STANFORD (US)—Health experts have called the water crisis in southern Asia the largest mass poisoning in history. While more than 140 million people drink arsenic-contaminated groundwater each day—and thousands die each year from chronic exposure—the source of the poisoning has been difficult to trace. New clues found in an isolated region of Cambodia could help millions tap safer water sources.
“How does the arsenic go from being in the sediment loads, in solids, into the drinking water?” asks Stanford University soil scientist Scott Fendorf.
More than 15 years ago, scientists pinpointed the source of the contamination in the Himalaya Mountains, where sediments containing naturally occurring arsenic were carried downstream to heavily populated river basins below.
One mystery lingered: Instead of remaining chemically trapped in the river sediments, arsenic was somehow working its way into the groundwater more than 100 feet below the surface. Solving that mystery could have significant implications for policymakers trying to reverse the mass poisoning, explains Fendorf, a professor of environmental Earth system science and a senior fellow at Stanford’s Woods Institute for the Environment.
To find out, he launched a field study in Asia in 2004 with two Stanford colleagues: Chris Francis, an assistant professor of geological and environmental sciences, and Karen Seto, now at Yale University. The initial study was funded with a two-year Woods Institute Environmental Venture Projects grant.
Five years later, the research team appears to have solved the arsenic mystery and is working with policymakers and government officials to prevent the health crisis from escalating.
“The real thing is, how do we help the people who are there?” Fendorf asks. “But first, we have to understand the coupling of hydrology—the way the water is flowing—with the chemistry and biology.”
Finding a study site
Arsenic-laden rocks in the Himalayas feed into four major river systems: the Mekong, Ganges-Brahmaputra, Irrawaddy, and Red. Epidemiologists first identified arsenic poisoning in the 1980s in the Ganges-Brahmaputra Delta in Bangladesh. The sudden occurrence of the disease was linked to the increased use of wells for drinking water.
Scientists had long assumed that the contamination process occurred deep underground, in buried sediments that release arsenic into aquifers 100 to 130 feet below the surface. But Fendorf and his colleagues had data suggesting otherwise. They suspected that the arsenic actually dissolved at a much higher depth, very close to the surface. “As the water starts to move down into the soil, it picks up arsenic. That was our hypothesis,” he says. “We needed to follow the chemistry of the surface water as it moved down into the groundwater.”
Fendorf and his colleagues began their fieldwork in the Brahmaputra River basin of Bangladesh. However, creating a hydrology model was a challenge, because the landscape was dotted with irrigation wells that alter the natural path of water. “When you draw out how the water might flow, it looks like spaghetti,” Fendorf explained. “Before we even started we said there is no way this is going to be possible.”
The researchers needed a less-developed site that was chemically, biologically and geologically similar to Bangladesh. The Mekong River in Cambodia offered a perfect alternative. Its headwaters are only 100 miles away from those of the Brahmaputra River. “All the chemistry up in the Himalayas is similar,” Fendorf adds. “The transport down the big river system is very similar as well.”
More importantly, the Cambodia site was mostly undeveloped. “Cambodia had been under a 35-year civil war that had really repressed its development, so it was in essence Bangladesh 40 or 50 years ago,” he explains. “In some ways it would actually be setting the clock back and getting a snapshot back in time. By virtue of having this more simplistic system, we could really track the entire water flow.”
The new field site was located just south of Cambodia’s capital, Phnom Penh. Fendorf hired local workers to drill wells at three different depths throughout the 20-square-mile site. Testing the water for dissolved arsenic at various depths allowed the researchers to pinpoint where the toxin was migrating into the aquifer. To observe solids, they also installed water-sampling devices a foot or two below the surface. The data they collected allowed them to put together a model of arsenic cycling in the river delta.
“We found out that, sure enough, within the first 2 to 3 feet from the surface, arsenic was coming out of the solids—that is, the sediments transported down from the Himalayas—and into the water, and then it migrated down into the aquifer,” Fendorf says. Aquifers are the source of drinking water for people who use wells throughout Cambodia, Bangladesh, Myanmar, India, and Vietnam.
The results confirmed Fendorf’s hypothesis: Arsenic contamination was occurring near the surface and, in fact, would take at least 100 years to reach the aquifer below. The Stanford team also showed that the 100-year-scale cycling of arsenic into the aquifer was a natural process that had been occurring for thousands of years, preceding any human influence. “We showed that there is a perpetual source of arsenic that replenishes from the surface,” Fendorf says.
Digging for answers
Understanding the area’s hydrology will allow developers to strategically install wells that draw from areas free of dissolved arsenic, providing clean, drinkable water. Such targeted excavation can be extremely accurate, Fendorf says.
But what if a village needs a well but is unable to find an arsenic-free location to install it? Fendorf has proposed several solutions, including installing arsenic filters, collecting rainwater, and purifying surface water. Each option has pros and cons, he says. Fendorf and Stanford post-doctoral scholar Matt Polizzotto have proposed finding the best option on a village-by-village basis.
In late March, Fendorf cohosted a four-day meeting on arsenic poisoning in Siam Reap, Cambodia, with about 60 experts, including government officials, scholars, NGOs, and funding agencies, such as the World Bank. The meeting was convened by the American Geophysical Union and the Woods Institute.
The goal moving forward, he says, is “to converge on a resolution, as a scientific body, on what we agree about the problem, what remains unresolved, and what needs to be done to fill the gap.” Until that process is complete, he plans to move ahead with efforts to provide clean water through the most workable method. “If we can give people a clean well or a rainwater harvesting unit, that’s going to go a lot further, in the short term at least, than any of our study results,” Fendorf adds.
World Health Organization’s summary on arsenic in drinking water: http://www.who.int/mediacentre/factsheets/fs210/en.
Stanford University news: http://news.stanford.edu