Building on an idea developed by medicinal chemists, Johns Hopkins researchers have devised a new mathematical tool that accurately predicts how long certain pollutants – including pesticides and pharmaceuticals – will remain in soil.
The work is timely because researchers and public officials have become increasingly concerned about pharmaceuticals and personal care products that have been detected in soil and water. Environmental engineers are seeking better ways to track these emerging pollutants, which tend to be more complex and water-soluble than previous contaminants of concern, such as chlorinated solvents and petroleum byproducts.
This new modelling approach is important because environmental regulators and cleanup consultants need to know the extent to which hazardous contaminants will linger on a piece of land and the rate at which they will migrate toward critical water resources and supplies. The new approach will help them decide whether the pollutants need to be removed and how best to accomplish this, the researchers say.
“If we release chemicals into the environment, we need to know what will happen to them,” said Thanh Helen Nguyen, a graduate student who played a leading role in adapting the tool and demonstrating its effectiveness. “For many years, we’ve made predictions with a method that doesn’t work very well on many chemical pollutants in soil. This new tool does a much better job.”
Nguyen, who is working toward her doctorate in the Department of Geography and Environmental Engineering, described the improved pollution predictor during an August 26 presentation in Philadelphia at the 228th national meeting of the American Chemical Society.
Although her own training is in geology and environmental engineering, Nguyen said 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, meaning those lacking an electrical charge, 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, said, “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.”
Nguyen agreed. “Over the past several decades, more than 90 equations involving the old octanol approach have been developed, and those equations do not work very well on many chemicals,” she said. “More people should be using this new tool because it’s easier and more accurate.”