Johns Hopkins doctoral student Thanh Helen Nguyen (left) and her faculty advisor William P. Ball have devised a new math tool that predicts how long pollutants will remain in soil. Photo by Will Kirk.Nguyen says the new tool is based on a breakthrough by chemists who study how medications move from the bloodstream into human tissue. At an American Chemical Society meeting last year, Nguyen heard a lecture in which Kai Uwe Goss, a senior research scientist at the Swiss Federal Institute of Environmental Science and Technology, suggested that this approach might be used to predict the behavior of soil pollutants. Nguyen took up the challenge and started to collaborate with Goss and her doctoral advisor on the new approach, supported by a National Science Foundation grant. She focused on the fate of non-ionic chemicals, including some solvents, pesticides and pharmaceuticals. Through intentional or accidental dumping, such contaminants often wind up in soil. Before approving new pesticides or making cleanup decisions, public officials need to know how long these chemical squatters will stay in the dirt. This requires an understanding of how these pollutants interact with soil, which is a mixture of minerals and natural organic matter, such as decayed vegetation. Charged chemicals usually cling to the mineral content, but non-ionic chemicals tend to make themselves at home in the soil's natural organic matter. For many years, environmental chemists have made predictions about how long the non-ionic pollutants will stay there by using octanol, an organic solvent, as a chemical stand-in for natural organic material. "But this technique doesn't work very well for polar pollutants that interact with surrounding solids in a more complex way," Nguyen said. To find out if the medicinal chemists' technique would yield better results, the doctoral student gathered 359 data points from published experiments involving 75 chemical pollutants. She then borrowed a medicinal chemist's method of converting each of the 75 pollutants to a mathematical representation. "We worked with these numbers and came up with a very simple equation that predicts what fraction of these non-charged chemicals will make their home in the soil rather than water under any given set of conditions," Nguyen said. "The equation works very well with complicated chemical structures like pesticides and pharmaceuticals." Her faculty advisor, William P. Ball, a professor in the Department of Geography and Environmental Engineering, says, "We've had a generally positive reaction to this technique so far." He added that the researchers' goal is "to move this into the mainstream so that more practitioners and regulators in the environmental engineering field can take advantage of it."
