When I was a growing up in Peterborough, “acid rain” was a major environmental concern, particularly in New England, where most lakes, ponds and streams are poorly buffered because of the region’s geology.

The acid rain problem resulted from the burning of high-sulfur fossil fuels, chiefly coal, for electric power generation. This converted chemically reduced forms of sulfur common in some coal deposits, like the mineral pyrite (ferrous iron sulfide) and organic sulfur moieties to oxidized, sulfuric acid.

Sulfuric acid is a strong acid that requires substantial quantities of base compounds to neutralize. Another strong acid that also contributed to the acid rain problem was nitric acid, which was primarily released by automobile exhausts.

The bedrock geology of New England chiefly consists of crystalline, aluminum-bearing silicate rocks (i.e., granite, schist, and gneiss). These crystalline rocks possess poor acid-buffering capacities compared to carbonate rocks such as limestones and dolostones, both of which are rare in New England. The increased acidity from acid rain made lakes and streams more corrosive to the local bedrock, facilitating their chemical weathering and dissolution, which released higher amounts of aluminum into these freshwater systems.

Aluminum is particularly toxic to fish, as it hampers their ability to regulate the composition of their internal body fluids, as well as suffocates fish by sticking to their gills.

Introduction of the Clean Air Act placed limits on the amount of sulfur that could be emitted by coal- and oil-fired power plants, which dramatically lowered the release of sulfuric acid to the atmosphere. The result was the near-elimination of the human-produced, acid rain problem across large portions of the globe.

Consequently, many of the hardest hit lakes in New England have largely recovered (i.e., surface water pH returned to pre-acid rain levels). In turn, populations of native wildlife, including brook trout, that had become extinct in acidified watersheds in northern New England and New York have since rebounded.

Rain in equilibrium with the atmosphere is slightly acidic because of the presence of carbon dioxide. When I started my doctoral studies in 1990, the concentration of carbon dioxide in the atmosphere was around 350 parts per million by volume (i.e., 0.035% by volume). Although this does not sound like much, rain in equilibrium with this level of carbon dioxide has a pH of around 5.7. Neutral pH is equal to 7.

The continued burning of fossil fuels, along with deforestation, has pushed the concentration of carbon dioxide in the atmosphere to about 419 parts per million (0.0412 % by volume). Besides warming of global temperatures (i.e., global warming), the continued rise in atmospheric carbon dioxide has decreased the pH of the surface ocean. Long-term records from the Bermuda Atlantic Time Series and the Aloha Station north of Hawaii clearly show that the rise in atmospheric carbon dioxide has lowered seawater pH by 0.3 to 0.4 pH units over the past 40 to 50 years.

This “ocean acidification” is of critical concern for many reasons, including its potential to negatively impact shellfish fisheries.

Carbon dioxide is a weak acid that readily dissolves in water. Once in water, it dissociates, producing acid that shifts the buffering capacity of the water to lower values, changing the relative proportions of bicarbonate and carbonate anions. Shellfish like clams, mussels, oysters, scallops and so forth require the carbonate anion to form their shells.

When carbon dioxide dissolves in waters, it lowers the amount of carbonate anions, making it harder for shellfish to make their shells. Hence, one possible outcome of ocean acidification could be the collapse of important shellfish fisheries. Although scientists have been studying ocean acidification for at least the past 30 years, we know much less about how freshwater ecosystems will respond to rising atmospheric carbon dioxide concentrations.

My research group at the University of Massachusetts initiated an investigation of how New England lakes may respond as atmospheric carbon dioxide concentrations continue to rise over the 21st century. Two lakes from the Monadnock region, Nubanusit and Dublin Pond, were including in our study. Atmospheric carbon dioxide concentrations are expected to rise to at least 600 parts per million by the year 2100 but could go as high as 1,100 parts per million by the end of the century.

Our study indicates that like the ocean, New England lakes will acidify during the 21st century, and that pH decreases in these lakes will be of the same magnitude as the ocean. However, because these lakes already have lower pH and acid-buffering capacities than the ocean, lake pH values will be much lower (more acidic) in 2100 than the ocean.

Dublin Pond currently has a pH of about 6.5, whereas the pH of Lake Nubanusit averages about 5.8. If atmospheric carbon dioxide increases to 1,100 parts per million by the year 2100, the pH of Dublin Pond could drop to 6.2 and that of Nubanusit may reach as low 5.4. For context, the pH of black coffee is about 5.

In addition to the negative impacts on freshwater calcifying organisms like clams, mussels and crayfish, these pH decreases will also increase the bioavailability of toxic trace metals like aluminum, lead and mercury. As we describe in our recent study, brook trout are one of the most sensitive organisms to aluminum toxicity. Acidification of local lakes from rising atmospheric carbon dioxide will increase the toxic stress from aluminum to these important sport fish.

Increases in bioavailable aluminum concentrations was one of the outcomes of watershed acidification during the 1970s through 1990 owing to acid rain, which led to the local extinction of brook trout from many New England lakes.

Our research shows that continued fossil fuel consumption will lead to acidification of New England lakes, which could also translate to economic losses for local tourism that relies on these lakes for recreational activities. Thus, the improvements in water quality and recoveries of numerous watershed that followed the implementation of the Clean Air Act could potentially be reversed by rising atmospheric carbon dioxide from fossil fuel consumption.

Hopefully, these issues will be addressed in any future amendments of the Clean Air Act.

Karen Johannesson is a professor of geochemistry in the School for the Environment at the University of Massachusetts Boston as well as in the Intercampus Marine Sciences Graduate Program of the University of Massachusetts system. Her research focuses on the chemical speciation and biogeochemical cycling of trace elements in the environment.