Environ. Sci. Technol. 2003, 37, 2620-2621
Response to Comment on “Comparison between Donor Substrates for Biologically Enhanced Tetrachloroethene DNAPL Dissolution” We welcome the comments by Lutes et al. (1) to our paper (2), as this has helped to focus on some important issues regarding biological remediation of chlorinated solventcontaminated sites. They indicate that our “statements could potentially mislead engineering practitioners”. This was certainly not our intention, rather we believe it is important in introducing a new technology that possible limitations as well as potential benefits be presented so that practitioners are not in fact misled. First, our paper addressed a new effortsthe potential for enhancing the degree of dissolution of the dense nonaqueous phase liquid (DNAPL) or source of PCE contamination in aquifers. We were not studying contaminated plumes that emanate from such sources, which for the most part is what Lutes et al. (1) and the references they provide address. In recent studies, we (3) and others (4, 5) have found that bacterial dehalogenating activity near a DNAPL can increase the rate of dissolution considerably. We are attempting to better understand the processes and limitations involved in the event that it might be applied for full-scale application. Our studies suggest that with biologically enhanced dissolution of DNAPL the time for plume remediation may be reduced by a factor of 3 or more, which could lead to significant cost reductions. One of the basic differences in using biological reductive dehalogenation for enhanced DNAPL dissolution versus plume management is the chlorinated solvent concentrations being addressed. In the references supplied by Lutes et al. (1), field chlorinated-solvent concentrations on the order of 10-20 mg/L and less were generally being treated. In Figure 1 presented in their response as a case study in point, the sum concentration of chlorinated ethenes was on the order of 65 µM or less, which translates into an equivalent PCE concentration of about 11 mg/L. The total dissolved ethene concentrations that we reported were over 2500 µM or 40 times higher. An objective now is to achieve even higher dissolution. While the amount of donor Lutes et al. (1) added to achieve their field results was not given, their Figure 1 shows that their additions increased the total dissolved organic carbon in the groundwater by about 400 mg/L or more. A typical chemical oxygen demand (COD) to carbon ratio for organics is about 3. Thus, the amount of organics they added perhaps increased the groundwater COD by about 1200 mg/L. This would be difficult to be “mitigated by aerobic zones downgradient”, which they suggest. If we added a comparable donor organics to PCE ratio for DNAPL dissolution, the groundwater COD increase would be 40 times higher or 48 000 mg/L. It is obvious that this could result in severe groundwater quality problems such as very high oxygen demand, low pH, aquifer clogging, high methane production, high iron and manganese concentrations, and generally unusable water. Even while trying to limit the amount of donor added, we found some such problems occurred in our columns, here with only 3.7 mM pentanol, representing a soluble COD of about 770 mg/L. We felt it important to report such problems so that those wishing to try this approach might understand their cause so that they can better attempt to avoid them in their designs. 2620
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
Even the references provided by Lutes et al. (1) with plume management indicate that field problems have occurred with the typical molasses concentrations that they have used, such as low pH (6), and methane concentrations that are of environmental concern (7, 8). In their 13-month pilot study under a building in New York, they warned (8), “... extreme fermentation conditions can generate unwanted intermediate products such as ketones, and can also kill bacteria near the injection well due to the extreme acidity created”. They measured methane concentrations in the treated water as high as 55 000 µg/L, well above saturation values, and stated that “Monitoring during full scale operations may indicate a necessity for active soil gas venting possible off-gas treatment”. Here, the chlorinated solvent concentrations addressed were only about 3 mg/L. Similarly, concern over methane production was expressed for a bioremediation system they designed for Southeast England (7) to treat a groundwater plume under an active manufacturing facility with chlorinated solvent concentrations on the order of 5-30 mg/L. For this reason, they reduced the amount of substrate added, even though resulting dehalogenation rates were lower. They stated because of “... the caution employed here in dosing due to vapor concerns, the England rates are acceptable”. Thus, they too have recognized that high substrate concentrations can result in environmental problems at times, even when dealing with order of magnitude lower chlorinated solvent concentrations than in our study. Concerning other issues raised, we recognize that with enhanced dissolution high solution concentrations of chlorinated solvents resulted that do not meet regulatory requirements and have recognized that downgradient treatment would be required (3). However, downgradient treatment of resulting higher concentrations that occur over a much shorter time period is likely to be cost-effective. That is our hope. Lutes et al. (1) questioned our evidence for column clogging. We found with the oleate column, which had visible gas bubbles throughout the column, that we could not add a second dosage of oleate after 300 days of operation because clogging made it impossible to pass a solution through the column readily enough. Pressure readings were not necessary to tell us we had a problem. However, we recognize such a column with one-dimensional flow will show clogging more readily than will a three-dimensional groundwater system where flow can more easily take pathways around a clogged section. The difficulty here with DNAPL dissolution is that such diverted flow may bypass the DNAPL zone, which would reduce dissolution effectiveness. We do agree with Lutes et al. (1) that dehalogenators normally can compete with methanogens when given high substrate concentrations. However, our data demonstrated that, in the pentanol-fed column, the methanogens consumed the pentanol substrate even before it could reach the first DNAPL droplets, and this “was the major problem” that we reported. It is apparent that such a problem could also occur in the field, especially when attempting to achieve enhanced DNAPL dissolution. Because of the potential problems that can result with adding excess substrate to groundwater, we feel it is prudent to be cautious on this issue. Too frequently, we as a profession have solved one environmental problem only to create another. Our objective in groundwater remediation is to protect a resource, not just to remove a toxic compound from it regardless of the consequences to water quality. We are supportive of biological remediation but feel that it should be done responsibly. 10.1021/es030395x CCC: $25.00
2003 American Chemical Society Published on Web 05/03/2003
Literature Cited (1) Lutes, C. E.; Liles, D. S.; Suthersan, S. S.; Lenzo, F. C.; Hansen, M.; Payne, G. C.; Burdick, J. V.; Vance, D. Environ. Sci. Technol. 2003, 37, 2618-2619. (2) Yang, Y.; McCarty, P. L. Environ. Sci. Technol. 2002, 36, 34003404. (3) Yang, Y.; McCarty, P. L. Environ. Sci. Technol. 2000, 34, 29792984. (4) Carr, C. S.; Garg, S.; Hughes, J. B. Environ. Sci. Technol. 2000, 34, 1088-1094. (5) Cope, N.; Hughes, J. B. Environ. Sci. Technol. 2001, 35, 20142021. (6) Payne, G. C.; Suthersan, S. S.; Lenzo, F. C.; Burdick, J. S. In Anaerobic Degradation of Chlorinated Solvents; Magar, V. S., et al., Eds.; Battelle Press: Columbus, OH, 2001; pp 53-60. (7) Burdick, J. S.; Jacobs, D. L.; Rowland, P.; Lock, G. In Remediation of Chlorinated and Recalcitrant Compounds-2002, Proceedings
of the Third International Conference; Gavaskar, A. R., Chen, A. S. C., Eds.; Battelle Press: Columbus, OH, 2002; Paper 2H46. (8) Burdick, J. S.; McGuinnes, K. A.; Rodriquez, E.; Wanamaker, D. J. In Remediation of Chlorinated and Recalcitrant Compounds-2002, Proceedings of the Third International Conference; Gavaskar, A. R., Chen, A. S. C., Eds.; Battelle Press: Columbus, OH, 2002; Paper 2B-34.
Yanru Yang and Perry L. McCarty* Department of Civil and Environmental Engineering Stanford University Stanford, California 94305-4020 ES030395X
VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2621
