Alkalinity generation

This chapter summarizes water chemistry changes and effects of acidification on biogeochemical processes. We focus on major ions and nutrients, discuss internal alkalinity generation and sediment ion-exchange processes, and present preliminary recovery models. Results for trace and minor metals and other chemical constituents are presented elsewhere (2-4). 

If we omit the ions contributing little or no net gain (i.e., Fe2+, NH4+, and probably Mn2+) to the alkalinity generated in the hypolimnion during August 1990, the decrease in sulfate between the epilimnion and hypolimnion was responsible for about 50% of the increase in alkalinity the increase in Ca2+ was responsible for 30%. Increases in Mg2 and K+ contributed 7 and 6%, respectively. The increase in Al3+ contributed about 5%, but this cannot be considered to be mitigative because of the toxicity of aluminum to aquatic biota. The relative contributions of each ion to the hypolimnetic alkalinity are similar to the relative contributions to whole-basin alkalinity as determined by ion budgets. 

Results of the four models (Figure 9a) illustrate the effect on predicted recovery rates of including various alkalinity-generating processes. Models 1 and 2 probably yield upper and lower limits of the time required to recover to the preacidification alkalinity level. Model 3 probably yields an underestimate of recovery time, in that it does not consider the need to neutralize acidified surficial sediments (and restore base cations on sediment-exchange sites that have been lost during the last years of acid loading). Model 4 probably yields the most accurate estimate of recovery time, but it does not provide a functional relationship for the cation-production term. Based on Model 4, the north basin will reach 50% of the preexperimental alkalinity concentrations in 3-5 years and 90% in 8 years. Complete recovery is predicted to occur in 12.5-15 years. 

The ferrous iron supply stems from reductive dissolution of ferric oxides, another alkalinity-generating process ... 

Several whole-lake ion budgets have shown that internal alkalinity generation (IAG) is important in regulating the alkalinity of groundwater recharge lakes and that sulfate retention processes are the dominant source of IAG (3-5)1 and synoptic studies (6-9) have shown that sulfate reduction occurs in sediments from a wide variety of softwater lakes. Baker et al. (10) showed that net sulfate retention in lakes can be modeled as a first-order process with respect to sulfate concentration and several "whole ecosystem" models of lake acidification recently have been modified to include in-lake processes (11). 

Watzlaf, G. R. and R. S. Hedin. 1993. A method for predicting alkalinity generated by anoxic limestone drains. In Proceedings of the 1993 West Virginia Surface Mine Drainage Task Force Symposium, April 27-28, Morgantown, WV. 

Sigg, L., Johnson, C. A., and Kuhn, A. (1991) Redox Conditions and Alkalinity Generation in an Anoxic Lake, Mar. Chem. 36, 9. 

Wetlands of humid climatic zones often emit H2S, as is evident from the rotten-egg odor of marshes and swamps. As long as the dominant exchangeable base cations are Ca " and Mg ", which is the case in most freshwater wetlands, H2S formation should not cause the soils to become strongly alkaline. In these nonsodic soils, alkalinity generated by reduction forms precipitates of Ca (and Mg) carbonates. The low solubility of these carbonates prevents the pH from rising much above 8. In sodic soils, however, reaction 7.64 causes alkalinity to build up in the form of soluble Na carbonates (see Chapter 8, Section 8.1). 

Thiamine Alkaline generation methods Alkaline degradation to and then acidification H2S... 

Norton S. A., Cosby B. J., Fernandez 1. J., Kahl J. S., and Church M. R. (2001) Long-term and seasonal variations in CO2 Unkages to catchment alkalinity generation. Hydrol. Earth Syst. Sci. 5, 83-91. 

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