Major and trace element geochemistry in upper Ganga river in the Himalayas, India

Abstract

Chemical weathering and resulting water compositions in the upper Ganga river in the Himalayas were studied. For the first time, temporal and spatial sampling for a 1 year period (monthly intervals) was carried out and analyzed for dissolved major elements, trace elements, Rare Earth Elements (REE), and strontium isotopic compositions. Amounts of physical and chemical loads show large seasonal variations and the overall physical load dominates over chemical load by a factor of more than three. The dominant physical weathering is also reflected in high quartz and illite/mica contents in suspended sediments. Large seasonal variations also occur in major elemental concentrations. The water type is categorized as HCO3−–SO42−–Ca2+ dominant, which constitute >60% of the total water composition. On an average, only about 5–12% of HCO3− is derived from silicate lithology, indicating the predominance of carbonate lithology in water chemistry in the head waters of the Ganga river. More than 80% Na+ and K+ are derived from silicate lithology. The silicate lithology is responsible for the release of low Sr with extremely radiogenic Sr (87Sr/86 Sr>0.75) in Bhagirathi at Devprayag. However, there is evidence for other end-member lithologies for Sr other than carbonate and silicate lithology. Trace elements concentrations do not indicate any pollution, although presence of arsenic could be a cause for concern. High uranium mobilization from silicate rocks is also observed. The Σ REE is much less compared to other major world rivers such as the Amazon, perhaps because in the present study, only samples filtered through <0.2 μm were analysed. Negative Eu anomalies in suspended sediments is due to the excess carbonate rock weathering in the source area.