Weathering acidic lakes

Calculating the bicarbonate concentrations with (3), it is possible to plot the measured average pH values of Table 2 against the calculated loglHCOs ]. The result is shown in Fig. 5a. For pH >5.5 a linear relationship, very close to that reported in the literature, can be observed (pH = loglHCOa ] + 11.2). However, for acid lakes the calculated bicarbonate concentrations seem to be too low. It is reported that at pH<6 the release of metals from soils or sediments as a consequence of weathering processes becomes more and more important. Consequently aluminium hydroxides can influence alkalinity. In [18] the equation for calculating alkalinity was modified as follows ... 

Acid rain increases the acidity of some types of soil, resulting in the removal of essential nutrients from the soil. The loss of nutrients adversely affects the area s vegetation, leaving trees and other plants with less resistance to disease, insects, and bad weather. Acid rain also increases the acidity of streams, rivers, and lakes, which can kill or harm aquatic life. As Figure 26-8 shows, damage to trees and to outdoor surfaces can be extensive. The acid in precipitation reacts with CaC03, the major component of marble and limestone. What products are produced by this reaction ... 

Case Study Chemical Weathering of Crystalline Rocks in the Catchment Area of Acidic Ticino Lakes, Switzerland... 

In Table 5.4 the contributions of the individual weathering reactions were assigned and combined in such a way as to yield the concentrations of Ca2+, Mg2+, Na+, K+, and H+ measured in these lakes the amounts of silicic acid and aluminum hydroxide produced and the hydrogen ions consumed were calculated stoichiometrically from the quantity of minerals assumed to have reacted. Corrections must be made for biological processes, such as ammonium assimilation and nitrification and the uptake of silicic acid by diatoms. Some of the H4Si04 was apparently lost by adsorption on aluminum hydroxide and Fe(III)(hydr)oxides, but the extent of these reactions was difficult to assess. 

Giovanoli, R., J. L. Schnoor, L. Sigg, W. Stumm, and J. Zobrist (1989), "Chemical Weathering of Crystalline Rocks in the Catchment Area of Acidic Ticino Lakes, Switzerland," Clays Clay Min. 36, 521-529. 

Sulphate, beside nitrate, is the main acid anion in acid deposition. The trend is significantly negative in 14 lakes. The absence of decreasing sulphate concentrations in the other lakes may be caused by weathering of geogenic sulphur. In fact, in most of these lakes sulphate concentrations are relatively high. 

For example, lakes in drainage basins of easily weathered soils such as calcareous rocks (calcite, dolomite, gypsum, halite) generally have high values of pH, alkalinity, total dissolved solids, conductivity, and hardness. Such is the case for lakes located in cavities formed through the gradual dissolution of water-soluble rocks (solution or Karst lakes). On the other hand if the drainage basin is in an acidic rock basin (i.e., where silicates predominate and are difficult to weather), the water has an acidic pH, low alkalinity, and low total dissolved solids. 

Stauffer R. E. (1990) Granite weathering and the sensitivity of Alpine Lakes to acid depositon. Limnol. Oceangr. 35, 1112-1134. 

Simple steady-state models have been used to determine critical loads for lakes and forest soils. The basic principle is that primary mineral weathering in the watershed is the ultimate supplier of base cations, which are required elements for vegetation and lake water to ensure adequate acid-neutralizing... 

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