Chapter 7 - Chemical and isotopic variability of Bhagirathi river water (Upper Ganga), Uttarakhand, India

Abstract

High-altitude Himalayan ecosystems are the water tower of Asia and provides water for more than 1 billion population. The Ganga, one of the longest rivers in India also originates from such high-altitude mountainous region of Gangotri Glacier. Bhagirathi river is one of the largest and major source streams of the Ganga fluvial system. Changing temperature of the earth’s surface has the potential to affect the hydrologic and thermal regimes of river water. Here, we have reviewed and compared the changes in major ionic and stable isotopic composition of Bhagirathi river water over the past 2 to 3 decades (1989 to 2015). This review suggests that the calcium (Ca2+) is the major cation followed by magnesium (Mg2+), and sulphate (SO42−) and bicarbonate (HCO3−) are the dominant anions in the Bhagirathi river water. Significant long-term changes in major ionic chemistry, particularly calcium and sulphate were observed. Overall, the dissolved salt content of the calcium increases and a decreasing trend in sulphate content were observed. The increasing and decreasing trend in ionic concentration is more pronounced during the last one decade. The concentration of other ions, like sodium, magnesium, potassium, chloride, fluoride and nitrate are very low throughout the review period and no significant changes were observed. Unlike, in river water chemistry, long-term isotopic variability in Bhagirathi river was not observed. However, the δ18O values of river water was found to vary significantly from upstream to downstream and from pre- to post-monsoon seasons. Our review-based observation suggests that (1) recent changes in climate may be influencing the hydrological processes of the basin which in turn affecting the stream chemistry of the Bhagirathi stream water, (2) due to very high short-term isotopic variability in precipitation and mixing of different components, long term changes in stable isotopic signature of Bhagirathi river were not observed.