Lithological control on groundwater chemistry in the Trans-Himalayan Indus River basin aquifers, India

The trans-boundary Indus River basin aquifers span 16 million hectors across six countries.  Although extensive research on groundwater quality and quantity has been conducted in its middle and lower reaches, the high-altitude aquifers have gone unnoticed until recently. The goal of the submitted work is to understand the groundwater – aquifer matrix reactions responsible for observed water chemistry, through geochemical mass – balancing and ⁸⁷Sr/⁸⁶Sr isotopic systematics. Located at average altitude of 3500 m these shallows of the Indus Basin are devoid of any significant anthropogenic interferences, and provide a unique opportunity to study the processes of water – rock interaction.

Mildly reducing to oxidizing Ca – HCO₃, Ca – Mg – HCO₃ waters were collected from a variety bedrock (from ultrabasic to acidic and from carbonate to siliciclastic) and over-burden aquifers (fluvial, fluvio – glacial, aeolian, lacustrine). Sodium normalised mixing diagrams suggest a dual pathway of carbonate and silicate weathering.  Thermodynamic calculations show waters are nearly saturated in calcite, oversaturated in Fe oxy(hydr)oxides, and in equilibrium with kaolinite. Groundwater Sr varies from 57 to 3416 μg/L and ⁸⁷Sr/⁸⁶Sr ratios from 0.7075 to 0.7275. Groundwater Sr/Ca and ⁸⁷Sr/⁸⁶Sr ratios are significantly higher than those of typical carbonates, suggesting a dominance of silicate weathering. Scattering in Sr–solute relationships, lack of a linear trend between 1/Sr and ⁸⁷Sr/⁸⁶Sr and correlations of ⁸⁷Sr/⁸⁶Sr with other solutes and indicators of silicate weathering (SiO₂/TDS, Na + K/total cationic charge) points towards derivation of solutes from multiple silicate sources. Mass – balancing suggests, a variety of silicate minerals (serpentine, olivine, chlorite, pyroxene, and biotite, plagioclase and alkali feldspars) have weathered to kaolinite, vermiculite and illite. Groundwater ⁸⁷Sr/⁸⁶Sr ratios in granitoid, siliciclastic, and ophiolitic aquifers matches well with their aquifer matrix values establishing them as their solute sources. Strong mismatch between aqueous and solid phase ⁸⁷Sr/⁸⁶Sr signatures in basaltic aquifers suggests solutes in them is derived from more radiogenic Himalayan sources.