In the Northeast, lakes follow a seasonal cycle. In the summer they become stratified: cold water on the bottom is topped by a warm layer. The two don't mix and have different properties. For example, the top layer is flush with oxygen while the bottom layer has none.
The mystery that Senn and Hemond were exploring occurs in the bottom layer, which complicates sampling. "If the water samples are exposed to oxygen during sampling, the chemistry could change before we'd have a chance to measure it," Senn said. So he and Hemond developed a filtration unit that allowed them to filter water samples while they were still deep within the lake, preventing oxygen contamination.
The two had a hunch that nitrate pollution was responsible for the anomalous forms of iron and arsenic (nitrate is common in U.M.L.). So over several months they tracked the concentration of nitrate, along with the concentrations and chemical forms of arsenic and iron. Higher nitrate concentrations did indeed coincide with the previously unexpected particulate forms of iron and arsenic. Culture studies in the lab, and thermodynamic calculations, further supported the feasibility of the reactions observed in the field.
The work not only aids scientists' understanding of how arsenic moves through polluted, stratified lakes, but also has implications for other pollutants, including phosphate and toxic metals (e.g. lead, cadmium and zinc). These contaminants are similarly adsorbed onto iron oxides, so they are expected to have similar behaviors in lakes. Hemond further expects that nitrate pollution could be important in groundwater or marine systems contaminated by arsenic.
The two researchers emphasize the significant contributions of earlier students and other faculty. "I came in after a lot of people had already done much work to identify t
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Contact: Elizabeth Thomson
thomson@mit.edu
617-258-5402
Massachusetts Institute of Technology
29-Aug-2002