Buffering of pH Depressions by Sediments in Streams and Lakes

Abstract

We have evaluated buffering of pH depressions in streams by sediments by examination of water chemistry data from experimental acidifications of small streams that have low acid-neutralizing capacity, low pH, and a variable history of impact from acidic precipitation. Neutralization of acidity is dominated by the release of Ca and Al from stream sediments and vegetation and by dissolution of aluminum hydroxide solids. Aluminum is released from vegetation, primarily below pH 5, by cation exchange and possibly dissolution of a poorly crystallized or amorphous Al(OH)3 phase. This cation exchange reservoir is easily depleted. Inorganic substrates release Al by dissolution of an Al(OH)3 phase. The reservoir for Al(OH)3 is large in streams with a pH between 5 and 6.Lake sediments neutralize acidity in overlying lake water primarily by (1) release of Ca by desorption (cation exchange), and (2) S04 reduction and net storage of S in the sediment. Nitrate reduction may also be significant. Paleolim- nological assessment of the effectiveness of these two processes for recently acidified lakes indicates that the total increase in sediment alkalinity generation is in the range of 0 to 5 μeqL−1 lake water for typical drainage lakes. The importance of sediment-generated alkalinity increases as the ratio of lake area: watershed area increases. Batch studies and microcosm studies of alkalinity generation greatly overestimate the role of these two processes, especially base cation release from sediments.KeywordsLake SedimentBase CationStream SedimentAcidic PrecipitationAcidic LoadingThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.