Arsenic has a bad reputation-for good reason. In part, it’s a potent poison, as everyone familiar with murder mysteries knows. But while it’s rarely used as a weapon in the real world, it does turn up as a contaminant in the environment-and mere traces of the stuff ingested from, say, water, can eventually lead to cancer. Arsenic taints more than 700 hazardous-waste sites across the nation, including lakes and rivers in or near many major cities. Typically the toxin comes from waste dumped decades ago by manufacturers of insecticides and other chemicals.

The question becomes what to do with sites contaminated with arsenic-a metal that can combine with oxygen, carbon, and other elements into various compounds, some that are more toxic and that travel more easily through the environment than others and hence are more likely to be ingested. Ideally, health officials would like to remove arsenic from water and soil. At the least, they’d like to minimize its exposure to humans. But neither can be adequately accomplished until researchers learn the basics of how arsenic moves in the environment and how compounds containing it change over time.

Harry Hemond, a professor of civil and environmental engineering at MIT, has been examining such matters for almost 10 years. Hemond, who also directs the Ralph M. Parsons Laboratory, which conducts studies in water resources and environmental engineering at MIT, has been researching arsenic in the Aberjona Watershed, a 25-square-mile polluted stretch of water north of Boston.

In this aquatic ecosystem Hemond finds that arsenic often cycles between two compounds. One, arsenite, is a highly toxic substance that dissolves and moves easily in water and that humans can thus ingest. The other, arsenate, is a less soluble and less toxic variety that iron-rich particles tend to absorb before settling deep in lake sediments.

Factors such as temperature, dissolved oxygen, organic matter, and microbe populations determine the relative proportions of arsenate and arsenite in water, Hemond says. He and his students have therefore been sampling lakes in the watershed under many conditions.

The group is constantly developing new techniques to obtain results that are as accurate as possible. For instance, David B. Senn, an environmental-engineering doctoral student, has designed a pump to collect and filter water samples from murky, oxygen-free lake bottoms without introducing any oxygen. Because the highly toxic arsenite differs chiefly from its chemical cousin arsenate in containing less oxygen, any addition of that element could lead to unrealistically high readings of the safer compound. Typically, water-filtering devices allow some air-and hence oxygen-to enter the samples. But Senn’s apparatus starts out filled with nitrogen. That physically prevents any arsenic-oxygen reactions when researchers introduce lake water into the instrument. The fact that the MIT device filters below the water surface rather than in a laboratory, as is the norm, also helps, Hemond explains.

Through its years of sampling work Hemond’s group has discovered that arsenic levels in bottom waters can change dramatically. In 1992 and 1994, the team found roughly the same amount of arsenic along the bottom of a lake that receives Aberjona drainage. But sediment levels skyrocketed in 1993. Thus, says Hemond, health researchers can’t “just grab one sample” to analyze an area’s level of contamination. The fluctuations could reflect the activity of organisms that absorb and release the metal, he suggests.

Indeed, Hemond’s group has discovered that bacteria play a major role in converting arsenate to arsenite. In 1994, then-graduate student Dianne Ahmann-now an assistant professor of microbial ecology at Duke University-and Franois Morel, a professor of geochemistry at Princeton University, found in sediment samples a bacterium that takes up arsenate, uses its oxygen for energy, and releases arsenite. Ahmann’s lab is studying the potential of the microbe, named MIT-13, for remediating arsenic-laden sites so that technicians do not have to excavate large quantities of contaminated soil. Engineers might be able to treat small, isolated areas with MIT-13 to mobilize the arsenic. Although that would result in a more toxic compound, engineers could then pump it out and purify it for reuse in chemicals employed in industries that today are subject to tight environmental regulations. Or engineers could pump out, concentrate, and dispose of the arsenic in designated landfills.

Meanwhile, Hemond’s lab, in its continuing quest to catalog environmental factors that influence arsenic, has started investigating other life forms and physical processes that may affect the pollutant. For example, Laurel Schaider, an undergraduate student in environmental-engineering science at MIT, is working to cull from sediment a bacterium that is apparently converting the more toxic arsenite to arsenate. Hemond says that beyond identifying the species, determining how quickly it oxidizes arsenite and how it can grow are necessary steps for indicating whether it could be a catalyst useful in a “low-cost arsenic removal system for small villages” where drinking water contains the metal.