Reductive Biotransformation of Explosives Compounds in Groundwater

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ABSTRACT: Earth Tech Inc. has successfully completed two bench-scale studies demonstrating the treatment of explosives contaminated groundwater by amendment with Hydrogen Release Compound (HRC). The first of these two bench scale studies was intended to demonstrate the removal of explosive contaminants, with the second designed to assess transformation product formation and accumulation. Groundwater used in these bench scale studies was obtained from groundwater monitoring wells located at the Pueblo Chemical Depot, in Pueblo, Colorado. Groundwater from this portion of the site contains various explosive compounds including, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), 4-amino-2,6- dinitrotoluene (4-Am-2,6-DNT), 2-amino-4,6-dinitrotoluene (2-Am-4,6-DNT), and 2,4-dinitrotoluene (2,4-DNT), as well as nitrate. Biotransformation of these compounds is achieved by cometabolic reduction resulting from the addition of HRC to the groundwater.

Results of the first bench scale study showed that treatment was achieved as quickly as 14 days in test reactors, with strongly reducing conditions (<-400mV) being achieved within 7 days. The efficacy of removal was greater than 95% for most compounds, as determined by Earth Tech’s on-site laboratory performing U.S.E.P.A. Method SW8330 analysis. Since early attempts to describe the reduction pathway of RDX included byproducts of environmental concern such as the mono-, di-, and trinitroso deriviatives (MNX, DNX, and TNX) of RDX, n-nitrosodimethylamine (NDMA), and hydrazine, a second study was performed to evaluate the potential for accumulation of these compounds. Samples were collected at a point believed to represent the end of the reductive treatment process and analyzed for these potential byproducts. Post treatment analyses for these byproducts confirm that these compounds are either not produced or are further reduced under continued reducing conditions. These results indicate that HRC may be suitable for in situ treatment of explosives contaminated groundwater. Successful in situ treatment of explosives contaminated groundwater by HRC could provide a cost-effective alternative to other in situ treatment technologies such as chemical oxidation, or ex situ ‘pump and treat’ strategies.

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