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Measuring Groundwater Contaminants
Through Image Analysis Tools
 EPA groundwater research programs help to ensure the safety of U.S. water resources by treatment and removal of hazardous contaminants. Groundwater that has become contaminated by chemicals that are sparingly soluble in water, such as chlorinated solvents, presents a serious ecological and human health risk. Groundwater remediation scientists at EPA’s National Risk Management Research Laboratory are using absorption and light refraction analysis to determine the saturation distribution of common groundwater contaminants in order to facilitate research on the development of effective methods for remediating sites impacted by these chemicals. BackgroundAny liquid contaminant that is denser than water is referred to as a DNAPL (dense nonaqueous phase liquid). Contamination of the subsurface environment by DNAPLs is a serious environmental challenge since these liquids have the potential to migrate downward and accumulate in aquifers. While conventional technologies for DNAPL mitigation such as excavation or pumping and treating, have demonstrated some success, the search continues for more effective remediation technologies with lower life-cycle costs. Designing effective remediation requires accurate knowledge of the subsurface DNAPL content and its spatial distribution. To this end, numerical models are developed to assist in evaluating and predicting the impact of these contaminants. These numerical models are usually validated against quantitative laboratory experiments. In addition to model validation, high-quality laboratory data also improve our understanding of contaminant behavior in the subsurface environment which, in turn, helps in designing an effective remediation technology. However, only a limited number of laboratory techniques can be used to accurately measure DNAPL and water content in physical models (flow chambers) and some of them entail risks. For example, techniques to quantify fluid content using X-ray or gamma ray measurements are slow and costly, and present the potential hazards of working with high-energy sources. An alternative technique that has been gaining popularity is the use of light transmission visualization (LTV) to quantify DNAPL saturation. Risk Management ResearchEPA researchers have developed a modified LTV method that takes into account both absorption and refraction light theories to measure DNAPL distribution in two-dimensional laboratory aquifer models. This innovative approach analyzes digital images of the models pixel-by-pixel to determine DNAPL saturation and its spatial distribution. Using this LTV technique, the saturation of pure (undyed) DNAPL was measured where previous approaches required the addition of a dye to the DNAPL. While the addition of dye enhances visualization, it can also alter the chemical and physical properties of the DNAPL. In addition, the NRMRL method has eliminated the need for the tedious process of developing a calibration curve, another drawback of older lab methods. The EPA approach can provide a more accurate assessment of the behavior of multiphase liquids (in this case, tetrachloroethylene) in porous media. Laboratory investigations by the LTV team are being used to evaluate the relationship between contaminant mass in source zones and the rate of contaminant release from these zones. This relationship is fundamental to the design of effective and cost-efficient remediation strategies at hazardous waste sites. By developing a method that advances the state-of-the-science for understanding and remediating DNAPL source zones, the National Risk Management Research Laboratory (NRMRL) research team has also advanced the EPA mission for protecting human health and the environment.
When Researchers Talk. . .Sometimes research ideas are generated by friendly exchanges between scientists. That’s what happened when Subhas Sikdar, associate director for science in the National Risk Management Research Lab, posed an idea to his colleague Dibaker Bhattacharyya (“DB”), a chemical engineer and faculty member of the University of Kentucky Center of Membrane Sciences. When Sikdar challenged DB to develop a membrane that could capture toxic metals with capacities of the order of one pound of metal captured per pound of absorbent material used—up to a 1000 percent increase in efficiency over existing methods—DB took the idea back to his laboratory. In due time, he more than met the challenge. With subsequent support from EPA, the Department of Defense and industry, he developed membranes that capture toxic metals such as mercury, lead, cadmium, barium and others from contaminated water with very low levels of these pollutants. The research collaboration also addressed the removal of polychlorinated biphenyls (PCBs) from water. Although PCBs were banned in the 1970s, they persist in soils and groundwater, creating a health hazard to humans and animals. DB’s research applied catalytic properties to nanotechnology materials, essentially multiplying the surface properties of the membrane, expanding its filtration and holding capabilities and allowing further treatment to render toxics like PCBs harmless. For a complete discussion of this research, go to the University of Kentucky's Odyssey web site. To learn more about current EPA nanotechnology research, see EPA’s Draft Nanomaterial Research Strategy (NRS) (PDF) (76 pp, 1.2 Mb) ContactJane Ice, NRMRL Office of Public Affairs (513) 569-7311
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