Heavy metal contamination in the sediments of the Brahmaputra river

The river Brahmaputra carries around 73 million tons of dissolved material annually, which accounts for approximately 4% of the total dissolved flux into the oceans (Singh et al., 2005). The dissolved chemical load and sediment flux of the Brahmaputra River has significantly higher rates of physical and chemical weathering than other large Himalayan catchments.

However, the river sediments act as both source and sink for heavy metals. Therefore, contaminants may eventually pass through the food chain and result in a wide range of adverse environmental effects. The sediments have been contaminated by heavy metals when rocks are disintegrated through natural or anthropogenic process.

The weathering of silicates exposed on the continents is the largest sink of atmospheric CO2 on geological time scales. The weathering and mineralogical studies of sediments are helpful in understanding different sediment sources, environmental parameters influencing the weathering of source rocks, duration of weathering, transportation and post-depositional processes, element distribution pattern and evaluating the environmental conditions existing in an area.

However, the unusual phenomenon occurred in the Brahmaputra River turning its colour to muddy indicates the need to monitor water quality and metal pollution assessment (Tsering et al., 2020).

This study ‘Geoenvironment and weathering of silicate minerals in sediments of the Brahmaputra River, India: Implications for heavy metal pollution assessment’ published in the journal 'Geosystems and Geoenvironment' does a systematic assessment of the heavy metal contaminations and silicate mineral distribution due to weathering in sediments of the Brahmaputra River, which helps to assess the eco toxic potential of the river sediments.

Experimental methods

The samples were collected in pre-monsoon period using grab sampling method from fifteen locations of Arunachal Pradesh and Assam, i.e., Pasighat (S1), Guijan (S2), Raumeria (S3), Dibrugarh (S4), Dikhomukh (S5), Nimatighat (S6) Dhansirighat (S7), Kaziranga (S8), Silghat (S9), Laharighat (S10), Gagalmari (S11), Hatisila (S12), Pandu (S13), Jogighopa (S14) and Dhubri (S15) of the Brahmaputra River from upstream to downstream at a depth of 2–5 ft. from the surface of each sampling locations. To eliminate the possibility of bank materials of the local origin, special care was taken on the sample collection by collecting them as far away from the banks as possible.

The suspended particles were separated by gravimetric method using Whatman filter paper (40 μ). The wet samples were allowed to dry and the moisture contents were removed by heating the samples at temperature 110°C for 10 min.

 The heavy metals composition of sediment samples were determined using a  Philips MagiX PRO wavelength dispersive X-ray fluorescence spectrometer with a rhodium anode X-ray tube was used, which may operate at up to 60 kV and current up to 125 mA, at a maximum power level of 4 kW.  The precision and accuracy of the data is ±2%, and average values of three replicates were taken for each determination.

The oxides of major elements (SiO2, Al2O3, Fe2O3, MgO, MnO, TiO2, CaO, Na2O, K2O, and P2O5) were measured in the homogenized powdered bulk samples after drying to determine the loss of ignition (LoI) by igniting the samples at 850 °C. The degree of contamination in the sediments is determined with the help of the parameters: enrichment factor (EF), contamination factor (CF), index of geo-accumulation (Igeo) and pollution load index (PLI) using the standard methods.

Conclusion

The nature of weathering is estimated using plagioclase index of alteration (PIA), chemical index of alteration (CIA), chemical index of weathering (CIW) and index of compositional variation (ICV).

The EF suggests moderate enrichment of Cu (2.40 ± 0.50) in the Brahmaputra River sediments. Moreover, CF suggests the study area is moderately contaminated by the elements Cu, Pb, Cr, Mn and Ni which have been attributed mainly to dispersion from the mineralized zone of the upper catchment area.

The Igeo indicates the sediments are unpolluted to moderately polluted by Cu. The PLI indicates sampling sites have no overall pollution.

The full paper can be accessed here