Tracing hydrological, lithological and geothermal sources of Himalayan river system: a case study from the Teesta River Basin

The Teesta River Basin is situated in the northeastern Himalayas and is known for its stunning natural features, including diverse ecosystems, glaciers, hot springs, and perennial river streams. The composition of isotopes (δ¹⁸O, δD, and ⁸⁷Sr/⁸⁶Sr) can serve as potential indicators of fluvial and lithological feeds as the river flows from the high-altitude glacial sources to the downstream of a basin. This study employs a quantitative approach to analyze the sources of water in the Teesta River system, focusing on its flow, lithological composition, and geothermal origins. A Monte Carlo-based three-endmember isotope mixing model is constructed to determine the contribution of source endmembers to the discharge of the river. Results show a maximum mean contribution from groundwater (48 ± 6.0%) with other inputs from rainfall (37 ± 4.3%) and snow/glacial melt (15 ± 5.8%). A state-of-the-art basin-specific Monte Carlo-based four-endmember strontium (Sr) model is also developed to segregate the contribution of dissolved Sr flux from different lithologies. The model shows the maximum mean contribution of Sr from silicate lithologies (177 ± 138.2 nm). The river samples collected from glaciated locations show maximum Sr contribution from carbonate lithologies. Moreover, two natural thermal springs at North Sikkim are investigated for stable isotopic composition and heavy metals. Arsenic contamination (67.05 ppb) with a high amount of tin (2095.35 ppb) is found in the thermal spring near the Changme glacial snout, which hints at an isolated geothermal system with an arsenic source.