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III. Environmental Setting, Impacts,and Mitigations K Hydrology,Geohydrology, and Water Quality The existing surface hydrology of the project site has been modeled with the SCS TR-20 hydrologic computer model,as required by the San Joaquin County drainage standards (R.W. Beck and Associates, 1992a). The hydrology of the 270 acre area is represented as four hydrologic areas (see Figure III.K.1). The areas include the existing landfill surface(A), proposed lateral expansion areas(B),closed depressions that do not contribute surface runoff to the downstream reach of Little Johns Creek(C),and the floodplain(D). The 126.4 acre and 6.5 acre parcels on the north side of the existing landfill were not included,assuming that they do not presently contribute runoff to Little Johns Creek on the site(See Table III.K1). The same is assumed with the project. Flood Hazards Flooding in the French Slough watershed occurs in response to high precipitation intensity and/or duration events in the contributing drainage basin, and localized drainage and channel capacity problems. The San Joaquin County Flood Control District channel projects along the South Fork . of Little Johns Creek are generally maintained at a 30 to 50 year level of protection(San Joaquin County, 1992). The most recent Federal Emergency Management Agency(FEMA)maps for the project area indicate that portions of the project site are subject to flooding during the 100-year storm (Figure III.K.2). The existing Little Johns Creek channel would convey floodwaters,and serves as the floodway. The area between the channel and Austin Road,on the northeast portion of the project site would be subject to shallow inundation,with an average depth of one foot. Another area of shallow inundation is mapped in the southwest comer of the site. It is uncertain whether excavation in the borrow area or localized grading in the vicinity of the channel has altered the existing flood inundation areas from those represented on the 1988 FEMA maps. GeohydrQLQgy Groundwater is water below the surface in zones of saturation where all void space within sediments or porous bedrock(aquifers)are filled with water. An unconfined aquifer is one with relatively permeable materials(sediments or rock)above it,and relatively impermeable materials below it. In addition to extensive groundwater zones in unconfined aquifers,localized perched water bodies may exist above impermeable materials near the surface. The water table is the height of the top of the saturated zone in an unconfined aquifer. A confined aquifer is one that III.K.4