Connecting groundwater age to subsurface weathering reactions at the catchment scale using silicon isotopes and reactive transport modeling

Fluid-mineral interactions taking place within the natural reactor at the Earth’s surface, the Critical Zone (CZ), are fundamental processes that regulates Earth’s surface conditions and terrestrial weathering fluxes across multiple spatiotemporal scales. Dissolution, precipitation and chemical reaction networks established through fluid-mineral interactions generally take place in the subsurface, and their extent is largely dictated by both the pathways of infiltrating water and the timescales of fluid transport. Deriving a quantitative understanding of how subsurface fluid residence times relate to weathering reaction rates remains a key challenge. This study seeks to better address this unknown by applying advanced geochemical tracers of weathering (silicon stable isotopes, δ³⁰Si) and groundwater ages tracers, along with reactive transport modeling approaches to a well-characterized natural system.

Our work focuses on Sagehen Creek basin, a small (27 km²) montane catchment situated in the Central Sierra Nevada of Northern California, USA. Sagehen Creek hosts robust, multi-decadal hydrologic and geochemical records of groundwater sourced from 12 naturally occurring springs. Over the course of a water year, > 80 spring water samples were collected at a bi-weekly frequency to develop a comprehensive geochemical (δ³⁰Si and dissolved solutes) and groundwater age tracer (CFCs, SF₆) dataset. Preliminary results from the field data show spring δ³⁰Si signatures to exhibit a strong correlation with groundwater ages over decadal timescales where the oldest springs have the lowest δ³⁰Si (+0.16 ± 0.08 ‰) and the youngest, the most elevated δ³⁰Si (+1.45 ± 0.07 ‰). This result suggests that weathering reaction progress varies as a function of mean groundwater ages and evolving transit time distributions (TTDs). A series of 1D isotope-enabled reactive transport models (RTMs) were developed to identify the major hydrogeochemical factors underlying the observed relationship between δ³⁰Si and groundwater ages. The leading framework generated from our preliminary RTM efforts centers on secondary mineral precipitation reactions and stable isotope equilibration. Younger groundwaters reflect early reaction progress dominated by active secondary mineral precipitation, which produce elevated δ³⁰Si due to kinetic effects. Older groundwaters on the other hand, reflect late stage, (near)equilibrium conditions for secondary mineral reactions, facilitating continued isotope exchange between minerals and the surrounding fluids, and thereby producing low δ³⁰Si values. Together, these preliminary results provide new constraints on the links between subsurface fluid residence times, weathering reaction progress, and solute generation in catchment-scale CZ systems.