The terrestrial water storage, both surface and groundwater, has been decreasing in Chennai and neighbouring districts since 2002. The decline is steady, but not alarming, according to a study done by the Department of Geology, Anna University.
The total storage in Chennai and neighbouring districts has been decreasing by 0.12 cm every year in an area of 8,242 sq. km., with Pazhaverkadu to the north and close to Puducherry to the south.
Satellite data
The study, which was taken up along with the National Geophysical Research Institute-CSIR, Hyderabad, has integrated the satellite data extracted from GRACE (Gravity Recovery and Climate Experiment) and the data obtained from the Public Works Department. GRACE was a joint mission of NASA and the German Aerospace Centre. “The satellite gravity data have been globally used in various applications for monitoring changes in water storage on a large scale. Our study attempts to explore the changes in surface and groundwater storage over a region to facilitate a rapid assessment and chalk out comprehensive projects for conservation,” said L. Elango, professor, Department of Geology, Anna University.
Rainfall has slightly increased over the districts around Chennai since 2002 and groundwater storage, too, has improved after June 2015. However, the overall groundwater level in the past 18 years has been dipping by 0.011 cm every year, said Samurembi Chanu, a research scholar who was part of the study.
Diverse water usage patterns and tremendous changes in land use and land cover have led to variations in groundwater storage. This has meant less area is available for rainwater percolation.
“The satellite data and the groundwater measurement of the PWD had a reasonable correlation. But we found that groundwater mass in coastal places had increased. While the physical measurement of groundwater indicates fluctuations, satellite data show the magnitude of seawater intrusion,” Prof. Elango said. For instance, the groundwater at Minjur and Tamaraipakkam would remain at the same depth. But seawater intrusion had occurred at Minjur, indicated by an increase in groundwater weight. Seawater is denser than freshwater, and has more gravitational pull.
According to the study, seawater intrusion in the Ponneri-Minjur belt was recorded to a stretch of 15 km till 2015. However, it has come down by a few kilometres after the 2015 floods, and groundwater quality has improved by 20% in these areas owing to interventions like check dams across waterways. The rise in the sea level by about 2 mm every year also resulted in seawater ingress into the coastal aquifers. “While the aquifers may have slightly recovered with the 2020 rain, we need to continue interventions like check dams and storage structures to improve groundwater recharge. Rainwater harvesting must be implemented on a larger scale,” Prof. Elango said.
