Seasonal hydrochemical dynamics and index-based ecological risk assessment of heavy metal pollution in the Turag river, Bangladesh

The Turag River, a major tributary of the Buriganga in Dhaka, Bangladesh, is experiencing severe ecological degradation associated with industrial, domestic, and agricultural discharges. This study assessed seasonal hydrochemical dynamics and heavy metal contamination of surface water at 20 sites during dry and wet seasons, analyzing major ions, physicochemical parameters, and trace metals (Pb, Cd, Cr, Ni, Fe, Mn, Cu, Zn). Dissolved oxygen declined to 0.9 mg L⁻¹, while BOD (15 mg L⁻¹) and COD (176 mg L⁻¹) exceeded guideline thresholds, indicating substantial organic loading and oxygen depletion. Pb (0.072 mg L⁻¹) and Fe (6.945 mg L⁻¹) concentrations exceeded drinking-water benchmarks, suggesting potential exposure concern. Multivariate analyses (PCA, FA, HCA) revealed patterns consistent with industrial effluents, domestic sewage, and geogenic contributions. Index-based evaluations (WQI = 52.75; HPI = 87.7; HEI = 6.30) classified the river water as poor to very poor quality, particularly during the dry season. Ecological risk indices (EHI = 5.40) and HHRA screening metrics indicated non-carcinogenic risk potential at some locations, particularly for children; screening-level carcinogenic risk for chromium was estimated under a conservative Cr(VI) assumption, and uncertainty is discussed due to lack of Cr speciation. Regression analyses confirmed strong relationships among HEI, WQI, BOD, COD, EC, and metal concentrations, with bootstrap-validated models (R² > 0.80) identifying BOD, COD, and Pb as influential predictors of water quality. These findings underscore the need for improved effluent control, continuous monitoring, and adaptive management strategies aligned with SDG 6 objectives. • Seasonal analysis reveals severe dry-season degradation in Turag River water. • Critically low DO (0.9 mg/L) and high BOD/COD indicate ecological stress. • Pb and Ni dominated ecological/non-carcinogenic screening risk; Cr(VI) cancer risk was screening-level under a stated assumption. • WQI, HPI, HEI, and HI classify river water as poor to very poor quality. • Pollution patterns were consistent with industrial and urban inputs, highlighting the need for targeted mitigation and regulatory action.