SITE INFORMATION AND CORRESPONDENCE

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1 ti DRAFT benzene (0.85 to 2.2 ppb), toluene (0.74 to 4 ppb), ethylbenzene (0.63 ppb), and xylene (0.52 to i 4.8 ppb) since 1994 (during 7 of 42 sampling events). Concentrations of TPH-g and benzene in 1 groundwater samples from wells peripheral to the former pump island have declined from relatively high concentrations (3,300 to-110,000 ppb and 160 to 330 ppb respectively) reported through 1995, to very low concentrations ("non-detect" for TPH=g and 1 ppb to "non-detect" for benzene) since 1997 (the past nine 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 is stable, concentrations of residual petroleum hydrocarbons in groundwater are decreasing, and that residual concentrations in excess of Basin Plan objectives are limited to less than 60 feet off-site with respectto toluene, ethylbenzene, and xylene, about 70 feet with respect to benzene, and an estimated 150 feet with regard to TPH-g. The introduction of petroleum hydrocarbons into the subsurface causes rapid changes in the,prevailing groundwater geochemistry due to microbially mediated reactions between the t hydrocarbons and naturally occurring inorganic compounds. Aerobic respiration, denitrification, Cp n Fe (III) reduction, sulfate reduction, and methanogenesis are largely responsible for these st� changes, and each leaves behind a unique biogeochemical signature. Analyses of the groundwater samples collected from site wells in March 1998 show that concentrations of r e{S educed iron (ferrous) are greater, and.nitrate and sulfate concentrations are significantlyificantl less in e� l� Y � l � n WI W yds 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 l� concentrations of less than 0.1 ppm while groundwater samples within the plume have reported 6I 8,q concentrations ranging from 0.5 to 3.4 ppm. Up-gradient concentrations of nitrate and sulfate are about 70 ppm and 170 ppm respectively while concentrations of these constituents at the core of the plume are "non detect" and in down-gradient groundwater nitrate concentrations are "non detect"while sulfate concentrations are only 10 to 20 ppm. Thus, the data demonstrate that the dissolved phase hydrocarbon plume in site groundwater is undergoing biodegradation via iron sulfate and nitrate reduction (i.e.; anaerobic biodegradation). Measurements of dissolved oxygen in groundwater samples collected from site wells in February 1999 provide the basis to conclude that aerobic biodegradation is also acting to I 1,0 V, s , �� '-f-��- ��►�. n�- et�_I�� -�o SIX, �