ac detective
Environmental chemis-tree reveals contamination Trees pinpoint toxic pollution sites. rees, the giants of the plant world, stand silently by as we go about our lives. They are not usually noticed, except maybe when their branches encroach on a power line or when they display their brilliant fall colors and then drop their leaves. But trees “notice” and record human activities simply by interacting with their environment and by absorbing many of the pollutants that we dump into the water and release into the air. Two new studies in Environmental Science & Technology demonstrate how scientists are using trees as sentinels to detect air and water pollution. In the first, Ronald Hites and postdoc Xinghua Qiu of Indiana University sampled the bark from trees in the northeastern U.S. and found Dechlorane Plus (DP), a flame retardant that is manufactured nearby (Environ. Sci. Technol. 2008, DOI 10.1021/es072039a). In the other study, Ellen Graber and colleagues at Israel’s Agricultural Research Organization and Ben Gurion University of the Negev collected tree cores from around Tel Aviv and showed that the presence of chlorinated volatile organic compounds (Cl-VOCs) in the trees indicated that there were Cl-VOCs in the subsurface (Environ. Sci. Technol. 2008, DOI 10.1021/es072014b). The idea of using trees to detect environmental contamination isn’t new. Graber has been working with trees for ~8 years, and Hites published his first paper on tree bark >20 years ago. Hites says that the idea to work with trees came to him after a colleague pointed out the similarities between the rings of a tree and the layers of sediment in core samples taken from lakes. The allure of using trees was immediately evident. “It’s hard to take sediment cores. You’ve got to get a boat, or in some cases it pays to wait until the lake freezes and sample the sediment through a hole in the ice,” he says. “My students didn’t like it.” Trees, on the other hand, are easy 558
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The bark and cores of trees contain pollutants released into the environment.
to sample. Hites started out looking for polychlorinated biphenyl (PCB) contamination in tree cores, but he didn’t find much. “In retrospect it makes sense, because the inside of the tree runs on water . . . and PCBs aren’t soluble in water,” he says. “They’re partitioned strongly to anything that’s lipophilic.” Luckily, the bark, which is designed to keep water in and rain out of the tree, is full of lipids, and Hites’s group eventually figured out that sampling the bark gave a much better picture of PCB levels. The bark is quite rough and porous and easily soaks up vapor- and particle-phase compounds from the atmosphere. It’s also more accessible than the cores; this allowed the group to collect data from all over the world. “It turned out that it was easy for people to sample bark for us and mail it back,” Hites says. “We ended up with bark samples from anywhere we could find somebody on email or friends or colleagues or friends of friends who were going somewhere.”
Tracing DP pollution to a single source
Building on that early PCB work, Hites’s group has expanded the number
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of compounds that they’ve identified in tree bark, the latest of which is DP. The scientists originally stumbled across this compound in their samples while looking for other contaminants. A student noticed two large, unidentified GC peaks with the same molecular weight, and the mass spectra of both peaks indicated the presence of 12 chlorines. It wasn’t difficult to figure out that these peaks were probably the two conformations of DP. “There’s not a big universe of industrial compounds with that molecular weight that have 12 chlorines,” says Hites. “So we were able to focus on it pretty easily.” DP is manufactured by OxyChem (formerly known as Hooker Chemical, most famous for its involvement in the Love Canal incident) at a single plant in Niagara Falls, N.Y. To confirm the identity of their GC peaks, the researchers needed a sample of the pure compound, but this was easier said than done, because Hites had published some earlier papers on environmental contamination by this company’s products. “Getting the authentic sample from OxyChem was a little tricky,” he says. “They know me.” Eventually, though, his group was able to obtain a sample and establish the identity of the mystery peaks. In their latest study, Hites and colleagues wanted to confirm that the DP found in the environment was coming from OxyChem’s manufacturing facility and to map the compound’s geographical distribution. They sampled bark from a wide range of sites throughout the eastern U.S. and found that the highest concentrations of the chemical were centered in Niagara Falls. Concentrations dropped off rapidly as they moved away from the manufacturing plant. By using a simple Gaussian diffusion model, the scientists calculated that the source’s location was
