Paper B-04, in: V.S. Magar and M.E. Kelley (Eds.), In Situ and On-Site Bioremediation—2003. Proceedings of the Seventh International In Situ and On-Site Bioremediation Symposium (Orlando, FL; June 2003). ISBN 1-57477-139-6, published by Battelle Press, Columbus, OH, www.battelle.org/bookstore . A HYDROGEN RECIRCULATION SYSTEM FOR THE TREATMENT OF TCE-IMPACTED GROUNDWATER Carol E. Aziz (ceaziz@gsi-net.com), Shahla K. Farhat, Elaine A. Higgins, and Charles J. Newell (Groundwater Services, Houston, TX) Jerry Hansen (Air Force Center for Environmental Excellence, San Antonio, TX) ABSTRACT: To speed the rate of reductive dechlorination of trichloroethylene (TCE)- impacted groundwater at the Old Jet Engine Test Site (OJETS), Offutt AFB, Nebraska, an in situ pilot-scale dissolved hydrogen addition system was installed. The test area consisted of an injection well, a recovery well, and five monitoring wells within an 8.5 m x 4.3 m (28 ft x 14 ft) test plot. Groundwater was pumped from the recovery well at a flow rate of 1.5 L/min (0.4 gpm), amended with hydrogen at 20 mL/min, and injected into the injection well, thereby creating a recirculation system. The results of the first 6 months of operation are presented in this paper. After 6 months of operation, the oxidation-reduction potential and dissolved oxygen of the groundwater decreased. Despite background sulfate levels of 280 mg/L, the mean TCE concentration declined 63% from 2.9 µmol/L to 1.07 µmol/L. Significant production of cis-1,2-dichloroethene (cDCE) was observed within the test area (i.e., 0.7 to 3.0 µmol/L), indicating that the hydrogen was promoting reductive dechlorination. The cDCE:TCE ratio increased more than ten fold from 0.25 to 2.8, despite decreasing groundwater temperatures. In summary, the dissolved hydrogen recirculation system was effective in removing TCE from groundwater without significant vinyl chloride production. INTRODUCTION The major contaminants of concern at the OJET site are TCE and its daughter products. Although these constituents can undergo natural attenuation, the rate can be severely limited by inadequate quantities of electron donor. At sites where natural dechlorination is occurring, organic substrates such as aromatic hydrocarbons (BTEX), landfill leachate, or natural organic matter can provide a source of dissolved hydrogen, produced through slow fermentation of these organics. The hydrogen is then rapidly utilized as an electron donor by naturally occurring bacteria to achieve reductive dechlorination of chlorinated compounds in the subsurface. Direct hydrogen addition eliminates the rate-limiting step (i.e., slow fermentation), by providing naturally occurring dechlorinating bacteria with substantive quantities of hydrogen. Because hydrogen is an ideal electron donor for anaerobic bacteria, dechlorinating microorganisms compete for dissolved hydrogen with other bacteria, such as methanogens, sulfate reducers, nitrate reducers, in the subsurface. Both laboratory and field studies (Carr and Hughes, 1998, Newell et al., 2000; Newell et al., 2001) have shown that the populations of dechlorinating microorganisms can be successful at competing for hydrogen in a hydrogen-rich environment This result can be attributed to the dechlorinators having: (i) a higher maximum utilization rate and (ii) a higher yield.