SITE INFORMATION AND CORRESPONDENCE

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Ms. Linda Turkatte December 14, 1995 Page 4 kt = In 0.0056, t = On 0.0056)/k. Substituting decay constant of -0.04 and -0.0035 produces t values of 130 days and 1,481 days (4.1 years). This suggests that benzene concentrations should have been reduced to below the laboratory detection limit of 0.005 since 1986 (more than 9.5 years, or 3,500 days). Similarly, groundwater contamination originally detected would be expected to fall below laboratory detection limits in 166 to 1,899 days (5.2 years). A review of historical groundwater analytical results indicate that dissolved concentrations of benzene decreased from 382 ppb on April 24, 1986 to <0.50 ppb on May 25, 1995, a period of 3,316 days. No benzene or other gasoline constituents were detected in downgradient wells MW2 or MW4 in two sampling rounds (May 1993 and May 1995). Conclusions Hendrix and Son Trucking, Inc. immediately discontinued operation and removed the underground tank and leaking line upon discovering a leak in 1986. A soil boring/monitoring well was installed directly over the location of the line leak. Analytical results of soil samples collected beneath the line leak indicated that gasoline constituents, including benzene, had impacted soil beneath the site to a limited extent, although no TPHG was detected in any soil samples. Benzene impaction was apparently limited to clay soils between approximately 18 and 50 feet below grade. Based on this observation it was estimated that approximately 115 cubic yards of soil had been impacted by an estimated 0.18 pounds of benzene. Since the source of the leak was immediately removed and well MWI. was placed directly over the location of the leak, we can assume that the documented concentration of benzene at 30 feet (0.89 ppm) was the maximum concentration in the soil. Based upon this maximum documented concentration of benzene in soil, the fact that the source of the leak had been removed, and estimated benzene degradation rates from numerous independent studies in the literature, it was calculated that the benzene in the soil would degrade to a level below standard laboratory detection limits in approximately 4.1 years. Making similar assumptions pertaining to groundwater, it was calculated that the benzene in the groundwater would degrade to a level below standard laboratory detection limits in approximately 5.2 years. Analytical results of groundwater samples collected from well MWI. over time and the absence of contaminants in downgradient wells tend to support the calculated degradation rates. groundze\hendriz\phs1295.jpl