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• • 2.4 HYDROGEOLOGY 2.4.1 REGIONAL GROUNDWATER CHARACTERISTICS Regionally, groundwater in the vicinity of the landfill is approximately 10 to 20 feet above mean sea level (MSL) and is reportedly flowing to the north or northeast toward pumping centers located west of the Stockton Central Business District. The Stockton region has experienced an overdraft situation from high water usage of the area groundwater. The U.S. Geological Survey (USGS) investigated chemical water quality conditions in San Joaquin County and published their findings in 1981 (SWAT, Kleinfelder, 1988). The USGS study indicates that the Cove Contractors site is located in an area that is transitional between a regional sodium/chloride water type (west of the site) and a regional calcium-sodium/bicarbonate water type (east of the site). The USGS study also found that electrical conductivity of the groundwater around the site is approximately 500 to 1,000 pmhos/cm. The site is located in a transitional area between a region west of the site with electrical conductivities greater than 1,000 pmhos/cm and a region east of the site with electrical conductivities under 500 Nmhos/cm. Total dissolved solids (TDS) concentrations in the USGS study area varied from 50 milligrams per liter (mg/L) to 3,520 mg/L, with a mean concentration of 463 mg/L, suggesting that the Cove Contractors site also appears to be in a transitional TDS zone. Regional chloride concentrations in the vicinity of the site appear to be approximately 100 to 250 milligrams per kilogram (mg/kg). The site is, again, in the transition zone between higher chloride concentrations west of the site and lower chloride concentrations east of the site. Manganese (Mn) and iron (Fe) were both detected in the groundwater in the USGS study area. Mn was detected at concentrations exceeding the United States Environmental Protection Agency (USEPA) and California Secondary Maximum Contaminant Limit (MCL) concentration of 0.05 mg/L in three wells located within approximately three miles of the Cove Contractors site. Fe was detected at a concentration exceeding the Secondary MCL of 0.3 mg/L in one groundwater monitoring well located approximately three miles south of the Cove Contractors site. Groundwater quality appears to vary regionally. 2.4.2 SITE GROUNDWATER CHARACTERISTICS Logging of the soil profile conducted by Miller Brooks has indicated that the upper portion is characterized by fine-grained materials comprised of clay and silt,which are underlain by more sandy materials. Groundwater was encountered within the sandy materials at a depth of approximately 50 ft bgs under semi-confined conditions. Static water levels measured after installation and development of groundwater wells are approximately 35 ft bgs. Groundwater flows to the northeast. 4 AI/LLER BROOKS P:\707 Cove Contractors\Feasibility Study and Corrective Action Plan\Corrective Action Plan_Final.doc