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wwccAS oEsr,AScaaulul,4 The natural background concentration of methane is less than 0 032 mg/L, based on data from upgradient well MW-9 The highest methane concentrations were detected in MW-2A at a concentration of 0.38 mg/L, which also has elevated BTEX concentrations This is a strong ' indication that anaerobic biodegradation of the BTEX compounds is occurring at the site through microbially bioremediation 2.2.7.6 Oxidation Reduction Potential Oxidation Reduction (redox) potential is a measure of the relative tendency of a solution to accept or transfer electrons The redox potential of a groundwater system depends on which electron acceptor is being reduced by microbes during BTEX oxidation The redox potential ' ranged from -112 to 126 millivolts (mV). The lowest redox potential was measured in well MW-2A, which as expected, coincides with the highest BTEX concentration, low DO, and elevated Iron, sulfate and methane concentrations, all of which are consistent with anaerobic biodegradation 2.2.7.7 pH ' The pH of a solution is the negative logarithm of the hydrogen ion concentration Groundwater at the site is slightly acidic (6 64 to 6 71) Microbes capable of degrading petroleum hydrocarbons generally prefer pH values in the 6 to 8 range, the pH of groundwater at the site is suitable for microbial activity 2.2.7.5 Total Dissolved Solids and Salinity Total Dissolved Solids (TDS) concentrations at the site range from 0 78 to 1 44 grams per Liter (g/L) The salinity ranged from 0 89 to 1 04 ppt This indicates that the Zones 3 and Zone 4 groundwater at the site is slightly brackish and not desirable as drinking water 2.2.8 Groundwater Use In the area surrounding the site, deep groundwater is utilized as a drinking water source by the ' California Water Service Company (CWSC) CWSC has three production wells within 5,000 feet of the site (Figure 1) Well 28 is located at the corner of Sierra Nevada Street and Fremont Street approximately 4,800 feet northeast of the site and is screened from 208 to 425 feet bgs Well 37 is located on Anderson Street between Center Street and El Dorado Avenue approximately 3,200 feet south of the site and is screened from 136 to 472 feet bgs Well 53 is located on Aurora Street between Fremont Street and Lindsay Street and is screened from 250 ' to 530 feet bgs Wells 28 and 53 are down-gradient of the site, however these wells pump water from a much lower aquifer Groundwater from the Zone 4 aquifer is not presently being used as a source of potable water The relatively low hydraulic conductivity of the Zone 4 aquifer suggests that it is not likely that Zone 4 water will be a future source of potable groundwater LTR0JECTWANJ0QIHSA}ACF10N?L 14