Pollution status and ecological risk assessment of heavy metals in surface water and sediment of an ecologically critical lake of Dhaka, Bangladesh

Hatirjheel Lake, a vital urban water body in Dhaka City with significant ecological and environmental importance, is undergoing a gradual decline in water and sediment quality. The primary goal of this study was to comprehensively evaluate the pollution status and ecological risk associated with heavy metals in sediment of the lake. This was accomplished by analyzing key physico-chemical parameters, including pH, salinity, dissolved oxygen (DO), electrical conductivity (EC), and total dissolved solids (TDS) in water, along with the concentrations of major heavy metals (As, Mn, Pb, Co, Cu, Cr, Hg, Ni, Zn, and Cd) in both water and sediment. Most of the evaluated water quality indicators, such as pH (7.12 ± 0.48), salinity (1.6 ± 0.06), EC (804.8 ± 36.54), and TDS (402.9 ± 19.19), complied with the permissible standards established by the WHO and USEPA, while DO (3.59 ± 0.97) levels were notably below the surface water threshold. All the metal concentrations were significantly lower than both the international recommended limit and background reference values, except for slight exceedance of Mn (72.94 ± 57.47) in water and Zn (128.14 ± 93) in sediment. The geo-accumulation index (Igeo) indicated minimal metal accumulation, and the enrichment factor (EF) suggested a predominantly geogenic or natural origin for the analyzed metals. The pollution indices, including degree of contamination (CD), nemerow pollution index (Np), and pollution load index (PLI) showed only mild pollution due to the synergistic effect of heavy metals. The analysis of principal component analysis (PCA) and hierarchical cluster analysis (HCA) demonstrated that Pb, Cd, Co, Cu, Zn, and Mn primarily originated from urban runoff and wastewater inputs, while Cr and As were governed by distinct lithogenic or redox-controlled processes. While Cd and Hg exhibited a slight ecological risk, the overall potential for ecological consequences was well below the critical limit. These findings provide a robust scientific baseline for urban lake management in rapidly growing megacities and highlight the importance of continued monitoring to prevent future deterioration under increasing anthropogenic pressure.