Understanding How Nested Reaction Fronts under Watersheds Impact Flow, Transport, and Geomorphological Evolution: the Shale Hills Example

Abstract:

Flow and transport of water and solutes within the Critical Zone are coupled to regolith formation. For example, the depth to unweathered bedrock is often related to both near-surface interflow and deep groundwater flow paths. Here, we explore the coupling of flow and reaction at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a first-order catchment developed on shale in Pennsylvania. One important chemical reaction in the watershed is weathering of clay that releases magnesium (Mg) to solution. Two major flowpaths occur in the catchment: 1) deep deoxygenated groundwater flow in the permanently saturated zone, ranging from deeper than 15 m beneath the ridge to 4 –10 m beneath the valley, and 2) oxygenated interflow which transports water through the shallow subsurface. Interflow, mostly constrained to the upper ~6 m, largely leaves the catchment as stream flow. The interflow contribution to the stream carries 80% of the Mg flux from clays. Conversely, 20% of the Mg flux from the catchment is released by chlorite oxidation at 10 to 15 m depth and leaves as deep groundwater flow. Reaction fronts for feldspar and illite lie as nested fronts above the chlorite front. The deepest reaction under the ridge, dissolution of pyrite, is attributed to diffusion of oxygen into rock at the permanent water table. This front is manifested as an increase in sulfur and ferrous iron in drill core material from just below the water table at tens of meters of depth beneath the ridge. In contrast, S and Fe(II) concentrations indicate pyrite oxidation proceeds to 6-8 m under the valley due to oxygen-rich interflow. It is likely that the depth to which oxygenated interflow water penetrates under the valley governs the dissolution rate of pyrite. Pyrite oxidation is thus the first of a cascade of reactions that lead to regolith production under the catchment. In turn, transformation of bedrock to regolith impacts porosity and permeability and water flowpaths. Geomorphological incision at the valley floor may even be initiated by the mixing of interflow and groundwater flow under the channel that causes pyrite oxidation.