, Groundwater Gradient Map). ' 4.0) QUARTERLY GROUNDWATER SAMPLE RESULTS On November 4, 1999, BEII contracted Del-Tech to collect groundwater samples from monitoring wells MW-1 through MW-7 for laboratory analysis. Groundwater parameters (pH, temperature, and conductivity) were measured before, during, and after well purging (See ' Appendix A for Groundwater Parameters). Groundwater samples were analyzed for TPH-d, TPH-g, TPH-m (total petroleum hydrocarbons as motor oil), BTEX, and MTBE using EPA methods 8015 modified for diesel, 8015 modified for gasoline, 8015 modified for motor oil, 8020 for BTEX, and 8260 for MTBE, respectively. Laboratory analysis results for all monitoring episodes are presented in Table 1. Recent analytical reports are included as Appendix B. Groundwater collected from all monitoring wells identified dissolved TPH-d and TPH-m concentrations in monitoring well MW-3 exclusively as 12,000 pg/L using EPA method 8015. All other monitoring wells identified non-detect (<50 µg/L) dissolved TPH-d and TPH-m concentrations. In addition, laboratory analysis of groundwater samples identified no detectable •