Tuesday, April 22, 2008

Those City Water Wells

Last week I came across a small report on groundwater levels in Pune and finally got down writing about it. The caption read: "Well" done! Water table has gone up in Pune. I am not sure how much significance to assign to this euphoric caption and particular findings detailed in the press report. One usual quibble I have regarding reports such as these is that a meaningful comparison and context is never given. Water table has gone up compared with what? The last measurement? A five year average? A ten year average ? What is the variation shown by the groundwater system over the last 100 years and how does this latest data point fit in? There are apparently some old observation wells that the Groundwater Survey and Development Agency (GSDA) monitors. This forms the basis of these periodic announcements of water table levels. The city has grown enormously over the last few decades and extrapolating from these few monitoring wells to the entire city does not seem to me a very sound idea. Basalt aquifers such as the ones that underlie Pune are quite heterogeneous in their groundwater transmissivity and storage capacities. What is happening to the water table in a few observation wells may not apply some distance away. Without understanding the variability of the system and its causes, simply reporting that the water table has gone up or gone down by a certain amount makes little sense.

The wells referred to in the report are shallow dug wells, which lots of old bungalows used to have when I was growing up in the 1970's. There are still some 5000 odd remaining according to the news report. I remember peering over the side to see how deep the wells were. The water table was shallow about 15-20 feet below surface and the wells always had plenty of water. But even then they were not being used as a drinking water source. Their use was for washing and for the garden. On Ananta Chaturdashi day many well owners used to let neighbors use the well for immersion of their Ganesh idol. It used to be quite a festive occasion. Over the last 3 decades or so their use as a water resource within the city has declined as the water has got more polluted. Still as a geologist I don't miss an opportunity to peer over the side. These dug wells tap the shallow groundwater system in the basalts. Image to the left below is a photo of a dug well I took a couple of days ago. The image to the right is a schematic which shows the hydrogeology of these wells (Source: ACWADAM).

In the photo you can see that the well is lined. Below that level is the basalt rock which has a slab like appearance. That is due to horizontal sheet joints or fractures along which groundwater flows. Basalt is a volcanic crystalline rock which has a tight interlocking crystal fabric. Water is not stored and transmitted through the rock matrix i.e. spaces between crystals but along cracks and fractures that develop in the basalt. Vesicular amygdaloidal basalt is a type of basalt that has horizontal sheet joints. Another type of basalt known as compact basalt usually develops vertical joints. Basalts with horizontal sheet joints have higher transmissivity and storage capacity and yield more water than compact basalt. These controls are shown in the schematic in the figure to the right (above)

This shallow groundwater system within the city is underutilized today. The trend has been to fill up these wells as bungalows make way for bigger apartment complexes and to bypass the shallow system and tap the deeper aquifers using bore-wells. Many new apartment complexes have bore-wells and utilize the water for washing and the garden. There is an assumption that this deeper aquifer contains cleaner water but that might not always be true. As the graphic to the left shows (source: ACWADAM) deep aquifers are replenished through the shallow system by penetrating vertical fractures. There is a real chance of contamination of the deeper system as well and using that water without adequate treatment is a health risk.

The population of Pune metropolitan area today is around 3 million or so. The city is fortunate to have lots of big surface water reservoirs (Khadakvasala, Panchet, Varasgaon) and the water supply per person is about 2oo litres per day according to the Pune Municipal Corporation Env. Status Report 2004. These reservoirs have adequate capacity to meet projected water demands over the coming decades and the PMC claims to be building water supply infrastructure to supply a population of about 6 million by 2025. Besides surface water the Municipal Corporation keeps talking about using these groundwater aquifers as a supplemental water resource to be used for non-domestic purposes. Rain water harvesting, artificial recharge of aquifers, sustainability, these are the latest buzzwords the city government likes to throw around. But I see two problems for any sustainable use of these systems. The first is a lack of basic science. City officials admit that they don't have any rigorous quantitative estimates on how much water is there and how much extraction is possible given natural replenishment rates. The second problem has to do with how to regulate the use of this resource. Currently groundwater use is not regulated in India and allows the owner of the land to extract as much groundwater as needed. Without changes in the regulatory framework, there will be overexploitation and the danger of degradation of the groundwater system.

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