Charles Driscoll
Acid Rain Revisited

Acidic deposition, or more commonly, acid rain, refers to elevated inputs of sulfate, nitrate, hydrogen ion and other solutes by precipitation, gas, particle and/or cloud deposition, originating from emissions of sulfur dioxide, nitrogen oxides and other materials. There has been concern over the effects of acidic deposition since it was first observed in eastern North America in the 1960s. Since that time research has advanced our understanding of this problem. As the Clean Air Act is being considered for reauthorization, it is an appropriate time to synthesize previous research and make projections on the response of ecosystems to potential future changes in emissions. In this presentation I will discuss the sources, ecological effects and management options of acid rain. I will highlight long-term measurements that have been made at the Hubbard Brook Experimental Forest (HBEF), NH. Long-term measurements of bulk deposition and stream water chemistry were initiated at the HBEF in the early 1960s. Since that time, additional measurements have been made at the site to better understand the structure, function and change in the northern hardwood forest and associated aquatic ecosystems in response to air pollution. Long-term measurements at the HBEF show a strong relationship between declines in concentrations of sulfate in precipitation and emissions of sulfur dioxide in the northeastern U.S. airshed. Trends in concentrations of nitrate in precipitation are consistent with decreases in nitrogen oxide emissions from electric utilities. Stream water chemistry measurements show long-term decreases in concentrations of sulfate, and calcium, and increases in pH in response to these changes. Long-term mass balance calculations and model hindcasts using the biogeochemical model PnET-BGC provide evidence of the long-term chemical consequences of acidic deposition. Loss of soil available calcium is consistent with delays in the recovery of the acid-base status of stream water and long-term declines in sugar maple. To test these hypotheses an experimental addition of calcium silicate was made to watershed 1 at the HBEF in 1999. This treatment has improved the base status of soil and drainage waters, and may have improved sugar maple regeneration. Model predictions of the response of soil and surface waters to anticipated future changes in acidic deposition will be presented.