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.

Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

“…The flux of a weathering-derived solute carried by a river is equal to the product of the measured flow rate (discharge) and the measured solute concentration. Characterizing the degree to which these quantities covary is important for accurately quantifying fluxes (Stelzer & Likens, 2006) and may also hold information about the underlying hydrochemical processes (Anderson et al, 1997;Clow & Mast, 2010;Diamond & Cohen, 2018;Godsey et al, 2009;Herndon et al, 2018Herndon et al, , 2015Hoagland et al, 2017;Hunsaker & Johnson, 2017;Knapp et al, 2020;Maher, 2011;Samanta et al, 2019;Torres et al, 2015;Winnick et al, 2017;Wymore et al, 2017). For example, these so-called concentration-discharge (C-Q) relationships may reflect changes in the path and/or rate of water flow through the landscape, affecting the availability of different mineral surfaces for reaction and/or the total amount of time over which reactions can occur.…”

The Role of Sulfuric Acid in Continental Weathering: Insights From Dissolved Major Ions and Inorganic Carbon Isotopes of the Teesta River, Lower Brahmaputra System

“…Major ion chemistry of the filtered samples was analyzed at IISER‐Pune following the analytical protocol of A. Samanta et al. (2019). The alkalinity concentrations of the unfiltered samples were measured using the gran‐titration method.…”

“…Galy & France-Lanord, 1999;Hren et al, 2007;Rai & Singh, 2007; A. Samanta et al, 2019;Sarin et al, 1989;Singh et al, 2005). These studies have estimated high silicate weathering rates for the Brahmaputra basin (5.5 × 10 5 mol/km 2 /year; Singh et al, 2005), which are mainly regulated by physical weathering rates of the basin and disproportionally higher weathering around the eastern syntaxes compared to that in other parts of the Himalaya (Hren et al, 2007;Singh et al, 2005).…”

“…Chemical weathering processes can be mediated by several natural acids (e.g., carbonic [H 2 CO 3 ], sulfuric [H 2 SO 4 ], nitric [HNO 3 ], and organic acids) and each weathering reaction influences the carbon cycle differently (A. Galy & France‐Lanord, 1999; Huh, 2010; Lerman et al., 2007; Perrin et al., 2008; Spence & Telmer, 2005). For instance, the weathering of silicate minerals mediated by H 2 CO 3 acts as a primary sink for atmospheric CO 2 over geological timescales (Bickle et al., 2005; Dalai et al., 2002; Gaillardet et al., 1999; A. Galy & France‐Lanord, 1999; A. Samanta et al., 2019), whereas the H 2 SO 4 ‐mediated weathering of carbonates releases CO 2 to the atmosphere (Calmels et al., 2007; Das et al., 2012; S. L. Li et al., 2008; A. Samanta et al., 2021; Spence & Telmer, 2005; Torres et al., 2014). Organic acids, another proton‐donor for rock weathering in natural systems, are formed in a closed carbon system involving atmospheric CO 2 , water, and energy (Adeleke et al., 2017).…”

Monitoring Large-Scale Inland Water Dynamics by Fusing Sentinel-1 SAR and Sentinel-3 Altimetry Data and by Analyzing Causal Effects of Snowmelt

“…As the youngest major river among the world [50], the Brahmaputra River has at least three characteristics: abundant tributaries, highly dynamic fluvial activities, and a varying river width, as shown in Figure 1b. Since the sediments of the Brahmaputra riverbed are composed of medium-to-fine sand and silt that is uniformly graded and has poor transport resistance [50], the riverbed and banks area are considerably mobile, which leads to frequent morphological changes affected by fluvial processes [50,52,53]. Therefore, river braiding, division, and shifting often happen [54].…”