CAREER: Unlocking the Isotopic Signatures of Weathering Recorded in Rivers Through Isotope-Enabled Reactive Transport

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The percolation of water through the shallowest layers of the solid Earth is vital to sustaining life on this planet. Moisture delivered to soil irrigates the crops we grow and sustains natural ecosystems. Water that reaches further into the subsurface recharges aquifers and supplies the baseflow to streams and rivers. This transit through the shallow Earth also introduces water as a reactive agent capable of weathering the minerals that compose the crust, allowing chemical transformations that convert rock to soil and set the chemical signature of water resources. Thus, water is an integrator of this “critical zone”, i.e., the surface of the Earth where we live and draw resources. Such vital transformations are largely hidden from direct study because they occur below our feet in the near surface. Thus, we rely upon models that describe the rates and pathways of fluid drainage through the critical zone, coupled to the chemical reactions that occur between water, minerals and life, to predict what takes place where we cannot make direct observations. These models are then checked against more accessible samples often using the chemistry of streams and rivers which drain watersheds. Using novel instrumentation and model development, this project will offer the first direct validation of these reactive transport models based on fluids and solids collected within the transition from soil to stream. The broader impacts of this study include the improvement of predictive models for the chemical signatures of water quality and critical zone functioning. The capability to develop and deploy such models is vital to the advancement of watershed management and critical zone science. In pursuit of this goal, the project will create an open access online training platform designed to educate the next generation of Earth scientists in the development of state-of-the-art reactive transport models. The teaching modules will be deployed as part of an NSF-RCN award and will feature ADA-compliant web design with both self-guided and classroom integration options. This resource will foster the expanded use and equitable availability of next generation quantitative models and promote international collaborations across the global critical zone community.Predictive models for the tight coupling between fluid transport and chemical reactivity in critical zone weathering profiles remain largely validated against easily accessible observations such as the fluids that emerge as spring water and baseflow to streams and rivers. These integrated signatures of fluid draining landscapes offer a vital means of testing forward models and an increasing emphasis is now being placed on the use of riverine stable isotope ratios (e.g., 7Li, 30Si, 27Mg, 44Ca) for their exquisite sensitivity to specific components of chemical weathering. Integration of these promising tools into reactive transport simulations could hold the key to high fidelity predictive models for the coupling of chemistry and fluid flow draining intact weathering profiles. This proposal will generate three major advancements towards this goal: (1) quantitative interpretation of solute isotope signatures as a function of fluid travel time; (2) validation of isotope signatures in the zone of weathering between infiltration and discharge; and (3) predictive models for the weathering processes recorded by the isotope ratios of rivers draining watersheds. This will be accomplished through leveraging of a series of laboratory column experiments designed to support model development and in turn application to a novel field-scale instrumentation capability which allows direct collection of fluid samples in the partially saturated section of the critical zone where fluid drains through regolith before emerging as the baseflow to streams. The results of this work will test a set of hypotheses that link theory to observations of isotope ratios in the solutes derived from chemical weathering.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.