Episodic acidification

In addition to the long-term acidification of waters, "episodic" or short duration events can have severe consequences for living organisms. In Canada, large amounts of the total annual precipitation are deposited through storm events or as snowfall. The large volumes of pollutants which can be released... 

Because of decreased SO2 emissions, the importance of nitrogen for freshwater acidification becomes more and more determinant. Therefore, uncertainties in the evolution of nitrogen emissions make it difficult to predict future trends in freshwater recovery. On the other hand, climate warming may increase the number of alkaline rain episodes (containing Saharan dust) and enhance weathering reactions, which buffer better acid precipitation, increasing alkalinity and pH. 

Lepori L, Barbieri A, Ormerod SJ (2003) Effects of episodic acidification on macroinvertebrate assemblages in Swiss Alpine streams. Lreshw Biol 48 1873... 

It has been estimated that 1.4-7.4 times as many streams in the eastern United States undergo episodic acidification than are chronically acidic (108). Similarly, the number of episodically acidic Adirondack lakes is estimated to be 3 times higher than the number of chronically acidic lakes (108). Wigington et al. (109) reported that acidic episodes occur in a wide range of geographic locations in the northeastern, southeastern, and western United States, as well as in Scandinavia, Europe, and Canada. 

Figure 16. Outflow chemistry from two snowmelt seasons (1986 and 1987) at Emerald Lake, a high-elevation lake in the Sierra Nevada Mountains of California. Maximum N03 concentrations are coincident with ANC minima during the early stages of snowmelt in 1986 and with a rain-on-snow event in 1987. Nitrate episodes are smaller in magnitude than at sites in the eastern United States, hut western lakes may he more susceptible to episodic acidification because they have a lower baseline acid-neutralizing capacity than most eastern lakes. (Reproduced with permission from reference 180. Copyright 1991 American Geophysical Union.)...

The state of episodic acidification in the Sierra Nevada Mountains (and the rest of the West) therefore remains uncertain because few data exist and the data that are available indicate ANC depressions to a value of 0 xequiv/L, but not below. For the time being, most western sites would be classified as Stage 0 watersheds. However, in systems as acid-sensitive as those at high elevations in the West, episodic acidification resulting from N03" may be possible in the watersheds that are only seasonally N-saturated. 

Wigington, P. J. Davies, T. D. Tranter, M. Eshleman, K. Episodic Acidification of Surface Waters Due to Acidic Deposition Acidic Deposition State of Science and Technology Report 12 National Acid Precipitation Assessment Program Washington, DC, 1990. 

The acidification problem in both the United States and Canada grows in magnitude if episodic acidification (brief periods of low pH levels from snowmelt or heavy downpours) is taken into account. Lakes and streams throughout the United States, including high-elevation western lakes, are sensitive to episodic acidification. In the Mid-Appalachians, the Mid-Atlantic Coastal Plain, and the Adirondack Mountains, many additional lakes and streams become temporarily acidic during storms and snowmelt. Episodic acidification can cause large-scale fish kills. ... 

For example, approximately 70 percent of sensitive lakes in the Adirondacks are at risk of episodic acidification. This amount is over three limes the amount of chronically acidic lakes. In the mid-Appalachians, approximately 30 percent of sensitive streams are likely to become acidic during an episode. This level is seven times the number of chronically acidic streams in that area,... 

The impact of nitrogen on surface waters is also critical, Nitrogen plays a significant role in episodic acidification and new research recognizes... 

Aluminum occurs ubiquitously in natural waters as a result of the weathering of aluminum-containing rocks and minerals. Of the known geochemical responses to environmental acidification, the best documented is the mobilization of aluminum from terrestrial to aquatic environments (Campbell et al. 1992). This mobilization of aluminum is often episodic in nature and is associated with pH depressions (acidification) occurring during the spring snowmelt or associated with erosion from specific storm events (Campbell et al. 1992 Nelson and Campell 1991 Rosseland et al. 1990). 

Aggrading forests contribute to surface-water acidification in a number of ways. Water discharge decreases because of enhanced evapotran-spiration, causing evaporative concentration of pollutants. Hydrological pathways become modified (Waters and Jenkins, 1992), and coniferous afforestation commonly raises DOC. Dry deposition of acidifying pollutants to a forest canopy increases as the canopy develops. Total deposition of sulfur and other pollutants in throughfall in forests is typically several times higher than bulk deposition outside the forest (Beier et al., 1993 Hansen et al., 1994 Rustad et al., 1994). Episodic acidification caused by marine aerosols... 

Figure 11 Experimental episodic acidification and recovery of a low-ANC stream in Maine, USA showing Ca and SO4 dynamics. Change in concentration of SO4 (solid line) and Ca (dashed line), 50 m downstream from an HCl addition that ended at 6 h (vertical line). Note adsorption and desorption of SO4 and desorption and adsorption (after 8 h) of Ca (S. A. Norton, unpublished).

Reinhardt R. L., Norton S. A., Handley M., and Amirbahman A. (2003) Mobilization of and hnkages among P, Al, and Fe during high discharge episodic acidification at the Bear Brook Watershed in Maine, USA. Water Air Soil Pollut. (in press). 

Tranter M., Davies T. D., Wigington P. J., and Eshleman K. N. (1994) Episodic acidification of fresh-water systems in Canada— physical and geochemical processes. Water Air Soil Pollut. 72, 19-39. 

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