Acid Rain Watershed Interactions Revisited

Until recently, the scientific consensus has been that acid-rain-watershed interactions are those of processes which predominate in deep, well-drained mineral soil and the waters derived from them. In such watersheds alkaline minerals dissolve to naturally impart alkalinity to surface waters and normally acid components in rain would be neutralized. Furthermore, the sulfate of the sulfuric acid component of acid rain can be retained in the watershed by reaction with iron and aluminum oxide residues left behind by the dissolving of silicate minerals. These processes effectively minimize the impact of acid rain on the acidity of runoff. The situation is different in the watersheds of acid lakes and streams. The hydrologically most important landscape elements in such watersheds receiving acid rain in North America and Europe are highly acidic, organic rich soil and their associated vegetation. These materials are poor in alkaline minerals, metal oxide residues and runoff is poor in alkalinity; thus the acidity of such waters was asserted to be from acid rain, the consensus being that but small additions of acid can easily acidify runoff from such materials. The more water received from such landscape units the more “sensitive ” the watershed was believed to be to acidification by acid rain