Water security through conjunctive use
INDUKANTH S. RAGADE & SEKHAR RAGHAVAN
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Enrichment of the shallow layer to its full potential will take care of Chennai's entire needs
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TOWNS AND cities without perennial rivers nearby get their water needs from rainwater collected in artificial reservoirs and from groundwater. In Chennai, for instance, rainwater is collected in four large reservoirs and groundwater tapped from the Araniar-Kortallayar basin and the southern coastal belt.
Actually, enough water to meet the city's needs can be tapped from rain alone but sadly the storages are woefully inadequate. The city is therefore forced to tap groundwater extensively to cover the shortfall. The extent to which groundwater can be tapped in a particular area depends on the nature of soil below the ground there.
Soil made of sand or sand mixed with reasonable amounts of silt or clay known as `alluvium' and layers of loose or broken rock called `weathered rock' are both favourable for holding and yielding water. Solid rock also often holds water in fissures or large cracks formed due to past geological processes but this is not connected to the water in the upper layers.
Groundwater tapped from alluvium and weathered rock is generally good in quality. If the soil is substantially clayey, neither yield nor quality is good. The quality from fissures varies. In Chennai, alluvium occurs up to depths ranging from 20 ft to even 200 ft followed by solid rock. Weathered rock is also encountered in some areas.
In the past, people drew water from the alluvium through open (dug) wells. With the rising population, groundwater levels kept going down and tubewells, deeper than open wells, came into vogue. With the proliferation of flat complexes, the water in the alluvium in many places went to the bottom practically and borewells — 200 ft deep and more — that tapped water from the rocky layer became common.
As this water is often brackish, builders install reverse osmosis (RO) systems to reduce the salt content and offer this combination as a `complete solution' to the water problem. This is, however, misleading.
Borewells with RO may offer water security today but the consequences are: a) Like petroleum, the water-stock in the fissures is limited and will run out soon; b) RO is an inefficient process — 20 to 45 per cent of the total water drawn is rejected in the form of concentrated salt solution; and c) pumping costs are steep.
Total neglect
The switchover to deep borewells has resulted in the total neglect of the rich potential of the shallow layer which holds water in many places because of the rainwater harvesting measures implemented by many residents of Chennai in 2003. Builders and borewell diggers unfortunately ignore this layer and fix plain casing pipes without any slots at shallow depths for their borewells, thus effectively ignoring the good quality water often available there.
This layer may be dry at present, but it can be charged with rainwater and the water table enriched substantially by comprehensive rainwater harvesting i.e. tapping not only the rain falling on the terraces but also that falling in open spaces around the buildings.
Chennai's average annual rainfall is such that if an entire neighbourhood practises comprehensive rainwater harvesting, the shallow layer will be enriched sufficiently to meet people's entire needs. This may seem a wild claim but the experience of the authors in numerous houses and apartment complexes is the basis for this statement.
The enrichment of the shallow layer to its full potential will, no doubt, take some time. Till then, we can practise conjunctive use of groundwater i.e. a) rejuvenating the shallow alluvial layer by comprehensive rainwater harvesting; b) tapping it during the monsoon and for some period thereafter; and c) switching to the deeper borewell whenever the shallow aquifer dries up.
The dug well plays a vital role in this and so people should not thoughtlessly fill up dug wells (even if they are dry) but clean them and deepen them if necessary and build the rainwater harvesting system around them. Even those wells already filled up can be revived by removing the filled-up material.
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