THE INFLUENCE OF PERCOLATION RATE ON THE WEATHERING RATES OF SILICATES IN AN E-HORIZON OF AN UMBRIC ALBAQUALF

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 l arge undisturbed soil column (length 30 cm and a surface area of 113 c m(2)) was percolated with a HCl/H2SO4 solution. Percolation rates duri ng 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 t o simulate the percolate from the soil column. During the experiment w eathering rates decreased by a factor of 5.6 upon a reduction in perco lation rate. This reduction could only be partly explained by changes in saturation indices or by depletion of reactive components. The resu lts could be simulated by assuming the following relationship between weathering and percolation rate: weathering flux = weathering rate x ( H)(gamma) x (percolation rate)(beta). This phenomenon is probably caus ed by the fact that at low percolation rates water stagnates in part o f the pores and weathering rates reduce due to saturation, whereas in other pores water is quickly replaced and weathering rates remain cons tant. Results showed that the best fit to the observed data was obtain ed with a value of 0.5 for gamma and 1.2 for beta. The obtained relati onship was utilized to scale the weathering rate obtained from the col umn experiment to field conditions with lower average soil water fluxe s and to laboratory experiments (pH-stat) with much higher percolation fluxes. Estimated weathering rates for field conditions and pH stat e xperiments were close to observed values.