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MOUNTAIN HOUSE NEIGHBORHOODS K AND L INITIAL STUDY 5.ENVIRONMENTAL CHECKLIST B.HYDROLOGY AND WATER QUALITY � ) 700-Year Floodplain Current Federal Emergency Management Agency(FEMA) maps for San Joaquin County designate roughly half of Neighborhoods K and L(the portions of the site located at or below 10 feet above msl)as being within the 100-year flood hazard zone for Old River(see Figure 5.8-1). The flood zone forms a band about 1,500 #� to 2,000 feet wide along the Old River levee at the northern edge of the Mountain House community(FEMA, 2009). Groundwater The Department of Water Resources(DWR)defines state groundwater basins L based on geologic and hydrogeologic conditions. According to the DWR, Neighborhoods K and L are located within the Tracy groundwater subbasin. The subbasin has an area of approximately 540 square miles and is drained by the San Joaquin River and Corral Hollow Creek. Primary water-bearing formations in the subbasin include semi-consolidated deposits of clay, silt, and gravel of the Tulare Formation,flood basin deposits, and older and younger alluvium(DWR, 2006). Generally, groundwater flows as a subdued reflection of the surface topography. Hydrographs for the Tracy groundwater subbasin indicate that the majority of water levels in wells within the subbasin have remained relatively stable over F ' time(DWR,2006). During subsurface investigations conducted in 2007 and 2010,groundwater was encountered in all soil borings in Neighborhood K at depths of 3 to 12 feet below the ground surface (bgs)and in Neighborhood L at 4.5 to 14.5 feet bgs(Engen, 2007, 2010). The water quality of the Tracy groundwater subbasin is somewhat impaired. Areas of poor water quality exist throughout the subbasin and elevated levels of chloride and nitrate have been encountered in the vicinity of the City of Tracy. According to the 1994 MEIR, elevated levels of total dissolved solids(TDS), nitrates, and sulfides have been encountered in groundwater in and around the Mountain House community.The high levels of TDS may be the result of saltwater intrusion from the Delta. Relatively high levels of nitrates may be the result of poor livestock management in the surrounding area and/or releases from household septic systems. Project Studies i The applicant has completed the following studies related to storm water L ' drainage for Neighborhoods K and L: (7 ■ Pacific Advanced Civil Engineering(PACE), Inc.,April 2002, Master Drainage Plan. Pacific Advanced Civil Engineering (PACE), Inc., September 28,2006. Storm Wafer Master Plan Update,Addendum Il. 010 Fill (quill) 5-105