Temporal Variations in Water Chemistry of the (Lower) Brahmaputra River: Implications to Seasonality in Mineral Weathering

Abstract

Dissolved major ions and Sr concentrations of the Brahmaputra River at Guwahati, India, have been investigated on weekly basis for one year to understand the seasonality in weathering pattern and relative contributions from possible solute sources. Comparison of major ion data sets from present and earlier studies for this location shows no appreciable change during last ~50 years. Elemental concentrations and Ca/Na* (Na* = Na − Cl) ratio of the Brahmaputra covary (inversely) with the water discharge; the degree of seasonality, however, is less pronounced compared to other Himalayan (e.g., Ganga and Salween) rivers. The monthly averaged Ca/Si ratios of the Brahmaputra (3.7 ± 0.2), which is lower by ~2 times than those reported earlier for the Ganga outflow (6 ± 1), show minimal (~6%) seasonal changes. Seasonal variability in Na*/K with higher ratios during non‐monsoon period has been attributed to proportionally higher Na supply from hot springs and/or alkaline salts. The silicate‐derived cations (Cats) and Sr (Srs) have been estimated using an inversion method. Although these estimated values broadly show seasonal changes, the average Cats and Srs values for the monsoon (26 ± 4% [Cats]; 26 ± 6% [Srs]) and non‐monsoon (27 ± 3% (Cats); 24 ± 4% (Srs)) seasons are statistically same. These estimates indicate a weak runoff‐weathering linkage for the Brahmaputra river. Outcomes from this study suggest that the chemical weathering intensity of this basin is more dominated by regional rapid weathering around the eastern syntaxis than the climatic (runoff) parameters.