MODEL FOR SIMULATING ROCK-WATER INTERACTIONS IN A WEATHERING PROFILE SUBJECTED TO FREQUENT ALTERNATIONS OF WETTING AND DRYING

Physically based equations for unsaturated groundwater flow and solute transport have been coupled with kinetic rate laws for mineral dissol ution-precipitation, and mass balance/mass action equations for aqueou s species, in a numerical model that is capable of simulating rock-wat er interactions in a weathering profile subjected to fluctuating bound ary conditions. A numerical experiment was conducted to demonstrate ho w incipient soil development may proceed in a warm subhumid environmen t. The simulation involved a hypothetical coarse-textured parent mater ial that was subjected to frequent wetting and drying during an annual water cycle. The hypothetical weathering profile evolved rapidly; dis solution of primary minerals (enstatite, forsterite, and diopside) and precipitation of secondary clay-minerals (kaolinite and Camontmorillo nite) occurred monotonically despite the abrupt fluctuations in soil-m oisture content. In contrast, the activities of aqueous species and di ssolution-precipitation rates of calcite were very sensitive to the ch anging moisture conditions in the upper part of the profile. Although the simulation involved numerous simplifying assumptions, reasonable r esults were achieved and the calculated (from the model) rate of chemi cal denudation fell within the range of contemporary denudation rates determined from the dissolved loads of rivers. (C) 1998 Elsevier Scien ce B.V. All rights reserved.