Mechanisms controlling the chemical weathering flux and CO2 consumption rate of Cauvery river, South India: Role of secondary soil minerals (in weathered profiles) versus primary minerals and anthropogenic sources

Abstract
Hydrochemical assessment have been carried out for a tropical, east flowing Western Ghats river Cauvery, southern India, to understand the dissolved load sources, acquisition processes and their controlling factors. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are estimated along with quantification of source wise solute load contribution towards total solute load of the Cauvery River Basin (CRB). Atmospheric input, anthropogenic activities and water-mineral interaction processes are identified as the major solute sources. Using the chemical mass balance forward model, source wise solute load contributions are estimated to be 13%, 32%, 47% and 8% from atmospheric input, anthropogenic activities, silicates and carbonates weathering respectively. It’s found that, chemical weathering followed by anthropogenic activities are the controlling factors for the solute load of CRB with minor influence of atmospheric input. Weathering index calculated for CRB (Re > 3), suggest incomplete weathering of drainage rocks resulting in formation of secondary soil minerals along the river course. Further, detailed analysis of chemical weathering mechanisms is accomplished via end-member mixing analysis approach (EMMA) by using Ca/Na and Mg/Na ratios of different end-members including primary minerals form country rocks and secondary soil (weathered profile) minerals. End-member mixing diagram referred as modified Na-normalized Ca versus Mg, reveal that chemical weathering of secondary soil minerals is as significant as primary minerals and source wise solute load contribution to the river is almost equal from both sources primary and secondary. At outlet of the basin (Musiri), SWR and associated CCR values are estimated to be 12.9 t.km-2.y-1 and 3.3 × 105 mole.km-2.y-1 respectively. Results indicate that average SWR values estimated for the east flowing Cauvery river are several times (~ 4) lower than the average SWR values of west flowing Western Ghats rivers, even though the associated CO2 consummation rates are comparable for both river systems.
Keywords: Cauvery river, solute acquisition mechanisms, chemical weathering, anthropogenic sources, primary minerals, secondary soil minerals, silicate weathering, CO2 consumption rates.