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Quarterly Groundwater Monitoring Report Frontier Transportation 0399-085 April S, 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 heterogeneous 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. BEII encountered shallow subsurface water between 6' and 10' bg. Data collected in December of 1998 identified the flow of groundwater in a generally western direction beneath the site (See Figure 5, Groundwater Gradient Map), 4.0) QUARTERLY GROUNDWATER SAMPLE RESULTS On February 23, 1999, BEII collected groundwater samples from 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, BTEX, and MTBE using EPA methods 8015 modified for diesel, 8015 modified for gasoline, 8020, and 8260, respectively. Laboratory ' analysis results for all monitoring episodes are presented in Table 1 and Table 2. Recent analytical reports are included as Appendix B. Groundwater collected from MW-1, MW-2, MW-4, MW-5, MW-6, and MW-7 identified no detectable (<500 ,ug/L) dissolved TPH-d concentrations using EPA method 8015. In monitoring well MW-3, groundwater analysis identified an increase in TPH-d concentrations from 1,300µg/L in December 1998 to 3,600µg/L in February 1999. However, TPH-d concentrations identified in MW-3 were not representative of true dissolved diesel concentrations. Laboratory analysis results reported the TPH-d concentration as a diesel and motor oil mixture in December ' 1998 and as a gasoline and diesel mixture in February 1999, but not as diesel exclusively. • 2