When everything changes: Watershed scale biogeochemical impacts of mountaintop mining
Mountaintop mining (MTM) for coal dramatically restructures landscapes throughout more than 3000 km2 of Central Appalachia. To mine shallow coal seams, MTM operators remove overlying bedrock with explosives and machinery. Some of this unconsolidated bedrock is deposited into headwater valley fills, while most is used to reconstruct the landscape. The porous, unweathered material is rich in acid generating pyrite, and neutralizing calcareous bedrock, which react to generate a distinct chemical signal in streams draining MTM landscapes. Alkaline mine drainage is characterized by elevated pH, electrical conductivity, anions (SO42-), cations (Ca2+, Mg2+) and metals and metalloids (Fe, Se) with strong correlations between conductivity and individual elements.
This unique pollution signal has been studied primarily at baseflow. However, MTM alters stream biogeochemistry and hydrology at both baseflow and stormflow. Using a paired watershed approach we ask: 1) How does mining affect watershed shape and subsurface structure, and 2) What are the consequences of this large-scale watershed restructuring for base- and stormflow biogeochemistry and hydrology? We address these questions using a pre- and post-mining terrain analysis of a mined and unmined watersheds of similar size (~.75km2 ) and aspect paired with high-frequency measurements of stream discharge and conductivity.
The mined watershed has been dramatically altered with reconfigured watershed boundaries shrinking the watershed by 32% from 97 to 66ha. More than 11 million m3 of spoil material has been deposited into the watersheds headwater valley, with an average of 12m removed from adjacent ridges. Together, these activities flatten the watershed with slopes having shifted from a distinctly unimodal distribution with a mean slope of 20˚ to a bimodal slope distribution with modes at near 2˚ (flattened ridges) and 22˚ (valley fills) and a mean of 12˚. The reference watershed has a unimodal slope distribution with a median slope of 19˚.
Streamwater conductivity from the mined watersheds remained 24-32x higher than unmined reference watersheds throughout the study. Before leaf-off (Aug-Oct), mined watershed’s exhibited mean conductivity of 1,600 (± 141 SD) µS/cm while unmined watersheds averaged 65 (± 10 SD) µS/cm. In winter months (Nov-Feb), mean conductivity values declined in the mined and unmined watersheds to 1141 (±68) and 34 (± 3) µS/cm respectively. During all storm events, both mined and unmined watersheds show dilution of the conductivity signal, suggesting low conductivity water is rapidly routed to streams during storms in both systems.