Understanding the Concentration-Discharge Relationship of Chloride and Magnesium in Shale Hills Using RT-Flux-PIHM

Abstract:

A number of solutes have been found to be “chemostatic” in US watersheds, meaning the concentration of these solutes only change slightly while stream discharge varies by up to more than three orders of magnitude. To understand complex hydrogeochemical processes at watershed scale, here we use RT-Flux-PIHM, a newly developed code that adds a multi-component reactive transport (RT) module to Flux-PIHM, a hydrological land-surface model. The model was calibrated using hydrological and water chemistry data at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) and was used to understand the watershed dynamics for chloride (Cl) and magnesium (Mg). Shale Hills is a V-shaped watershed with a first order stream underlain by Rose Hill shale in central Pennsylvania. Both Cl and Mg are found to be “chemostatic” in the stream water in SSHCZO. The use of RT-Flux-PIHM helps us validate and visualize this solute watershed dynamic. We found that the watershed is hydrologically more connected (between hillslope and stream) during wet spring and winter seasons, which leads to quicker release of Cl. In the dry summer, however, the watershed is much less connected and high concentration of Cl is trapped along planar hillslopes. The stream mostly drains from swales and valley flows with relatively low Cl concentrations. Large rainfall events right after summer flush out and dilute the “old water” with high Cl concentration ([Cl]). Thus, this seasonal hydrologic connectivity controls the relative stable stream [Cl] despite of changes in stream discharge. Mg is originated from clay dissolution and groundwater influx and is also buffered by cation exchange reaction, which maintains relatively uniform Mg concentration across the watershed. In the wet season, higher discharge and therefore more diluted groundwater influx is compensated by faster dissolution and quick release of Mg from cation exchange sites. The opposite occurs in the dry summer. The balance of these multiple processes maintains relatively stable [Mg] concentration explaining its chemostatic behavior. This work demonstrates the potential of RT-Flux-PIHM as an integration tool for resolving long-standing puzzles at the interface of hydrology and geochemistry: e.g., the “double paradox”.