ARCHIVED REPORTS XR0011902 - Laserfiche WebLink

Home Browse Search

-- v ATC A S S O C I A T E S I N C years. This assumed the well was screened in the shallow aquifer, a worst case scenario (Smith, 1997). 3.0 DISTRIBUTION OF PETROLEUM HYDROCARBONS 3.1 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 6W 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%. Cross section locations are shown on Figure 3. Cross sections of the site are shown on Figures 4 and 5. These figures illustrate the estimated vertical extent of soil contamination. Based on the analytical results from soil samples, the lateral extent of soil contamination beneath the site appears to extend from the former UST locations north (downgradient) less than 100 feet as indicated by soil samples collected from boring MW-4. Soil samples collected from borings B1/MW-1, SBI, SB6, MW-4, and MW-5 appear to define residual petroleum hydrocarbon b' constituents in soil. The lateral extent of TPHg and benzene contamination in the soil is shown on Figures 6 and 7, respectively. As indicated on Figure 6, the highest concentrations of TPHg are present in soil north to northwest of the former location of the gasoline USTs. Similarly, as indicated on Figure 7, the highest concentrations of benzene are present in soil north to northwest of the former location of the gasoline USTs, in the vicinity of MW-3. The eastern boundary of residual soil contamination has been estimated based on analytical soil data from borings SB 1 and MW-4 located 40 feet east and 120 feet north-northeast from the former USTs, respectively, and on a consistent groundwater gradient flow direction to the north- northwest. It is noteworthy to mention that MTBE was not detected in any of the soil samples analyzed above laboratory detection limits by EPA methods 8020 or 8260. Furthermore, the highest concentration of benzene in soil detected during the various subsurface investigations was only 15 milligrams per kilogram (mg/kg). This sample (SB3-9.0) was collected from soil boring SB3lMW-3 located approximately 60 feet downgradient of the former USTs at a depth of 9 feet bgs. No benzene was detected in this boring's underlying soil sample at 21.5 feet bgs. wA95122Vepotts\Request for NFAdoc 7