Long term changes of chemical weathering products in rivers heavily impacted from acid mine drainage: Insights on the impact of coal mining on regional and global carbon and sulfur budgets

The long term impacts of acid mine drainage (AMD) on stream chemistry and regional carbon and sulfur budgets were explored using watersheds of Pennsylvania underlain by extensive coal deposits. Areas of these watersheds have been mined for 200 yr, yet mining activity decreased to < 2% of peak by the late 1900s. A unique aspect of this study was the coupling of 100 yr of data on stream chemistry measurements with detailed coal mining data, which allowed for new budgets of the impact of mining on regional and global budgets. The Lackawanna River and upper Schuylkill River, both ~ 900 km2 watersheds, witnessed dramatic changes in pH, alkalinity, calcium, magnesium and sulfate. Sulfate fluxes from these watersheds, for instance, were 4–12 times higher in the 1940s than they are currently. Fluxes of sulfate and magnesium from the Susquehanna River at Danville, the major tributary to the Chesapeake Bay, are currently 32 and 70% of what they were in the 1940s, while alkalinity fluxes have doubled and pH has recovered 0.8 pH units. The direct impact on regional carbon budgets through the degassing of CO2 from carbonates was intense during the height of AMD but the long term regional impact is modest, resulting in the loss of ~ 3.1 Tg of carbon to the atmosphere over the last century. During the 1940s, the export of AMD derived sulfate to the 29,000 km2 portion of the Susquehanna River studied here was twice as large as the current input from SOx deposition to the entire 71,000 km2 Susquehanna watershed. This is surprising, comparing the small spatial footprint of AMD to the large footprint of the entire Susquehanna watershed. Normalizing these export rates to coal production data we estimate that global sulfur releases from AMD could account for 28–40% of riverine sulfate derived from pyrite oxidation, and be equal to ~ 20% of anthropogenic S from atmospheric deposition. This study emphasizes the potential importance of AMD to global S budgets, particularly since coal mining is predicted to increase by 50% in the coming century.