Mechanisms controlling the dissolved load, chemical weathering and CO_2 consumption rates of Cauvery River, South India: role of secondary soil minerals

摘要

Hydrochemical assessment of the Cauvery River Basin (CRB), an east flowing Western Ghats (WG) river, is carried out to understand the dissolved load sources and controlling mechanisms with quantification of source-wise input to the dissolved load. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are evaluated on account of silicate basement of CRB comprising of granulites and supracrustal rocks. The source-wise solute load contributions estimated using the chemical mass balance model signify that 68 of total load is from chemical weathering followed by anthropogenic (18.5) and atmospheric (13.5) inputs respectively, indicating that chemical weathering is the major solute load controlling mechanism for CRB. The intensity of silicate chemical weathering occurring in the CRB is measured by degree of weathering index (Re). It is found to be higher ( 3), suggesting an incomplete weathering of drainage rocks (primary minerals) resulting in formation of soils comprising of secondary minerals including oxides, alumino-silicates and clay minerals (kaolinite, montmorillonite and smectite). Detailed understanding of the mechanism of chemical weathering is carried out using Ca/Na and Mg/Na elemental ratios of different end-members including primary minerals from rocks and secondary minerals from soils. The Na-normalized mixing diagram reveals that chemical weathering of secondary minerals is dominating and the solute load contribution to the total dissolved load is significantly higher (35.5) from secondary minerals than primary minerals (23.5). The SWR and associated CCR are estimated to be 13 t km(-2) year(-1) and 3.3 x 10(5) mol km(-2) year(-1), respectively, at outlet (Musiri) of the CRB. Results additionally signify that SWR of the east flowing WG river, Cauvery, are several times (similar to 4) lower than the average SWR of west flowing WG rivers even though the associated CCR are comparable for both river systems.