Recharge wells and their role in aquifer managementMay 16, 2010
The importance of recharge wells
At the last count, the Bangalore Water Supply and Sewerage Board had listed nearly 110,000 borewells as having been drilled in the city. At least another 110,000 may exist because the BWSSB counts only those borewells where it has provided a water connection.
With such huge numbers, an average pumping of 2,000 litres per day per borewell means well over 400 million litres per day is being sucked out from the aquifers.
The groundwater is topped up whenever it rains. In a year, six to 10 per cent of the rainfall goes below one metre and recharges the aquifer. In Bangalore this would mean that the natural recharge would be about 200 to 250 million litres equivalent.
From tank beds, the average recharge rate is 10 mm to 20 mm per square metre per day. This means about 10 to 20 litres of water recharges the aquifer every day for every square metre of tank area when the tank is full and de-silted.
To recharge the aquifer and to reduce the overdraft of groundwater we need recharge wells. Typically, in the Bangalore context, they are three ft. in diameter and about 20 ft. deep. Recharge rates vary depending on the thickness of the weathered rock overburden and the nature of it. On the IIM premises in South Bangalore recharge rate of 1,000 litres per hour has been observed in recharge wells. In North Bangalore’s Vidyaranyapura, similar rates have been observed in wells which are just 12 ft. deep, probably due to the lateritic overburden.
Existing wells in several parts of the city which have been recharged also show such rates of recharge.
Compare the recharge rate of a tank at 20 litres per day per square metre to that of a good recharge well which is 24,000 litres per day and the difference becomes stark. Recharge wells outperform tanks by a factor of 1200.
If two lakh recharge wells — one for each borewell in the city — pick rainwater from 200 square metres of roof area and recharge the aquifers, close to 32,000 million litres of water will reach the aquifers annually.
How is a recharge well made?
A pit of four ft. diameter is dug till the depth required, and three ft. diameter precast concrete rings are lowered and placed one on top of each other. The sides are then packed with gravel as each ring is placed. The well is covered on top by a steel grill or a concrete slab for security and safety. Clean storm water or rooftop rainwater is then led into the well to recharge it during the rains.