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Arsenic contamination (>10 μg/L) has been reported in 10 out of the 15 major river basins of Bihar, affecting 22 districts and 87 blocks, with over 10 million people impacted. This study aims to comprehensively assess the spatial and temporal trends of arsenic pollution, along with the hydrochemical characteristics and mobilization factors, based on past documented data. The Ganga stem and northern river basins, especially Bhojpur district (1,861 μg/L), have the highest arsenic levels, while most southern basins report levels below 10 μg/L. The study finds that shallow aquifers (<90 m) in younger, calcareous alluvial formations of fine to medium sands show higher arsenic contamination, whereas deeper aquifers (100–300 m) in older alluvium composed of medium to coarse sands have lower contamination. Groundwater levels have steadily declined across all basins, with the most decline observed in the Kamla-Baghmati and Mahananda Basins, influencing arsenic mobilization. Contaminated aquifers predominantly have Ca–Mg–HCO3, Ca–HCO3, and Ca–Na–HCO3 hydrochemical facies. Processes such as silicate weathering, calcite and dolomite dissolution, and Fe-Oxyhydroxide reduction play a major role in shaping the hydrogeochemistry and facilitating arsenic mobilization. A well-structured research strategy is critical for addressing arsenic contamination and safeguarding public health in Bihar's major river basins.

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  • It deals with the basin-wise assessment of arsenic contamination level in Bihar.

  • It also deals with the basin-wise assessment of geological and geographical characteristics and depth-to-water level behaviours.

  • It also deals with the basin-wise assessment of hydrochemical characteristics and mobilization mechanisms of arsenic and its correlation with influencing factors.

  • It also integrates all aspects of arsenic pollution.

arsenic contamination, calcite dissolution, depth to water level, Fe-oxyhydroxide reduction process, hydrochemical facies, river basins of Bihar, silicate weathering
© 2025 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).