Limestone: one answer for acid lakes
Much of the United States east of the Mississippi River as well as eastern Canada and southern Norway and Sweden, consists of acid-sensitive soils and lakes. The bedrock of these regions is low in base metals (calcium and magnesium) and cannot buffer (neutralize) new acid deposits. Many of the lakes in these acid-sensitive regions have become acidified by airborne pollutants, deposited mainly as acid rain and snow.
Now, encouraged by tests in Sweden, pressure is building in the United States for experiments using lime to offset acidification of sensitive lakes. However, the chemistry involved is complex. The Swedish experience suggests that liming may help cure the acid-lake problem but that possible adverse side effects have not been adequately identified. Rushing to use liming as a easy solution to acid rain thus could be an ecological mistake.
Acid precipitation results when airborne sulfur and nitrogen oxides, emitted primarily by power plants, industrial processes, and motor vehicle exhausts, combine with moisture in the air to form sulfuric and nitric acids. These then precipitate out of the atmosphere in rain and snow (or sleet, hail, mist, fog, dew, or frost). A similar result occurs when dry sulfate particulates combine with moisture at ground level.
Acidification is measured on the so-called pH scale. Values 0 to 7 indicate acidity, 7-14, alkalinity, 7, neutrality.
The accumulation of the man-made acids in acid-sensitive lakes and streams from wet fall and dry fall pollution causes drastic reduction of fish stocks and destroys other forms of aquatic life. Salmonids (salmon and trout) cannot survive in low pH waters (below 5.5) and are the first to disappear. Other fish, invertebrates, and phytoplankton soon follow.
The acidity mobilizes metals (e.g., aluminum, cadmium, lead, zinc, and mercury) in the water. These metals are toxic to fish and inhibit their reproduction. At extremely low pH values (3.5 to 4.0), the water becomes very clear due to the precipitation of dissolved organic matter. In the final phase, sphagnum moss blankets the lake bottoms and prevents essential nutrients from reaching any remaining aquatic organisms.
Since 1977 the Swedish government has supported the systematic liming of 700 of Sweden's 20,000 acid-sensitive lakes to counteract acidification and its harmful effects on aquatic life. In the Swedish program during 1977-79, 120,000 tons of limestone was applied directly into the targeted lakes at a cost of $50 to $75 a ton. The liming agent itself is cheap; most of the cost is entailed in transporting and spreading the agent.
One Swedish liming expert, William Dickson, estimated that 0.3 to 0.5 million tons of liming agent are needed yearly to neutralize the acid deposition in Sweden's acid-sensitive region (100,000 square kilometers). The Swedish government has recently increased significantly its level of support of the Swedish liming program and may soon double current levels. Some Swedish scientists believe that most of Sweden's acid-sensitive lakes will eventually be limed.
The Swedish liming program has had largely favorable biological effects, including fish reproduction, an increased number of plankton species, increased decomposition of organic matter, and successful recolonization after restocking of eliminated fish species and certain invertebrates. In one study, six Swedish scientists report that liming measures caused no negative ecological effects other than damage to acidophilic white mosses. In certain lakes, a single lime application may show neutralizing effects for 5 to 10 years. Waters with short turnover times must receive more frequent lime applications on their shores and in feeder streams.
In the Swedish liming experience, the main problem has been to maintain an acceptable pH level at high flow, i.e., during the spring snowmelt. The first snowmelt contains very high concentrations of acid. This cold acid meltwater settles on top of the lakes instead of mixing with the higher pH waters below.
Another major problem is the vulnerability of aquatic organisms to toxic metals present in the water during a neutralizing transition period before the metals precipitate to the lake bottom. For example, aluminum leached from the soil is highly destructive to fish gills when waters are in the pH range of 4.5 to 6.0. Because of this, liming has sometimes killed salmon and trout instead of saving them.
In short, the possible negative effects and institutional implications of liming have not yet been adequately explored, although the Swedish liming experience is an appropriate starting point. A recent report by the US National Academy of Sciences concluded that, ''Due to high costs and logistical difficulties, lime cannot be applied to the vast areas that are currently endangered by acidification.'' In view of the successful Swedish experience, this seems a curious and questionable assertion.
Compared with emissions reduction, liming must be regarded, at least potentially, as a cost-effective strategy to mitigate the effects of acid deposition, and to save biologically endangered aquatic life and habitats. According to one source, pressures are already building up within the Reagan administration, the electric power industry, certain state fisheries authorities , and Capitol Hill to undertake broad-scale liming experiments and operations. Liming is likely to loom even larger as decisionmakers become increasingly aware of the damage to aquatic ecosystems from acid deposition.
The relative success of full-scale liming abatement programs in Sweden appears to be leading to a sort of conventional wisdom -- that liming is an acceptable, cost-effective measure to reverse the acidification of lakes and streams. On the face of it, however, liming suggests a characteristic of many other quick answers: an inadequate assessment of possible side effects. In this case, proponents of liming seem willing to gamble on gross improvements without adequate regard for the complexity and delicate balance of limnological systems and possible bad effects of the treatment.
The environmental implications of full-scale liming need to be more fully understood before liming is embraced to mitigate the effects of acid rain. Moreover, officials need to question whether liming would in fact relieve current pressures on polluters to abate acid-causing emissions. Would it, paradoxically, be a license to pollute? Notwithstanding its cost-effectiveness, liming would seem a far less attractive strategy if it inhibited acid-rain abatement efforts, let alone if it actually resulted in increased acid deposition.