Rock-water interactions and chemical weathering are critical factors that determine the quality and quantity of solute transport in river catchments. However, most studies in this field have been limited to temperate regions and data from the tropics and subtropics are scarce, particularly for small rivers that are more responsive to environmental and climate changes. In this study, we investigated chemical weathering, its causal mechanisms and controlling factors that determine changes in the quality and quantity of solutes in two small, tropical mountainous rivers-the west-flowing Thuthapuzha river and the east-flowing Bhavani river-that drain through contrasting climate and geologic gradients across the southern part of the Western Ghats, in India. This region is an elevated passive continental margin and an ecologically sensitive area. Hydrochemical analysis of river water revealed that the cationic and anionic concentrations in the samples are in descending order of importance Ca2+ > Na+ > Mg2+ > K+ and HCO3- > Cl- > SO42-. Principal component analysis (PCA) indicated that geogenic factors (geology, chemical weathering and leaching/erosion) have a significant role in determining the hydrochemical characteristics of the rivers. To discriminate the relative contributions of silicate weathering in the river basins, we performed a detailed analysis of the major ion ratios along with forward and inverse modelling. The weathering index computed from the molecular ratio of cations and silica concentrations suggested that silicate weathering leads to kaolinite formation in the soil profile. The carbon dioxide consumption rate (CCR) estimated for the Thuthapuzha river, which is draining the humid western flank of the Western Ghats, was 2.18 x 10(5) mol center dot km(-2) year(-1), whereas the Upper Bhavani river, which is draining the semi-arid area of the Western Ghats was 1.39 x 10(5) mol center dot km(-2) year(-1). Our study highlights the interplay of climatic factors and the geologic characteristics of the terrain that significantly influence the nature and characteristics of solute transport through the rivers in the study area.
