Geothermal fluxes of alkalinity in the Narayani river system of central Nepal

Abstract : Numerous hot springs flow within the steeply incised gorges of the central Nepal Himalayan front. The spring fluids have total dissolved solids (TDS) up to 7000 mg/L and Na⁺, and K⁺ typically comprise >50% of the cationic charge, indicating that high-temperature silicate alteration is the dominant source of hot spring alkalinity. HCO₃^- is normally the dominant anion. Sr isotope ratios from the hydrothermal fluids are similar to the range of values found in the host rocks and imply significant fluid-rock interaction with local lithologies. To determine the impact of the hydrothermal solute load on the local and regional river chemistry, we use a chemical mass balance approach to quantify the hot spring discharge. The springs are ubiquitously enriched in germanium (Ge) with high but variable Ge/Si. Himalayan rivers upstream of the hot spring zones have Ge/Si systematics like other unpolluted rivers, but downstream they are highly anomalous, with Ge/Si from 2 to 20 μmol/mol. Ge and Si appear to behave conservatively during mixing of spring and river, and the large disparity between river and spring [Ge] and Ge/Si ratios makes germanium an effective tracer of hot spring input. We use the Ge/Si mass balance to estimate the spring flux to individual river systems. Our results show that the premonsoon spring flow over the entire Narayani basin is about 2 m³/s (with a factor of 2 uncertainty), or 0.5% of the total Narayani river discharge. We estimate that the springs provide 25 (±15)% of the silicate-derived alkalinity to the Narayani system during the low-flow season from October to May. Available monsoon season data indicate that the spring flux increases during the monsoon by a factor of 2-3, but this increased flow is diluted by the up to 10× increase in overall river flow. The annual river discharge-weighted mean spring flux is 3.0 ± 1.2 m³/s for the Narayani; hydrothermal alteration contributes ∼10% of the annual flux of silicate alkalinity to this large river system.