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Hydrogeologic Report, North County Sanitary Landfill, San Joaquin County, Cal- ifornia, EMCON Associates, September 1987. 4.3.1 Regional Geology The landfill development site is located near the northeastern edge of the San Joaquin Valley The valley is a deep, asymmetrical structural trough filled with sediments derived from adjacent mountain ranges. The geology of the eastern San Joaquin Valley is characterized by alluvial sediments deposited by rivers and streams draining the Sierra Nevada to the east. The landfill site is situated on the western edge of the Arroyo Seco pediment (Piper et al. 1939). This geomorphic feature was produced by the Mokelumne River and other streams originating in the Sierra Nevada. The sediments consist of sand, gravel, silt, and clay. The youngest geologic units underlying the site are serniconsolidated alluvial de- posits of the Turlock Lake and Riverbank Formations. The Turlock Lake and 40 Riverbank Formations consist of arkosic alluvial deposits of gravels, sands, silts, and clays. These deposits show coarsening upward patterns, typical of pro - grading alluvial systems along the western Sierra Nevada. E 4.3.2 Site GeolqM The geologic units beneath the proposed Landfill Development Site were char- acterized by reviewing existing local data and drilling six borings (P-1 through P-3 and G-1 through G-3) ranging from 40-1/2 to 215 feet in depth (see Appendix A).. In addition to the borings drilled under the supervision of EMCON, previous drilling was conducted by J.H. Kleinfelder and Associates both on the same property and on the half section parcel immediately to the west (Kleinfelder, 1980 and 1986). Data from these earlier borings was also analyzed in studying the subsurface geology underlying the site. PJ9 9390218A.00W 20 Rev. 0 July 20, 1989