THE DEPENDENCE OF CHEMICAL WEATHERING RATES ON PHYSICAL AND GEOCHEMICAL HETEROGENEITIES

Mineral chemical weathering plays an important role in earth system formation and functioning. Chemical weathering rates measured in field studies have been reported to be orders of magnitude lower than those measured under well-mixed batch reactors. Physico-geochemical heterogeneities can contribute to such rate discrepancy as it controls preferential flow paths and the available mineral surface area for reactions to occur. Here we applied reactive transport modeling to 1) understand the dependence of chemical weathering rates on spatial heterogeneity and 2) quantify the relation between weathering rate and measureable heterogeneity parameters. Two important subsurface reactive minerals with widely different weathering rates, magnesite and plagioclase, were considered. Their mineral abundance distributions were inversely correlated with hydraulic conductivity distribution. Multiple realizations of heterogeneous hydraulic conductivity distributions were generated by the Gaussian sequential simulation method, differing in length scale, hydraulic conductivity variance σ_k², and correlation length in the longitudinal and transverse directions (λ_L and λ_T). Chemical weathering rates of both magnesite and plagioclase were shown to be a function of σ_k² and anisotropy ratio of λ_L and λ_T. Large σ_k² and anisotropy ratio resulted in smaller weathering rates. Sensitivity analysis indicated that this dependence is highly related to the length scale, flow velocity and mineral kinetics, which can be integrated by Damköholer number (Da). The dependence of weathering rates on spatial heterogeneities weakened with increasing Da in specific scales of interests when Da values are smaller than a critical value. Beyond the critical Da value, the heterogeneity effects are negligible.