Exploring the impact of heterogeneity on weathering in fractured rocks

Abstract

Fractured rock is ubiquitous throughout the Earth's surface and near-surface environments. Fractures act as preferential flow pathways through an otherwise relatively impermeable medium, delivering reactive fluids and transporting solute through the subsurface. The ubiquity of fractures in the natural environment and the important role they play in rapid delivery of fluid to the subsurface makes flow, transport, and reaction in fractures an important field of study.

This study uses numerical simulations to better understand how heterogeneity associated with fractures impacts mineral dissolution rates. In addition, the rate of mineral dissolution impacts the extent of reaction front propagation, which has important implications for anthropogenic use of fractured rocks as repositories for geologic storage of CO2 or nuclear waste. In 2 and 3 dimensional models, the rate of mineral dissolution is impacted more by the fluid flow rate than the structure of the subsurface fracture pattern, additionally, the reaction over geologic timescales becomes transport-limited. The findings from the numerical studies are then applied to a field site with unique subsurface structures across a north and south-facing hillslope in the Gordon Gulch catchment of the Boulder Creek Critical Zone Observatory. The presence of groundwater wells across the catchment allows us to determine that water draining the fractured bedrock is saturated with respect to the major minerals and therefore reaction is transport-limited at the field scale. This study shows that mineral dissolution in heterogeneous domains leads to depletion of minerals in fast flowpaths over relatively short geologic timescales, but reaction over long geologic timescales occurs in protected regions, or matrix, that are transport-limited.