Most Rocky Mountain lakes sampled by the Western Lake Survey are not acidic, although they are very sensitive to acidification. Because of a lack of historic data, indirect methods must be used to estimate past acidification. Lakes having ANC of ≤ 200 µeq L−1 are estimated to have lost no more than 5 µeq L−1 in the Bitterroot Range, 10 µeq L−1 in the Colorado Front Range, and 12 µeq L−1 in the Wind River Range. Present (1989) data are inadequate for determining whether episodic acidification occurs.
Concentrations of SO42− and CI− in dilute Rocky Mountain lakes are controlled primarily by the concentration of these ions in wetfall. Evapotranspiration, dryfall, anion adsorption, SO42− reduction, and mineral weathering rarely are half as important as wetfall in controlling the lake concentration of these ions.
Lake concentrations of major cations, silica, and ANC are not consistent with a simple stoichiometric weathering of the granitic minerals common to the lake watersheds. In particular, lake concentrations of sodium and silica are smaller than would be expected from such stoichiometric weathering. Preferential dissolution of common granitic minerals, atmospheric sources of readily weathered minerals, or trace amounts of readily weathered minerals within the watersheds may be the source of most of the major cations within the lakes.
KeywordsDust Silicate Shale Sulfuric Acid Cretaceous
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