Temporal and spatial heterogeneity of mineral dissolution rates in fractured media

Abstract

Physical heterogeneity in the subsurface creates preferential flowpaths that have the potential to impact mineral dissolution rates. Previous studies using numerical simulations have found that physical heterogeneity can reduce the mineral dissolution rate by an order of magnitude relative to homogeneous simulations. These findings indicate that the long-studied difference between laboratory dissolution rates, and field dissolution rates could be the result of processes caused by heterogeneity in the subsurface, specifically variable fluid flowpaths controlling the approach to fluid saturation. In this study, we investigate the behavior of mineral dissolution rates in heterogeneous fractured rock domains through time. Domains containing albite and non-reactive quartz evolve through 1 million years as rainwater percolates through the system and allows for albite dissolution and secondary mineral precipitation. Fracture density and orientation vary in the domains to determine the importance of fracture topology on mineral dissolution rates. In addition, the volumetric flow rate through each domain is systematically varied to determine the impact of changing flow conditions relative to reaction rates.Temporal analysis of the simulation results indicates that domain-averaged dissolution rates decrease with time, similarly to what has been observed in field systems and long-term laboratory experiments. Spatial analysis of the mineral dissolution rates throughout the simulations, indicates that fractures remain reaction limited through most of the simulation, while the matrix is transport limited. Since most of the domain consists of matrix, the domain is transport limited as a whole. However, speciation of the flux-weighted fluid leaving the domain indicates that the fluid is undersaturated with respect to albite, potentially leading to an interpretation that dissolution within the domain is not transport limited. The direct comparison of the spatial distribution of rates and the integrated flux signal appears contradictory, however, this approach reveals that the transport-limited signal that persists through most of the domain is diluted by the higher volume of water that leaves the system through fast-flowing fracture pathways compared to the mass of solutes diffusing out of disconnected portions of the domain. This parsing of the domain into transport-limited regions and kinetic-limited regions, induced by physical heterogeneity, has the potential to further explain the differences between laboratory dissolution rates and field dissolution rates. It is possible that mineral weathering is dominated by transport-limited conditions in many field systems with disconnected fluid pathways where reaction rates are slow due to a buildup of solutes, despite integrated signals in streams indicating far-from equilibrium conditions for dissolution reactions of primary minerals.