Microbial processes ultimately determine whether arsenic builds to dangerous levels in groundwater, according to researchers at the University of Illinois at Urbana-Champaign. Remediation may be as simple as stimulating certain microbes to grow.
Arsenic contamination is a serious threat to human health. In the Ganges Delta of Bangladesh, for example, chronic exposure to arsenic has been linked to serious medical conditions, including hypertension, cardiovascular disease, and a variety of cancers.
“The threat extends to Central Illinois, where there are very high levels of arsenic contamination in a number of wells,” said Craig Bethke, professor of geology at Illinois and corresponding author of a paper in the November issue of the journal Geology. “We also discovered important links between the amount of organic material dissolved in the groundwater and the concentrations of sulfate and arsenic.”
The researchers analyzed water from 21 wells at various depths in the Mahomet aquifer, a regional water supply for Central Illinois. “The Mahomet aquifer was produced by a glacier, which pulverized and homogenized the sediments,” said Bethke. “As a result, arsenic sources that leach into the groundwater are pretty uniformly distributed.”
Surprisingly, however, arsenic concentration varied strongly from well to well, said Bethke. “Concentrations may reach hundreds of micrograms per liter in one well—which is enough to make people very sick—but fall below detection limits in a nearby well.”
The concentration of arsenic varied inversely with the concentration of sulfate, the researchers found. Methane concentration also varied with the sulfate content. “We believe this reflects the distribution of microbial populations in the aquifer system,” said researcher and graduate student Matthew Kirk. “Our analyses suggest the aquifer is divided into zones of mixed microbial activity, some dominated by sulfate-reducing bacteria, others by methanogens.”
Sulfate-reducing bacteria reduces sulfate to sulfide, which reacts to precipitate arsenic, leaving little in solution. If the bacteria run out of sulfate, methanogenic bacteria take over as the dominant metabolic force, said Kirk. Because methanogenic bacteria don't produce sulfide, there is no precipitation pathway for the arsenic, which accumulates to high levels in the groundwater.
What does this mean to people living in Illinois? “The majority of wells in Central Illinois belong to individual homes and farms,” said Bethke. “Lacking effective water treatment and testing, private wells are more at risk of arsenic poisoning.”
There is good news, however. The researchers' findings suggest that groundwater contaminated with arsenic might be easily identified and remediated. “Unlike detecting the presence of arsenic— which generally requires a sensitive laboratory analysis—testing for sulfate is simple and straight-forward,” said Bethke. “If all waters containing sulfate are safe, as in our dataset, then measuring sulfate level would be an easy but reliable field test to identify safe drinking water from unsafe.”
And adding sulfate to naturally contaminated groundwater might be a simple but effective method to sequester the arsenic, said Kirk. “The bacteria are already present, so all you have to do is stimulate them.” Sulfate salts, he said, are inexpensive, readily soluble, and easily obtained. According to Bethke, the most important potential practical application to the public works industry is the possibility of reducing arsenic levels of groundwater in situ—within the aquifer before the groundwater is pumped—by stimulating the bacteria.