Geological controls on groundwater chemistry in the Himalayan Indus River basin aquifers, India

The upper Indus River basin aquifers spanning over the Himalayan, Karakoram, Hindu Kush ranges is a vast water scares region, which have escaped the notice of the groundwater scientists until recently. The work presented here aims to decipher the processes of groundwater-rock interactions in the shallow Trans-Himalayan Indus River basin aquifers of India. Located on the Indus-Tsangpo suture zone, the area provides a unique opportunity to study water-rock interaction processes in one of the coldest and highest inhabited regions of the world.

Alkaline to circumneutral groundwater are collected from wells mostly located in the meta basics and associated volcanoclastic of the Dras Volcanics (DV) and the granitoids and their extrusive equivalents of the Ladakh Plutonic Complex (LPC). Waters mostly belong to Ca-HCO₃ and Ca-Mg-HCO₃ facies. Ca-Na-HCO₃ occurs as minor facies. Among bivalent cations Ca and Mg shows high degree of correlation, with a low Ca/Mg ratio. Ca-Mg-HCO₃ relations suggest bivalents come from Ca-Mg pyroxenes and calcite of the meta-basalts, and calcic plagioclase. Ca-Mg pyroxenes are sourced from the DV, while Ca-feldspars only from the LPC, given the ones in DV are albitised. Decreasing trends of calcite saturation with Ca/Mg ratio hints secondary calcite precipitation. Among monovalent ions, Na + K versus SO₄^2- + Cl^- relations suggests, waters owe their Na content to silicates or cation exchange reactions when the ratio >1, and to inputs from saline springs or compounds when the ratio falls below unity. Nearly 60% of samples have Na in excess of Cl^- (Na*), but only a minority of them correlates well with dissolved silica. However, thermodynamic calculations suggest waters are mostly in equilibrium with kaolinite, along with some Ca -, Na – smectites and in disequilibrium with all sorts of feldspars suggesting both Na- and K-feldspar weathering from both meta-basics and felsic lithologies. The absence of Na*-Si correlativity indicates simultaneous Na addition through ion-exchange processes or dissolution of non-halite hydrothermal precipitates; borax, trona, burkeite, being the most common. Lack of co-relation between Cl^- and SO₄^2- suggest dissimilarity in their provenance.  High Ca/ SO₄^2- ratio precludes inputs from gypsum or anhydrite, so SO₄^2- can only stem from sulphide oxidation or dissolution of sulphates like thenardite or jarosite, which are known to occur in vicinity of local hot springs.