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This report was prepared for the Air Force Center for Environmental Excellence (AFCEE) by Parsons Engineering Science, Inc. (parsons ES) as an update to the Final Intrinsic Remediation Engineering Evaluation/Cost Analysis (EE/CA) for Underground Storage Tank (UST) Site 870, Hill Air Force Base (AFB), Utah (parsons ES, 1995). The EE/CA was conducted to evaluate the use of intrinsic remediation with long-term monitoring (LTM) for remediation of fuel hydrocarbon contamination dissolved in groundwater at UST Site 870. This update summarizes the results of the third sampling event performed as part of longterm groundwater monitoring at the site. This sampling was conducted in September 1998 by researchers from the US Environmental Protection Agency (USEPA) National Risk Management Research Laboratory (NRMRL) Subsurface Protection and Remediation Division. The original EE/CA sampling event was conducted in August 1993 with the first long-term sampling occurring in July 1994 and the second long-term sampling occurring in September 1995. The main emphasis of this summary is to evaluate changes in dissolved benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations, plume extent, and natural attenuation mechanisms through time. Results, calculations, and predictions presented in the EE/CA are used as the basis for comparison. UST Site 870 is located in the southwestern comer of Hill AFB, Utah and encompasses the area immediately downgradient from and adjacent to the former location of UST 870 (Figure 1). The Base fuel tank farm and a portion of the Patriot Hills Base housing area located southwest of the tank farm are also included with the site. Hill Field elementary school is located immediately southwest of the housing area near the southwestern property boundary of the Base. Potential sources of contamination at UST Site 870 include: UST 870, leaky piping associated with the UST, surface spills and releases from tank farm operations since the 1940s, and a faulty 6-inch-diameter pipe located behind the pump facility Building 870 (pending repairs). Additional site information, including site background, geology, and hydrogeology, is provided in the EE/CA (Parsons ES, 1995). In 1993 and 1995, bail-down tests and pilot scale bioslurping occurred at a monitoring well located near the source area (Leeson et al., 1996). However, being pilot scale and. lasting only a few days, these activities did not remove a significant volume of free product. Additionally, on-going free product recovery using sorbent pads has occurred on a monthly basis in seven of the most contaminated site wells since 1996 (Greeley, 1999). Free product recovery via the sorbent pad technique has resulted in the removal of approximately 3 to 5 gallons per month, totaling approximately 110 to 180 gallons of free product.


Prepared for: Air Force Center for Environmental Excellence, Brooks Air Force Base, San Antonio, Texas and Hill Air Force Base, Ogden, Utah