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GEOMATRIX suggests that the intermediate zone is semi-confined, and that migration of groundwater between these zones is inhibited. ' 2.5 Regional Groundwater Quality The Stockton area is located adjacent to the San Joaquin River Delta, which is influenced by ocean tides and salt water incursions from San Francisco Bay into the river delta region. Saline groundwater beneath the western Stockton area has been well documented since the early 1920s (DWR, 1967; ' B&C, 1985, SJCFCWCD, 1992, 1995, 1996, 1997; USGS, 1981). Groundwater in the vicinity of the Facility has been classified as a sodium-chloride type water with a chloride content greater than ' 250 milligrams per liter (mg/E) (USGS, 1981). A detailed groundwater investigation of San Joaquin County indicates that seawater had intruded into the river delta region and western Stockton area as ' early as 1931 (DWR, 1967). Between 1953 and 1963, saline groundwater with a chloride content in excess of 300 mg/E migrated eastward beneath the Stockton area at a rate of 140 to 150 feet per year. Selected sections of the 1967 DWR groundwater investigation report are presented in Appendix A. tTwo water supply wells (IN6-IOQ7 and IN6-34A3), located northeast and southeast of the Facility, ' respectively, are routinely monitored for water quality by the SJCFCWCD (Figure 4). Well IN6-1OQ7 has had an average chloride content of approximately 600 mg/E since 1970 (Figure 5). Well IN6-34A3 has had an average chloride content of approximately 1,740 mg/E since 1984 (SJCFCWCD, 1997). The SJCFCWCD uses a chloride concentration of 300 mg/E (equivalent to ppm) to define brackish groundwater within the Stockton area affected by saltwater intrusion beneath the San Joaquin River ' Delta. A location map of the saline front prepared by the SJCFCWCD indicates that the Charter Way facility has been located behind the saline front since at least 1980 (Figure 6). This suggests that the elevated concentrations of chloride beneath the Facility and vicinity may be a result of natural ' background conditions. Selected sections of the 1991 SJCFCWCD groundwater quality report are presented in Appendix A. ' The City of Stockton sewage treatment facility disposal (oxidation) ponds are located approximately ' 3,500 feet upgradient(west)of the Charter Way Facility(Figure I). In addition, property in agricultural production is located adjacent to the Facility. Either of these operations could be upgradient sources of nitrogen compounds in groundwater. As part of this characterization, Geomatrix will review available groundwater quality data for the area to identify background concentrations of nitrogen compounds in groundwater upgradient of the Facility. i2.6 Local Groundwater Quality Groundwater quality to a depth of about 150 feet bgs has been evaluated beneath and in the vicinity of ' the Facility during previous investigations. Following is a brief description of observed groundwater quality by area. i:�raohrnc�s�v�oocur.�rm�e�v-n aac 6