To Chile's deserts, mountains, and snow to see how arsenic flows

The Atacama Desert in northern Chile is rugged, barren, and isolated — the driest place in the world. At more than 4,500 meters above sea level, roads are nonexistent and water is scarce. The few rivers that do cross this desert — including the Río Loa — are the only sources of fresh water for nearby cities and villages, but the water is tainted with toxic levels of arsenic.

Such contamination happens naturally — the Loa originates high in the Andes, in bubbling pools of water heated by the Puna-Altiplano volcanic complex. That complex holds the El Tatio geyser field, which discharges considerable amounts of arsenic into the river, leading to major public health problems for the people farther down the mountain who rely on the river as their only source of water.

Aaron Packman, associate professor of civil and environmental engineering, and Jean-François Gaillard, professor of civil and environmental engineering, are working with researchers from Pontificia Universidad Católica de Chile (PUC) to understand how arsenic propagates down the river and to develop strategies that will protect local communities from this natural hazard.

"It's at the fringe of where people can live," Packman says. "It's a very unusual site and a good test case for evaluating how we can best manage water resources and cope with naturally occurring sources of water contamination."

The partnership with PUC developed several years ago when Gaillard had a graduate student from PUC. Field work in the area started in earnest in 2005 when Gaillard and Packman investigated the sites. Since then, several Northwestern students have participated in sample collection and analysis efforts, working closely with their counterparts from PUC.

"This effort involves a lot of water resource issues — contaminants, chemistry, and public health," Packman says.

When consumed in high amounts, arsenic has been known to cause skin disease and cancer. The World Health Organization recommends a limit of 0.01 mg/L (10 ppb) of arsenic in drinking water. In the Río Loa, arsenic levels are routinely more than 100 times greater than this. Arsenic-tainted drinking water is also a problem in the United States, often near mining sites, in industrial areas, and from hydrothermal sources — but nowhere in the United States is arsenic such a widespread problem as it is in northern Chile.

A Río Loa excursion
In 2007 Packman, Gaillard, their students, and their Chilean collaborators Pablo Pastén and Gonzalo Pizarro set out on a 10-day field expedition to characterize arsenic distributions along the entire Río Loa, the longest river in Chile. The extreme conditions and isolation of the region made the work challenging. The group rented trucks in Santiago, got a block of hotel rooms in a nearby city, set up a makeshift lab, then took their trucks up into the mountains. The group split up, taking water and sediment samples from different areas along the river. In some places, particularly in the Río Salado tributary, the "salty river" that contains very high arsenic concentrations, no life could be found in or around the river. At one point they found they could drive no higher in the mountains — snow blocked the roadway.

The group tested some samples on site and brought others back to the temporary labs at the hotel. Many samples were shipped back to McCormick as well, and both students and professors are still testing the samples. "The follow-up analysis takes years," Packman says.

Analyzing the results
The group has discovered that arsenic in the Río Loa behaves differently than arsenic in the United States. In most locations here, arsenic in water is mainly found in areas with high amounts of iron. In the Río Loa, arsenic appears to be found where calcium is high. The group also learned that arsenic is highly mobile, and its concentration is controlled by dilution from other rivers and by evaporation, which is quite severe because of the extremely dry climate around the Río Loa. Most recently, the group has begun to scan arsenic distributions in river sediment using X-ray tomography at Northwestern's Synchrotron Research Center and at the Advanced Photon Source at Argonne National Laboratory.

Packman says he hopes the research will spur student groups to develop new processes for removing arsenic from the water. A new program funded by the Chilean Science Foundation will support
the design of local treatment methods for villages located along the Río Loa and other arsenic-bearing rivers in northern Chile. This will allow the collaboration between McCormick and PUC to continue: McCormick has a study abroad program with PUC in which Northwestern students combine academic study with research experience, and Marco Alsina, from PUC, is visiting Northwestern as a predoctoral fellow.

"This collaboration allows students to see the field component of environmental engineering research," Packman says. "They take the knowledge gained from the field to help provide solutions to the world's most pressing problems, like access to clean drinking water for everyone."

—Emily Ayshford