ARCHIVED REPORTS XR0011898 - Laserfiche WebLink

Home Browse Search

("%TC A S S O C I A T E S I N C . On March 9, 1999, ATC supervised the advancement of three additional soil borings (MW-4, MW-5, and MW-6) to evaluate the extent of impacted soil and groundwater northwest of the former USTs. These soil borings were completed as groundwater monitoring wells to depths of approximately 26 feet below ground surface (bgs). Wells MW-4, MW-5, and MW-6 were placed in locations where elevated concentrations of petroleum hydrocarbons were detected during the subsurface investigation performed in September 1996. Of the soil and water samples collected from these wells/borings, only low to nondetectable concentrations of total petroleum hydrocarbons as gasoline (TPHg), toluene, ethylbenzene, and xylenes were detected from water samples obtained from wells MW-4 and MW-5. The very low to nondetectable concentrations of petroleum hydrocarbons currently detected at these locations appear to confirm that intrinsic bioremediation is reducing the petroleum hydrocarbon plume beneath the site. This is especially evident in the vicinity of well MW-4 where free product was detected in September 1996. This work is summarized in ATC's Summary Report of Additional Subsurface Investigation dated May 12, 1999. Since the installation of the groundwater monitoring wells in July 1995, sixteen quarterly groundwater monitoring events have been performed at the site. The groundwater gradient has been predominantly north-northwest during these monitoring events. Groundwater elevation data is summarized in Table 4. SCOPE OF WORK In order to evaluate the level of risk residual petroleum hydrocarbons at the site pose to human health and the environment, ATC evaluated contaminant transport potential of affected soil and groundwater at the site. DISTRIBUTION OF PETROLEUM HYDROCARBONS Soil Based on the analytical results from soil samples collected on July 18, 1995, September 24, 1996, October 2, 1996, and March 9, 1999, the vertical extent of soil contamination beneath the site appears limited to above 20 feet bgs. Three soil samples, B1-21.5, B2-25.5, and B3-26.5 were collected on July 18, 1995, from depths below 20 feet bgs and none of the samples contained detectable concentrations of petroleum hydrocarbon constituents suggesting that residual petroleum hydrocarbons appear to attenuate rapidly with depth. Field observations, PID measurements, and laboratory analytical data indicate the majority of petroleum hydrocarbon-affected soils are present at depths from approximately 6 to 12 feet bgs. Residual hydrocarbons at these depths are most likely attributed to the sorption characteristics of v the clay strata noted above ± 10 feet bgs. Laboratory analysis indicated the percent organic matter of the upper soil column ranges from 19% to 23%. w:\17861\repoAs\mo&1ing.doc 3