The principal demonstration objectives for this project included (1) to select and optimize RDX-degrading microbial cultures for use in aerobic bioaugmentation at the UMCD, (2) to compare in situ RDX biodegradation rates for aerobic bioaugmentation to those for biostimulation, and (3) to quantify and compare costs of RDX remediation in groundwater and time-to-complete at UMCD using aerobic bioaugmentation, conventional P&T, and both anaerobic and aerobic biostimulation without bioaugmentation. The performance objectives were:
(1) Use aerobic bioaugmentation to degrade RDX to <2.1 micrograms per liter (µg L)-1,
(2) Find the RDX removal rate for aerobic bioaugmentation comparable to removal rates for aerobic and anaerobic biostimulation,
(3) Find the RDX mass removed per mass of substrate added enhanced for aerobic bioaugmentation compared to aerobic and anaerobic biostimulation, and
(4) Bioaugmentation culture will remain viable and will retain RDX-degrading capability over time in situ.
Several strains of RDX-degrading bacteria were initially evaluated in laboratory studies (Phase I) to assess RDX degradation rates on various substrates, as well as their growth, viability, and transportability under simulated field conditions. Field testing was conducted with selected strains to evaluate the transportability of RDX-degrading strains (Phase II). An extensive series of field tests (Phase III) were conducted to compare the rate and extent of RDX degradation following bioaugmentation to two conventional treatments: aerobic biostimulation and anaerobic biostimulation.
Biostimulation was accomplished by five injections of six cubic meters (m3) of site groundwater containing 0.25–1 millimolar (mM) fructose into two adjacent wells over 24 days to stimulate the growth of indigenous organisms with the ability to degrade RDX. After push-pull tests (PPTs) were conducted in all wells to measure RDX degradation rates, six additional, higher concentration (15–24 mM) fructose additions were used to create anaerobic conditions in those same wells. Average RDX degradation rates (all wells combined) for aerobic and anaerobic biostimulation were 0.49 and 0.67 day-1, respectively.
Three additional wells were bioaugmented by injecting 6 m3 of site groundwater amended with RDX, tracer, fructose, and 108 cells milliliter (mL)-1 of Gordonia sp. KTR9 KanR (KTR9). Rates of RDX degradation were measured three times—once immediately following initial bioaugmentation with KTR9 (first test) and twice more over a period of 130 days. The results indicated that aerobic bioaugmentation achieved a rate and extent of RDX degradation larger than aerobic biostimulation and comparable to anaerobic biostimulation, while requiring substantially less added substrate. The average RDX degradation rate (all wells combined) for aerobic bioaugmentation was 1.2 day-1.
The cost-benefit analysis completed for this demonstration was based on groundwater remedy optimization work completed at UMCD. Cost estimates were developed for the following four UMCD groundwater remedy optimization scenarios:
(1) Installation of additional extraction wells for enhanced P&T,
(2) Enhanced P&T followed by anaerobic biostimulation in the remaining smaller plume footprint,
(3) Enhanced P&T followed by a combination of anaerobic biostimulation and aerobic bioaugmentation in the remaining smaller plume footprint, and
(4) Enhanced P&T followed by a combination of anaerobic biostimulation and aerobic biostimulation in the remaining smaller plume footprint.
KTR9 (and other flavodoxin cytochrome P450 gene [xplA] gene-containing microbes) are able to utilize RDX as a nitrogen source for growth and thus promote RDX degradation; however, these bacteria are not able to use (or degrade) trinitrotoluene (TNT). Therefore, Scenarios 3 and 4 include application of aerobic bioaugmentation or aerobic biostimulation for the distal RDX plume only. Anaerobic biostimulation effectively degrades both RDX and TNT and is therefore well-suited for remediation of comingled explosives present near the source area. Assuming a 1.4% discount rate, the total estimated costs to implement Scenarios 1–4 were approximately $11.9M, $10.3M, $10.7M, and $9.6M, respectively. By including aerobic bioaugmentation as part of the bioremediation strategy at UMCD, this has the potential to save over $1M in costs, preserve aerobic groundwater quality over a large portion of the distal RDX groundwater plume, and achieve cleanup in 15 years compared to Scenario 2, which is predicted to achieve cleanup in 30 years.
Aerobic bioaugmentation satisfied the performance objectives and is considered the first successful demonstration of bioaugmentation for treatment of RDX-contaminated groundwater plumes. Demonstration results are being used to optimize the existing P&T groundwater remedy at UMCD by supporting incorporation of bioaugmentation into a full-scale remediation program. Cost and performance data from this demonstration concerning the utilization of aerobic bioaugmentation for full-scale RDX groundwater treatment will benefit other U.S. Department of Defense (DoD) sites with large RDX plumes as well, including Milan Army Ammunition Plant, TN; Fort Wingate, NM; former Hastings Naval Ammunition Depot, NE; former Nebraska Ordnance Plant, NE; and Massachusetts Military Reservation.
Although the aerobic bioaugmentation demonstration was considered successful, it is not possible based on demonstration results alone to know if aerobic bioaugmentation would provide sustained, more-cost-effective RDX removal compared to biostimulation. Therefore, as with all bioremediation remedies, a phased and flexible approach should be accounted for during design. Specific design elements of the amendment injection and circulation system should include the ability to: