ARCHIVED REPORTS XR0008264 - Laserfiche WebLink

Home Browse Search

fZXsJWw Working To Restore Nature Where, Cx = The concentration at a distance x downgradient of the source, Co = The source concentration, In e = 1 M = The decay constant (percent per day), v = The groundwater velocity. To estimate the maximum longitudinal downgradient distance that benzene would be expected to travel, Equation 5 was used with the following estimated parameters: Co = 1 7 ppb, based on the expected benzene concentration in groundwater at the source, m = 0.3 percent per day, based on numerous independent determinations cited above, this value represents the lowest (slowest) published decay rate constant for benzene, v 0.008 ft/day, calculated assuming highest reasonable hydraulic • conductivity value from the literature (1 gpd/ftp), the site groundwater gradient (0.006), and an effective porosity of 10% Based upon these input parameters, Equation 5 predicts that no benzene will be detectable (at MDL of 0.5 ppb) at a distance of 4 feet downgradient of the source. Assuming a groundwater velocity of 0 008 ft/day and a retardation factor for benzene of 100, it would take in excess of 130 years for the contaminant plume to travel 4 feet CONCLUSIONS/RECOMMENDATIONS Field observations and analytical data indicate that vertical migration of residual contaminants has not occurred beyond 14 feet below surface grade and that lateral migration was not extensive Previous investigations indicated that the main concern was the soil in the vicinity of the former diesel fuel tank pit. Analytical results of excavation confirmation samples indicate that impacted soil has been removed to the maximum possible extent without causing structural damage to the existing building It was previously estimated that approximately 1.1 gallons of diesel fuel and 0 00018 gallons of benzene remained in soil beneath the existing building RESNA determined, through mathematical modeling, that these residual contaminants did not pose a significant threat to groundwater beneath the site. At the request of PHS/EHD, RESNA reevaluated the mass of contaminants beneath the site under the assumption that soil contamination may extend from Adlhthe former tank locations to downgradient well MW1, resulting in a highly conservative 1500541RAD2-994 DOC 5