Environ. Sci. Technol. 2005, 39, 1199-1200
Response to Comment on “Addition of Carbon Sorbents to Reduce PCB and PAH Bioavailability in Marine Sediments: Physicochemical Tests” Weber (1) raises several points concerning the potential environmental impact, implementation feasibility, and cost of a new approach for dealing with persistent organic contaminants in sediments. Our proposal is based on the original and novel concept that adding activated carbon to field sediment reduces the availability of contaminants such as PCBs, thus reducing uptake by benthic biota and release to water (2). We appreciate that questions will arise in any innovative approach for dealing with challenging environmental problems, but this should not forestall research and development. Our objectives were to establish that the treatment works under laboratory conditions as a proof-ofconcept. Our prior research with sediment from Hunters Point, San Francisco Bay, CA, and Milwaukee Harbor, WI, provided particle-scale understanding of the distribution of PCBs (3). This work showed, for example, that about 68% of the PCBs and 89% of PAHs in Hunters Point sediment were associated with black carbonaceous particles, principally char. Desorption tests revealed that hydrophobic contaminants associated with black carbonaceous particles in the sediment were much less bioavailable than hydrophobic contaminants associated with the heavier, mineral fraction of the sediment (4, 5). Thus, it is important to understand that our proposal (2) for activated carbon addition to sequester PCBs in sediment is based on our own observations from nature. In essence, we are enhancing a natural process of repartitioning and sequestration of hydrophobic contaminants into carbonaceous sorbents. Several recent studies (e.g., ref 6) report that naturally occurring soot carbon, with diameters orders of magnitude smaller than the carbon we are using, causes strong sorption and reduced bioavailability of PCBs and PAHs in sediments. Contrary to the speculation of Weber (1), sorbent black carbonaceous particles incorporated into the sediment do not easily resuspend into the water column and raise contaminant availability. Weber (1) is concerned that activated carbon amendment may constitute an “enriched and readily available route for entry of potent contaminants in aquatic food chains”. In fact, our available data show the opposite. McLeod et al. (7) conducted tests to assess the bioavailability via diet of 2,2′,5,5′tetrachlorobiphenyl and benzo[a]pyrene from different carbonaceous particle types to clams (Macoma balthica) collected from San Francisco Bay. The PCB and benzo[a]pyrene bound to activated carbon exhibited less than 2% absorption efficiency and were up to 60 times less available to clams than the same contaminants associated with other types of carbonaceous matter. Our work with two other benthic infauna (Neanthes arenaceodentata and Leptocheirus plumulosus) has shown that bioaccumulation of PCBs in Hunters Point sediments amended with activated carbon is reduced by more than 80% (Millward et al., submitted; (10)). The differing reactivity of organic sorbent materials and tools to measure such differences are discussed in an NRC report on the bioavailability of contaminants in soils and sediments (8). Weber suggests the concept is infeasible (1). We disagree with such sweeping generalizations. As a practical matter, 10.1021/es047983a CCC: $30.25 Published on Web 01/12/2005
2005 American Chemical Society
any sediment management plan must be evaluated on its own merit in consideration of site-specific conditions, and that is no different in this case. At present, we cannot possibly answer all the hypothetical questions that may be raised, preferring instead to focus on the problem at hand and work with responsible parties, public groups, and regulators to conduct closely monitored field trials at Hunters Point. As described in the original paper (2), the contaminated sediments at Hunters Point include a tidal mudflat, and we envision mixing activated carbon into the biologically active layer of sediment, the upper 0.3-0.5 m, at low tide when the treatment area is exposed. We performed an initial assessment of bringing large-scale mixing equipment to the site at high tide and demonstrated the feasibility of mixing dry carbon into the sediment at low tide. We observed no “slurrylike nephelometric layers swirling” on the incoming tide. As described in the paper (2), Hunters Point sediment at South Basin is cohesive, and the site is slightly depositional and protected from severe hydrodynamic forces. We measured the erosion rate and critical shear stress for incipient particle motion of sediment mixed with activated carbon. Results from Sedflume experiments show that the critical shear stress is at least 0.4 Pa for surficial sediment and mixing with activated carbon did not decrease stability (9). Hydrodynamic modeling using Tidal Residual Intertidal Mudflat (TRIM-3D) showed that the critical shear stress might be exceeded only for short periods during extreme storm events (9). Thus, measurement and modeling indicate that cohesive sediment at South Basin will maintain activated carbon in place under normal conditions with only minimal erosion during extreme storm events. We do not advocate “broadcasting activated carbon upon earth’s open waters” as Weber suggests (1). The costs of the proposed in situ stabilization technology for Hunters Point are attractive. While detailed cost estimates are not possible, we can estimate cost ranges based on our experience to date. Compared to dredging and disposal, the costs for in-situ treatment of the biologically active layer of sediment with 2-3% activated carbon may be a factor of 5-10 times less than dredging and disposal. Furthermore, dredging at this site may be unacceptable to certain stakeholders, and capping would not be allowed without first dredging to prevent San Francisco Bay filling. In summary, we find the concerns of Weber (1) largely based on hypothetical issues and a tortured reading of the paper (2). We acknowledge that the concept of in-situ treatment of sediment by amendments to sequester persistent organic contaminants requires further development and study, including laboratory tests, mechanistic process understanding, and field demonstration and assessments at various sites. Our results to date are promising, and we continue to work on the development and advancement of the concept.
Literature Cited (1) Weber, W. J., Jr. Comment on “Addition of carbon sorbents to reduce PCB and PAH bioavailability in marine sediments: physicochemical tests”. Environ. Sci. Technol. 2005, 39, 1197. (2) Zimmerman, J. R.; Ghosh, U.; Millward, R. N.; Bridges, T. S.; Luthy, R. G. Addition of carbon sorbents to reduce PCB and PAH bioavailability in marine sediments: physicochemical tests. Environ. Sci. Technol. 2004, 38 (20), 5458-5464. (3) Ghosh, U.; Zimmerman, J. R.; Luthy, R. G. PCB and PAH speciation among particle types in contaminated harbor sediments and effects on PAH bioavailability. Environ. Sci. Technol. 2003, 37 (10), 2209-2217. VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(4) Ghosh, U.; Gillette, J. S.; Zare, R. N.; Luthy, R. G. Microscale location, characterization, and association of polycyclic aromatic hydrocarbons on harbor sediment particles. Environ. Sci. Technol. 2000, 34 (9), 1729-1736. (5) Talley, J. W.; Ghosh, U.; Tucker, S. G.; Furey, J. S.; Luthy, R. G.; Particle-scale understanding of the bioavailability of PAHs in sediments. Environ. Sci. Technol. 2002, 36 (3), 477-483. (6) Accardi-Dey, A.; Gschwend, P. M. Assessing the combined roles of natural organic matter and black carbon as sorbents in sediments. Environ. Sci. Technol. 2002, 36 (1), 21-29. (7) McLeod, P. M.; van den Heuvel-Greve, M. J.; Allen-King, R. M.; Luoma, S. N.; Luthy, R. G. Effects of particulate carbonaceous matter on the bioavailability of benzo[a]pyrene and 2,2′,5,5′tetrachlorobiphenyl to the clam, Macoma balthica. Environ. Sci. Technol. 2004, 38 (17), 4549-4556. (8) National Research Council. Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications; The National Academies Press: Washington, DC, 2003. (9) Zimmerman, J. R. In Situ Stabilization of Persistent Organic Contaminants in Marine Sediments. Ph.D. Thesis, Stanford University, Stanford, CA, December 2004. (10) Zimmerman, J. R., Werner, D., Ghosh, U., Millward, R. N., Bridges, T. S., Luthy, R. G. The Effects of Dose and Particle Size on Activated Carbon Treatment to Sequester PCBs and PAHs in Marine Sediments, accepted, Environ. Toxicol. Chem., 2005.
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John R. Zimmerman and Richard G. Luthy* Department of Civil and Environmental Engineering Stanford University Stanford, California 94305-4020
Upal Ghosh Department of Civil and Environmental Engineering University of Maryland Baltimore County Baltimore, Maryland 21250
Rod N. Millward and Todd S. Bridges Environmental Laboratory U.S. Army Engineer Research and Development Center Vicksburg, Mississippi 39180 ES047983A
