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

Abstract

Wetland sediments act as major sinks for trace metals and play an important role in regulating metal mobility and ecological risk in aquatic ecosystems. This study investigates the physicochemical characteristics, heavy metal distribution, geochemical speciation, and ecological risk of sediments collected from 40 wetlands in the Cauvery delta, India. Sediment pH ranged from 8.1 to 9.5, indicating moderately to strongly alkaline conditions, while electrical conductivity varied between 0.13 and 1.98 mS cm⁻¹. Total organic carbon ranged from 0.72–2.85%, and organic matter content from 1.24–4.91%, reflecting spatial variability in organic accumulation and depositional processes. Calcium (0.51–2.04%) and magnesium (2.44–5.50%) concentrations indicate carbonate-rich sediments that influence buffering capacity and metal adsorption. Among the analysed metals, Fe showed the highest concentration (9,335–31,876 mg kg⁻¹; mean 24,127 mg kg⁻¹), followed by Pb, Ni, Cr, Cu,  and Cd. Sequential extraction using the Tessier procedure revealed that Fe, Ni, and Cu were mainly associated with residual and Fe–Mn oxide fractions, indicating lithogenic origin and limited mobility. In contrast, Pb, Cr, and, particularly, Cd were enriched in the exchangeable and carbonate-bound fractions, suggesting greater mobility and bioavailability. Ecological risk indices, including contamination factor (CF), pollution load index (PLI), and geo-accumulation index (Igeo), indicated generally low contamination levels across most wetlands. However, localized Cd enrichment in parts of Karur and Tiruchirappalli districts suggests anthropogenic inputs, likely associated with agricultural runoff and fertilizer application, highlighting the need for continued environmental monitoring and management.