Bioventing systems at Alaska air force base achieve cleanup closures

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The U.S. Air Force and Alaska Department of Environmental Conservation (ADEC) collaborated in installing and operating seven full-scale bioventing systems over the last 10 years to address hydrocarbon and VOC contamination at the King Salmon Airport (KSA) in Alaska. Five of the systems achieved cleanup closure within five years of operation, and another is currently under closure review. The remaining system will continue operating to address light non-aqueous phase liquid (LNAPL) for an additional five years, after which sampling data will be reviewed to determine if regulatory cleanup standards have been met.

The KSA is located on the Alaska Peninsula adjacent to Bristol Bay and Katmai National Park and Preserve, approximately 280 miles southwest of Anchorage. KSA had been used as a World War II military fuel and support base and subsequently became part of a permanent state-wide air defense system. Hazardous materials formerly used and stored at the facility include diesel fuel, gasoline, oil, antifreeze, cleaning solvents, pesticides, and electrical transformers containing polychlorinated biphenyls (PCBs). Storage tanks were suspected to be the source for up to 400,000 gallons of fuel released into the subsurface.

Investigations under the U.S. Department of Defense Installation Restoration Program during the late 1980s indicated soil and ground water contamination at several KSA sites, which were segregated into seven treatment zones. High concentrations for contaminants of concern found in soil during 1994 and 1998 remedial investigations included 19,200 mg/kg for diesel-range organics (DRO), 15,000 mg/kg for gasoline-range organics (GRO), 27 mg/kg total BTEX (benzene, toluene, ethylbenzene, and xylenes), 0.26 mg/kg trichloroethene (TCE), and 2.19 mg/kg PCE.

A seep collection system was installed in 1994 to capture any source material and treat contaminated ground water before it reached an adjacent surface water body (Eskimo Creek). The collection system was shown over years to be ineffective in reducing the time to achieve the remedial action objective (RAO) of removing subsurface free product. Evaluated or implemented treatment methods to address the problem included a reactive iron wall, in situ biological treatment, bio-slurping, air sparging, bioventing, diversion walls, phytoremediation, capping, and monitored natural attenuation. Bioventing was selected in five of the seven treatment zones as a lower cost alternative with a shorter duration compared to other remedial approaches.

Six bioventing systems were installed in 1998 through 2001. Each system consisted of 1-11 venting wells extending 14-34 feet bgs with a 10- to 20-foot radius of influence per biovent. Equipment for each system consisted of an enclosed motor/blower assembly generating approximately 5-15 cfm of continuous air flow into each venting well. Four to eight vapor points per site were installed for periodic monitoring of soil gas.

Operation of the first system began in 1999, with the last going into operation in 2002 as part of a remediation process optimization (RPO) for an existing SVE system. The switch from SVE to bioventing in this treatment zone was estimated to save $2.8 million in project costs, including an annual $890,000 for operation and maintenance (O&M).

An additional RPO in 2004 resulted in expansion of the site’s radar approach control building (RAPCON) bioventing system to provide additional source treatment of diesel-range organics and free product. Expansion involved installation of four bioventing wells, which effectively enlarged the treatment area (Figure 2). Results of ground water sample analysis showed that bioventing was accelerating attenuation. At one RAPCON monitoring well, GRO and TCE pre-treatment concentrations in 1996 were 13.071 mg/L and 0.0589 mg/L, respectively. During a 2006 ground water monitoring event, GRO and TCE concentrations in the same well had reached established cleanup goals, with concentrations of 0.41 mg/L for GRO and 0.0159 mg/L for TCE.

Follow-up monitoring was conducted approximately two months after shutting down each of the five bioventing systems achieving cleanup objectives. Activities included installing two or three soil gas monitoring points and conducting a soil gas survey at existing vapor monitoring points to determine oxygen utilization. From each soil gas location, one soil sample was collected to evaluate soil gas total petroleum hydrocarbons and target contaminant concentrations. Appropriateness of system shutdown and site closure was determined through application of ADEC’s '350 determination,' otherwise known as the 'ten-times rule.' This determination allows for a site to be considered closed once contaminant concentrations attenuate below 10 times the established cleanup level; it can be applied only to soil-impacted areas where ground water is non-potable.

RPO in 2004-05 indicated marginal performance of the sixth bioventing system, which had operated intermittently since 2001 in the vicinity of another building. Operational problems involved blower and motor overheating and inability to inject adequate oxygen, partially due to surface water infiltration. Less permeable soil, water-saturated soil, and temporary leaks in system piping combined to limit oxygen delivery in this system. Piping repairs and diversion of air flow to air injection wells within the treatment area were implemented but had limited effect. To determine whether NAPL-saturated soil could be limiting bioventing success, six direct push borings equipped with an in-situ probe for measuring laser-induced fluoroescence (LIF) were advanced. LIF results showed low average saturation (2-4%) in the smear zone at a depth of 10-12 bgs. Based on recommendations from the RPO team and finalization of RPOs, ADEC recently granted approval for conditional closure with long-term monitoring and institutional controls.

The seventh bioventing system, which continues to operate, combines a bioventing curtain with a carbon-based pump and treatment system. This system was installed in 2003 to reduce LNAPL mobility and reduce continuing migration of hydrocarbons and TCE into the onsite creek. The bioventing curtain focuses on remediating the smear zone and augmenting natural attenuation, which was found to be occurring despite the cold climate. It consists of wells distributed across a contaminant plume perpendicular to ground water flow, in order to oxygenate ground water before entrance to the creek. This combined treatment strategy allowed complete shutdown of the ground water seep collection system, consequently reducing annual O&M costs by approximately $300,000 and potentially reducing long-term project costs by an estimated $40 million.

Abandonment of all KSA bioventing wells will be completed under a future base-wide well abandonment and maintenance effort. The U.S. Air Force Center for Engineering and the Environment (AFCEE) estimates that installation and operation of the five bioventing systems reaching cleanup closure cost approximately $1.2 million and accelerated cleanup closure by decades when compared to monitored natural attenuation. More information about AFCEE's bioventing initiative, which has involved more than 150 bioventing systems at more than 30 sites, is available at www.afcee.af.mil/resources/technologytransfer/.

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