For a city that is facing a drought situation with water reserves dangerously low, water managers are constantly scrambling for solutions to store, harness and use water. Top on the list of many tools available to them are check dams across the rivers to prevent flood water from draining into the sea.
These structures have an impact on the groundwater quality in a radius of up to 3 sq.km. and help in significantly raising the water level, says a study by Anna University.
The groundwater table in the vicinity of the 260-metre check dam constructed across the Arani river in Paleswaram, Tiruvallur district, was studied for nearly five years to determine the structure’s influence on groundwater levels.
The team from the Department of Geology, Anna University, studied the groundwater in 33 wells near the check dam built in 2010.
According to the study, groundwater levels in the wells located in a radius 1 sq.km. increased everytime there was water flow in the river near the check dam.
Groundwater level, which was at 12 metres below ground level in 2010, rose to 8 metre below ground level last year.
S. Parimala Renganayaki, who was part of the study team, said there was an rise in water levels by three to four metres every time there was water flow in the river. “We found that the river had flow till May due to the check dam and places such as Pudupalayam, Rallapadi and Periapalayam had groundwater recharge,” she said.
The water quality within a few kilometres from the check dam also changed.For instance, the chloride concentration, which was 400 mg/litre, reduced to 200 mg/litre and the quality of water became suitable for drinking in some places, the study reported.
L. Elango, professor, Department of Geology, Anna University, who spearheaded the study, said nearly 63% of the water stored in the check dam led to groundwater recharge. This works out to the tune of nearly 850 million litres, given the storage capacity. “However, we found microbial contamination in water near the check dam. But, this too decreased in wells located a little distance away from the river. Water must be pumped from wells located at least 100 metres away from check dam if it is to be used without treatment,” he said.
Considering the maintenance cost, life of the check dam and volume of water recharged, the study estimated that cost of 1,000 litres of groundwater recharged will be ₹1.20. More check dams will help a steady rise in the water table, he added. In and around Chennai, there are around 20 check dams across the Kosasthalaiyar and Arani rivers that help store water and recharge the water table for irrigation and drinking water needs.