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• Geosyntec'% consultants 4.3 Groundwater Analytical Results Field parameters collected during groundwater sampling at the Site included temperature, conductivity, dissolved oxygen, ORP, and pH. In summary, groundwater ranged in temperature from 19.2°C to 22.4°C. Conductivity ranged from 0.94 milli- ohms per centimeter(m-ohm/cm) to 2.52 m-ohm/cm. Dissolved oxygen concentrations ranged from 0.7 mg/L to 2.1 mg/L. ORP ranged from -159 milliVolts (mV) to -71 mV and pH ranged from 7.08 to 7.60. To assess groundwater impacts at the Site, groundwater analytical data were compared to established California MCLs (Cal EPA, 2008), including those for arsenic (0.01 mg/L), barium (2 mg/L), and nitrate as N (10 mg/L). Where MCLs were not available, groundwater data was compared to: California SMCLs (Cal EPA, 2006) including those for chloride (250 mg/L), iron (0.30 mg/L), manganese (0.16 mg/L), sulfate (250 mg/L), and TDS (500 mg/L); Central Valley water quality goals (Cal EPA, 2007) including the goal for sodium (30 mg/L); or the USEPA's Drinking Water Standards and Health Advisories (2012) including the health advisory for ammonia(30 mg/L). The laboratory analytical results for dissolved metals reported detectable concentrations of eight metals, including arsenic, barium, calcium, iron, magnesium, manganese, potassium, and sodium in the groundwater samples collected from GS$-1 through GSB-3. Reported arsenic concentrations ranged from below reporting limits to 0.018 mg/L. Reported barium concentrations ranged from 0.18 mg/L to 0.40 mg/L. Reported calcium concentrations ranged from 110 mg/L to 260 mg/L. Reported iron concentrations ranged from 0.41 mg/L to 8.5 mg/L. Reported magnesium concentrations ranged from 29 mg/L to 86 mg/L. Reported manganese concentrations ranged from 1.1 mg/L to 3.5 mg/L. Reported potassium concentrations ranged from 42 mg/L to 97 mg/L and reported sodium concentrations ranged from 35 mg/L to 190 mg/L. Groundwater data for the Site is shown on Figure 2. Although arsenic concentrations were reported above the MCL of 0.01 mg/L, review of recent Water Quality Consumer Reports for the City of Manteca (2012, 2013) have shown that arsenic concentrations range from 0.003 mg/L to 0.018 mg/L in local drinking water aquifers. As such, the reported arsenic concentrations at the Site appear to be within acceptable background concentrations for the area. All reported concentrations of iron, manganese, and sodium exceed the applicable screening levels (SMCLs for iron and manganese and the water quality goal for sodium). Barium concentrations reported in groundwater at the Site did not exceed the MCL. • Phase II ESA_1351 Dupont_FINAL_12302013 7 1/2/14