This summer has been a cruel one – all across the city, borewells have been running dry – many for the very first time since the borewell was drilled. The reason for this is the long dry spell that we’ve had – the weakening of the monsoon in Sept – Oct last year followed by the non-existent NE monsoon and the delayed pre-monsoon this year. The groundwater table has fallen precipitously with extraction continuing to remain the same, with minimal or no recharge of the groundwater table.
In this scenario, many people whose borewells have run dry (and many who fear that they may do so eventually) are very keen to recharge their borewells, hoping that by doing so with the monsoon rains this year, they may not have to face a similar situation next year.
There are several service providers who have made a name for themselves doing direct borewell recharge, especially in the dry rural areas of Karnataka and Maharashtra. There are also NGOs that provide this as a free, voluntary service.
Borewell recharge can be done either directly or indirectly. Direct borewell recharge involves digging a pit around the borewell – the depth of which can vary from 8 to 20 feet. At the bottom of this pit, the borewell casing is punched with small holes or slits, and is wrapped tightly with several layers of fine mesh. The pit is then filled with several layers of graded gravel (jelly stones) and sand, or sand and charcoal. This serves to filter the water that flows into the pit before it enters the borewell through the punched holes.
In urban areas, rainfall runoff water from the nearest rooftop is channelled into this pit, and thence enters the borewell. Usually, surface runoff from rain falling on the paved areas around a building is not used, and is not advisable to be used, as it carries with it petrol, diesel, brake oil, carwash detergent, floor cleaner, weedicides and pesticides from the garden etc, aside from fallen leaves and silt. Maybe even dog and cat poop.
In rural areas, the surface runoff from the fields is used to recharge the borewell. Since this contains huge amounts of silt, there is a holding tank built upstream of the borewell pit, that acts as a giant silt trap. The rainfall runoff from the fields is channelled into this tank, in which the silt settles, and then the clean supernatant water is channelled into the borewell pit. In practice, it doesn’t always function this way – if there is heavy or prolonged rain, the holding tank fills up very quickly, and muddy water flows into the borewell pit, as there isn’t enough time for the silt to settle in the tank.
While the graded gravel, sand and charcoal filter out a significant amount of the silt in the pit, some mud – especially the fine particles of clay – will enter the borewell through the fine mesh and the punched slits. Over a period of time, this clay will settle at the bottom of the borewell, and may even block the bottom- most fissures that feed the borewell. They may even pass through the fissures and enter the deep aquifer from which the borewell draws water – an aquifer deep within the rock, in which water has accumulated after being filtered through the various layers of the topsoil, the weathered zone and the bedrock over countless years.
At least this clay just muddies the water in the deep aquifer. Along with it, the runoff water from the fields contains significant amounts of fertilizer, pesticides and weedicides that go into the borewell, and from there into the deep aquifer, contaminating it forever. There is no filter that can filter out the chemicals in the water...
Direct borewell recharge is so attractive because it gives immediate results. After a single good rain, a dry borewell miraculously comes to life. This method is sought after not only by people whose borewells have run dry, but also by those whose borewells have water – they wish to ensure that their borewells’ yields will continue to be good in future if it is recharged in this way.
A wiser approach to borewells would be:
If it is functioning, don’t disturb it
If it never yielded water, forget about it
If it yielded water and has run dry due to groundwater depletion, wait for the rain to recharge it
And here, we can certainly give the borewells a helping hand – by doing indirect recharge through recharge wells. A recharge well is a structure that is dug along the path of flowing runoff water, such that the water enters it and percolates into the ground. For the geology of Bangalore and surrounding areas, it has been found that a depth of at least 20 feet is needed for a recharge well to send water into the weathered zone, rather than into the topsoil. Consistent recharge over some years has been shown to revive shallow borewells, and also dry open wells.
The best place to store harvested rainwater is the shallow aquifer. And the way to do it is to dig recharge wells.