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amec ji wells MW-5A and MW-56 and 0.023 downward between wells MW-513 and MW-5C, consistent with the vertical gradients previously measured (AMSC, 2010b). 4.5 BACKGROUND GROUNDWATER QUALITY The quality of groundwater beneath the western portion of the City Stockton, where the site is located, is being impacted by seawater intrusion from the Sacramento/San Joaquin River Delta (USGS, 2006). Seawater intrusion results in elevated concentrations of total dissolved solids (TDS). The site is located within the eastern edge of the seawater intrusion front (defined by the 300 milligrams per liter[mg/L] TDS contour) (CDM, 2001). It has been estimated that the seawater intrusion front is migrating to the east at approximately 150 to 250 feet per year(CDM, 2001). The electrical conductivity (EC) values measured in the field for the groundwater samples collected at the site in March 2010 ranged from 1,000 to 1,288 micromhos per centimeter (AMEC, 2010b). Using an estimating method for TDS in the groundwater samples collected at the site using their EC measurements (Todd, 1980), TDS concentrations for groundwater samples collected in March 2010 are estimated to be 641 to 826 mg/L. These values indicate that intrusion of seawater in the Sacramento/San Joaquin River Delta has had a significant impact upon groundwater quality beneath the UniFirst site and vicinity. The quality of groundwater beneath the western portion of the City of Stockton also has been affected by point-source contaminants. A Phase I Environmental Site Assessment was performed for the site in 2001 (EECI, 2001). The results of the assessment indicate that within a 2-mile radius of the site there are potential sources of contaminants to groundwater that are not associated with the site. Groundwater samples from at least four municipal supply wells southeast(cross-gradient) of the site have been reported to contain detections of CVOCs and petroleum hydrocarbon constituents (EECI, 2002). 4.6 BENEFICIAL USES OF GROUNDWATER Shallow groundwater in the western downtown Stockton area, in which the site is located, is not used for domestic or municipal water supply. The groundwater beneath this area has high TDS and chloride concentrations as a result of seawater intrusion from the Sacramento/San Joaquin River Delta (to the west of the site). The poor groundwater quality of western Stockton has resulted in importation of drinking water from eastern Stockton through the municipal water distribution system (CDM, 2001). AMEC_ Geomatrix, Inc. 1A1 3000s1l 3290\Archive\1 3290-49.DOC 5