Zinc In Sludge From Spanish Reservoir Break Equals Mine's Annual Output, Researchers Find

October 01, 1998

River Water Almost As Acidic As Vinegar, Columbia Team Says

Researchers from Columbia University's Lamont-Doherty Earth Observatory believe that the collapse last spring of a retaining wall holding tailings from a Spanish zinc mine has dumped an amount of zinc into rivers nearly equal to the mine"s total annual output.

The accident last April 25 at the Los Frailes zinc mine in Aznalcollar, about 50 miles south of Seville, released some 5 million cubic meters of acidic sludge into the Guadiamar River, which one week after the spill had an acidity approaching that of vinegar, the Columbia researchers say. Sludge in the Guadiamar also contaminated the Guadalquivir, the river that runs through Seville in southwestern Spain. The work was published in the Sept. 22 issue of EOS, the weekly journal of the American Geophysical Union.

Alexander van Geen, a research scientist at Lamont, in Palisades, N.Y., and Zanna Chase, a Columbia graduate student, measured the amount of zinc in water and sediments in the rivers downstream from the Los Frailes mine early last May. They concluded that the sludge released from the retaining pond may have contained as much as 120,000 tons of zinc, comparable to the mine's annual output of 125,000 tons, as well as high quantities of sulfates, contributing to the sludge's acidity.

At high concentrations, zinc, like many heavy metals, can be harmful to the environment, Ms. Chase said. Plants and nitrogen-fixing bacteria are particularly sensitive to zinc pollution, causing concern about possible crop losses. Algae also is affected negatively by excess zinc, she said, posing a possible threat to fish populations that depend on algae as a food source.

In wildlife studies since the mine accident, fish caught near the mouth of the Rio Tinto were found to be highly contaminated with zinc, Dr. van Geen reported. While emergency dikes successfully diverted the contaminated sludge away from DoÒana National Park, a nearby wildlife reserve, vegetation and wildlife in other areas were exposed to the pollution, he said.

Dr. van Geen plans to compare the nearby Tinto-Odiel watershed, a historically polluted area, to the two rivers contaminated by the mine spill. The Tinto and Odiel rivers are located in a Spanish region known as the Iberian pyrite belt, which has been heavily mined for thousands of years, Dr. van Geen said. The Rio Tinto is chronically acidic and contaminated with both zinc and iron; its name means "tinted river." Data from past Tinto-Odiel contamination may help determine what effects the mine accident will have on the Guadiamar and Guadalquivir rivers, the Lamont researchers suggested.

Mined ore is ground and the useful zinc is separated from rock by floating the pulverized ore in a reservoir. Once the metal is removed from the pool's surface, what is left is a reservoir filled with highly acidic water and mining residue, often containing toxic minerals that failed to separate in the process.

The researchers took measurements at distances of up to 40 kilometers, or about 25 miles, from the mine. Ten kilometers, or about 6 miles, from the mine, the amount of zinc found in river sediments was 100 times the amount found in uncontaminated river banks, the Columbia researchers said. Zinc concentrations downstream were less than they expected, however, leading to the conclusion that carbonates in local rock may have buffered the water, reducing its acidity and causing some zinc to be precipitated from the tailings.

Dr. van Geen will return to Spain in December to conduct more tests on water quality in the rivers affected by the mine spill.
This document is available at http://www.columbia.edu/cu/pr/. Working press may receive science and technology press releases via e-mail by sending a message to rjn2@columbia.edu.

Columbia University

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