Seasonal disparity in the co-occurrence of arsenic and fluoride in the aquifers of the Brahmaputra flood plains, Northeast India

Arsenic (As) and fluoride (F⁻) in groundwater are increasing global water quality and public health concerns. The present study provides a deeper understanding of the impact of seasonal change on the co-occurrence of As and F⁻, as both contaminants vary with climatic patterns. Groundwater samples were collected in pre- and post-monsoon seasons (n = 40 in each season) from the Brahmaputra flood plains (BFP) in northeast India to study the effect of season on As and F⁻ levels. Weathering is a key hydrogeochemical process in the BFP and both silicate and carbonate weathering are enhanced in the post-monsoon season. The increase in carbonate weathering is linked to an elevation in pH during the post-monsoon season. A Piper diagram revealed that bicarbonate-type water, with Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations, is common in both seasons. Correlation between Cl⁻ and NO₃ ⁻ (r = 0.74, p = 0.01) in the post-monsoon indicates mobilization of anthropogenic deposits during the rainy season. As was within the 10 µg L⁻¹ WHO limit for drinking water and F⁻ was under the 1.5 mg L⁻¹ limit. A negative correlation between oxidation reduction potential and groundwater As in both seasons (r = −0.26 and −0.49, respectively, for pre-monsoon and post-monsoon, p = 0.05) indicates enhanced As levels due to prevailing reducing conditions. Reductive hydrolysis of Fe (hydr)oxides appears to be the predominant process of As release, consistent with a positive correlation between As and Fe in both seasons (r = 0.75 and 0.73 for pre- and post-monsoon seasons, respectively, at p = 0.01). Principal component analysis and hierarchical cluster analysis revealed grouping of Fe and As in both seasons. F⁻ and sulfate were also clustered during the pre-monsoon season, which could be due to their similar interactions with Fe (hydr)oxides. Higher As levels in the post-monsoon appears driven by the influx of water into the aquifer, which drives out oxygen and creates a more reducing condition suitable for reductive dissolution of Fe (hydr)oxides. An increase in pH promotes desorption of As oxyanions AsO₄ ³⁻ (arsenate) and AsO₃ ³⁻ (arsenite) from Fe (hydr)oxide surfaces. Fluoride appears mainly released from F⁻-bearing minerals, but Fe (hydr)oxides can be a secondary source of F⁻, as suggested by the positive correlation between As and F⁻ in the pre-monsoon season.