Heavy-metal bioavailability, water quality, and sediment characteristics in relation to freshwater snail distribution in Alaro River, southwestern Nigeria

This study assessed surface water and sediment physicochemistry, quantified Cd, Co, Cr, Pb, and Ni, and evaluated snail distribution and metal bioaccumulation along the Alaro River, Southwest Nigeria. Samples were collected and analysed at three stations using standard methods to provide an integrated view of contamination and bioavailability in a river impacted by urban and industrial activities. The mean surface water metals values (mg/L) were Cd (0.03 ± 0.05), Pb (0.17 ± 0.12), and Ni (0.06 ± 0.05) (all > WHO limits); while electric conductivity (EC) ranged from 389 to 1592 µS/cm. Sediment had the highest concentrations of metals (mg/kg), Pb (17.67±23.69) > Ni (9.29±13.88) > Cd (6.06±11.25), confirming it as a metal sink. Seasonally, pH, EC, phosphate, nitrate, and chloride were significantly higher in the dry season (p < 0.05), whereas sulphate, Cd, Pb, and Ni peaked in the wet season (p < 0.0001). A total of 1,114 freshwater snails (11 species) were recorded, including Melanoides tuberculata and snails of public health concern such as Bulinus globosus and Lymnaea natalensis. Metals were significantly higher in viscera than shells (p < 0.05), indicating consumer risk and the need for stricter waste regulatory enforcement and public health interventions in River Alaro.