Tracing chemical weathering dynamics during seasonal variability and storm in the Gaoping (Kaoping) and Choushui Rivers, Western Taiwan: Insight from Li isotope

Elemental compositions and lithium isotopes were analyzed in river water collected from two catchments—Gaoping River (GPR) during seasonal changes in 2016–2017 and an intense rainfall event (IRE), and Choushui River (CSR) during a storm in 2004—to evaluate weathering intensity and water-rock (W/R) interaction time in small mountainous rivers (SMRs) under varying climatic and hydrological conditions. Both catchments exhibited a kinetically limited weathering regime, but the degree of limitation intensifies during extreme discharge events due to the shortened W/R interaction time, which restricts the extent of chemical reactions. By integrating elemental compositions with Li isotopes, we further constrain the sources of weathering-derived solutes. In addition to upstream flow and groundwater, soil pore water stored in the wet season, characterized by prolonged water–rock interaction and elevated δ⁷Li, emerges as a potential contributor to weathering fluxes in both the GPR and CSR catchments. The influence of pore water is particularly evident in the CSR, where storm-driven discharge mobilized silicate-rich (older) solutes, as indicated by high δ⁷Li and ⁸⁷Sr/⁸⁶Sr ratios in river water. Riverine Li isotopic compositions, together with weathering indices, indicate that both the CSR and GPR systems undergo incongruent weathering with moderate to high intensity. The lack of a direct correlation between Li isotopes and CWR suggests that silicate weathering is not the sole contributor to riverine solutes. Therefore, it remains essential to disentangle the relative contributions of silicate weathering and other potential end-members (e.g., carbonate and sulfide dissolution) in shaping riverine elemental compositions. A more comprehensive source apportionment of weathering solutes is necessary to evaluate the implications of the substantial weathering fluxes from SMRs for the carbon cycle and long-term climate regulation.