LITHOLOGICAL CONTROL ON GROUNDWATER CHEMISTRY IN THE HIGH-ALTITUDE INDUS RIVER BASIN, INDIA

The trans-boundary Indus River basin aquifers spans 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 understand the groundwater – aquifer matrix reactions responsible for observed water chemistry.

Low temperature and near - neutral groundwater were sampled from wells drilled into the greenschist meta – volcanics of Dras Formation (DF) and granitoids of Ladakh Batholith (LB). Dras Formation waters are of Ca-HCO₃ and Ca-Mg-HCO₃ type, while in LB they belong to the Ca-Na-HCO₃ facies. Sodium normalised mixing plots puts LB waters in the global silicate weathering field, whereas DF samples are transitional between silicate and carbonate dissolution domain. Alkaline earth cations (Ca and Mg) are supplied through pyroxene and calcite dissolution in the DF while, feldspar weathering gives way in the LB. Alkali cations, especially Na has diverse sources. It is taken to originate from silicate weathering if its molar ratios with Cl^- is greater than 1, and from mixing with hydrothermal solutions if Na/Cl^- <1. Source of Na in the DF is albitised plagioclase, and both kind of feldspars along with limited exchange of Ca for Na in the LB. Stability diagrams constructed for Na, Ca, K bearing alumino -silicate indicates waters are in equilibrium with kaolinite and disequilibrium with all feldspars and mica, suggesting their active weathering. Stabilisation of kaolinite on a large scale is unlikely under the arid climate of the study site, due to insufficient leaching activity leading to low Al³⁺ concentrations. This is attested by only a few samples exhibiting Na: 2SiO₂ molar ratios and the almost lack of kaolinite in the aquifer sediments. Feldspars are most likely to weather to smectite yielding a lower Na/Si. Samples with highest Na/Si ratios are possibly impacted by dissolution of non – halitic salts such as borax, trona, burkeite, commonly found as hydrothermal precipitates. Lack of Cl - SO₄^2- correlativity attests to their dissimilar provenance. High Ca/ SO₄^2- precludes gypsum or anhydrite dissolution as a SO₄^2- source, leaving behind sulphide oxidation or thenardite or jarosite dissolution as its source.