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' Quarterly Groundwater Report . Frontier Transportation ' 0600-091 June 20, 2000 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 and results are tabulated in Table 1. ' In January 2000, BEII advanced two borings (B-5 and B-6), installed four additional groundwater monitoring wells (MW-8 through MW-11), and collected nine soil vapor samples for a soil vapor survey as part of an additional site assessment (See Figure 2, Site Layout Map for ' well locations). Soil sample analysis identified no detectable concentrations of petroleum hydrocarbons with the exception of boring B-6. Maximum contaminant levels in B-6 were identified 100 mg/kg as TPH-g, 2,343 pg/kg as BTEX, and 10,600 µg/kg as MTBE at 10' bg. ' Maximum soil vapor concentrations were identified at sample point V5 at 4' bg. Groundwater samples were subsequently collected from all monitoring wells. 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. 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 May of 2000 identified the flow of groundwater in a generally southwestern direction beneath the site (See Figure 3, Groundwater Gradient Map). 1 4.0) QUARTERLY GROUNDWATER SAMPLE RESULTS ' On May 2, 2000, BEH contracted Del-Tech to collect groundwater samples from monitoring wells MW-1 through MW-11 for laboratory analysis. Groundwater parameters (pH, temperature, and conductivity)were measured before, during, and after well purging (See ' Appendix A for Monitoring Well Field Logs). Groundwater samples were analyzed for TPH-d,