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4.8 – Water Draft Environmental Impact Report February 2021 14800 W. Schulte Road Logistics Center 4.8-2 Tracy Subbasin The Tracy Subbasin is largely located in San Joaquin County and primarily bounded by the San Joaquin River on the north and east and the County line on the south and west. Within the Tracy Subbasin, historical groundwater levels have shown declines due to seasonal and local pumping influences. Water uses in the Subbasin include agricultural, municipal, industrial, domestic, and native vegetation and aquatic species. Some water is also being used for managed habitats, mostly for migrating birds. Some water purveyors rely exclusively on either groundwater or surface water, but most rely on a combination of surface water and groundwater. Groundwater Quality in the Tracy Subbasin Groundwater quality in the Tracy Subbasin is variable. Good quality water, from a salinity aspect (TDS) being below the recommended drinking water standard, is locally present in both the confined and unconfined aquifers in the southern portion of the Subbasin. In the remaining portions of the Subbasin, groundwater quality is marginal to poor due to naturally-occurring high concentrations of salts from various sources, and is part of the reason that the cities have obtained surface water supplies. The concentration of the other naturally occurring constituents varies widely over the Subbasin and also with depth at any given location. This may affect the supply, beneficial uses, and potential management of groundwater in the Subbasin. Local occurrences of PFAS, uranium, nitrates, manganese have been detected above the MCL, as discussed in Chapter 4. Although these elements and compounds may have been detected, the community water systems only supply drinking water that meets all water quality standards. When an element is detected above the MCL, the wells have been brought offline until treatment or remediation has been implemented to meet the drinking water standards. The concentration of the naturally occurring elements varies widely over the Subbasin and also with depth at any given location. Groundwater quality in the Subbasin has locally exceeded the maximum contaminant levels (MCL) for drinking water for specific elements, some exceedances are scattered and some are clustered. Poor groundwater quality has been noted in the following general areas: • Salinity, as represented by TDS, is high in both the Upper and Lower aquifers with a few areas with good quality water. • Elevated concentrations of sulfate are present near the foothills in both the Upper and Lower aquifers potentially as a result of recharge water originating from the Coast Ranges. • Elevated concentrations of arsenic are only in the Upper aquifer and within the Delta area and not in the Lower aquifer. • Boron is present in the Upper aquifer. Most elevated concentrations are present in the non -Delta areas and in the northern portions of the Delta area. In the Tracy Subbasin there are a few large and known grou ndwater contamination sites that could affect supply and beneficial uses of groundwater in the Subbasin. The most significant of these sites are former Occidental Chemical Corporation site, Sharpe Army Depot site, and the Army Tracy Depo. Cleanup activitie s have been in progress for multiple years and contaminants appear to be contained, although off site at some locations, based on reports submitted for regulatory purposes. There are over 100 small sites that may present threats to local groundwater quality. These sites may have leaking underground storage tanks, improperly stored pesticides, leaking dry cleaning solvents, or other point sources of