Geochemistry,dissolved elemental flux rates,and dissolution kinetics of lithologies of Alaknanda and Bhagirathi rivers in Himalayas,India |
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Authors: | Sri Krishan Yadav Govind J Chakrapani |
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Institution: | (1) Department of Earth Sciences, Indian Institute of Technology, Roorkee, 247667, Uttarakhand, India; |
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Abstract: | Alaknanda and Bhagirathi (AB) river basins in the Himalayan region in India expose lithologies comprising mainly of granites,
low–high-grade metamorphics, shales and carbonates which, in conjunction with the monsoon rains and glacial melt, control
water chemistry and dissolved elemental flux rates. In the present study, we monitored two locations: (a) Srinagar on the
Alaknanda river and (b) Maneri on the Bhagirathi river for daily variations in total suspended sediments, major ions and dissolved
silica over one complete year (July 2004–June 2005). Based on long-term discharge data, discharge-weighted composition and
dissolved elemental flux rates (with respect to Ca, Mg, HCO3, Si) of the river were estimated. The information thus obtained has substantially added up to the existing chemical data
of these rivers and has refined the flux rates. Our high-frequency samples provide informations such as (a) water chemical
compositions that show a large temporal and spatial variation and (b) carbonate lithology that controls water chemistry predominantly.
The dissolution kinetics of various lithologies namely leucogranite, gneiss, quartzite, phyllite and shale of the AB river
basins were studied through batch experiments at controlled temperature (25 and 5°C) and pH (8.4) condition. In laboratory,
these lithologies undergo slow rates of dissolution (10−13 to 10−15 mol/m2 s), while field weathering rates based on dissolved elemental flux rates in the AB rivers are much higher (10−8 to 10−9 mol/m2 s). Extremely high physical weathering rates in AB rivers, which enhance chemical weathering significantly, mainly attribute
this wide discrepancy in laboratory-derived rates of representative basin rocks and dissolved elemental fluxes in the field.
However, laboratory-simulated experiments facilitate to quantify elemental release rates, understand the kinetics of the dissolution
reactions, and compare their roles at individual level. |
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