Increased acidity in rain arises from release of sulfur and nitrogen compounds from fossil fuel power plants. The deposited rainwater tends to negatively affect the health of a host of ecosystems, including streams and forests. In the United States, remarkable reductions of acidity arose after the passage of the US Clean Air Act Amendments of 1990, leading to adoption of technologies to reduce power plant emissions and phasing out of dirtier plants. However, acid rain remains a problem for many parts of the country.
Robison and Scanlon  document and project an additional stressor from projected anthropogenic climate change on a stream in the southern Appalachian Mountains. Many of these streams have failed to recover quickly as soils are continue to release adsorbed sulfate. A simulation model was used to demonstrate that projected temperature and precipitation patterns will likely offset more than two-thirds of otherwise projected improvements in stream acid-base chemistry.
Robison, A. L., & Scanlon, T. M. . Climate change to offset improvements in watershed acid‐base status provided by Clean Air Act and Amendments: A model application in Shenandoah National Park, Virginia. Journal of Geophysical Research: Biogeosciences, 123. https://doi.org/10.1029/2018JG004519
—Ankur Rashmikant Desai, Editor, JGR: Biogeosciences