Ecotoxicological Insights into Heavy Metal Dynamics in Cauvery Delta Wetland Sediments: Geochemistry, Speciation and Ecological Risk Indices

Wetland sediments in deltaic ecosystems act both as sinks and secondary sources of heavy metals, thereby influencing contaminant mobility, bioavailability, and ecological risk. The present study investigated the distribution, speciation, mobility, and ecological risk of Fe, Ni, Pb, Cu, Cr, and Cd in surface sediments collected from 40 wetlands across the Cauvery Delta, India. The study integrates total metal concentration analysis, sequential extraction, contamination indices, risk assessment models, and multivariate statistical techniques to evaluate sediment quality and identify dominant sources of contamination. Sediment physicochemical properties indicated predominantly alkaline and carbonate-rich conditions, which strongly influenced metal partitioning and mobility. The overall decreasing order of mean metal concentrations was Fe > Pb > Ni > Cr > Cu > Cd. Contamination factor (CF), Pollution Load Index (PLI), and geoaccumulation index (Igeo) revealed generally low to moderate contamination for most metals, while Cd exhibited localized enrichment and emerged as the principal contaminant of concern. PLI values ranging from 1.07 to 1.29 indicated slight but consistent cumulative pollution across all wetlands. Sequential extraction analysis demonstrated that Fe was predominantly associated with stable residual fractions, whereas Cd showed comparatively higher association with exchangeable and carbonatebound fractions, indicating enhanced mobility and bioavailability. Risk Assessment Code (RAC) and Potential Ecological Risk Index (PERI) further identified Cd, Pb, and Cr as ecologically significant metals with elevated remobilization potential under changing environmental conditions. High RAC and PERI values were particularly observed in wetlands located within Karur and Tiruchirappalli districts, indicating localized anthropogenic influence. Pearson correlation analysis and principal component analysis (PCA) suggested that Fe and Cr were primarily influenced by lithogenic processes, whereas Cd, Pb, and Cu were associated with mixed anthropogenic inputs including agricultural runoff, fertilizer application, urban discharge, and localized industrial activities. The findings demonstrate that although most wetlands remain moderately contaminated, Cd exhibits elevated mobility and ecological sensitivity, warranting targeted monitoring and management in identified hotspot regions. This study provides a comprehensive regional-scale assessment of heavy metal behavior in deltaic wetland sediments and contributes important baseline information for future wetland conservation and pollution management strategies.