Abstract

Inferences of weathering rates from laboratory and field observations suggest significant scale and time‐dependence. Preferential flow induced by heterogeneity (manifest as permeability variations or discrete fractures) has been suggested as one potential mechanism causing scale/time‐dependence. We present a quantitative evaluation of the influence of preferential flow on weathering rates using reactive transport modeling. Simulations were performed in discrete fracture networks (DFNs) and correlated random permeability fields (CRPFs), and compared to simulations in homogeneous permeability fields. The simulations reveal spatial variability in the weathering rate, multidimensional distribution of reactions zones, and the formation of rough weathering interfaces and corestones due to preferential flow. In the homogeneous fields and CRPFs, the domain‐averaged weathering rate is initially constant as long as the weathering front is contained within the domain, reflecting equilibrium‐controlled behavior. The behavior in the CRPFs was influenced by macrodispersion, with more spread‐out weathering profiles, an earlier departure from the initial constant rate and longer persistence of weathering. DFN simulations exhibited a sustained time‐dependence resulting from the formation of diffusion‐controlled weathering fronts in matrix blocks, which is consistent with the shrinking core mechanism. A significant decrease in the domain‐averaged weathering rate is evident despite high remaining mineral volume fractions, but the decline does not follow a  urn:x-wiley:00431397:media:wrcr22368:wrcr22368-math-0001 dependence, characteristic of diffusion, due to network scale effects and advection‐controlled behavior near the inflow boundary. The DFN simulations also reveal relatively constant horizontally averaged weathering rates over a significant depth range, challenging the very notion of a weathering front.

Citation

Pandey, S. And Rajaram, H. (2016): Modeling the influence of preferential flow on the spatial variability and time-dependence of mineral weathering rates. Modeling the influence of preferential flow on the spatial variability and time-dependence of mineral weathering rates. DOI: 10.1002/2016WR019026