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ACIDIFICATION OF SURFACE WATERS
Natural pH Variations
Acidity is measured as pH which is a key parameter in water quality. pH is closely linked to biological productivity in aquatic systems and is a limiting factor for certain water uses. The pH scale is logarithmic with a pH of 7.0 being neutral. Each whole unit on the scale represents a multiplication factor of 10. Thus, water with a pH of 5.0 is 100 times more acidic than water with a pH of 7.0.In the absence of strong acid anions such as SO4-- and NO3-, rain water is naturally acidic (pH = 5.7). This acidity is caused by the dissolution of atmospheric CO2. After weathering reactions, natural pH levels in rivers are generally close to neutrality.
Most average annual pH values are between 6.5 and 8.3. Values of pH do not generally display strong variability at individual stations. In the GEMS/WATER data bank, the global median pH value is 7.7.
When weathering is limited and TDS are low, the major dissolved load is often dominated by dissolved organic acids resulting from soil leaching. Under these conditions pH values lower than 4.0 have been measured. Conditions such as this can be found downstream from peat bogs and other wetlands in many parts of the world. In Central Amazonia, and particularly within the Rio Negro basin, the so-called 'Black Waters' have a natural pH below 5.0. Other south American rivers are more neutral. African values used here come mostly from the Nile basin and are not representative of the whole continent.
In eutrophic rivers or downstream from lakes and reservoirs where chlorophyll maximums may exceed 100 mg m-3, pH increases as a result of bicarbonate assimilation processes by aquatic plants. Values exceeding 8.5 are quite common in such waters. Under unusual conditions pH values can increase and decrease by one pH unit in a single day and values can exceed 9.0 at midday. The Loire River (France) is an example of this kind of phenomenon where maximums of 200 mg m-3 of chlorophyll have been recorded. In rivers such as the Rhine that are polluted by organic wastes, algal production is counter-balanced by bacterial degradation and the CO2 concentration may keep the pH close to normal values (Ref. 19).
Alkalinity
Within the usual range of pH values of rivers, from 6.4 to 8.3, the bicarbonate ion (HCO3-) is the most common carbonate species found in natural waters. Concentrations of the bicarbonate ion are strongly related to Ca++ concentrations which reflect the weathering of limestones (CaCO3) and dolomites (CaCO3, MgCO3). When these rocks are present the risk of acidification is low.The distribution of bicarbonate follows the same pattern as that of the Ca++ ion (see figure). In streams (<100 km2), HCO3- concentrations naturally range from 0 to 350 mg L-1, while in major rivers (>100 000 km2), the concentration ranges from 10 to 170 mg L-1 (Ref. 4).
Acidification
The natural acidity of rain water is increased by the presence of sulphur dioxide (SO2) and nitrogen oxides (NOX) which are atmospheric pollutants originating mainly from fossil fuel combustion. These compounds are likely to be carried by winds over long distances from urban, mining, thermo-electric power plants and industrial emission sources. During rainfall the acidic pollutants are washed out as sulphuric and nitric acids over vast areas and may affect pristine areas located hundreds or thousands of kilometres away from pollutant sources. Acidified waters are characterized by a major decrease in biological density and diversity.Regional areas at risk from acid rain have been estimated by combining both the source areas (use of sulphur-bearing coal, major cities, oil refineries, various industries) and the occurrence of sensitive soils found in wet and humid regions. Most crystalline shields and non-carbonated sedimentary rocks can be considered as being sensitive to acid precipitation. The presence of geologically sensitive areas downwind of existing major emission sources lead to three problem areas where acidification is a major issue: Southern Scandinavia, North-Eastern USA / Eastern Canada, and China. Projected rapid increases in emissions creates potential future problem areas in Nigeria, India, Venezuela, Southern Brazil and South-East Asia.
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