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IL Laguna Formation The Laguna Formation is Pliocene -Pleistocene in age and consists of discontinuous lenses of stream -laid sand and silt with lesser amounts of clay and gravel. The formation thickens from approximately 400 feet in the Mokelumne River area to approximately 1,000 feet in the Stockton area. Groundwater is under unconfined to locally semi -confined conditions within this unit. Regionally, groundwater yields of 1,500 gpm have been reported from highly - permeable layers, but average yields are approximately 900 gpm (DWR, 2003). Mehrten Formation'. The Mehrten Formation is exposed in the easternmost part of the subbasin where it forms 1 readily -identifiable, nearly flat-topped hills. The formation is late Miocene to Pliocene in age and is composed of moderately- to well -indurated andesitic sand to sandstone interbedded with conglomerate, tuffaceous siltstone, and claystone. The Mehrten Formation is approximately 400 feet thick in eastern surface outcrops to over 600 feet thick in the subsurface near Stockton. The top of the Mehrten Formation occurs at depths of approximately 800 to 1,000 feet in the �I Stockton area. The formation is reportedly semi -confined, at least locally in the Stockton area, due to the inferred extensive fine-grained beds in its upper part. Regional studies indicate that Cthe Mehrten Formation sand commonly yields groundwater from wells at 1,000 gpm. The } Mehrten Formation is considered to be the oldest fresh water -bearing formation on the eastern side of the basin (DWR, 2003). U 2.3 LOCAL GEOLOGY AND HYDROGEOLOGY Figure 3 illustrates the traces of the two geologic cross-sections prepared from logs of borings drilled at the Site. Figures 4 and 5 present the two cross-sections, illustrating the lithology and stratigraphy beneath the Site. Cross-section A -A' has a general west -to -east orientation through the long axis of the former gasoline UST excavation and is generally parallel to the direction of groundwater flow. Cross-section B -B' has a general south -to -north orientation through the short axis of the former gasoline UST excavation and is generally perpendicular to the direction of groundwater flow. Soil encountered in the sixteen borings drilled during this investigation consisted of clay, silt, sand, and gravel. Silty gravel (as an aggregate fill material) was primarily encountered in the former gasoline USTexcavation, with a layer of clayey silt at approximately 10 feet BGS (9 feet msl), and silty sand from approximately 24 to 27 feet BGS (-5 to -8 feet msl). In most of the borings, silt with varying amounts of clay and sand was primarily encountered, with lenses of silty sand occurring primarily between approximately 7 and 10 feet BGS (12 to 9 feet msl), 23 and 25 feet BGS (4 to -6 feet msl), and 45 and 50 feet BGS (-26 to -31 feet msl). Saturated, flowing sand was encountered in some of the borings at depths greater than approximately 50 feet BGS (-31 feet msl). In the boring for well MW -9, the sand extends to a depth of at least 75 SL_AIR SCM and RI wP 3 J