In-situ bioremediation via direct hydrogen addition has the potential to become a simple and low-cost treatment approach for sites contaminated with chlorinated solvent compounds (PCE, TCE, etc.). Based on the results of recent research, the role of hydrogen as an electron donor is now widely recognized as the key factor governing the dechlorination of chlorinated compounds (Holliger et al., 1993; DiStefano et al., 1992; Maymo-Gatell et al., 1995; Gossett and Zinder, 1996; Smatlak et al., 1996; Hughes, Newell, and Fisher, 1997). Because of hydrogen's low cost, its ability to be delivered safely and inexpensively in a variety of ways, and its ability to promote rapid dechlorination, direct hydrogen addition represents a potentially superior approach for managing and remediating chlorinated solvent plumes.
Recent laboratory column studies sponsored by Groundwater Services, Inc. (GSI) and conducted by Dr. Joseph Hughes at Rice University show the potential for directly adding hydrogen, as an electron donor, to aid in the microbially mediated reduction of chlorinated compounds. In Hughes' laboratory system, hydrogen has been shown to support the transformation of PCE to reduced end products. This work has led to the development of a patent for the process of in-situ biodegradation of chlorinated aliphatic hydrocarbons by subsurface hydrogen injection (U.S. Patent No. 5602296; Hughes, Newell, and Fisher, 1997). This process involves the subsurface delivery of dissolved hydrogen using: i) low-flowrate sparge wells, ii) introduction of hydrogen releasing compounds, iii) operation of closed-cycle circulation cells, iv) placing hydrogen-generating electrodes in the subsurface, or v) a number of other methods.