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Quarterly Groundwater Monitoring Report i Frontier Transportation 0899-139 August 13, 1999 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 Iheterogeneous mix of gravel, sand, silt, and clay (Reference; Site Characterization Report, Twining Laboratories, October 16, 1996). BEII encountered clay and a heterogeneous mix of clay from 0'to 25' bg in January of 1998. Two cross sectional depictions of the site are provided as Figure 3 and Figure 4. 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 G and 10'bg. Data collected in July of 1999identified the 1 flow of groundwater in a generally western direction beneath the site (See Figure 5, Groundwater Gradient Map). Previous groundwater gradients were identified in the southwest direction indicating a modulating gradient beneath the site. 4,0) QUARTERLY GROUNDWATER SAMPLE RESULTS On July 13, 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, and 8260, 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 low level dissolved TPH-d and TPH-m concentrations(:5260 µg/L) using EPA method 8015. In addition, laboratory analytical results identified no detectable dissolved concentrations of TPH-g above 50 pg/L from groundwater samples collected from MW-2, MW4, MW-5, MW-6, and MW-7 using EPA • 2