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San Joaquin River Hydrologic Region California's Groundwater San Joaquin Valley Groundwater Basin Bulletin 118 Specific yield values for water bearing deposits in the San Joaquin Valley and Delta area range from about 7 to 10 percent. Older Alluvium. Older alluvium consists of loosely to moderately compacted sand, silt and gravel deposited in alluvial fans during the Pliocene and Pleistocene. The older alluvium is widely exposed between the Coast Range foothills and the Delta. The thickness of the older alluvium is about 150 feet. It is moderately to locally highly permeable. Flood Basin Deposits. Flood basin deposits occur in the Delta portion of the subbasin,in the northern two-thirds of the basin. They are the distal equivalents of the Tulare Formation and older and younger alluvial units and consist primarily of silts and clays. Occasional interbeds of gravel occur along the present waterways. Because of their fine-grained nature,the flood basin deposits have low permeability and generally yield low quantities of water to wells. Occasional zones of fresh water are found in the basin deposits,but they generally contain poor quality groundwater. The maximum thickness of the unit is about 1,400 feet. Younger Alluvium. Younger alluvium includes those deposits that are accumulating or would be accumulating under natural conditions. It includes sediments deposited in the channels of active streams as well as overbank deposits and terraces of those streams. They are present along the channel of Corral Hollow Creek and consist of unconsolidated silt,fine-to medium- grained sand,and gravel. Sand and gravel zones in the younger alluvium are highly permeable and,where saturated,yield significant quantities of water to wells. The thickness of the younger alluvium in the Tracy Subbasin is less than 100 feet. Groundwater Level Trends Review of hydrographs for the Tracy Subbasin indicate that except for seasonal variation resulting from recharge and pumping, the majority of water levels in wells have remained relatively stable over at least the last 10 years (DWR unpublished data; San Joaquin County Flood Control unpublished data). Groundwater Storage Groundwater Storage Capacity. There are no published groundwater storage values for the entire basin; however, (Hotchkiss and Balding 1971), estimated the groundwater storage capacity for the Tracy-Patterson Storage Unit at 4,040,000 af. This storage unit includes the southern portion of the currently defined Tracy Subbasin from approximately one-mile north of Tracy to the San Joaquin-Stanislaus County line. Since the Tracy Subbasin comprises roughly one third of the Tracy-Patterson Storage Unit, it can be inferred that the approximate storage capacity of the southern portion of the Tracy Subbasin is on the order of 1,300,00 af. Groundwater in Storage. There are no published data available on.the amount of groundwater in storage for this Subbasin. Last update 1120105