Geochemical fractionation and bioavailability of heavy metals in the middle gangetic plain: Assessing ecological risks in water and sediment systems

The Gangetic Plain, one of the world's most fertile regions, is vital to food and water security in densely populated areas. However, metal contamination in sediments and water poses significant challenges, owing to intensified industrial and agricultural activities and periodic flooding. The ecological risks imposed by metals in the Middle Gangetic Plain remain underexplored because of limited data on their bioavailability across varying sediment depths. This study examined the geochemical fractionation, bioavailability, and potential risks of metals in sediments using the sequential extraction (modified BCR) method and assessed pollution indices and health hazards in both groundwater and Ganga River. The results revealed a greater percentage of bioavailable fractions in younger alluvium deposits, which was attributed to elevated silt and organic matter. The metal concentrations in the groundwater were influenced by hydrogeochemical dynamics, sediment texture, and anthropogenic inputs. Fine-grained sediments enriched with organic matter facilitate metal leaching from sediments under redox conditions, serving as a key process controlling metal concentrations in groundwater. Pollution indices revealed that, although the average metal concentrations remained below background levels, As and Cr concentrations exceeded the recommended threshold levels within the upper sections of the core and at depths of 27-30 m bgl. Most water samples were classified as having unacceptable pollution levels. Health risk assessments have highlighted ingestion as the primary exposure pathway, with children particularly vulnerable to toxicity and carcinogenic effects. This study offers a framework for identifying and assessing the risks of multi-metal contamination in industrial regions with shallow water bodies.