When you consider the scale of the United States and Canada, it might seem like an impossible task to rein in damaging pollution across the continent. But the story of acid rain shows that it’s sometimes possible to solve such vast environmental challenges in a relatively short timeframe.

Over the course of 20 years, the EPA’s Acid Rain Program was a success. By 2010 sulfur dioxide emissions were 12 million tons lower than in 1980, and emissions of nitrogen oxides were reduced by 4 million tons between 1995 and 2010 (Figure 13.30). One of the major contributors to these decreases was that electrical utilities switched from using high-sulfur coal from Appalachia to low-sulfur coal from the western United States, particularly from Wyoming’s Powder River Basin (see Figure 9.31, page 287). As predicted, acid rain also decreased (Figure 13.31). Precipitation in the eastern portion of the country became less acidic between 1994 and 2009 and acid rain (pH <5.3) was essentially eliminated from the central part of the country to the Pacific Coast.

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With reduced acid rain, lakes and streams that had been acidified during the 1970s and 1980s began to recover. Consider an example from the Adirondack region of New York State. Prior to the start of the Acid Rain Program, 284 lakes out of nearly 2,000 being monitored there were highly acidified. Within 2 to 3 years, that number dropped by 32% to 192 (Figure 13.32). By 2007, 132 of the originally acidified lakes had recovered. Streams in the Adirondack region are also recovering, although at a slightly slower rate.

Up near Sudbury, Clearwater Lake is also showing remarkable recovery. Beginning in 1972, pollution controls decreased the SO₂ emissions of the smelters in the Sudbury areas by more than 90%. In response, the pH of Clearwater Lake, and of other lakes in the area, increased (Figure 13.33). After 1999 the pH in Clearwater exceeded 6.0, a threshold allowing for full recovery of the ecosystem. The concentrations of heavy metals, such as nickel and copper, have also decreased following pollution control (Figure 13.34).

However, recovery of the biological community has been uneven. The phytoplankton and zooplankton of many lakes have bounced back, but neither invertebrates that live at the bottom of the lake nor fish are back to natural levels yet. Factors that may retard recovery in certain areas include competition with established acid-tolerant species. Once a community of organisms is established, in this case of acid-tolerant species, they can persist in an ecosystem and compete with individuals of less tolerant species when physical conditions change. Predation by high numbers of invertebrate predators, which have been released from control by plankton-feeding fish eliminated by acid rain, can also reduce the chance of successful colonization by small organisms. In addition, limited dispersal slows colonization by some organisms. Another problem has been high heavy metal concentrations and reduced concentrations of calcium, Ca²⁺.

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Restoring the forests that once covered the Sudbury landscape is a huge challenge—much greater than restoring a forest following logging or fire. The main reason for the difficulty is that the damage done to these ecosystems has seriously impaired the very foundation of terrestrial ecosystems: their soils. Succession in such circumstances takes a very long time unless helped along by active restoration methods. One of the main treatments used at restoration sites in the most disturbed areas has been spreading crushed limestone on the soils. Limestone adds the critical Ca²⁺ ions stripped from the soils by acid rain and can neutralize acidic soils, resulting in higher soil pH. Liming at the restoration sites has raised average soil pH to 6.3, compared with 4.3 at unrestored sites. Some sites were limed and planted with a single species of pine. Others were limed, fertilized, and planted with three to five tree species.

As a result of active restoration, the number of plant species at the restored sites averaged more than 3 times greater, compared with unrestored sites in the most disturbed zone (Figure 13.35). The number of plant species at the most species-rich restoration site was comparable to that at the low disturbance site, approximately 36 kilometers from the decommissioned smelter. However, approximately 30% of the plant species at restored sites were invasive non-native species, which have high dispersal rates and had colonized the restoration sites on their own. Meanwhile, all plant species at unrestored, control sites were native. Researchers suggest that it may take decades or even centuries for the plant communities at the restored sites to return to a composition close to the original community before disturbance.