Predicted response of stream chemistry to acid loading tested in Canadian catchments

Abstract

SHORT-TERM acidification of lakes and streams can cause bio-logical damage by lowering pH and increasing concentrations of inorganic aluminium^1–4. Storms laden with acids and sea salts, rapid melting of acidic snow and remobilization of acids stored in catchment soils can cause episodes of acidification lasting from hours to months. These episodes can help to reveal the mechanisms that regulate catchment runoff chemistry^5–7. Here we use extreme, climatically triggered acidification episodes in 18 intensively monitored streams in Canada to test a geochemical theory⁸ that predicts the chemical response of catchments to changes in acid loading. At all 18 catchments, changes in base cation (Ca²⁺, Mg²⁺, Na⁺, K⁺, NH⁺₄) concentrations offset about 75–95% of the observed changes in acid anion (SO²⁻, NO⁻₃, Cl⁻, OA⁻) levels; increases in hydrogen and aluminium ions and decreases in bicarbonate accounted for the remaining 5–25%. In response to equal acid anion increases, however, some catchments released over 35 times more H⁺ or 50 times more inorganic aluminium than others. The observed chemical responses to shifts in acid anion loading agreed with a priori geochemical predictions derived⁸ from the chemical composition of runoff, indicating that catchment vulnerability to acidification can be assessed, in advance, directly from surveys of lake and stream chemistry.