Major ion chemistry and silicate weathering rate of a small Western Ghats river, Sharavati, southwestern India

Tropical, small mountainous rivers draining granite/granitic gneiss exhibit intense weathering rates and associated carbon dioxide sequestration, which has implications on the global CO₂ budget. However, there is paucity of data from these catchments. This study aimed to understand silicate weathering rates (SWR) and CO₂ sequestration rates (CCR) in a small tropical mountainous river, Sharavati in the southwestern India. Bicarbonates, Cl⁻, Na⁺, Ca²⁺ and silica, are predominant in the river, indicating their source from catchment rocks and atmosphere. Groundwater shows a similar abundance of major ions, indicating identical sources. Intense chemical weathering due to hot and humid climate, heavy monsoonal rains and associated river discharge are the main controlling factors of major ion chemistry in the Sharavati river. The presence of clay mineral kaolinite in the catchment corroborates with the above controlling factors. The calculated silicate weathering rate (SWR) is 27 t km⁻². y⁻¹ and associated carbon dioxide consumption rate (CCR) is 3.9 × 10⁵ mol km⁻². y⁻¹. When compared to other small tropical river basins having similar climate and lithology, CCR of Sharavati is comparable to Jiuhua Mountain rivers, (South China) and twice that of Sorocaba River (Brazil). CCR is 3.9 times higher than the global average on account of the peculiarity of the terrain. It can be concluded from a comparative study of small tropical rivers, that rainfall and runoff are the main parameters controlling the weathering rates irrespective of catchment lithology.