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.