Acidification of surface waters and the loss of forest soil productivity remains a serious threat to the northeastern United States and eastern Canada. Though the topic is no longer a front page story in the daily papers, the scientific community continues to research and publish journal articles. Without a doubt, few environmental problems have been studied to the extent of "Acid Rain". In the 1980s the subject was relatively controversial because the political climate did not look favorably on the emission cut backs and further restriction to utilities thought necessary to improve air quality. Since acid deposition was looked upon as a northeastern regional problem and not a national problem, the Clean Air Act was delayed for nine years. In 1981, Congress launched the National Acid Precipitation Assessment Program (NAPAP). This Program spent over $500 million and supported thousands of scientists who conducted research, monitoring and assessment studies that contributed to our understanding of this complex issue.

Vermont was one of the first states to begin precipitation and lake monitoring programs specifically to assess acidification. This 20 year monitoring history has established Vermont's place as a significant contributor of information and knowledge of the ecological effects of acid rain. This knowledge eventually lead to the controls mandated in the 1990 Clean Air Act Amendments. Title IV of the Amendments required sulfur dioxide (SO2) emissions to be cut 10 million tons below 1980 levels by the year 2000. These reductions were implemented in two phases. Phase I, effective January 1,1995, required the largest fossil-fuel fired plants to reduce emissions. In addition, it mandates nitrogen oxide emissions be reduced by two million tons. Phase II, effective in the year 2000, includes the smaller fossil-fuel plants and the large coal-burning power plants.

Significant reductions in SO2 emissions and deposition have been recorded throughout the northeastern United States and the Atlantic Provinces of Canada. No trends have been observed in nitrogen oxides, but levels are expected to improve following the required reductions. According to Vermont's Volunteer Acid Precipitation Monitoring Program (VAPMP), Vermont's precipitation remains unquestionably acidic with a mean pH of just below 4.4. In fact, 94% of all storm events have resulted in a pH less than 5.6, the theoretical pH of unpolluted precipitation. The VAPMP has observed no improvement in pH over time (1980-1999).

VAPMP Frequency Distribution for All Stations 1980-1998. 11,035 Observations

The reductions in SO₂ emissions have produced region-wide decreases in lake sulfate. No temporal trends have yet to be observed for nitrate (NO3), although seasonal NO3 spikes during spring runoff and snowmelt can lead to acute short-term acidification. The spring period of episodic acidification limits a water body's overall biological community. When the pH drops below critical levels in the springtime, only the most tolerant organisms can survive. Short-term exposure to low pH and elevated aluminum swept from soils can cause significant stress and increased mortality of some species of fish and other organisms.

It was anticipated that reductions in sulfate would increase buffering capacity and eventually improve pH. What was not fully predicted was the loss of soil base cations, predominantly calcium, occurring simultaneously with the sulfate declines. Calcium levels in water bodies are declining because of less input from atmospheric sources, and more importantly, the watershed. Simply put, as acid sulfate ion deposition decreases, less calcium ions from soils and bedrock are mobilized and deposited into lakes and streams. On a regional basis, the Adirondack lakes have observed no recovery. Fortunately, Vermont has observed limited improvement in buffering capacity in some lakes. However, these improvements in alkalinity have not had a positive effect on pH. It appears the majority of Vermont's impaired lakes have the potential to recover, but only if further reductions in both sulfur and nitrogen oxides proceed. Vermont has 37 lakes listed as extremely acidic. Lakes with acid neutralizing capacity values of less than 2.5 mg/l are considered extremely acidic.

Although 19 years of trend monitoring is relatively short, north-easterners and our neighbors in the eastern provinces of Canada have every reason to be optimistic that water quality in this region can improve with further reductions in air pollution. It is unlikely, however, that the most critically acidic (pH less than 4.5), lakes will fully recover. These severely damaged lakes and surrounding watersheds may lack the internal buffer generating capacity to recover even with reduced acid loading. As a result, biological recovery is in doubt. Former balanced ecosystems may never be re-attained. In fact, of all pollutants, international acid deposition is now recognized as one the most serious threats to biodiversity at the eco-region level. There are concerns that acidification, in combination with climate change, mercury (and other metals) deposition, stratospheric ozone depletion and increased ultra-violet radiation will have vast environmental implications.

Updated: June 2003