The coastline north of Chennai near Tiruvallur district continues to be affected by seawater intrusion and it has now extended nearly 18 km inland from the coast, reveals a study by Anna University.
The study, conducted in the Ponneri, Panchetti and Minjur coastal belt that forms part of the Araniar-Kosasthalaiyar belt, called it one of the worst-hit areas in the world in terms of seawater intrusion.
Seawater intrusion had extended from 4 km inland, found near Athipet during 1969, to nearly 18 km inland up to Vannipakkam in 2019, according to the study done by the Department of Geology, Centre for Climate Change and Disaster Management, Anna University, along with the Institute of Geological Sciences, Freie Universität Berlin, Germany.
“We noticed that the seawater ingress was more in the lower aquifer than the upper aquifer. This indicated overexploitation of groundwater and it had resulted in salinity in the coastal aquifer,” said Indu S. Nair, who authored the paper along with K. Brindha. Groundwater salinity increased by four times over the past decade. For instance, areas such as Kattur, Mouthampedu and Senganimedu have total dissolved solids, which denote salinity in water, of 8,000 mg/litre against the maximum permissible limit of 2,000 mg/l.
To assess factors contributing to groundwater salinity in coastal aquifers, the study adopted field investigation with a hydro-chemical modelling technique. Some of the indicators, such as electrical conductivity, sodium and chloride and isotopes in the groundwater, were studied to assess the extent of mixing of freshwater with seawater.
Groundwater samples were collected from 50 wells once in two months. It was found that concentration of sodium and chloride, which are indicators of seawater intrusion, had gone up by 10 times. For instance, chloride concentration had gone up to 7,000 mg/l in some areas closer to the sea compared with that of 1988. The permissible level is 200 mg/l, said L. Elango, professor, Department of Geology, Anna University, and one of the authors of the paper published recently in Hydrological Sciences Journal.
Dubious distinction
Low sodium-chloride ratio was another indicator of the extent of salt water intrusion in the region. The highest chloride and bromide ratio, which is used to determine groundwater quality, was found near Minjur that indicated extensive seawater intrusion, he added.
Groundwater in the northern parts of the study area, such as Ponneri, was better than those in the south, like Minjur. After 2015, groundwater quality improved slightly here. The study suggested steps such as construction of more check dams to recharge the water table.