Exploring riverine solute behavior, dynamics, and export into the Arabian Sea from a semi-arid catchment, Western India

The geochemistry of rivers draining into the Arabian Sea has received lesser attention than their eastern counterparts draining into the Bay of Bengal. The major ion concentrations in the third-major river (Mahi) draining into the Arabian Sea have been reported. Hydrochemistry of seasonally collected samples (n = 67) during two consecutive years reveals that the average solute export and the cationic charges are similar to 1.2 and similar to 4 times their corresponding global average values. Generally, lower solute levels (except Ca) in the rainy season reflect the dilution effect. In contrast, the higher intensity of carbonate weathering out-weighing the dilution effect, aided by CaCO3 precipitation in summer, dominates the Ca dynamics. Chemical weathering in the catchment is mediated predominantly by carbonic acid, as evident from the alkalinity/(Ca + Mg) and SO4/(Ca + Mg) molar ratios. Using a forward model, the estimated average contributions from various sources to the cationic composition are atmospheric (5 +/- 1%), evaporites and soil salts (27 +/- 9%), silicates (39 +/- 12%), and carbonates (29 +/- 13%). Runoff exerts the dominant control on the weathering rates, with the best estimated average silicate (11 t km(-2) yr(-1)), carbonate (22 t km(-2) yr(-1)), and evaporite and soil-salt (8 t km(-2) yr(-1)) weathering rates, which sums up to a dissolved chemical flux of similar to 41 t km(-2) yr(-1). The lower-bound estimate of CO2 consumption by silicate weathering of the Mahi basin is similar to 2.8 x 10(5) mol km(-2) yr(-1), translating into CO2 consumption of similar to 9.7 x 10(9) mol yr(-1). This is similar to 0.08% of the global average value (by silicate weathering) and is disproportionately higher than its fractional continental drainage area (similar to 0.03%). Integrating the data of Mahi with those available for the other two major rivers (Narmada & Tapti), this study underscores the prominent role of these rivers in supplying solutes to the Arabian Sea and controlling the biogeochemistry of the Arabian Sea.