Mountaintop mining “valley fills” can leave streams salty for decades

Leftover debris from mountaintop removal mines, like this one in Kentucky, is piled and revegetated in nearby valley fills (Credit: iLoveMountains.org via Wikimedia Commons)

Streams emerging from piles of rocky debris leftover from mountaintop mining operations often harbor salinity levels that are harmful to aquatic life, and many will take two or three decades to recover to safe levels.

That’s according to a recently published study of 137 so-called “valley fills” in the Appalachian Mountains of Virginia. Scientists evaluated water quality data collected before, during and after valley fill construction, in which excess rock and mining byproducts are pushed into valleys and covered in new vegetation.

Though specific conductance data — a surrogate for salinity — from each phase wasn’t available for every site, the analysis found that surface waters rose significantly after fill construction and replanting before beginning to taper off.

“The numbers are high and the amount of time it takes to come down is pretty high, but it’s not infinite,” said Daniel Evans, research associate at Virginia Tech and lead author of the study. “It doesn’t stay high for geologic time, thousands of years. It will come down, it just takes a long time.”

Some of the study’s sites were constructed recently and others as many as 20 years ago. In the years covered by the data, specific conductance at only 7 of the 137 sites had declined to levels determined to be reasonably safe for aquatic life.

The researchers used mathematical models to project how long it will take for the still-elevated sites to return to safe levels of salinity. The results differed depending on which sites were included in the model, but a conservative projection produced a range of 9 to 31 years.

That’s a big range, but the variation suggests that those sites on the short end of the spectrum could be used as a reference for better valley fill construction in the future. Part of the challenge is determining which characteristics of the low-salinity sites gave them the edge in keeping ions such as dissolved sulfate, bicarbonate, calcium and magnesium out of the water. For example, some types of mining spoils are more likely to leach ions than others, but data generally don’t exist on how various spoils are used at a given fill.

To learn more about controlling the exposure of certain spoil types to surface waters, Evans is working on proposed valley fill construction plans with researchers at the University of Kentucky. At their experimental sites, more reactive spoils are hydrologically isolated, while the top meter of the fill is reserved to rock types more conducive to tree growth.

They’re also trying to influence the flow of water so that it doesn’t run off too quickly, yet doesn’t hang around too long either.

That way, “you don’t have all the water infiltrating the bulk fill and sitting there like a tea bag seeping on those rocks,” Evans said. “The hope is that these types of projects will lead to some sort of management approach that can help mitigate the impacts from the mining.”

The study is published online in the Journal of the American Water Resources Association.

Top image: Leftover debris from mountaintop removal mines, like this one in Kentucky, is piled and revegetated in nearby valley fills (Credit: iLoveMountains.org via Wikimedia Commons)