CHENNAI: A comprehensive study by researchers from Anna University has raised concerns over increasing pollution and ecological stress in Tamil Nadu’s inland wetlands, warning that decades of pollutant inflow have turned wetland sediments into ‘legacy nutrient reservoirs’ that continue to degrade water quality even when surface water appears to meet regulatory standards.
The peer reviewed study, published in the international journal, Water, Air, & Soil Pollution, found that nutrient accumulation in sediments, severe oxygen depletion and organic pollution were posing growing ecological risks to inland wetlands across the State, highlighting the need for urgent restoration and improved pollution management.
Titled ‘Integrated Physico chemical and ecological risk assessment of inland wetlands in Tamil Nadu, India, reveals nutrient driven ecological stress’, the study was carried out by Anna University researchers Praveen Arjunan, Saranya Kuppusamy and S Kanmani. The team analysed water, sediment and surrounding soil samples collected from 10 major inland wetlands in Tamil Nadu.
The researchers reported critically low dissolved oxygen (DO) levels ranging from 0.1 mg/L to 4.0 mg/L, well below the 4-5 mg/L generally required to sustain healthy aquatic life. Biological Oxygen Demand (BOD) ranged between 0.17 mg/L and 4.5 mg/L, while Chemical Oxygen Demand (COD) reached as high as 420.2 mg/L, indicating significant organic pollution entering wetland ecosystems. Although nitrogen and phosphorus concentrations in surface water were largely within Tamil Nadu Pollution Control Board (TNPCB) limits, the study found that sediments had accumulated substantial nutrient loads over time.
Total nitrogen concentrations in sediments reached 5,387 mg/kg, while total phosphorus levels were recorded at up to 32.5 mg/kg. According to researchers, these sediments now function as ‘legacy nutrient reservoirs’, continuously releasing nutrients back into the water through internal nutrient cycling and worsening ecological degradation.
The overall ecological risk across the wetlands was moderate. However, researchers cautioned that several localised pollution hotspots require immediate intervention.
The study recommends moving beyond routine surface water monitoring and adopting sediment-focused restoration measures such as targeted dredging and internal nutrient control. It also calls for preventing untreated municipal sewage and agricultural run-off from entering wetlands through nature-based wastewater treatment systems, including constructed wetlands, and implementing site-specific ecological restoration plans.
Environmental expert and former Madras Institute of Development Studies (MIDS) professor S Janakarajan said that Pallikaranai marsh, one of Chennai’s last remaining wetlands, has been facing severe ecological degradation, and urged the State government to take immediate restoration measures. “We’ve been polluting the Pallikaranai wetland for more than 40 years. Solid waste dumping and sewage inflow have contaminated the soil, and heavy metals can now be traced in the groundwater in areas surrounding the marsh,” he opined. “Groundwater contamination has affected localities including Velachery, Guindy and several other areas located 40-50 sqkm area around the wetland.”
He pointed out that Pallikaranai marsh once spread across more than 50 sq km, but only about 5-6 sq km now remain. “The wetland has shrunk drastically over the years due to encroachments and developmental activities. Even after Pallikaranai was declared a Ramsar site, there is still no clearly demarcated boundary for the wetland. The remaining area must be protected without further delay,” he said.
Calling for immediate conservation measures, Janakarajan urged the State government to implement measures to prevent garbage dumping within and around the marsh and undertake large scale planting of indigenous tree species along its boundaries in the width of 50 metres. “If native trees are planted in a 40-metre wide buffer around the wetland and further dumping of waste is prevented, nature has the ability to heal itself over time. Protecting the remaining wetland has to be made a priority,” he added.
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