Of PCBs and the river
Environmentalists claimed victory when EPA approved dredging the Hudson River to remove polychlorinated biphenyls. But will the cleanup operations reduce exposure of humans to PCBs or not?
The Bush administration isnt usually recognized for having a strong environmental bent. But on August 1, 2001, it approved one of the largest cleanup operations in U.S. history, ordering General Electric to remove thousands of pounds of polychlorinated biphenyls (PCBs) from the bottom of New Yorks Hudson River. The move is a major step toward the biggest environmental dredging effort ever attempted. It also caps a long-standing and emotional battle between GE, which has resisted the cleanup, and a variety of advocacy groups that have pushed hard for remediation since a 200-mile stretch of the river was designated a Superfund site in 1981. The affected portion of the river stretches from the town of Hudson Falls to the rivers outfall in New York City.
The PCBs came from two capacitor manufacturing plants operated by GE from 1947 to 1975. Together, these plants released 400800 tons of PCBs (mainly under the Monsanto Co. brand names Aroclor 1242 and 1016) into the river during that time. The releases were legal at the time they occurred: PCBs were not banned by the Environmental Protection Agency from manufacture or release until 1997. Today, the residual contamination from PCB production plagues GE and other companies not only on the Hudson but also at numerous other sites around the world.
Questions over motivation
According to the skeptics, the PCBs should be left in place, where they will eventually degrade, be covered over by clean sediment, or both.
Bonnie Bellow, spokesperson for EPA Region II, which includes New York State, says the agency has studied the issue carefully and concluded there is a strong likelihood the river will benefit from the cleanup. Our proposed plan for the Hudson is based on a sound premise, she says. If you reduce sediment PCB concentrations, you will ultimately reduce the levels in fish.
Scientists from around the world will be watching as the cleanup progresses. Despite the existence of numerous aquatic sites contaminated with PCBs, the experience with removal by dredging is limited. A recent report issued by the National Research Council (NRC) concluded that the current database on the success or failure of dredging is not sufficient to draw strong general conclusions as to its applicability in particular situations (1, p 222). According to the NRC, more high-quality monitoring data are needed to characterize the effectiveness of dredging for reducing environmental risk. Presumably, the Hudson River cleanup will provide ample opportunity to gather the needed data.
Questions about toxicity
But the human data are shrouded in mystery: Despite years of study, scientists still do not know exactly how PCBs affect humans, particularly at low levels of exposure. This is mainly because epidemiology studies that investigate the risk to humans have some basic shortcomings, including difficulties in exposure assessment, small sample sizes, and lack of suitable control populations.
GE has tried to exploit this uncertainty to its advantage. We believe there is no credible evidence that PCB exposure in humans is associated with increased deaths from cancer or any other disease, says GE spokesperson Mark Behan.
The company has had trouble selling this view to the public. Hudson River communities are worried about PCB toxicity and angered by fishing bans and reduced property values. For the citizen groups who spent decades advocating the cleanup, EPAs order is a long overdue vindication. We see this as a tremendous victory, says Kit Kennedy, senior attorney with the Natural Resources Defense Council in New York. It sends a signal that we have the technology to dredge contaminated sediments safely, accurately, and precisely; and it shows that the EPA isnt going to ignore the problem.
Some critics of the plan argue that dredging could have the opposite effect: raising PCB levels in fish rather than decreasing them. This is because buried sediments will be resuspended in the water column as the dredge rips across the bottom, making the pollutants more available to biota. In its report, the NRC describes several instances in which dredging has caused fish PCB levels to rise because of resuspended sediments. Dredging at a General Motors PCB site in Massena, NY, for example, resulted in a slight increase in PCB concentrations in fish tissue the year after dredging was completed (1, p 275).
Controls for minimizing resuspension are increasingly available. Silt curtains made of vinyl or polyurethane can be used to increase the sediment residence time around the dredgerhead and encourage settling. Hydraulic dredges that use vacuum suction cause less resuspension than mechanical dredges that scoop sediments out of the river. And cameras and global positioning satellites help target hot spots and thereby reduce unnecessary removals.
Furthermore, postdredging elevations in fish are likely to be transient, says John Farrington, Dean of Graduate Studies at Woods Hole Oceanographic Institution (MA), and chair of the NRC Committee on Remediation of PCB-Contaminated Sediments. PCB concentrations in fish are a function of the levels in water and sediment combined, he said. If you reduce sediment PCB levels, water levels should fall, and eventually levels in biota should follow.
The caveat here is the duration of the cleanup. Dredging the Hudson could take years: EPA says up to 5, and GE says at least 10. And PCB levels in fish are likely to be higher than they already are throughout the cleanup and for an unknown period afterward. Bob Gibson, a scientist with GE Corporate Environmental Programs, says that once the cleanup begins these elevated PCB levels will increase exposures among anglers who eat their catch.
Natural attenuation: Too slow?
But like everything else associated with the Hudson River cleanup, natural attenuation is complex, and the potential reduction in PCB risk is uncertain. The driving mechanisms are deposition with clean sediments and biodegradation, in which chlorine atoms on the PCB molecule are replaced with hydrogen. Both processes have their shortcomings: Sedimentation is vulnerable to removal of dams, flooding, and storms that re-expose buried PCBs to the water column. PCB degradation, also known as weathering, tends to be incomplete and does not always reduce the toxicity of the original mixture. In some cases, weathering might actually heighten toxicity because less completely chlorinated molecules lose chlorine atoms first, leaving the more highly chlorinated and more toxic molecules behind (1, p 23).
PCB weathering is a two-step process (3). First, an anaerobic reaction strips chlorine atoms from the biphenyl ring and replaces them with hydrogen. Anaerobic transformations are most effective with more highly chlorinated PCBs, for example, the commercial Aroclor 1254 and 1242 preparations. Chlorines at the meta and para positions on the ring are cleaved first because they are farthest from the connecting bond. Ortho-substituted chlorines are more resistant. As the chlorine atoms are removed, the PCBs become increasingly soluble and rise into the water column, where they can be further degraded by aerobic processes. Theoretically, biphenyls that are only meta- and para-substituted can be completely degraded. Laboratory studies have shown that once the chlorine atoms are gone, microbes can break the residual biphenyls down to carbon dioxide and methane (4). But according to the NRC report, the presence of ortho-substituted chlorines in most environmental mixtures could make it nearly impossible for PCBs in sediments to break down completely (1, p 42).
Richard Bopp, professor in the Department of Earth and Environmental Sciences at Rensselaer Polytechnic Institute (NY), says that in the best circumstances, the loss of chlorine atoms from biodegradation could reduce the mass of PCBs in the Hudson by 1015%. But the drop in mass will not necessarily result in a proportional drop in toxicity. This is because the total number of PCB molecules in the ecosystem remains constant even after the chlorine atoms have been removed. Furthermore, biodegradation drops off sharply once PCBs fall to a certain concentration in the sediment (3). Rates of dechlorination are highest when environmental levels range from hundreds to thousands of parts per millionsimilar to those found in Hudson River hot spots. According to the NRC, the process slows as the contamination approaches 50 ppm, at which it mysteriously stops (1, p 332). The reason, says Danny Reible, professor of chemical engineering at Louisiana State University, Baton Rouge, may be that microbial colonies cannot be sustained when PCB levels are too low.
The debate rages on
Reible, who is also a member of the NRC Committee on Remediation of PCB-Contaminated Sediments, takes a more moderate view of the Lake Champlain cleanup. Id be surprised if there were enough monitoring data to determine how successful that remediation really was, he says. Echoing the NRCs conclusions, Reible says the extent to which a dredging operation can be judged successful depends on the interpretation of PCB sampling data before and after the remediation. In some instances, pro- and antidredging stakeholders have reviewed the same cleanups and arrived at completely different conclusions. To determine how this could happen, the NRC undertook what it describes as an impartial evaluation and concluded that stakeholders may disagree about remediation goals and assessment measures; post-monitoring data are often sparse compared with preremediation data; and it is sometimes the intention of reviewers, agencies, and industries to support their preferences, and that might lead to more conflict (1, p 276).
According to Reible, the true test of the Lake Champlain cleanup will come with extensive sampling over time. But he acknowledges that in areas where buried contaminants are vulnerable to reexposure, dredging could be a viable option. Some buried sediments are a ticking time bomb, he says. If conditions allow you to actually get at the sediments, dredging should be effective.
Toward the future
Woods Holes Farrington points out that each contaminated area is unique and that a remediation that succeeds in one location may not necessarily succeed in another. The key to selecting the appropriate remedy, he says, is to define the problem in the context of a risk-management framework, which is conveniently provided in NRCs report (1, pp 5267).
Peter Lehner, Chief of the Environmental Protection Bureau at the New York State attorney generals office, says that for other underwater sites contaminated with persistent organic pollutants, the Hudson River cleanup sets an important example. Says Lehner, This [environmental dredging operation] is the biggest yet, and well be studying it closely. Well learn a lot from this one.
Charles W. Schmidt is a freelance science writer based in Portland, ME (email@example.com).