regions, so far, are southern and western Norway, the Swedish west coast, central Scotland, the Northeastern United, States, Ontario, and the Canadian provinces of Nova Scotia and Newfoundland. Acidified waters are also found in areas as widely separated as Northern Florida, Colorado, and the Puget Sound Basin. Obviously, this is not a local or regional problem. This acidification is not like the natural, slow process by which some lakes and streams, especially those with brown waters, are acidified as a consequence of normal biological changes, e.g., bog formation. Instead, this acidification has been very rapid by comparison, happening almost simultaneously in these widely separated areas, in the span of a few decades, and occurs primarily in clear waters or even causes clearing of brown waters. The number and size of areas being acidified are increasing and the regions affected are determined by their sensitivity to acid inputs.

Sensitive Areas

Several factors interact to make the waters of a region susceptable to acidification due to inputs of strong acids. The most important of these are (1) proximity to emissions sources, (2) regional meteorological patterns, (3) bedrock geology, and (4) topography. The most heavily impacted regions of Europe (Fig. 1) and North America (Fig. 2) are located down wind of large regions of emissions in Great Britain and Northern Europe, and of the central industrial region in the U.S. and Canada. The importance of geographic location and wind direction is illustrated by the fact that the very sensitive waters of northern Minnesota and southwestern Ontario are not yet as severely impacted as the waters of the Northeast and the Maritime provinces. There is evidence that acidification (loss of alkalinity) is occurring in ths region. Much larger areas of Europe (Fig. 9), eastern North America (Fig. 10), and western North America are underlain by sensitive bedrock and are suspected as being sensitive areas.

The most heavily impacted areas of Europe and North America lie on hard bedrock (predominantly intrusive or eruptive bedrock, undifferentiated precambrian granite and gneisses). These materials provide little acid buffering to incoming acidity. In mountainous regions (e.g., southern Norway and the Adirondacks) the terrain forces air masses to rise, causing cooling and precipitation formation, so that such elevated areas tend to have greater wet deposition of atmospheric contaminants due to "rain-out" and "wash-out" effects.

Figure 9. Regions in Europe with acidic intrusive or eruptive bedrock together with undifferentiated precambrian granites and gneisses.

Figure 10. Regions in North America with acidic intrusive or eruptive bedrock together with undifferentiated precambrian granites and gneisses.

Of course, local factors such as soil type and origin, soil depth, vegetation, land use, and position in the watercourse (head-water versus downstream) all contribute to the susceptability of acidification of any particular stream or lake. Thus, in a region receiving highly acidic rain, such as the Adirondack Mountains, lakes which appear to be quite similar and are located very near to each other may have very different susceptabilities and chemistry. The exact mechanisms operating at the local level and controlling the sensitivity of a specific water to acidification, are largely unknown. If we wish to predict the rate of acidification of a lake or stream, or its recovery to a less acid state following reduction in acid loading, these local factors must be understood in greater detail.

Temporal Trends in Aquatic Chemistry

Acidification is defined as a long-term loss of alkalinity, the principal chemical buffering component. Decreased alkalinity and pH values have been reported for many lakes and streams in Norway, Sweden, Canada and the United States. The comparison of data obtained one or more decades ago to current data has been criticized on the grounds that methods used earlier altered the chemistry of the sample or were lacking in accuracy. For example, Zimmerman and Harvey (1979) note that older data for alkalinity were determined by fixed end point potentiometric or colorimetric titrations. In systems of less than · 300 micro equivalents of alkalinity per liter (where species other than those of the carbonate system are significant) these fixed end point titrations cannot be expected to give accurate results. The error is an over-estimate of about 35 micro equivalents per liter, and the error is so indeterminant "that little hope exists for any mathematical salvaging operations," according to these authors. This substantial and helpful critique of temporal comparative studies may call into question much of the chemical evidence which indicates that lakes have been acidified by acidic precipitation.

There is, however, another independent line of evidence, based on the aquatic biota. Fish have disappeared from many lakes and streams in which acidification is reported, and from many others where no chemical data from an earlier period are available. Previously the fish were present; now they are absent, because the water is too acidic and perhaps because the concentrations of materials related to watershed acidification (e.g., aluminum) are too high (Almer et al., 1978; Cronan and Schofield, 1979). In fact, many waters formerly renowned for their good fishing are now barren and so acidic that fish