Salty Streams

One of the most persistent and concerning impacts of mountaintop mining is its effect on streams in mined areas. Numerous studies have shown that water coming out of mined sites is far saltier –containing higher concentrations of ions– than water coming out of unmined sites. In order to understand why this happens, we need to dive into the process of mountaintop mining.

The first step in mountaintop mining is the removal of vegetation and topsoil. Explosives are then used to expose seams of coal as much as 300 meters (the equivalent to five Duke Chapels) below the surface. Next, the coal is excavated and the now-exploded rock is used to regrade the mountain or is pushed into the adjacent valley to create a “valley fill.” Water moving through these valley fills becomes enriched with solutes as it quickly weathers the fractured rock. This ion rich water is then deposited into streams below valley fills.

An EPA illustration of the process of valley fill creation

Scientists can understand ion content in these streams by measuring how well the water can serve as a medium for electricity—its conductivity. Study after study has found a remarkable increase in conductivity in streams draining water from valley fills, and some have found sustained high levels of conductivity as much as thirty years after valley fill construction.

A valley fill
A valley fill with a sediment pond intended to allow mining salts to settle before run-off reaches the streams below

This rise in conductivity is not limited to small streams directly below valley fills. In rivers downstream of mining operations, rises in conductivity have also been observed. The graph below compares conductivity in 1973-1974 with conductivity in 2011-2012, a period during which there was an 11% change in mined-area in this river’s watershed. The red line indicates the EPA’s conductivity threshold—300 μS/cm—above which the agency has determined there can be consequences for aquatic life. Of the 164 days for which we have conductivity data between 1973-1974, 90 days have conductivity measurements below this threshold. For the same period in 2011-2012, only 7 days have conductivity measurements below this threshold.

Data from the US Geological Survey (USGS)

Of particular concern is the impact of increased conductivity on the organisms without backbones living at the bottom of streams and rivers: benthic macroinvertebrates. In this case “macro” simply means that the organisms are visible without the use of a microscope. Many benthic macroinvertebrates have an external “skin” that is permeable to ions, making it difficult for them to regulate their internal ion concentrations, which can have fatal consequences. Mining-impacted streams with high conductivity levels have experienced an elimination of many sensitive species. This local extinction is also hypothesized to have grave consequences for other organisms such as salamanders and freshwater fish, which feed on benthic macroinvertebrates. This is of particular importance because Central Appalachia is home to one of the most diverse arrays of salamanders and freshwater fish in the world.

A variety of benthic macroinvertebrates
A variety of benthic macroinvertebrates

As compelling as the data illustrating the rise of conductivity and its consequences are, I am left with a nagging question: why should I care about the conductivity of these mining-impacted streams? I am not a resident of West Virginia nor am I a sensitive, benthic macroinvertebrate. I am, however, a consumer of coal. The average American, in fact, consumes about three tons of coal per year. Consequently, we all play a role in the transformation of these ecosystems, and it is up to each of us to critically consider the ecological consequences of mountaintop mining.



Bernhardt, E. S., Lutz, B. D., King, R. S., Fay, J. P., Carter, C. E., Helton, A. M., Campagna, D., Amos, J. (2012). How many mountains can we mine? Assessing the regional degradation of central Appalachian rivers by surface coal mining. Environmental Science and Technology, 46, 8115-8122.

Hitt, N. P., Chambers, D. B. (2014). Temporal changes in taxonomic and functional diversity of fish assemblages downstream from mountaintop mining. Freshwater Science, 33(3), 915-926.

Palmer, M. A., Bernhardt, E. S., Schlesinger, W. H., Eshleman, K. N., Foufoula-Georgiou, E., Hendryx, M. S., …Wilcock, P. R. (2010). Mountaintop Mining Consequences. Science, 327, 148–149.

Pond, G. J., Passmore, M. E., Pointon, N. D., Felbinger, J. K., Walker, C. A., Krock, K. J. G., … Nash, W. L. (2014). Long-Term Impacts on Macroinvertebrates Downstream of Reclaimed Mountaintop Mining Valley Fills in Central Appalachia. Environmental Management, 54, 919-933.

US Environmental Protection Agency (EPA) (2011). The Effects of Mountaintop Mines and Valley Fills on Aquatic Ecosystems of the Central Appalachian Coalfields; US EPA: Washington, DC.