Correspondence Comment on “Comparison between Donor Substrates for Biologically Enhanced Tetrachloroethene DNAPL Dissolution” Yang and McCarty’s recent paper (1) makes discussion statements that we believe are inadequately supported by the data presented and in disagreement with published bench-scale work of other groups. These statements also disagree with our field observations in a population of over 140 sites. Thus, these statements could potentially mislead engineering practitioners. Their experimental design and results section focuses primarily on enhancement of DNAPL dissolution. These sections are essentially sound, although the enhancements achieved are modest. However, the majority of their discussion focuses on competition between methanogens and dehalorespiring organisms in chlorinated aliphatic hydrocarbon (CAH)-degrading systems and on the negative aspects of methanogens, especially aquifer clogging. The authors’ bench-scale systems never achieved substantially complete mineralization of the CAHs to ethene, a widely accepted measure of success in CAH bioremediation. Thus, despite highly efficient utilization of substrate, their treatment results would be unlikely to meet cleanup goals or satisfy regulators. The authors do not present quantitative data (such as decreased column flow rates, increased column influent pressures, etc.) as to the extent of cloggingstheir primary evidence for this seems to be the appearance of gas bubbles. We have conducted more than 140 field-scale enhanced CAH biodegradation/anaerobic metals precipitation projects using soluble carbohydrate substrates (2-6) during which highly methanogenic conditions are often induced. Our field data (such as potentiometric surfaces, substrate indicator parameter concentrations at downgradient monitoring wells, injection well pressures, travel times, and treatment efficiency) indicate that clogging in highly methanogenic systems is not a widespread problem at field scale. Any clogging that has been observed has been localized around injection wells and could be economically remedied with occasional well redevelopment. We suggest hydrostatic pressure of the water column probably makes bubble formation less likely at field scale. The authors assert that, with a pentanol feed, methanogenic competition for substrate with dechlorinating organisms “was the major problem”. Other workers have published conflicting results for similar substrates. For example, Carr and Hughes (7) compared methanol to other substrates and concluded that “Dechlorination and methanogenesis occurred simultaneously and neither process appeared to be inhibited by the other”. Fennel and Gossett (8) submitted a comment on the Carr and Hughes paper in 1999, and their comment and the response at least partially explains the reasons for conflicting bench-scale results of various workers on this issue. Their published debate revolved around whether such competition would likely be detrimental to the performance of actual field-scale remedial systems. While Yang and McCarty provide additional bench-scale data, we believe that field experience under a variety of conditions suggests that the position of Carr and Hughes is correct, that competition from methanogens is not a significant concern. For example, at a site in southwestern Ohio, we have conducted high-rate enhancement of reductive dechlori2618
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
FIGURE 1. Long-term operation of ERD at a site in southwest Ohio. nation in a barrier for 2.5 yr (Figure 1). Groundwater flow is approximately 1 ft/day. The graphed monitoring location is located 100 days downgradient from the injection well line. Contaminant concentrations flowing into the reactive barrier have been similar to those seen in the pretreatment data, a continuous influent of more than 10 µmol of PCE + TCE + cis-DCE + VC. Through the period of treatment shown here, PCE and TCE are the dominant influents. The spike in cisDCE at the 600-day mark is due to shutoff of the molasses injections (due to contractual interruptions). They were restarted, and the cis-DCE (as well as all other chlorinated alkenes) went back to less than detection (
