Hundreds of landfills on Department of Defense (DoD) installations have generated chlorinated aliphatic hydrocarbon (CAH) plumes in groundwater. Surface covers, which are intended to provide a barrier to prevent direct contact with waste material and minimize or eliminate infiltration of precipitation through the waste material (i.e., leachate formation), are the current method of choice to manage human and ecological risks at these sites. In some cases, impermeable covers may impede natural attenuation processes by reducing the quantity of organic-rich leachate that promotes the bioremediation of CAHs. This project conducted a pilot-scale field demonstration of a recirculation bioreactor at Landfill 3 (LF-03), Altus Air Force Base (AFB) in Oklahoma. The purpose of constructing and operating the bioreactor was to demonstrate that a combination of organic material addition and accelerated leaching can rapidly reduce source area concentrations of CAHs in groundwater at unlined, closed landfills. The results provide environmental engineers with an additional perspective on treatment of dissolved volatile organic compound plumes originating at unlined landfills (and at non-landfill source areas).
The objective of this project was to promote the development of alternative landfill closure designs and management strategies for DoD landfills that can reduce the time required for stabilization of the landfilled wastes and enhance the long-term natural attenuation of chlorinated solvent leachate plumes.
A literature review was first completed to determine how alternative landfill closure designs and management strategies can impact the natural attenuation and long-term risk of solvent plumes. Historical geochemical data for leachate-contaminated groundwater at a variety of DoD landfill sites also were evaluated to determine typical geochemical characteristics. Various leachate-control, recirculation, and landfill-cover options were then evaluated for their potential use under different climatic and hydrogeologic scenarios.
Specific objectives of the subsequent bioreactor demonstration at Altus AFB included:
Demonstrate construction techniques and the instrumentation of two types of bioreactor cells that can be used for unlined and closed landfills:
a. An active bioreactor that collects shallow groundwater and recirculates the groundwater through organic mulch to accelerate organic-rich leachate production and CAH biodegradation (Recirculation Bioreactor)
b. A passive bioreactor that relies on natural groundwater flow and infiltration moving through an organic mulch layer to produce an organic-rich leachate (Passive Bioreactor)
The primary regulatory driver for investigation and cleanup of hazardous waste at Altus AFB is Resource Conservation and Recovery Act (RCRA), Section 3008 (h). U.S. Environmental Protection Agency Region 6 is the primary regulatory agency for the Base. LF-03, also referred to as Solid Waste Management Unit 7, was included in the base-wide RCRA Facility Investigation, Investigation Analysis, and Corrective Measures Study (CMS). The CMS recommended anaerobic bioremediation as the groundwater remediation alternative for the site. These regulatory considerations contributed to elimination of the passive bioremediation cell scenario.
The bioreactor was constructed by excavating a 30-ft by 30-ft by 11-ft-deep portion of the landfill near the suspected TCE source area and backfilling the excavation with a mixture of organic material and sand. A groundwater extraction trench was excavated into the shallow aquifer downgradient of the reactor cell and backfilled with gravel. Groundwater from the trench was extracted and distributed within the bioreactor cell using a drip irrigation system. Groundwater monitoring wells were installed within the bioreactor cell and in the aquifer adjacent to and beneath the test cell for monitoring concentrations of CAHs and geochemical/microbial indicator parameters. Five performance monitoring events were completed during the approximately 24-month pilot test.
During the five performance monitoring events, the bioreactor removal efficiencies for TCE and total chlorinated ethenes (sum of TCE, DCE, and VC) from recirculated groundwater ranged from 97 to 100% and 76 to 96%, respectively. A source of residual TCE in the subsurface upgradient of the bioreactor and above-average precipitation during a portion of the pilot test caused an influx of dissolved TCE and an accumulation of TCE biodegradation daughter products (DCE and VC) in the monitored area adjacent to and beneath the bioreactor. Dissolved organic carbon (DOC) concentrations were elevated above the 20 mg/L threshold that is conducive for reductive dechlorination for approximately 6 months to a year in the deeper wells beneath the bioreactor and for almost the entire 2-year duration of the pilot test in the shallow wells adjacent to the test cell. The presence of high sulfate concentrations in groundwater at LF-03 likely reduced the effectiveness of the bioreactor but did not prevent reductive dechlorination from occurring. Because of a continuing TCE source upgradient of the bioreactor and the accumulation of daughter products in the aquifer beneath and adjacent to the bioreactor, the objective of reducing CAH concentrations by 90% was not achieved.
Implementation of the recirculation bioreactor technology requires excavation of vadose zone fill or waste, backfilling with a mulch/sand mixture, and installation of an infiltration gallery. In a full-scale implementation of this technology, it is likely that an area larger than the 900 sq ft tested in the pilot study would be excavated and that a significant percentage of the soil/waste would require off-site disposal. In addition, if a vadose zone source area is removed, it is possible that a portion of the spoils would require disposal in a RCRA Subtitle C facility. Each of these conditions would affect the overall cost of the bioreactor technology.
A cost analysis was performed to assess the impact of bioreactor size and off-site disposal requirements on the total net present value. The total net present value increases substantially with bioreactor size. For the small bioreactors evaluated (30 ft x 30 ft), the primary cost component was operation, maintenance, and monitoring (OM&M). For larger bioreactors, the capital costs contributed the most to the total net present value. In addition, the periodic cost of substrate replenishment represented a substantial portion of the total net present value. The greatest contributors to the capital cost were off-site disposal (transport and tipping fee) and bioreactor backfilling. If it is possible for the excavated soil to remain onsite, the capital costs would be substantially lower. However, it is expected that a technology which installs a bioreactor in a former source zone would result in off-site disposal of a significant percentage of the excavated material.
The cost analysis indicates that the mulch bioreactor technology has the potential for high costs to be incurred, depending on the size of the source area and the type of waste encountered. While this approach may be appropriate for well-defined, small, isolated source areas marked by shallow groundwater, it may not be the optimum approach for large landfills with multiple source areas.
Following completion of this demonstration project, Altus AFB has continued to operate the recirculation bioreactor and subsequently funded a project to add liquid carbon substrate and a bioaugmentation culture to the recirculation system. The goal of the follow-on project is to refresh the organic carbon supply and to determine if more complete and effective reductive dechlorination can be achieved.
Through this project, three decision trees also were developed to assist DoD remedial project managers in the selection of an appropriate remedial strategy for chlorinated solvent-contaminated landfills. These decision trees guide the user through the following remedy selection steps: (1) landfill screening, (2) detailed remedial alternatives assessment, and (3) landfill cover designs. The landfill screening decision tree assists the user in determining if engineered remediation of chlorinated solvents in groundwater is required or if cleanup goals can be met with natural attenuation. The remedial alternatives decision tree guides the selection of an appropriate remedial alternative when risk or time considerations dictate a more aggressive approach and indicates the conditions under which operation of the landfill as a bioreactor, with collection and recirculation of leachate-contaminated groundwater, is an appropriate alternative. The landfill cover decision tree facilitates selection of an appropriate cover for solvent-contaminated landfills.