Novel Geochemical Tools for Characterizing Hydraulically Active Fractures

Strong preferential flow paths, very low porosity, low storage, and high degrees of flow anisotropy make rock and aquifer properties of fractured bedrock very different from classically studied aquifer materials (uncemented and cemented clastic sediments). As a result, classic aquifer characterization and analysis tools more appropriately applied to porous media do not apply to fractured rock aquifers. The characteristics of hydraulically active crystalline rocks are primarily controlled by weathering and structure. Several studies have identified the importance of residence time, rather than physical structure as a controlling mechanism of weathering reactions, and that the relative analyte concentrations released vary depending on the system. This suggests that a correlation can be made between weathering product concentrations and the presence of advective flow within a fractured bedrock aquifer.

This project used modeling to simulate weathering under varying residence times and flow lengths to provide expected reaction product concentrations for correlation with the laboratory testing results. Specifically, the goal was to determine the minimum contact time of the fracture water with the mineral surface to result in measureable concentrations of weathering products. For this study biotite was the mineral used to represent the weathering of the fracture surface, and resulting weathering products of potassium, ferric iron oxide, sodium, magnesium, and silicic acid. Geochemical modeling of the weathering reactions and transport of the weathering products was completed in one dimension using CrunchFlow (Steefel 2009), a multicomponent reactive flow and transport model. The CrunchFlow simulations focused on the signatures of the water being transported by the fracture and the mineral weathering on the fracture surface. The results of this modeling effort were able to confirm the importance of residence time. However, further modeling is needed using kinetically controlled reactions in combination with the advective transport to better understand the timing of constituent release.