The influence of percolation rate on the weathering rates of silicates in an E horizon of an Umbric Albaqualf

Abstract

Weathering rates of silicates obtained from laboratory experiments are generally one or two orders of magnitude higher than field weathering rates. To obtain more information on this gap in weathering rates a large undisturbed soil column (length 30 cm and a surface area of 113 cm 2) was percolated with a HCl/H 2SO 4 solution. Percolation rates during the experiment were reduced in three steps from 0.89 to 0.15 cm d −1. A dynamic multi-layer model including transient flow, hydrodynamic dispersion, geochemical transformations and equilibria was utilized to simulate the percolate from the soil column. During the experiment weathering rates decreased by a factor of 5.6 upon a reduction in percolation rate. This reduction could only be partly explained by changes in saturation indices or by depletion of reactive components. The results could be simulated by assuming the following relationship between weathering and percolation rate: weathering flux = weathering rate × ( H) γ × (percolation rate) β. This phenomenon is probably caused by the fact that at low percolation rates water stagnates in part of the pores and weathering rates reduce due to saturation, whereas in other pores water is quickly replaced and weathering rates remain constant. Results showed that the best fit to the observed data was obtained with a value of 0.5 for γ and 1.2 for β. The obtained relationship was utilized to scale the weathering rate obtained from' the column experiment to field conditions with lower average soil water fluxes and to laboratory experiments (pH-stat) with much higher percolation fluxes. Estimated weathering rates for field conditions and pH stat experiments were close to observed values.