Transit Times and Reaction Rates: Coupling Hydrologic and Geochemical Perspectives in a Montane Watershed in the Central Sierra Nevada

Abstract

Water-rock interactions are drivers of silicate weathering fluxes in the critical zone and are sensitive to the residence times of groundwater in the subsurface. The chemical reactions that occur when rocks are exposed to water are largely controlled by the timescale of such interactions and pathways that water takes through the subsurface. Characterizing such water-rock interaction timescales remains a challenge as they occur mostly in the subsurface where direct observations are difficult to obtain. The incorporation of more advanced geochemical tracers of silicate weathering (δ³⁰Si and Ge/Si ratios) over traditional tools, such as concentration discharge (C-Q) relationships, can offer more insight into water-rock interaction behavior and reaction timescales driving observed stream dissolved solute fluxes. This study employs the use of δ³⁰Si and Ge/Si ratios on stream samples collected from Sagehen Creek, a small (27 km2) montane catchment in the Central Sierra Nevada, in order to better constrain the extent of weathering reactions with relation to groundwater residence times.

Sagehen Creek has a mediterranean climate of which 80% of its annual precipitation (850mm) falls as snow. Multiple studies have indicated that stream flow in Sagehen creek is largely groundwater dominated. The average ages of groundwater that support streamflow are decadal, indicating that streamflow is supported by waters with deep flow paths and long water-rock interaction timescales. Stream samples collected during the summer of 2023 (n=18) show a range of δ³⁰Si between 0.91 to 1.24‰, which is elevated compared to the lithology of the basin (-0.2‰). Ge/Si ratios are low (ranging from 0.23 to 0.33 pmol/µmol) with Ge concentrations ranging between 2.5 to 5.0 parts per trillion. Both δ³⁰Si and Ge/Si values suggest that stream waters in the basin are strongly controlled by incongruent weathering reactions. The enriched values of stream water δ³⁰Si and low values of stream Ge/Si imply that a substantial amount of Si released from primary minerals is being taken up in the formation of secondary minerals (clays). Our preliminary results imply a stream chemistry reflecting a system that is in a state close to chemical equilibrium where water-rock interactions are at sufficiently long timescales for highly evolved reaction progress.