Recharging bore-wellsMay 12, 2012
Time to give back to the bore-well
Almost ubiquitous all over the city the bore-well used to symbolize independence and self sufficiency in water for many households and apartments. Never mind if the city water supply did not give water on a day, switch on the pump and you had as much water as you needed. Given that the city utility did not give water for new constructions every house and apartment that was getting newly built drilled a bore-well almost even before the ‘Bhoomi Pooja’ and the excavation for the foundation. No more is this independence and self sufficiency true.
The city has now a staggering 370,000 bore-wells to 500,000 bore-wells depending on who is doing the guesstimating. Many of them are unfortunately also going dry. Those in the old part of the city like Rajajinagar or Jayanagar were drilled at a time when groundwater was available at an unbelievable 100 feet or at best 150 feet. These old bore-wells from the 80’s and 90’s are being affected first as the groundwater table declines in the city centre too.
Those on the periphery of the city are being drilled to depths of 1200 feet sometimes. Staggering and unimaginable depths which need lots of energy too to lift water from such levels. These newer bore-wells too are going dry faster simply because of the uncontrolled numbers in the periphery where the city water lines do not reach and therefore the dependence on groundwater is almost cent per cent.
So what needs to be done about it? At the individual building level every bore-well must have a recharge structure. Rainwater from the rooftop must be filtered and led in to the bore-well to recharge it. A slug test , filling in a certain volume of water and measuring how long it takes for the bore-well to absorb the given volume, establishes the acceptability and rate of recharge of bore-wells. This must be done first before the rooftop rainwater recharge system is put in place.
Recent slug tests have indicated a rate of recharge of 9000 litres per hour in some bore-wells tested. This will vary from bore-well to bore-well however and each should be tested before it is recharged. What a 9000 litre per hour recharge rate indicates is that a 100 sq mt roof area can be connected through a filter to the bore-well and it will be able to absorb the heaviest rain intensity in Bangalore which is 90 mm/hour. At any rate it can also handle the heaviest rain in a day reported which was 180 mm , this probably occurred over 6 hours.
While direct recharge of a bore-well may have a certain risk in terms of silting up or air locks it should not be such a problem for a dried up bore-well or one which has started to yield less water.
Indirect recharge through a recharge well adjacent to the bore-well is one another method that can be followed. The best however seems to be a combination of storage of rainwater into a sump tank and recharge of the excess water directly into the bore-well.
At larger scales, recharging storm-water through recharge wells and de-silting lakes are broader measures that need to be taken in a city fast crusting up and permitting no place for water to seep in to the ground.
At current costs the investments made by citizens and institutions in groundwater abstraction structures could be a staggering Rs 5000 crores in the city alone. All this would be dead capital if our groundwater runs out. It is in the interest of every groundwater user that recharge be taken up at a massive scale and that abstraction be limited. The entitlement of every groundwater user can be no more than what the user recharges and puts back into the groundwater bank. That is the law of nature and ecosystems. That is water wisdom.