Abstract

The spatiotemporal variations in solute concentrations within a watershed can provide valuable understandings on the hydrogeochemical processes in the system, e.g. the residence time of the water, retention of contaminants and weathering rates of minerals. However, such understandings are often prevented by the complexities in the process coupling and challenges in modeling such couplings at watershed scale.

This dissertation presents an integrated approach to study this system. A fully coupled finite volume hydrological, land surface and reactive transport model: RT-Flux-PIHM has been developed. Based on Flux-PIHM, which simulates the terrestrial water cycle and the surface energy balance, the additional RT module explicitly models mass transfer and geochemical reactions, including mineral dissolution, precipitation, and ion exchange. As such, RT-Flux-PIHM is the first numerical model that provides the integration of land surface, hydrological, mass transfer and biogeochemical reaction at large scale.

The model was verified and then applied at the Susquehanna Shale Hills watershed (0.08 km2), a National Science Foundation (NSF) Critical Zone Observatory (SSHCZO). Based on existing conceptual framework on major hydrogeochemical processes and extensive measurements at the site (Herndon et al., 2015; Jin et al., 2011a), RT-Flux-PIHM reproduces the spatiotemporal evolution of solute concentrations which matched field observations.

The chloride concentration is controlled by inputs from rain and the hydrological connectivity of watershed. The watershed is well connected in the wet seasons, which allows fast flushing of chloride. In contrast, the less connected watershed in the summer sees “trapping” of chloride in less connected area. Large rainfall events connect the whole watershed and wash out these “old water” pockets of high Cl concentrations – however, by the time the water emits at the stream mouth it is diluted significantly. This seasonal change in hydrological connectivity at the watershed scale essentially regulates the chloride concentration.

Existing studies show the slope of the log-log concentration discharge (CQ) plot of chloride is ranging from 0 to -0.25 however it is unclear what controls the variations. Numerical experiments are conducted to elucidate the hydrological controls on the Cl CQ relationship. The chemostasis of chloride is dependent on the capability of the watershed to effectively mitigate the concentration variations induced by transition between source waters, e.g., from valley floor and swale subsurface flow to upslope subsurface flow, or from subsurface flow to deeper groundwater flow. Larger water storage leads to more chemostatic behavior while larger precipitation level and coarser soil both lead to stronger dilution behavior. However, transport parameters such as macropore conductivity changed the Cl CQ slope only marginally.

Mg concentrations, however, are regulated by the interplay between clay dissolution and groundwater influx as sources and discharge as sink while ion exchange acts as the storage buffer. Faster clay dissolution in the wet season with more abundant water is accompanied by more diluted groundwater influxes to the stream at the mouth by more discharge. In the dry summer, the slower clay dissolution is accompanied by less diluted groundwater influxes at the stream mouth and lower discharge. Cation exchange buffers the Mg concentration by storing tens of times higher Mg on exchange sites than in pore water. Large rainfall events flush out significant amount of stored Mg on the exchange sites while also diluting the waters, leading to similar Mg concentrations in the stream waters in large and small rainfall events. In general, the multiple processes work together to generate the relatively consistent concentrations for both solutes.

In sum, the development of RT-Flux-PIHM enables studies on the hydrogeochemical dynamics at large scale, offering process-based modeling that integrates different processes while at the same time can separate and interrogate the importance of each mechanism.

Citation

Bao, Chen (2016): UNDERSTANDING HYDROLOGICAL AND GEOCHEMICAL CONTROLS ON SOLUTE CONCENTRATIONS AT LARGE SCALE. Doctor of Philosophy, Petroleum and Natural Gas Engineering, The Pennsylvania State University, p. 183.

This Paper/Book acknowledges NSF CZO grant support.