High Resolution River Chemistry Timeseries Reveal Hydro-Climatological Controls on Weathering Fluxes Across the Himalayan Mountain Range

Understanding chemical weathering process and its controlling factors is important for interpreting hydrochemical dynamics in a catchment system. This study analyses solute chemistry, river discharge, and precipitation data from the Kali-Gandaki, a trans-Himalayan River, over four annual cycles. River samples were collected weekly at two sampling locations, upstream (Lete) and downstream (Purtighat), complemented by grab samples from tributaries, springs and main river covering the entire catchment. Concentration-discharge relationships for most elements, including Na⁺, Ca²⁺, Mg²⁺, Li⁺, Sr²⁺, SO₄^2-, Cl^-, and HCO₃^-, show dilution behaviour during monsoon with highest elemental concentrations during baseflow season. However, K⁺ and Si concentrations exhibit chemostatic behaviour downstream. The Ca/Na and Sr/Na ratios increase with increasing discharge in Lete, indicating carbonate mineral dominance during high flow, while Sr/Na ratio decreases downstream, signifying an increased contribution of solutes from silicate rocks. Weathered solute budget from inverse modelling reveals seasonal and temporal variation, with the highest proportions of cations derived from carbonate and silicate occurring during the monsoonal period. Additionally, the relative contributions from these sources increase from upstream to downstream. Carbonate weathering rates at Lete and Purtighat are significantly higher than silicate rates, calculated  at (40±8) x10⁴ tons km^-2 yr^-1 and (73±8) x10⁴ tons km^-2 yr^-1 respectively, compared to silicate weathering rates of (2±0.1) x10⁴ tons km^-2 yr^-1 at Lete and (7±1) x10⁴ tons km^-2 yr^-1 at Purtighat. Weathering rates strongly correlate with discharge, explaining their peaks during the monsoon as well as in downstream. Moreover, the chemical weathering rates also show weak but positive trends with rainfall amounts. On the other hand, the consistent counter-clockwise hysteresis patterns of all elements (except SO₄^2- and Sr²⁺ in Purtighat), reveal the role of groundwater contribution. Groundwater has long transit times, with extended water-rock interaction periods, resulting in more dissolved elemental concentration compared to the rising limb of fast responding peak discharge. The effect of groundwater can also be supported by the relationship of elemental ratios, such as Na/Si and Li/Si which consistently show decreasing trend with increasing discharge across both sites. During high runoff, the river receives a greater proportion of water with shorter transit times, such as direct surface runoff or shallow subsurface flow. These waters have had limited time to interact with minerals, resulting in lower Na and Li concentrations relative to Si. However, after peak discharge, increased groundwater contribution allows prolonged interaction, enables the groundwater to reach chemical equilibrium with secondary silicate minerals and results in removal of silica from solution and high Na/Si and Li/Si. The findings emphasize the dual role of monsoonal rainfall in enhancing weathering processes, and groundwater contributions in maintaining elevated solute fluxes after the monsoon peak. This interplay underscores groundwater’s role as a buffer system, modulating river chemistry and ensuring consistent solute contributions across varying hydrological conditions.