September 2004, Issue 3

Removing Organic Chemical Contaminants from the Earth's Soils

Linda M. Abriola, PhD, a civil engineer whose research focuses on novel cost-effective interdisciplinary approaches to deal with groundwater contamination, was named dean of the School of Engineering in 2003. In addition to serving as dean, Abriola is a professor in the departments of Civil and Environmental Engineering and Chemical and Biological Engineering. While her main emphasis is on mathematical modeling of flow and transport of organic chemicals in porous media, she likes to work at the interface between disciplines. Currently, she is working with microbiologists to study organisms that show promise for transforming subsurface contaminants into less toxic compounds.

Abriola comes to Tufts from the University of Michigan at Ann Arbor, where she was the Horace Williams King Professor of Environmental Engineering and director of the Environmental and Water Resources Engineering Program. She obtained her doctoral degree in civil engineering from Princeton University in 1983, and has been a visiting associate professor in the Department of Petroleum Engineering at the University of Texas at Austin and a visiting scientist in the Department of Geotechnical Engineering at the Universitat Politècnica de Catalunya in Barcelona, Spain.

She describes the problem she has been working on for decades: "A type of contaminant that is especially troublesome on Superfund sites [the nation's worst toxic waste sites] is the group of chemicals known as dense non-aqueous phase liquids, or DNAPLs. These compounds are so-named because they are heavier than water, and when they are in contact with water they do not readily dissolve in it but instead remain as a separate liquid, or 'phase.' In addition to being pervasive in the environment, DNAPLs are problematic as contaminants because they are difficult to detect and treat, particularly in the subsurface environment. DNAPLs may be mobile in permeable materials such as porous soils, advancing quickly under the influence of gravity, but a fraction tends to remain behind as isolated globules of varying sizes. These isolated globules often serve as a continual source of soil and groundwater contamination, hampering clean-up efforts and raising concern about the potential for contamination of drinking water supplies. Additionally, some of the more volatile compounds existing as DNAPLs can enter and contaminate the vapor phase of soil systems. Examples of common DNAPLs include chlorinated solvents (e.g., dry-cleaning fluids), coal tars, PCBs, and certain pesticides."

In 1985 Abriola published the first mathematical model capable of describing the interphase mass partitioning and non-aqueous phase migration of organic contaminants in the subsurface. She became interested in the experimental aspects of environmental engineering when she realized that laboratory experimentation would allow her to test her model. After setting up a full experimental laboratory, she began testing the validity of her calculations and gaining new data to refine her model.

Laboratory soil column experiments have allowed Abriola's research group to gain a better understanding of the factors that influence the behavior of DNAPLs in the subsurface environment. Among their many findings have been (1) that the solubilization rates of entrapped DNAPLs can be predicted from easily measured parameters of flow rates and soil characteristics, (2) that the addition of surfactants to injected water can greatly improve the recovery of entrapped DNAPLs from soils, and (3) that the subsurface mobility of a DNAPL can be measured in terms of a newly defined quantity known as the "trapping number."

Field tests were the logical next step after mathematical modeling and laboratory experiments. In 1996, Abriola's group began field-testing a surfactant enhanced aquifer remediation (SEAR) technology near Wurtsmith Air Force Base in Oscoda, Michigan. The test site was contaminated with multiple pollutants, including the chlorinated solvent perc (i.e., perchloroethene).

Surfactants lower the surface tension of a liquid and are used as detergents (which dissolve oil in water) and emulsifiers (which suspend oil in water). Abriola's research team pumped an aqueous solution of the FDA-approved surfactant Tween 80 (commonly found as polysorbate 80 on the listed ingredients of whipped toppings and cake mixes) into the contaminated soil to demonstrate the ability of this surfactant to increase the aqueous solubility of perc under natural field conditions, and thereby increase contaminant removal. "It's like washing the soil, cleaning it with surfactants," Abriola says. Results from this field test indicate that SEAR was effective in recovering perc from a contaminated aquifer.

The use of surfactants has improved the conventional pump-and-treat method of aquifer remediation, but surfactant-enhanced recovery procedures still leave behind residual contamination. A second possible treatment phase is what Abriola calls a "polishing" step that involves stimulation of contaminant-degrading microbes. Her team is currently running experiments to see if various microbes that can degrade contaminants under ideal laboratory conditions can also do so under the harsh conditions that exist below the surface. "We're doing an experiment now where we're starving the microbes to see if they retain their ability to degrade the contaminant during periods of stress," Abriola says. Because bacterial physiology and gene expression can change depending on nutrient type and availability, these experiments are crucial steps in taking the technology into a real-world situation.

Abriola will be involved in Tufts' new multidisciplinary graduate program beginning in the fall of 2004 called "Water: Systems, Science, and Society" (WSSS). This program involves students from several schools of the University, including Engineering, the Friedman School of Nutrition Science and Policy, Arts & Sciences, Medicine, Veterinary Medicine, and the Fletcher School. The program's goals reflect Abriola's multidisciplinary research approach, and she plans to lecture in WSSS courses and to welcome WSSS graduate students into her laboratory.

Collaboration has always been key to Abriola's mode of research, and she plans to continue this approach with her new Tufts colleagues. Her current interuniversity research collaborators include microbiologists, geostatisticians, soil scientists, chemical engineers, and remediation specialists.


 

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