SITE INFORMATION AND CORRESPONDENCE

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lwftr/ 1%01-1 Ms. Linda Turkatte December 14, 1995 Page 3 (Spring, 1986) to more than 75 feet below grade (April, 1992). The most recently recorded depth to water was approximately 72.5 feet below grade (May, 1995). Figure 3 also shows the estimated extent of residual hydrocarbon contamination. Based upon analytical results of soil samples collected during drilling of well MW 1, it appears that residual gasoline constituents associated with the line leak existed only in clay soils between approximately 18 and 50 below grade. Based upon the literature, one can estimate transverse dispersion of contaminants to be ' approximately 1/8 that of longitudinal dispersion (Illinois State Water Survey, Bulletin 65, 1981). Therefore, the estimated maximum extent of impacted soil in 1986 was a zone roughly equivalent to a vertical cylinder 32 feet tall (between depths of 18 and 50 feet) by 11 feet in diameter in the vicinity of well MW L Based on this configuration, approximately 115 cubic yards of soil had been impacted by approximately 0.18 pounds (0.025 gallons) of benzene. A standard calculation sheet is included in Attachment B. Based upon analytical results, toluene, ethylbenzene, and xylenes were present at slightly smaller masses. However, no TPHG was detected in any samples. Natural in-situ biodegradation of hydrocarbon constituents over time is widely documented in the literature with the rate of decay limited by the available oxygen (Borden and Bedient, 1986). Decay constants have been determined at laboratory and field scales for certain gasoline constituents, including benzene. Benzene decay constant values from numerous independent studies reported average values ranging from 0.35% to 4.0% per day ( Howard and others, 1991). In order to predict the expected concentration at a given time, we can extrapolate from a known concentration using the following equation from the law of exponential change: Y = Yoe where yo is the original concentration, In a equals 1, k is the degradation rate (negative number for decreasing progression), and t is the elapsed time. For soil, we seek the value of t at which Yoe = 0.0056yo, because this will be the time when the concentration will be at the detection limit (the ratio of the detection limit for benzene in soil [0.005 ppm] to the initial maximum concentration of benzene in soil [0.89 ppm] is 0.0056). The Yo's cancel in this equation to leave e't = 0.0056, groundze\hendrix\phs1295.jp1