Study sheds light on mess in polluted streams

Downstream from mining sites, a suffocating gel forms in the water of creeks and rivers. A new study by an international team of researchers details the processes that make that gel and should advance our understanding of the damaging environmental effects of mine drainage and acid rain.

“This new nanoscale level of understanding of trace metal pollution of streams opens new doors for addressing the problem of contaminated waters in affected areas,” says Sonia Esperanca, program director in the National Science Foundation’s (NSF) division of earth sciences, which funded the research.

According to the team’s report in this Friday’s issue (Sept. 27) of the journal Science, the gel results when runoff made acidic by mining or acid rain collects aluminum from local soils and then mixes with stream water that is less acidic. In subsequent chemical reactions, aluminum molecules link together to form polymer gel.

Scientists call the gel “floc” and say its influence is widespread: Mining disrupts about 240,000 square kilometers of the Earth’s surface (about 93,000 square miles, an area roughly the size of Oregon).

The gelatinous floc is bad enough by itself; it gums up the gills of fish and suffocates them, and is equally deadly to other aquatic animals and plants. But it also possesses another dangerous quality: It binds to toxic metals, including mercury, lead and cadmium, and transports them far downstream.

“This combination of floc and metals pollutes streams,” said William Casey, a University of California at Davis geochemist and an author of the new report. “Bad things adsorb into this gel and then it travels forever.”

Knowing how floc forms at the molecular level may suggest some practical solutions, Casey said. One such solution might be to stop aluminum from migrating into streams.

“Now we know how these pollutants enter the watershed, how fast they move and perhaps how to prevent the reactions by cutting off the ingredients. Detailing the molecular pathways helps us better understand the pollutants’ source and their fate,” Casey said.