Acid Rain

Compared with the water of precipitation, that of lakes, ponds, streams, and rivers is relatively concentrated in ions, especially in calcium, magnesium, potassium, sodium, sulfate, and chloride. These chemicals have been mobilized from the terrestrial part of the watersheds of the surface waters. In addition, some surface waters are brown-colored because of their high concentrations of dissolved organic compounds, usually leached out of nearby bogs. Brown-water lakes are often naturally acidic, with a pH of about 4 to 5.

Seasonal variations in the chemistry of surface waters are important. Where a snowpack accumulates, meltwater in the springtime can be quite acidic. This happens because soils are frozen and/or saturated during snowmelt, so there is little possibility to neutralize the acidity of meltwater. So-called "acid shock" events in streams have Acid rain damage done to a piece of architecture in Chicago, Illinois. Photograph by Richard P. Jacobs. JLM Visuals. Reproduced by permission. been linked to the first meltwaters of the snowpack, which are generally more acidic than later fractions.

A widespread acidification of weakly-buffered waters has affected the northeastern United States, eastern Canada, Scandinavia, and elsewhere. In 1941, for example, the average pH of 21 lakes in central Norway was 7.5, but only 5.4-6.3 in the 1970s. Before 1950 the average pH of 14 Swedish water bodies was 6.6, but 5.5 in 1971. In New York's Adirondack Mountains, 4% of 320 lakes had pH less than 5 in the 1930s, compared with 51% of 217 lakes in that area in 1975 (90% were also devoid of fish). The Environmental Protection Agency sampled a large number of lakes and streams in the United States in the early 1990s. Out of 10,400 lakes, 11% were acidic, mostly in the eastern United States. Atmospheric deposition was attributed as the cause of acidification of 75% of the lakes, while 3% had been affected by acidic drainage from coal mines, and 22% by organic acids from bogs. Of the 4,670 streams considered acidic, 47% had been acidified by atmospheric deposition, 26% by acid-mine drainage, and 27% by bogs.

Surface waters that are vulnerable to acidification generally have a small acid-neutralizing capacity. Usually, H⁺ is absorbed until a buffering threshold is exceeded, and there is then a rapid decrease in pH until another buffering system comes into play. Within the pH range of 6 to 8, bicarbonate alkalinity is the natural buffering system that can be depleted by acidic deposition. The amount of bicarbonate in water is determined by geochemical factors, especially the presence of mineral carbonates such as calcite (CaCO₃) or dolomite (Ca,MgCO₃) in the soil, bedrock, or aquatic sediment of the watershed. Small pockets of these minerals are sufficient to supply enough acid-neutralizing capacity to prevent acidification, even in regions where acid rain is severe. In contrast, where bedrock, soil, and sediment are composed of hard minerals such as granite and quartz, the acid-neutralizing capacity is small and acidification can occur readily. Vulnerable watersheds have little alkalinity and are subject to large depositions of acidifying substances; these are especially common in glaciated regions of eastern North America and Scandinavia, and at high altitude in more southern mountains (such as the Appalachians) where crustal granite has been exposed by erosion.

High-altitude, headwater lakes and streams are often at risk because they usually have a small watershed. Because there is little opportunity for rainwater to interact with the thin soil and bedrock typical of headwater systems, little of the acidity of precipitation is neutralized before it reaches surface water.

In overview, the acidification of freshwaters can be described as a titration of a dilute bicarbonate solution with sulfuric and nitric acids derived from atmospheric deposition. In waters with little alkalinity, and where the watershed provides large fluxes of sulfate accompanied by hydrogen and aluminum ions, the waterbody is vulnerable to acidification.