A new assessment of 10 inland wetlands across Tamil Nadu has found that pollution risks are accumulating beneath the water surface, with oxygen depletion and nutrient-rich sediments emerging as major ecological concerns. The findings matter beyond wetland health: degraded water bodies can weaken natural flood storage, water-quality regulation and climate resilience as towns and cities expand.
Researchers from Anna University assessed water, sediment and surrounding soil samples from wetlands across northern, western and southern Tamil Nadu. The study, published in Water, Air, & Soil Pollution in May 2026, examined samples collected between May and July 2025 using a multi-matrix approach.The most significant warning came from dissolved oxygen levels. Measurements ranged from just 0.1 mg/L to 4.0 mg/L, indicating severe oxygen stress at several locations. Biological Oxygen Demand reached 4.5 mg/L, while Chemical Oxygen Demand rose to 420.2 mg/L, pointing to substantial organic loading within parts of the wetland system.
Yet surface-water readings alone could understate the problem. Nitrogen and phosphorus concentrations in water were generally within applicable Tamil Nadu Pollution Control Board limits, while sediments recorded total nitrogen of up to 5,387 mg/kg and phosphorus of up to 32.5 mg/kg. The researchers identified these sediments as long-term nutrient stores capable of feeding nutrients back into the water through internal cycling.That finding has an important implication for urban management. Cleaning visible pollution from a wetland does not necessarily remove its accumulated ecological burden. Nutrients trapped in sediment can continue influencing water quality, particularly when untreated sewage, agricultural runoff and organic waste keep entering the system.
The 10 sites included wetlands associated with the Pulicat complex, Barur Lake, Wallajah Lake, Singanallur Lake, Vagaikulam, Koonthankulam, Kadamba Tank, Nainar Kulam, Perungulam and Madakulam. The study found generally low to moderate ecological risk, but identified localised hotspots requiring targeted intervention. For Chennai, the findings offer a wider warning through the condition of Pallikaranai Marsh. Official wetland records describe the Ramsar site as a critical floodwater storage ecosystem serving a 250 sq km drainage area, while listing sewage, urban wastewater, solid waste, development pressure and climate-related stresses among its threats.The research therefore strengthens the case for monitoring sediment alongside routine water testing. Targeted remediation, tighter control of sewage and runoff, nature-based wastewater treatment and site-specific restoration could help prevent accumulated pollution from becoming a permanent ecological liability. For rapidly urbanising Tamil Nadu, protecting wetlands is no longer only a conservation question. It is increasingly tied to flood management, water security, biodiversity and the long-term resilience of the built environment.
