Bioremediating Explosives Contaminated Soil

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Courtesy of BioCycle Magazine

From the beginning of World War II until the 1970s, the site that is now the Naval Surface Warfare Center in Crane, Indiana (NSWC-Crane) was the home of a major bomb manufacturing operation. Explosive compounds such as TNT, Royal Demolition Explosives (RDX) and Her Majesty’s Explosive (HMX) were handled loose and drifted through air vents. Often, the material settled on the roofs of buildings, which were hosed off. The compounds contaminated the soil so that five decades later, the tainted earth still has a reddish hue and cannot support plant life.

In the 1990s, NSWC-Crane evaluated its 31 Solid Waste Management Units to determine which ones would be amenable for Interim Measures cleanup due to explosives contamination. Four sites on the base’s 98 square miles were identified as needing remediation — the Ammunition Burning Grounds (11 acres), Rockeye Munitions (one acre), Mine Fill A (two acres) and Mine Fill B (two acres). Based on the results of similar projects at other military installations such as the Umatilla Army Depot in Hermiston, Oregon and the U.S. Naval Submarine Base in Bangor, Washington, NSWC-Crane ran tests on several feedstock mixes and decided to use composting. It is estimated that cleanup will take between seven and ten years and project costs will total $26 million.

The decision to bioremediate rather than incinerate — the traditional route — is expected to save money. At Umatilla, for example, cleanup costs for 15,000 tons of explosives-contaminated soil would have been $7.9 million for incineration ($527/ton) versus $5.3 million for composting ($351/ton) using a complex mix of 30 percent contaminated soil, 21 percent cattle manure, 18 percent sawdust, 18 percent alfalfa, ten percent potato waste and three percent chicken manure.

At NSWC-Crane, incineration was estimated at $500/ton compared to an original estimate of $240/ton for bioremediation, using 60 percent straw, 25 percent contaminated soil and 15 percent chicken manure. “One of the big reasons the estimate was so high was that we thought it would take 20 to 30 days to compost,” says Dave Beall, case project manager for Morrison-Knudsen of Cleveland, Ohio, the company contracted to construct NSWC-Crane’s new composting facility and operate it for the first year. “In time, we got the composting time down to seven to ten days, because the chicken manure and straw heat up to 60ƒC within 24 hours.” Costs, in turn, have decreased to about $140/ton and the facility expects to save money on the back end eventually by using the compost as daily cover at NSWC-Crane’s solid waste landfill. (Previously, cover material was purchased.)

Different feedstocks and amendments such as corn stalks, alfalfa, hay, cow manure, turkey manure, potato peelings, sawdust, wood chips and biosolids were tested during a 60-day pilot composting project in 1997. The mixture of straw, contaminated soil and chicken manure was the most effective. “This mix costs the least, is the best at reducing the contaminants and is the quickest,” Beall explains. Straw and manure are purchased from area farms. 
 
COMPOSTING PROCESS

The Army Corps of Engineers conducted the bench-scale testing and EPA’s Region 5 office helped design the new composting facility and operational plans, as well as identify cleanup goals. Recently, TolTest Inc. of Toledo, Ohio came on board as the primary contractor for daily operations, including excavation of the explosives-contaminated soils. Approximately $3 million was spent on the three composting buildings, excavation and materials handling equipment, two walking floor tractor trailers and a decontamination facility for workers.

In April, 1998, the bioremediation project went full-scale. NSWC-Crane is cleaning material from the mine fills first, followed by the munitions site. Work on the Ammunition Burning Grounds will complete the project. The final segment will be the most challenging because of the size of the area and soil contamination levels. Unlike the other sites, where the explosive compounds settled into the soil, the military used to burn surplus and outdated bombs directly on the ground at the Ammunition Burning Grounds. (The current practice is to burn the materials in clay-lined steel pans.)

The explosives-contaminated soil is excavated and placed directly into a trommel screen. The screened soil is placed in a walking floor trailer and transported to the bioremediation station. Prior to leaving the excavation site, the trucks are washed to prevent the spread of contamination. Additionally, the rocks screened out of the soil are pressure washed.

The bioremediation facility is set on 5.5 acres near NSWC-Crane’s landfill. Three buildings measuring 300 by 70 by 18 feet each house two windrows. Amendments are loaded into a grinder-mixer in the proper proportions. The blend is discharged from the unit via a conveyor onto the floor of the composting building. Windrows measure 250 feet long and are comprised of 843 cubic yards (cy) of material. “The windrows are turned with a Scarab at least once a day,” says Christine Freeman, NSWC’s project manager. “There is an exception if the temperature is below the optimum range. In such a case, turning is delayed until the temperature reaches the optimum range.” Moisture content is checked three times/week and when needed, the piles are sprayed using a tankard system; the material is turned to distribute the moisture evenly. Windrows also are monitored daily for oxygen and temperature, and weekly for pH.

Prior to composting at a site, a windrow of materials from the most concentrated area is composted and undergoes toxicity testing. Samples are collected from five cross-sections of the windrow and sent to a laboratory for explosives analysis; all other constituents are ruled out by pre-excavation sampling. To reduce costs, field test kits are utilized before laboratory analysis to ensure the cleanup goals have been met. Prior to composting, explosives concentrations are as follows: TNT 3,790 mg/kg; RDX 15,300 mg/kg; and HMX 10,400 mg/kg. The goals following composting, based on EPA standards, are: TNT 15 mg/kg; RDX 4 mg/kg; and HMX 3,300 mg/kg. Once acceptable remediation has been achieved, the finished compost is used as daily cover at the landfill or backfill at the original excavation site. As of mid-April, 11,394 cy of contaminated soil have been remediated, resulting in a total composted volume of 45,576 cy.

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