Do past PFOA sources dominate the Arctic Ocean?
New modeling results suggest that ocean reservoirs could supply PFOA to the Arctic for many years.
Past discharges appear to be the dominant source of perfluorooctanoic acid (PFOA) to the Arctic Ocean, according to a new modeling study published today on ES&T’s website (DOI 10.1021/es070124c). But the new study, by University of Toronto global pollutant modeler Frank Wania (and funded by perfluorochemical manufacturer DuPont), fails to resolve the debate over whether current atmospheric sources or legacy oceanic sources are responsible for PFOA and other perfluorinated chemicals in Arctic wildlife.
Wania’s study adds strong support to the oceanic hypothesis, says oceanographer Robie Macdonald with the Canadian Department of Fisheries and Oceans. “The results, together with my experience of other chemicals’ behavior, suggest that the ocean transport route cannot be written off,” he says. The model predicts that even if atmospheric inputs were to cease, the oceans could continue to supply PFOA to the Arctic for decades to come.
But other scientists are more skeptical. “By including the atmospheric emissions of PFOA precursors, this article is a major improvement over previous modeling into the Arctic,” says Derek Muir with Environment Canada’s National Water Research Institute. But the model ignores the chemicals for which the most data are available—perfluorooctane sulfonate (PFOS) and its precursors, Muir adds.
The widespread environmental presence of PFOS, a breakdown product of chemicals formerly manufactured by 3M, prompted that company in 2000 to voluntarily withdraw products based on its chemistry. Recently, PFOS concentrations in Arctic ringed seals (Environ. Sci. Technol. 2007, 41, 42–49) and snow (Environ. Sci. Technol. 2007, 41, 3394–3395) have declined rapidly, suggesting to the authors of these studies the quick response of atmospheric transport, not the slow, lagging response of an oceanic mechanism.
Not necessarily, says Macdonald: “Simple monitoring of complex systems is risky business,” he says. “Just think about the number of transports and transfers that occur between emission and polar bear or seal.”
Wania used the global fate and transport model Globo-POP together with historical emissions estimates for PFOA and its precursors—the fluorotelomer alcohols (FTOH). He found that oceans are more efficient at transporting PFOA because the yield of the atmospheric reaction that converts FTOH to PFOA is low. Wania’s estimates for FTOH emissions are also much lower than estimates used by Ford Motor Co. atmospheric chemist Tim Wallington, who first modeled the atmospheric mechanism. “In my model, these lower emissions estimates lead to atmospheric concentrations in accord with measurements from remote locations, so they are about right,” Wania says.
But University of Toronto chemist Scott Mabury, one of the principal originators of the atmospheric theory, notes that the abundance ratios of the longer-chain carboxylates have a distinctive, atmospheric signature. “Longer-chain chemicals are important because, in wildlife, they are present at higher concentrations than PFOA,” he says. Atmospheric reactions can explain the relative abundances of these chemicals observed in the Arctic, but oceanic transport cannot, he adds. Wania says he did not model longer-chain carboxylates because data on direct emissions are lacking. “It is possible that direct inputs are the major explanation for PFOA in the Arctic, but for longer-chain carboxylates this may not hold true,” Wania says.
Muir attributes the rapid PFOS decline in seals to the fact that they are affected by shallow water that is dominated by atmospheric deposition and not by the Arctic Ocean as a whole. This is possible, says Macdonald, but other studies have not found strong evidence for such stratification.
Although scientists have differing views on the impact of the new modeling results, they agree on the best way to resolve the Arctic sources debate—measure perfluorinated acids in Arctic Ocean samples at different depths and in different seasons.
