Seasonal Variations of Major Ion Chemistry and Solute Fluxes of Meltwater of River Bhagirathi, a Himalayan Tributary, India

Abstract

AbstractRiver Ganga originates from the portals of the Gangotri glacier which is the largest glacier in the Ganga basin. Meltwaters of the Himalayan tributaries and streams originating from the glaciers and snow in the basin are characterised by its low ionic concentrations which dilute the solute and pollutant load in the river. Hence, any change in the quality and quantity of the Himalayan tributaries under the climate change regime will affect the quality parameters of the River Ganga as well. The stream runoff in the Himalayas tributaries has significant seasonal variations forced by the melting of the winter snow preceded by the summer monsoon rains and intervening dry periods forcing lean flows. In the present paper, seasonal variations of major ion chemistry and solute fluxes of meltwater of River Bhagirathi (River Ganga originating from glacier) have been discussed covering three consecutive melt seasons from 2014 to 2016. The results revealed that the anions for all sampling seasons in decreasing order were observed to be SO4−2 > HCO3− > Cl− > NO3−. Sulphate was the dominant anion accounting for 75.4% during rising limb of hydrograph, 75.2% during peak flow and 77.9% during falling limb of hydrograph of total anions (TZ−). Bicarbonate was the next dominant anion after sulphate accounting for 21% during rising limb of hydrograph, 22% during peak flow and 20% during falling limb of hydrograph of total anions. First possibility of acquiring SO4−2 content in the Gangotri glacier meltwater is the dissolution of sulphate minerals (gypsum and anhydrite), whereas the second possibility is sulphide oxidation. The cations for all sampling seasons in decreasing order were observed to be Ca2+ > Mg2+ > K+ > Na+. Calcium was the dominant cation accounting for 64.9% during rising limb of hydrograph, 69.1% during peak flow and 59.8% during falling limb of hydrograph of total cations (TZ+). Magnesium was the next dominant cation after calcium accounting for 19.8% during both rising limb of hydrograph and peak flow and 22% during falling limb of hydrograph of total cations. The seasonal variation of the dissolved ions in the Gangotri glacier meltwater indicates that generally dissolved ions concentration was low during the peak flow period and high during rising and falling limb period of hydrograph. Such dissolved ion concentration variations are indicative of climatic control on the intensity of different processes operating in the study area. Solute fluxes in glacial meltwaters usually increase with discharge in an aquatic system. The largest solute fluxes are transported by the most dilute meltwater as solute concentration is usually inversely related to discharge. Fluxes of all major dominating cations (Ca+2 and Mg+2) and major dominating anions (HCO3− and SO4−2) follow the same trend as that of discharge. Suspended sediment concentration and load are low during the low discharge period (falling limb) and high during the high discharge period (monsoon + Glacier melt). These seasonal variations in suspended sediment concentration during the study period may be attributed to the progression of subglacial drainage network development. The distributed channel system in the subglacial zone is dormant during the winter season with less water availability and high overburden pressure of glacier ice. As melt season progresses, more and more snow and glacier melt routed to the subglacial zone and the distribution channel spread to the higher elevations of the glacier. Higher sediment flux from the glacier is also indicative of good integration of the distributed system. These processes are also linked with the solute acquisition in the meltwater, which could be distinctly different for Alpine and Himalayan glacier systems.