The water position in Chennai has been satisfactory for several years now because of a couple of good monsoons, good supply from Andhra Pradesh and from the Veeranam Lake. The rainwater harvesting systems installed by many citizens have also helped in improving ground water levels. However, citizens should not presume that their water problems are solved for the following reasons;
Chennai’s water needs are mainly met by impounding the rainwater from the Araniar-Kortalayar rivers north of the city and the capacity of the reservoirs is woefully inadequate. It has been estimated that an entire year’s supply flows wastefully into the sea consequently. The quantum of supply from AP and the Veeranam Lake is uncertain.
Therefore citizens seriously need to consider measures to become more self-reliant in their water needs. For this, they need to know how to tap their ground water sources effectively and in a sustainable manner. Tips on this are given below based on actual experience:
Ground water is available in two unconnected layers:
- Layers at shallow depths, tapped by dug wells and tube wells
- Layers of water entrapped in rocks at deeper layers through bore wells
The depth at which rock occurs in any city varies from locality to locality. In Chennai, it occurs at about 5 ft in Little Mount, at 60 ft in Besant Nagar and at 150 ft in some places. In the last three decades, the traditional dug wells and tube wells tapping the shallow layer dried up and went out of fashion due to over-exploitation. Deep bore wells are now the fashion. Thus the shallow water layer has been totally ignored in the last two decades.
Dug wells are actually our insurance for the future, as they are sustainable, unlike the bore wells!
The (100 years’) average annual rainfall in Chennai is 125 cm and dug wells will be rejuvenated and made sustainable remarkably by charging them efficiently with this huge quantity. Costs are moderate and water quality will also be generally excellent. The rocky layers tapped by bore wells are not sustainable as it is difficult to charge them with rainwater. Quality is also often unsatisfactory and pumping costs are high. So, if an open well exists within the premises (even if it is dry), it should be retained and repaired, if necessary.
Deepening it with smaller diameter rings will help. As many as possible of the terrace down-take pipes should be connected to it directly. The open well will be revived sooner or later. If no dug well exists, seriously consider having one dug and connect all terrace pipes to it.
If a report of the soil investigation (up to about 30 ft depth) done usually for designing the foundation of a building is available, it will give an idea of the depth to which it should be dug. If not, one can get it done for a few thousand rupees. Initially, yield from the well may last only for some months after the monsoon. In this period, use well water. When it goes dry, switch over to the bore well. This is termed ‘Conjunctive Use of Aquifers’
Tubewell
If there is a neglected tube well, it may have now revived because of the recent rise in the shallow water table due to the spread of rainwater harvesting. It will be worth activating it once again. If there is no space for a dug well, then one can go in for a new tube well. Use the tube well water in the post-monsoon months. Switch over to the bore well if it dries up. While digging a new deep bore well, the bore well digger is likely to put a plain casing PVC pipe up to the rocky stratum.
In this shallow, non-rocky stratum, if reasonably sandy or gravelly layers are encountered, insist on a slotted PVC pipe for the favourable stretches instead of a plain pipe, so that the water in these layers can enter into the bore well through the slots to the bottom of the bore well and help sustain it.
It is hoped that readers will utilise these tips and obtain greater self-sufficiency in water. Much of what has been stated above is applicable to most cities of India.
For further details:
iragade@yahoo.co.in
sekar1479@yahoo.co.in
