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ES&T News
Arsenic and Old Landfills
Microbes doing the otherwise beneficial job of degrading organic waste in landfills could be releasing arsenic from the surrounding rocks and sediments by creating reducing environments that mobilize the metal, according to new research published in this issue of ES&T (pp 67–73).
The findings suggest that, in many parts of the U.S. and the world, landfill leachate can create an arsenic problem in groundwater that moves through rocks containing normal amounts of iron oxides and arsenic.
Data from Superfund sites reveal the magnitude of this problem, says the research’s corresponding author, geochemist Benjamin Bostick at Dartmouth College. Arsenic is a groundwater contaminant of concern at more than a third of U.S. Superfund sites—even though arsenic was never dumped, deposited, or used at many of these sites.
Microbial degradation is the cornerstone of the increasingly popular practice of monitored natural attenuation (MNA). The Superfund sites that use MNA are nearly 50% more likely to have excess arsenic contamination than sites that use other methods, Bostick notes.
Bostick and his Dartmouth colleagues show in a combined field and laboratory study that common soil microbes are oxidizing organic carbon and reducing Fe(III) that occurs in poorly crystalline iron oxide minerals. This process also releases arsenic that is adsorbed to the iron oxides and can result in high levels in the underlying groundwater down-gradient from a landfill site. It is the same mechanism—albeit on a much smaller scale and with far less dire consequences—thought to be responsible for the high arsenic levels in drinking water in Bangladesh.
The phenomenon is likely to be widespread at landfill sites, says hydrogeologist Charles Harvey at the Massachusetts Institute of Technology, who is studying the problem in Bangladesh. “You don’t need hazardous organic waste from a Superfund site. Every landfill has garbage, and that’s organic waste.”
“This problem is probably widespread,” agrees U.S. EPA environmental engineer Robert Ford. “It’s been identified in the Northeast [U.S.], but it will be recognized more and more in other parts of the country,” he predicts. Indeed, every scientist contacted for this article agreed that Bostick has identified a little-known problem likely to be occurring throughout the world.
For 10 years, Dartmouth scientists have monitored the Coakley Landfill, located near the town of Rye in southern New Hampshire. Coakley was listed as a Superfund site in 1983 because of leaking VOCs. It was also contaminated with arsenic and other metals, although the source of the arsenic was unknown. The landfill was capped in 1998 to limit the spread of the contaminants and encourage MNA. Since then, natural processes have reduced most of the organic contaminants to levels considered safe.
For example, concentrations of benzene, the only source of which was the landfill, decreased after capping. So did the concentrations of all the metals—except that of arsenic, which increased as much as 10-fold in some wells. This demonstrates that the arsenic isn’t coming from the landfill, Bostick says.
Instead, the arsenic is leaching out of the clay soils on which the landfill sits. This clay has concentrations of 20 ppm arsenic with 5% iron—most of which is in amorphous iron hydroxide minerals. Although these arsenic levels are high, they are not unusually high, Bostick says. He believes that benzene accumulated in the clay layer because it strongly partitions to clay. Capping the landfill set off the arsenic release process by limiting the supply of oxygenated water and thus creating reducing conditions, he says. Laboratory experiments further confirm the group’s hypothesis.
“It’s very significant to note that the landfill is not the direct source of the arsenic—it’s natural,” says University of Arizona environmental chemist Wendell Ela. “It means that many leaking landfills have, and will have, arsenic come out into groundwater,” he says.
Natural attenuation can still be an appropriate remedy for organic contaminants, Ela says. But when making such a decision, one needs to consider the iron content of the surrounding rocks, the mineralogy, and the likelihood that the subsurface geochemical environment could become anaerobic. “In some cases, we may find that natural attenuation needs to be augmented—for example, by aerating the site,” he says.
Janet Hering, a geochemist at the California Institute of Technology, agrees. The suitability of MNA for remediation of a particular site depends on a lot of things, she notes. MNA could even be suitable for arsenic remediation in situations in which the aquifer is oxidized and its arsenic concentration is low, says Hering. “MNA is not an easy solution,” she adds. “It has to be carefully evaluated.”
