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h' Quarterly Groundwater Report Frontier Transportation ' 0101-006 January 26, 2001 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-I I), 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 µg/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 1 I' 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 beneath the site between 6' and 10' bg. Data collected in December of 2000 identified the flow of groundwater in a generally western direction beneath the site (See Figure 3, Groundwater Gradient Map). The groundwater gradient is relatively slow at 0.002 ' ft./ft. Historic groundwater data identified a west or southwest flow beneath the site. 4.0) QUARTERLY GROUNDWATER SAMPLE RESULTS ' On December 28, 2000, BEII contracted Del-Tech to collect groundwater samples from monitoring wells MW-I through MW-I I for laboratory analysis. Groundwater parameters (pH, ' temperature, and conductivity) were measured before, during, and after well purging (See