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WATER WISE
Recharging defunct borewells
S. VISHWANATH
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Rooftop rainwater can be filtered and allowed to directly enter the casing of the borewells to build up the water table
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Novel idea: An infiltration pit around the casing of a borewell
By one estimate there are over 1,80,000 borewells in the city of Bangalore alone. Almost every new house or apartment drills a borewell as a water security measure and also since the Water Supply Board does not provide water for construction. On Sarjapur road to the east of the city, borewells have reached depths of 1,250 feet and yet have failed to strike water.
It is also most likely that one in every two borewells is now defunct mostly due to the lowering of the water table and also sometime because of problems with silt and pump failures. The yield of some borewells yield has declined dramatically and can no longer be pumped in a sustainable manner.
Can these borewells be used for a positive purpose by owners? Considering the investments made for their drilling how can they be converted from a dead asset to a live asset?
Rainwater harvesting offers one hope for these defunct or low yielding borewells. Rooftop rainwater is picked from a clean catchment, filtered to remove physical impurities and allowed to directly enter the casing of the borewell. It is important to ensure a byepass arrangement in case there is accidental pollution of water in the catchment.
In the case of Bangalore with 900 mm of rainfall, approximately 90,000 litres of rain will fall annually on a 100 square metre roof area. Much of this water can be recharged into defunct borewells. Since this rainfall is spread over nearly 60 rainy days it is reasonably well distributed and therefore the topping up of the underground waters is also well distributed.
Filters
Many rooftop rainwater filters are available in the market, including the rainy filter and the pop-up filter. They can be used while recharging the borewells. Many borewells are located in public places such as parks or open playgrounds. There is a potential to recharge them with flowing storm water. Of course there should be no garbage in the catchments. Clean stormwater can be filtered thoroughly through a recharge pit filled with materials such as stone, charcoal and sand and then allowed into borewells.
While these are direct recharge methods, several indirect methods too are available. These include using a surface pond, recharge trenches, recharge wells and landscaping details such as swales and berms.
The advantage of such indirect methods is that they filter water more thoroughly than the direct methods. The disadvantages are that they first have to make the soil moist before the water enters the shallow aquifer and water may or may not enter the secondary porosity or the crack or crevice from which the borewell draws its supply.
The direct method has better reach and effectiveness in placing water where the borewell itself draws the water.
Precautions
The defunct borewell will silt up because of reasons such as poorly done casing, the profile of the soil itself or the lack of adequate filtration of water before it enters the borewell.
The rate of recharge also varies widely with some borewells capable of guzzling water at enormous rates whereas others can only do so slowly.
The assumption here is that the defunct borewell has been given up as lost and therefore some efforts are made to try and revive it.
If proper precautions are not taken, borewell water may get contaminated. It is wise to remember that this water has stayed pure through millions of years and therefore great responsibility lies on us not to contaminate it.
A measured and reasonable demand, a commitment to recharge, taking as much water as has been recharged, these are all the mantras of water wisdom and wise use of water.
www.rainwaterclub.org
www.arghyam.org
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