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3.5 HYDROLOGY AND DRAINAGE EXISTING SETTING Drainage. The study area and project site are located within the San Joaquin River drainage system. Runoff from the entire area drains by overland flow generally to the northeast. Topography of the study area ranges from level terrain to one half percent slope. No streams or creeks occur in the study area or project site. In the vicinity of the study area, there is no integrated drainage system. Drainage is provided on a piecemeal basis by individual property owners, cities, and special districts. There is no evidence of significant erosion or gullying or of pooling or standing water on the project site or study area. Banta-Carbona Irrigation Canal. The Banta-Carbona Irrigation Canal, owned and operated by the Santa-Carbona Irrigation District, bisects the study area and borders the project site. The canal flows in a southeast to northwest direction and feeds areas to the north and west of the study area. The canal is concrete lined through the study area and eventually percolates into the ground one mile to the west. The District plans to utilize the canal through the next decade or until demand for irrigation in the area diminishes (Terrill, 1987). Flooding. The study area and project site do not contain any areas identified as occurring within the 100 year flood hazard zone. Surface Water Quality. Specific water quality data are not available for study area or project site surface water. Runoff from the study area and project site may contain sediment, organic litter, and minor amounts of pesticides and herbicides typical of agricultural land use. Groundwater Quality. Recent groundwater sampling in the vicinity of the study area revealed the presence of trichloroethylene (TCE) in shallow groundwater. The Department of Health Services (DOHS) speculates the Tracy Defense Depot (immediately northeast of the study area) to be the source of the contamination (Kaufman, 1987). Currently, there is no evidence of TCE contamination of private wells in the study area. The Regional Water Quality Control Board and DOHS plan to establish a monitoring and clean up program with the Tracy Defense Depot to reduce levels of TCE in groundwater. Other groundwater testing in the study area vicinity revealed nitrate levels at 30 to 35 milli rams er liter ( /1). The presence of nitrate in groundwa er is fair y common in in ensive agricultural areas. These levels are under the State of California's maximum allowable threshold for nitrates of 45 mg/1 (State of California, Title 22, 1987). The Tracy area has also experienced problems with salt water intrusion from nearby Delta sediments. Saline groundwater has migrated southeastward and has mixed with fresh water strata. At the present, this problem is not considered serious. However, with continual overdraft of underlying fresh water aquifers and subsequent declining water levels, the migration of saline waters could severely impact the utility of groundwater in the vicinity of Tracy. 3.5-1