Heavy metal contamination in river systems of upper Assam, India: A review

Heavy metals are toxic, persistent, and non-biodegradable pollutants that pose severe ecological and human health risks when released into aquatic ecosystems. Acid mine drainage (AMD), arising from the oxidation of sulfide minerals during coal and polymetallic mining, is a major source of heavy metal contamination in rivers. In Northeast India, particularly Upper Assam and adjoining Nagaland, coal mining has significantly impacted river systems such as Dikhow, Disang, Burhi Dihing, Dhansiri, Tirap, and Jhanji. This review focuses on the Dikhow River, where a sudden water color change and fish mortality event in April 2022 raised concerns of AMD-induced pollution, while also synthesizing available literature on other Upper Assam rivers. Findings reveal that cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), and lead (Pb) concentrations consistently exceed permissible limits, especially during the monsoon when AMD inflow is highest. The ecological implications include fish kills, loss of aquatic biodiversity, bioaccumulation in the food chain, and reduced water usability. This review highlights the urgent need for improved monitoring, metal fractionation studies in sediments, and the adoption of advanced remediation strategies including bioremediation using sulfate-reducing bacteria (SRB). Sustainable management of AMD-impacted watersheds is critical for protecting river health and dependent communities in Upper Assam.