Mountaintop Mining’s Impact on Watershed and Regional Scale Nitrogen Export

Abstract

Mountaintop removal coal mining with valley fills (MTM) is the largest contributor to land use change in the Central Appalachia Region. MTM uses explosives and draglines to uncover shallow seems of coal from mountaintops and ridgelines. The coal residues and overburden are disposed of into adjacent valleys forming valley fills. The large quantities of unconsolidated rock increase watershed storage potential and vastly increase rates of rock weathering via sulfuric acid generated by coal residues. This leads to high concentrations of coal and rock derived ions in receiving surface waters and causes a number of associated water quality impairments. Alongside known these known impairments, recent studies have also reported high dissolved nitrogen (N) concentrations in samples from affected streams. This project quantifies, for the first time, the magnitudes and persistence of this elevate nitrogen export from MTM affected catchments and investigates how MTM increases N inputs and alters catchment N cycling. Using two years of hydrologic measurements and regular baseflow and storm sampling near the Hobet Mining Complex in West Virginia, this research finds that annual mass flux of nitrate in mined catchments was 9 to 61 times greater than at a reference catchment. Further, the project finds that high levels of nitrate export during active mining declines after reclamation but can remain significantly elevated for at least several decades post mining. Analysis of nitrate isotopes from stream water finds baseflow at mined sits to be highly enriched in both δ15N and δ18O compared to the reference site. These isotopic values do not match the signature of any known potential nitrate source but do match with the results of fractionation from denitrification occurring in large pools of NO3-. A developed watershed nitrogen budget identifies mining explosives as a sizable mining input of N that could export 9 to 3716 kg N ha-1 but also suggests other sources including fertilizer, weathering derived rock N, and soil mineralization all may play a role in elevated export. Finally, an analysis of regional water quality and surface mining extent indicate a significant correlation between the cumulative extent of surface mining and annual mean nitrate concentrations in the mostly heavily mined regional basin.