biological productivity show a progressive increase in [Alk] and pH values: a sequence of small Alpine lakes represents a progression from acidic waters of the source stream emerging from a snow field through several lakes of lengthening water residence time, each occurring downstream of the preceding (Schnoor and Stumm, 1986). Concentrations of inorganic solutes in water emerging from snow fields and glaciers are generally very low (Raiswell, 1984).
On a physically larger scale, streams flowing through crystalline rock terrains have as a whole lower alkalinity and pH values than rivers draining volcanic rocks. Waters in carbonate drainage basins show the highest [Alk] and pH values of the three classes (Figure 2.8).
In acidic waters, up to a pH of about 5.5, that have not (or not yet) become neutralized by chemical weathering reactions, for all practical purposes an increase in dissolved CO2 does not affect the [Alk] or [Acy] value of the solution. In less acidic and near-neutral waters, at pH≥5.5, smaller changes in dissolved CO2 may more significantly affect the [Alk] and pH values of the solution. The range of dissolved CO2 values in rivers and freshwater lakes varies from some degree of undersaturation with atmospheric carbon dioxide to values higher than the saturation by a factor of about 3, corresponding to an equilibrium PCO2 = 1000 ppmv (Kempe, 1988). The higher PCO2 values in such rivers, computed from the carbonate- and hydrogen-ion equilibria, are associated with supersaturation of river waters with respect to calcite (CaCO 3), a phenomenon variably controlled by primary productivity or discharge of carbon dioxide-supersaturated waters from the subsurface (Holland, 1978, pp. 105-107; Kempe, 1988).
The weathering releases of dissolved materials from bedrock and soil to continental waters, viewed as a process of acid neutralization and mineral dissolution, are the basis of any conceptual model that ties the surficial transport fluxes into one interactive system. An increase in input of acidity to the bedrock should, in a steady state system, result in faster dissolution rates and greater amounts of solutes transported in flow. Because the residence time of surface runoff from the continents is short, months to years, the effects of higher dissolution rates might be expected to show in the runoff on time scales comparable to those of the runoff residence times. Although historical data on dissolved loads of streams and rivers in preindustrial times are difficult to come by, some modern analogues to the magnitudes of past changes in weathering are provided by chemical surveys of lakes over the past 40 to 60 yr.
Data on the pH of freshwater lakes on the Scandinavian shield, going back to late 1930s, indicate a continuous decrease in the lake water pH by about 1 pH unit in 25 yr. This suggests, as pointed out in an earlier section, that the rates of water-rock neutralization reactions do not always keep pace with an environmental change. Another set of historical records going back to the 1920s is the result of surveys of lake-water chemistry in a population of 145 lakes in Wisconsin and a number of lakes in the Adirondack Mountains of New York (Kramer et al., 1986): during the 50- to 60-yr period since the 1920s and 1930s, there have been very small changes in the mean values of alkalinity in the two regions. In the Wisconsin lakes, [Alk] has increased by about 0.04 x 10-3 eq/liter; in the Adirondack Mountains the change was a decrease by 0.04 X 10-3 to 0.07 X 10-3 eq/liter. These numbers, compared to the [Alk] values of >10-4 eq/liter shown in Figure 2.8, indicate a very small change over a half a century of accelerating acid-producing emissions.
Another longer-term effect may reflect the stabilizing capacity of the rock-soil-water system that tends to keep the weathering release rates nearly constant on time scales of 103 to 104 yr. The rates of soil formation on time scales of 103 to 106 yr, summarized by Brunsden (1979), range from 60 to 450 mm/1000 yr for soils in subtropical regions. Slower rates of formation characterize ferralites (15 to 45 mm/1000 yr) and duricrusts on granite (9 mm/1000 yr). These rates overlap the rates of chemical weathering given in Table 2.2. Thus, on time scales of 103 to 104 yr, changes in the rates of dissolution of the bedrock, which might have been caused by combinations of environmental and biological factors, could be in part taken up in a faster development of the residual soil profiles rather than only in higher solute concentrations in runoff.
One of the major trends of industrial times is an increase in the acidity of atmospheric precipitation caused by emissions of nitrogen and sulfur oxides from burning of fossil fuels. However, an additional component of potential acidity resides in the living and dead organic carbon on land. On a global scale, from tropical to cold climatic zones, carbon density in soils increases by a factor of about three, as shown in Figure 2.9. This trend runs opposite to the declines in net primary productivity and standing crop of the plant mass, from the tropics to the tundra, over a temperature range from about 25 to 5°C (Lieth, 1975; Post et al., 1982). The opposite trends of carbon storage in soils and in the living phytomass are shown in Figure 2.9 by the curve labeled carbon density ratio. The soil/biomass carbon ratio is generally higher in colder climates than in warmer. This trend also accounts for the longer residence times of carbon in colder soils, where net primary productivity is lower than in the tropics. The soil carbon is a potential source of additional acidity, both organic and inorganic, that in a warmer climate of the future may promote a faster development of the regolith and contribute to a greater dissolved load in riverine flow.