Appraisal of Some Current Hypotheses Describing Acidification of Watersheds

Abstract

There is an increasing awareness of the complexity of biogeochemistry and the resultant difficulties associated with modeling water chemistry and watershed acidification. The simple comparison of change in acidity over time is insufficient to prove an acid rain effect. The Henriksen model uses modern water chemistry data to estimate acidification by acid rain: base cations are the surrogate for preacidification alkalinity and nonmarine SO₄ is the surrogate for anthropic acid input. Both assumptions tend to overestimate the effect of acid rain; the errors can be particularly large for “sensitive” water. Ion exchange and SO₄ leaching models use inorganic chemistry to describe landscapes that give acid runoff. But these landscapes usually contain organic-rich acid soils and peats, hence these models overestimate the effect of acid rain by assuming that all export is ionic. “Sensitive” watersheds tend to have humic colored waters, and the assumption that waters consist of only inorganic solutions overestimates acidification. A hypothesis of the determining role of organic buffering and solubility in such cases is described and illustrated with preliminary data.