Acidification of freshwaters

Land, vegetation, and bodies of water are the surfaces on which acidic deposition accumulates. Bodies of fresh water represent the smallest proportion of the earth s surface area available for acidic deposition. Yet, the best-known effect is acidification of freshwater aquatic systems. 

Malcolm Cresser and Anthony Edwards Acidification of Freshwaters... 

M.S. Cresser and A.C. Edwards, Acidification of Freshwaters , Cambridge University Press, Cambridge, 1987. 

Ford J, Young TC (1994) In Steinberg CEW, Wright RF (eds) Acidification of Freshwater Ecosystems Implications for the Future, Wiley... 

Henriksen A. (1980) Acidification of freshwaters—a large scale titration. Ecological Impact of Acid Precipitation. SNSF Project, Oslo, 68-74. 

It is, however, acidification of freshwaters that commonly results in aluminium mobility resulting in ecological damage. This acidification is typically caused by two anthropogenic processes, acid rain and acid mine drainage. 

Acidification of freshwater is most marked in upland areas with high rainfall (hence high acid flux), steep slopes (resulting in a short residence time for water in the soil) and crystalline rocks (which weather, and supply cations, slowly). Thus, while acid rain is a widespread phenomenon, acidified freshwaters are less common and are controlled both by rates of atmospheric input and by rock types (Fig. 5.7). All weathering processes, except sulphide oxidation (see Sections 4.4.2 5.4.2), consume hydrogen ions, driving pH toward neutrality. Hence, mature rivers, which drain deeper, cation-rich lowland soils, have higher pH and lower aluminium concentrations. 

The effects of upland acidification of freshwaters can be dramatic. Between 1930 and 1975 the median pH of lakes in the Adirondack Mountains of northeastern USA decreased from 6.7 to 5.1, caused by progressively lower pH in rainwater (Fig. 5.7). The acidified lakewater killed fish and other animals by several mechanisms. The problem for fish is that the dissolved Al3+ in the acidic water precipitates as an insoluble Al(OH)3 gel on the less acidic gill tissues, preventing normal uptake of oxygen and suffocating the animal. Similar problems have occurred in Scandinavia and Scotland. In addition to problems in freshwaters, the loss of forests in high-altitude areas has been linked to acid leaching, which leads to impoverishment of soils coupled with direct loss of cations from plant leaves. 

Atmospheric sulfur from smokestacks can be carried hundreds of kilometers over flat terrain. In regions of sulfur-deficient soils, atmospheric sulfur at low concentrations can benefit plants. Benefits from the low concentration, however, must be weighed against the associated acidification of freshwater, phytotoxicity, health hazards, smog, and building deterioration at the higher concentrations near the sources. 

P. Brimblecombe Air Composition and Chemistry, Second Edition A. C. ChwabeAam Radioactive Aerosols M. Cresser, K. Killham, and A. Edwards Soil Chemistry and its Applications A. Edwards and M. Cres.s.er Acidification of Freshwaters R.M. Harrison and S. J. de Mora Introductory Chemistry for the Environmental Sciences,... 

Deposition of sulfur in regions where the soils are deficient in sulfur may be considered a beneficial effect all of the other effects of sulfur air pollution are considered adverse. They include effects on human health, materials degradation, vegetation, and atmospheric visibility, and acidification of soils, watersheds and freshwaters. Limitations on the scope of this paper will greatly restrict our discussions of the details of these effects see the References 1-11 for more detailed information. 

Sammut J., White I., and Melville M. D. (1996) Acidification of an estuarine tributary in eastern Australia due to drainage of acid sulfate soils. Mar. Freshwater Res. 47(5), 669-684. 

Bayley S. E., Parker B., Vitt D., and Rosenburg D. (1988) Experimental Acidification of a Freshwater Wetland. Wildlife Toxicology Fund, Final Report. 

The dry and wet deposition of acidic sulphur compounds arrives on the soil and on the vegetation. Most rain occurs over land surfaces and washes the pollutants absorbed on vegetation through the soil into freshwater systems. As the rain passes through the soil it is affected by many chemical reactions which can change its composition before it reaches a head-water stream or upland lake. These chemical reactions can be driven by the increased deposition of acidic sulphur compounds and can lead to the acidification of the soils and the freshwaters. Acidified soils are not as productive as weU-buffered soils and have depleted communities of flora and fauna. Acidified freshwaters have depleted fisheries and populations of insects, amphibians, mammals and birds. 

Selecting sensitive receptor sites In the analysis of environmental acidification from sulphur dioxide emissions, an important first step is the selection of possible locations for receptor sites which may be particularly sensitive to the deposition of acidic sulphur species. Table 1 contains the locations of arbitrary receptor sites which have been chosen to reflect the contribution from sulphur deposition to environmental damage caused by the acidification of soils, surface waters and freshwater ecosystems [14]. 

Based on the critical loads maps for the acidification of UK soils and freshwaters by the deposition of acidic sulphur compounds, critical loads of 5 kg S/ha/yr have been adopted to reflect the contribution to environmental damage caused by sulphur deposition in the regions with sensitive geologies selected in Table 1. These estimates are purely illustrative and have been developed to facilitate the analysis. In aU cases it is assumed that both wet and dry sulphur deposition are equally damaging and henceforward the term, total sulphur deposition, refers to their sum. 

Within the defined areas, critical loads are calculated for all major combinations of tree species and soil types (receptors) in the case of terrestrial ecosystems, or water biota (including fish species) and water types in case of freshwater ecosystems. These combinations include the great variety of different ecosystems, the sensitivity of which to both acidification and eutrophication inputs by atmospheric pollutants differs greatly, determining the necessary reduction needs when CDs are exceeded by modern deposition levels. 

Van Sickle, J., Baker, J.R, Simonin, H.A., Baldigo, B.P., Kretser, W.A. and Sharpe, W.F. (1996). Episodic acidification of small streams in the northeastern United States Fish mortality in field bioassays. Ecological Applications, 6, 408-421. Wright, R.F (2003). Predicting recovery of acidified freshwaters in Europe and Canada. Hydrology Earth System Science, 7, 429—430. 

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