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U INC • The late Pliocene Mehrten Formation is a volcanic (andesitic source) formation which is i characterized by abundant black volcanic sandstones and conglomerates with interbedded dark-colored mudstones and volcanic mud flow deposits The Miocene Valley Springs Formation, which overlies the deeper continental and marine deposits, is composed of volcanic (rhyolitic source) sandstones, mudstones, and ash to tuffaceous beds (Page 1986) The source of groundwater in the Lodi area is predominantly from the Late Tertiary and Quaternary non-marine sediments, which comprise the overlying alluvium, the Turlock Lake, and the Laguna Formations in the Lodi area (DTSC, 1994) Dale and others (1966) indicated that, although units of non-marine sediments could be differentiated and mapped on the surface by using physiographic and weathering criteria, the subsurface equivalents of these units cannot be mapped because there is no significant and distinctive physical appearance in lithology However, Shlemon (1971) mapped three Pleistocene river channels in the Mokelumne River area located approximately one mile north of the site The oldest channel deposits, which Shlemon relates to the Friant Pumice Member of the Turlock Lake Formation is present in the Lodi area at a depth of about 250 feet below ground surface and is reported to be approximately 600,000 years old The second channel deposit mapped by Shlemon is present at a depth of about 90 feet below ground surface in the Lodi area and is 75,000 to 300,000 years old The third channel deposit mapped by Shlemon is found at a depth of about 40 feet below ground surface and is greater than 27,000 years old These channel deposits all dip to the west and can be correlated to lacustrine clays found to the west and southwest 4.2 Local Geology and Hydrogeology Review of subsurface geologic logs from Lodi water wells and borings from the 1994 DTSC report of the Lodi Groundwater Site (DTSC, 1994) indicated that subsurface sediments in the Lodi area are comprised mainly of sands with discontinuous lenses of • 08220010 o11 25 {