Heterogeneous geochemistry of catchment acidification

Abstract

Many ions in catchment runoff are primarily controlled by heterogeneous reactions. The mass law governing heterogeneous equilibria leads directly to a simple, general expression for catchment acid buffering, which can be estimated from bulk catchment runoff composition. This analysis requires no particular assumptions or data regarding catchment minerals, water flowpaths, reaction pathways, or equilibrium constants. Thus, a catchment's vulnerability to acidification can be assessed directly from its runoff chemistry. Critical tests using data from four intensively studied catchments (Hubbard Brook, USA, and Sogndal, Risdalsheia, and Birkenes, Norway) reveal catchment-scale buffering behavior that is quantitatively comparable to that predicted from bulk runoff composition. Theoretical predictions and field data both indicate that reversing acidification at the acidified Norwegian catchments would require large decreases in sulfate concentrations. Long-term acidification will occur where leaching of base cations, accelerated by add anion loading, exceeds resupply from mineral weathering. Where the rate of depletion of adsorbed base cations can be estimated, the resulting acidification of runoff can be predicted analytically. Base cation leaching, partly offset by mineral weathering and acting in conjunction with declining acid anion loading, appears responsible for the long-term decline in base cation concentrations at Hubbard Brook. A general method is proposed for evaluating the effects of spatial variability in catchment buffering mechanisms. Spatially averaged catchment runoff will generally yield whole-catchment buffering predictions that closely approximate the spatially averaged buffering properties of a collection of geochemically diverse source regions. Mechanistic prediction of catchment buffering does not require identifying discrete source regions, characterizing their individual geochemical properties, or tracing the hydrologic flowpaths that connect them. This considerably simplifies the problem of predicting whole-catchment buffering behavior.