Water-specific toxicity factor and index for heavy metal risk assessment: application to urban lakes in Chennai, India.

Heavy metals released into aquatic environments disrupt ecological balance. Urban lakes in the Pallikaranai catchment, Chennai, India, face pollution pressures from rapid urbanization. This study introduces a water-specific toxicity factor and toxicity index to assess toxicity risks posed by heavy metals in surface waters to aquatic life. Results revealed considerable variability in in situ water quality parameters, including pH, electrical conductivity, and dissolved oxygen. pH was mostly neutral to alkaline, with some lakes experiencing anoxia while others exceeded 10 mg/L oxygen. Pb and Cu posed the highest toxicity risks in water, with several samples reaching Class II and III in the United Nations Economic Commission for Europe (UNECE) classification, indicating potential toxicity risks. In sediments, Cr and Ni posed the highest toxicity risks, exceeding the PEC in all samples and Cu and Zn frequently exceeding sediment quality thresholds. Exceeding the TEC indicates possible toxic effects, while surpassing the PEC suggests probable toxic effects to aquatic biota. Compared to other polluted urban water bodies globally, the Pallikaranai catchment exhibited relatively low water contamination and high sediment contamination, particularly for Cr, Cu, Ni, and Zn. The newly developed water toxicity index, based on UNECE guidelines, indicated varying levels of potential ecological risk across lakes. This study highlights the need for monitoring and effective pollution mitigation strategies to protect the aquatic ecosystems of the Pallikaranai catchment. The proposed toxicity index provides a practical screening tool for assessing and prioritizing heavy metal risks in urban aquatic environments.