Oxygen, deuterium, and strontium isotope characteristics of the Indus River water system

Understanding the sources and compositional characteristics of waters and sediments in the Indus River system is extremely important as its water availability is one of the primary factors for sustenance of the irrigation activities and the socioeconomic status of a very densely populated region of the world. Here we used stable isotopic compositions (δD and δ18O) and strontium isotopic ratio (87Sr/86Sr) in the Indus River water, its tributaries and its small streams (nallahs) in the Indian territory to understand the regional hydrology, water sources, and catchment processes (evaporation, transpiration, recycling, and mixing). The δ18O values in the Indus River system (IRS) ranges from −16.9‰ to −12.5‰ and δD from −122.8‰ to −88.5‰. The Indus River and its major tributaries (such as the Zanskar, Nubra and Shyok rivers) are characterized by relatively lower δ18O values, whereas TangTse and other small streams contributing to the Indus are relatively enriched in 18O. The local meteoric water line for the IRS was found to be δD=7.87×δ18O+11.41, which is similar to the Global Meteoric Water Line (GMWL) indicating meteoric origin of the water and insignificant secondary evaporation in the catchment. The Deuterium excess (d-excess) in the IRS varies between 6.5‰ and 14.9‰ with an average of 11.7‰, which is mostly higher than the long-term average for the Indian summer monsoon (~8‰). The higher d-excess value is because of the contribution of moisture from westerlies; a simple mass balance shows ~26% water in the main Indus channel is contributed by the westerlies originated from the Mediterranean Sea. The Sr isotope ratio in IRS varies between 0.70515 and 0.71291; wherein the Indus, and its tributary rivers Shyok and Nubra, are characterized by relatively high Sr isotope ratios (avg. 0.71086–0.71243) compared to the Zanskar and TangTse tributaries (Sr~0.709) because of the variation in silicate rock weathering component and carbonate rock weathering component ratios respectively.