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. [See related article, "Anti-bacterial additive found in
Maryland streams," in this issue.] 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 modeling 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 math 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 Aug. 26 presentation at the
228th national meeting of the American Chemical Society,
held in Philadelphia.
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 ACS 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
adviser on the new approach, supported by a National
Science Foundation grant.
She focused on the fate of nonionic 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
nonionic 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 nonionic 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, Nguyen 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
noncharged 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 adviser, William P. Ball, a professor in
DOGEE, said 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 agrees. "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."
Nguyen grew up in Vietnam and completed her
undergraduate studies at the Ivan Franko National
University of L'viv, Ukraine. Before enrolling at Johns
Hopkins, she earned a master's degree in earth and
environmental sciences at the University of Illinois at
Chicago.
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