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.