Flood-driven shifts in river ecosystem integrity: Heavy metal contamination and climate change impacts

This study investigates flood-induced alterations in the ecological condition of the Achankovil River network, with a focus on heavy metal contamination and climate-linked changes. Water samples collected before (April 2018) and after (September 2018) the 2018 flood were analyzed in conjunction with meteorological data, Gross Primary Productivity (GPP), Net Primary Productivity (NPP), land use, and a range of physicochemical parameters. Notable changes were observed post-flood: atmospheric temperature shifted from 29–34°C to 27–32°C; water depth increased significantly from 0.70 m to 4.73 m (p < 0.05); turbidity dropped from 5.3 NTU to 2.1 NTU; and pH declined from 7.86 to 5.19 (p < 0.05). Dissolved oxygen levels increased (from 2.8 mg/l to 4.5 mg/l), while BOD decreased (from 4.2 mg/l to 2.3 mg/l). Nutrients such as nitrate and phosphate exhibited post-flood elevation, with nitrate increasing from 1.2 mg/l to 2.7 mg/l. GPP increased from 0.82 to 2.82 mgC/l/d and NPP from 0.27 to 1.53 mgC/l/d (p < 0.05). Thermotolerant coliforms spiked up to 1600 MPN/100 ml, indicating widespread fecal contamination. Heavy metal analysis showed a general decrease in cadmium and chromium concentrations post-flood, while lead exhibited a slight increase at most stations, occasionally exceeding the BIS drinking water threshold of 0.01 ppm. These results highlight the substantial influence of flood events on riverine health, emphasizing the need for continuous monitoring and sustainable watershed management strategies.