Kinetics of Weathering in Soils from a Subalpine Watershed

Abstract

AbstractWeathering reactions were studied in samples of two subalpine soils from the Emerald Lake Watershed in Sequoia National Park, California (36°35′N, 118°40′W). Rates of weathering were measured as a function of pH and ionic strength. Reaction kinetics were characterized by rapid H+ consumption, and Si, Al3+, Ca2+, and Mg2+ release in the first week. This was followed by a period in which H+ consumption and cation release were a linear function of time for up to 4 wk. Based on reaction stoichiometry and equilibrium calculations, rapid short‐term H+ consumption in these soils was related to Al solubility. The rate laws for long‐term acid consumption were approximately first order with respect to H+ concentration in the pH range 3 to 5. The consumption of H+ was independent of NaCl concentration. These results indicate that the long‐term acid‐consumption kinetics of these soils are consistent with a surface‐reaction‐controlled mechanism. Although the distribution of products was slightly different, the fractional orders of the rate laws for Ca, Si, and Mg release in response to acid addition were similar in these two contrasting soils. Based on reaction rates and solution composition, long‐term acid consumption was attributed to weathering of plagioclase or hornblende.