Montana State
University |
Montana State
University |
by Carol Flaherty
12/96 BOZEMAN -- Montana State University scientists have developed techniques to revegetate the state's old mine and smelter wastes. However, a new study shows that the same soil amendments that allow plants to grow on these wastes may increase the arsenic that percolates down to ground water.
Bill Inskeep, a soil scientist at MSU-Bozeman, recently completed work that shows that adding lime to soil to reduce its acidity increases arsenic movement.
Mine wastes are highly acidic. Adding lime is a common way of reducing the acidity so these once-barren landscapes can support vegetation. Without vegetation, mine debris creates a health hazard when it blows.
However, reducing soil acidity appears to be the key that releases arsenic, says Inskeep.
Increases in arsenic mobility are a concern, because it is toxic at relatively low levels and also because the EPA is considering reducing how much arsenic is allowed in drinking water.
Arsenic is a fairly common trace element in mine waste. It is particularly problematic in some locations of the Clark Fork basin. There is arsenic in the waste material around Anaconda's Smelter Hill, where aerial fallout from the stack accumulated for many years, and under the water of the Mill Town Reservoir, downstream from many old mine sites. Scientists and the federal Environmental Protection Agency monitor water quality in both areas, and have found arsenic in some water wells near the reservoir.
Scientists had predicted that arsenic would be mobilized if soils were limed. However, little data supported that premise and less described how much arsenic moved in different soils.
Factors that influence arsenic movement include the degree of soil acidity, aeration and the presence of other elements such as phosphorus. Vegetation also immobilizes some arsenic, taking it up in its roots. There are now about 30 varieties of plants growing and reproducing on Smelter Hill.
With or without liming, arsenic can move through soil, says Inskeep. Chemical bonds determine how much and how quickly arsenic moves. When the chemical bonds are broken, as can happen when lime is added to soils that have arsenic, arsenic is freed to move through the soil system.
After Inskeep added lime to arsenic-contaminated soils in his laboratory study, the arsenic in water that percolated through the soil column increased 10 to 100 times. Variations in the quantity of arsenic mobilized were related to specific characteristics of the mine waste.
Inskeep's data support and add detail to our understanding of arsenic movement through soils. He recently received additional support from the EPA to study this in more detail. By understanding the processes controlling arsenic mobility, we improve our ability to predict where and how quickly arsenic will move. Such predictions may allow us to avoid some arsenic pollution.
But even though liming increases the potential for arsenic to percolate through soils, lime is the most practical amendment to reduce the mobility of other trace metals, such as copper, and to provide a more suitable environment for revegetation, says Dennis Neuman. He is one of the MSU-Bozeman scientists who worked to revegetate Smelter Hill.
So MSU mining reclamation scientists continue to look for new ways to immobilize arsenic and reduce wind-blown contaminants.
Send questions or comments to Carol Flaherty and the sources of this story at carolf@montana.edu.
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