Heavy metal accumulation in surface sediments of the Ganga River (India): speciation, fractionation, toxicity, and risk assessment Abstract We investigated the distribution of different fractions of eight heavy metals (Zn, Cr, Cu, Pb, Cd, Ni, Fe, and Mn) in the bed sediment of the Ganga River. The study was conducted during summer low flow (March to June 2017) considering a 285-km middle stretch of the Ganga River between the Allahabad upstream and the Varanasi downstream. To assess the metal levels from a toxicological perspective, we tested the relationships between metals and sediment microbial/extracellular enzyme activities. Most of the metals showed a large fraction in residual form. However, Zn, Pb, and Cd showed about 20–30% share in the exchangeable form. The total metal concentration poorly reflected the toxicity but the exchangeable fractions did show strong negative correlations (r = − 0.83 to − 0.63; p < 0.01) with microbial/enzyme activities. Also, the nutrients and organic carbon showed strong positive correlations (r = 0.62 to 0.89; p < 0.001) with microbial/enzyme activity. The phosphate showed a strong negative correlation (r = −0.82; p < 0.001) with alkaline phosphatase. The principal component analysis (PCA) and the indices such as contamination factor (CF), enrichment factor (EF), pollution load index (PLI), geoaccumulation index (Igeo), and risk assessment code (RAC) revealed moderate to severe contamination with strong anthropogenic influence. As per the United States Environmental Protection Agency, the metal concentrations at many locations were in the highly toxic range. The study has relevance from a toxicological perspective and for the management of the Ganga River.