Hubbard Brook LTER

Long-term Research shows Acid Rain a lingering problem

Figure1.

Many people believe that the problem of acid rain was solved with the passage of the 1990 Clean Air Act Amendments (CAAA). However, research from the Hubbard Brook Experimental Forest (HBEF) in New Hampshire and other study sites in the northeastern United States demonstrates that acid rain is still a significant problem. Acid deposition delivers acids and acidifying compounds to the Earth's surface, which then move through soil, vegetation, and surface waters and, in turn, set off a cascade of adverse ecological effects.Recent research shows that the ability of some ecosystems to neutralize acid deposition has diminished over time, delaying the recovery of forests, lakes, and streams. The alteration of soils by acid deposition has serious consequences for acid-sensitive ecosystems. Soils that are compromised by acid deposition are less able to neutralize additional amounts of acid deposition, provide poorer growing conditions for plants, and delay ecosystem recovery. Stream data from the HBEF reveal a number of long-term trends (see Figure 1). Specifically, the concentration of sulfate in streams at the HBEF declined 20 percent between 1963-1994. The pH of streams consequently increased from 4.8 to 5.0. Although this represents an important improvement in water quality, streams at the HBEF remain acidic compared to background conditions, estimated to be above 6.0. Moreover, acid-neutralizing capacity–an important measure of a lake or stream's susceptibility to acid inputs–has not improved significantly at the HBEF over the past thirty years. Three factors account for the slow recovery in chemical water quality at the HBEF and across the Northeast, despite the decreased deposition of sulfur associated with the CAAA.

First, acid-neutralizing base cations have been depleted from the soil due to acid deposition and, to a lesser extent, a reduction in atmospheric inputs of base cations. Second, inputs of nitric acid have acidified surface waters and elevated their concentration of nitrate in many regions of the Northeast. Finally, sulfur has accumulated in the soil and is now being released to surface waters as sulfate, even though sulfate deposition has decreased. In sum, long-term research suggests that deeper emissions cuts will lead to greater and faster recovery from acid deposition in the Northeast.