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P� A S S O C I A T E S I N C 2. Groundwater in the uppermost water-bearing zone appears to be confined to semi confined. Groundwater flow beneath the site is generally northwest with a magnitude of 0.003 ft/ft, or approximately 16 ft/mile. 3. Petroleum hydrocarbon-affected soils are present primarily in the vicinity of the capillary fringe at approximately 8 to 12 feet bgs. The highest concentrations of TPHg and benzene are present in soil northwest of the former location of the gasoline USTs. The horizontal extent of TPHg and benzene and the presence of these contaminants in the capillary fringe are indicative of groundwater impact and advection. r ace appear to be essentially defined both laterally and 4. Petroleum hydrocarbons in the subsurf vertically, as evidenced by analytical data from the site's perimeter borings and deeper soil samples, respectively. Free-product was initially observed on-site during the collection of a groundwater grab sample from soil boring SB2 in September 1996, however subsequent soil borings advanced near SB2, including FPB-1, FPB-2, and MW-4, did not reveal the presence of free-product. 5. 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). Historically, MTBE has been detected in groundwater samples collected from the site's borings/wells at concentrations up to 40 ug/L by laboratory EPA method 8020. However, the highest MTBE concentrations detected by EPA method 8260, a more reliable method, did not exceed 7.6 ug/L. A review of the most recent groundwater analytical results (June 1999) indicate that MTBE was not present in the dissolved hydrocarbon plume beneath the site (Table 2). 6. A groundwater receptor survey revealed thirteen wells are located within one half mile of the site. The on-site well is the closest groundwater receptor to the plume (within approximately 100 feet crossgradient of the plume). The estimated time for the hydrocarbon plume to reach the on-site domestic well by advection alone is 20 years. This assumed the well was screened in the shallow aquifer, a worst case scenario (Smith, 1997). 7. Based upon geotechnical and geochen&al data collected from groundwater and/or soil underlying the project site, intrinsic bioremediation is occurring in the contaminant plume beneath the site. The biodegradation processes in the first water bearing zone at the site have the capacity to degrade 52.1 mg/L of BTEX. The total BTEX detected during the April 1997 monitoring event was approximately 1.56 mg/L. As a result, the assimilative capacity is one order of magnitude greater than the BTEX that is present(Table 6). 8. Given the biodecay rates for TPHg (578 days) and benzene (408 days) the time required to biodegrade TPHg presently in groundwater to 100 ppb (taste, odor threshold) and benzene presently in groundwater to 1 ppb (MCL) has been estimated to be approximately 11 years. w:\95122Veports\ReVest for NFA doc 14 r