Models of transport and reaction describing weathering of fractured rock with mobile and immobile water

We propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone. A macropore network is assumed to be responsible for the transport of mobile water, which removes soluble weathering products at the interface of that network and the matrix. Conditions of infrequent rainfall (A) and of very frequent rainfall (B) are separately modeled, but both lead to a volumetric weathering rate with the general form v₀ exp(-Z\ξ), where the amplitude ν₀ and the equilibration length ξ depend on the pore geometry of regolith and on parameters describing transport across the macropore-matrix interface (A) or mineral dissolution (B). This is obtained with no assumption of steady state for regolith evolution. Extrapolating these end-members into intermediate conditions, the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields ξ in a variety of regolith types in the range from centimeters to meters. The velocity of the regolith-bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock. When the residence time of fluid in the weathering zone, τ_f, is large, the bulk weathering rate R_b is inversely proportional to this time, and for small τ_f, R_b reverts to the laboratory rate. Application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite.