SITE INFORMATION AND CORRESPONDENCE

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DRAFT concentrations of these constituents have declined to about 80,000 and 1,000 ppb, respectively, as-demonstrated by the monitoring results of the past year. Reported concentrations of TPH-g and benzene in groundwater from the monitor well (MW-4) immediately down-gradient of MW-2, have ranged from 69 to 460 ppb and "non-detect" to 2.0 ppb, respectively (32 sampling events over nine years) and concentrations in wells peripheral to the former pump island have been"non-detect" for at least the past six quarterly monitoring events. Similarly, concentrations of toluene, ethylbenzene, and xylene in groundwater samples from well MW-2 have historically ranged from 25,000 to 82,000 ppb, 2,700 to 9,900 ppb, and 14,000 to 37,000 ppb, respectively. However, concentrations of these constituents are less than MCLs (generally"non detect"to less than 10 ppb) in groundwater from all other site monitor wells. Thus the data clearly show the plume Js stable, concentrations of residual petroleum hydrocarbons in groundwater are decreasing, and that residual concentrations in excess of Basin Plan objectives are Iimited to less than 60 feet off-site with respect to toluene, ethylbenzene, and xylene, about 70 feet with respect to benzene, and an estimated 150 feet with regard to TPH-g. Natural attenuation includes physical, chemical and biologic processes. Biologic processes are the most important for reducing the mass of residual petroleum hydrocarbons and occurs under both aerobic and anaerobic conditions. The primary evidence showing that natural attenuation is occurring at the site is the documentation demonstrating that the plume is stable and concentrations of dissolved phase hydrocarbons are decreasing. Secondary evidence demonstrating that bioremediation is occurring are geochemical indicators. Analyses of groundwater samples collected from site wells in March 1998 show that concentrations of reduced iron(ferrous) are greater, and nitrate and sulfate concentrations are significantly less, in groundwater samples from wells within the plume when compared to samples from"background"wells. For example, groundwater samples from up-gradient and cross-gradient wells have ferrous iron concentrations of less than 0.1 ppm while groundwater samples within the plume have reported concentrations ranging from 0.5 to 3.4 ppm. Up-gradient nitrate and 3 One sample, collected in August 1992, had a reported TPH-g concentration of 3,000 ppb. 9