2008 Issue

www.utahengineerscouncil.org 38 BOMBS AWAY – By Mark F. Roginske, P.E.; Jesse A. Stewart, P.G; David W. Mills; J. Hhan Olsen, P.G. and Stacey Arens Scrap Iron May Prove Valuable Metal for Environmental Cleanup M ILITARY ENGINEERS FROM HILL AIR FORCE BASE (HILL AFB) ARE INVESTIGATING THE USE OF INERT PRACTICE BOMBS, REFERRED TO AS “BOMB DUMMY UNIT-33S”, AS A SOURCE OF IRON FORPERMEABLE REACTIVEBARRIERS (PRBS). PRBS are subsurface groundwater treatment systems designed to intercept contaminated groundwater and transform the contaminants into nontoxic end products. Granular cast iron is a reactive material used in PRBs to treat chlorinated solvents such as trichloroethene (TCE) in groundwater through chemical dehalogenation processes. The Bomb Dummy Unit-33 (BDU-33) is a 25–pound hardened cast iron practice bomb used for nearly a half century by the Air Force and other military branches for aircraft training exercises at military ranges including the Utah Test and Training Range (UTTR) in northwestern Utah (Figure 1). BDU-33 practice bombs are routinely cleared from military range target areas, demilitarized, and recycled. During the demilitarization process, the casing, tail fins, and spotting charge are removed from the BDU-33s and the cast iron shell is crushed to a size of approximately four-inch minus. The four-inch minus scrap iron is typically recycled. The idea of using iron from demilitarized practice bombs for PRB groundwater treatment systems was conceived in 2004 during the installation of a PRB in the Operable Unit 12 (OU 12) TCE-con- taminated groundwater plume at Hill AFB. Commercially-obtained granular iron used in the OU 12 PRB was a signifi- cant portion of the cost of the PRB installation. Hill AFB personnel, aware of the large quantities of BDU-33 practice bombs at the UTTR and other military ranges, questioned whether these practice bombs could be a less expensive source of iron for future PRBs. In 2006, funding was obtained to investigate the use of BDU-33 iron in PRBs and the Demili- tarized Bomb Munitions Iron (DBMI) project was formed. The DBMI project scope includes: h Evaluating the effectiveness of granular BDU- 33 iron to treat chlorinated solvents in bench- scale studies, h Determining the most cost-effective method for reducing the four-inch minus BDU-33 iron to the granular distribution (-8 to +50 United States Standard Mesh particle size) required for optimal iron/contaminant reactions, and h Testing the effectiveness of the granular BDU-33 iron to treat TCE-contaminated groundwater at Hill AFB’s Operable Unit 5 (OU 5). Two bench-scale column studies were performed to evaluate the effectiveness of BDU-33 iron for use in PRBs. The initial bench-scale column study compared reaction rates of commercially available iron from Connelly-GPM Inc. to that of BDU-33 iron using chlorinated solvent-contaminated groundwater collected fromHill AFB Operable Unit 1 (OU 1) and OU 5 (Figure 2). Since a method for reducing the four-inch minus BDU-33 iron to a granular distribution had not yet been identified, the granular practice bomb iron used in this column study was obtained by magnetic separation and sifting of bulk four-inch minus and smaller material consisting of iron, dirt, and concrete filler that was collected from the bottom of the crushing machine output hopper during the demilitarization process. Bench-scale testing of the BDU-33 iron using groundwater from OUs 1 and 5 indicated that the practice bomb iron could be used as a reactive iron material for PRBs at these sites. The study concluded that the reaction rate and cor- responding TCE-iron residence time for the BDU-33 iron was similar to that of the commercially-obtained granular iron. Figure 1. Stockpile of BDU-33 Practice Bombs Prior to Demilitarization at the UTTR