INTRODUCTION: Kolkata is now a great megalopolis with about ten million populations. This city is not only growing in size but also growing in population day by day. This growth of population has cast a great impact on the land on one hand and on the other hand the demand for drinking water has grown by many folds.
The history of Kolkata water supply starts from the initiation of the British settlement in 1717 and runs up to 1870 when Calcutta Municipality was establish. During the various phases of formation of Calcutta municipality, topmost priority was given to water supply problems. The water supply of the newly constituted municipality commenced in the year 1870 with a 6 mgd plant at Palta. To cope with the increasing demand of portable water, the capacity of the water treatment plant was augmented in stages. In addition, there is 60 mgd of unfiltered water supply and ground water supply of 22 mgd by 260 deep tube wells.
At the increase in demand of water more and more hand tube wells were installed during the last two decades. Apart from private initiative the corporation has also installed a number of deep tube wells fitted with hand pumps.
The actual number of private deep tube wells is yet to be known. But withdrawal of ground water has increased to a great extent during the last two decades. This unaccounted has developed two major groundwater problems: -
a) Depletion of water level to the tune of 10-12mbgl at places.
b) Deterioration of water quality.
Geologically and geomorphologically Kolkata belongs to the lower deltaic plain of the Ganga-Padma river system.. The surface material is clay and clay loam. This clay extends up to a depth of 10 to 25 m bgl in most of the area. Below this clay bed a fine sand bed is found which extends up to a depth of 30-to 35-metre bgl. Below this level another clay, dark brown to grayish brown in colour occur up to a depth of 60 to 100-metre bgl. From this depth another sand zone occur which comprises of fine, medium and coarse sand and extends up to a depth of 120 to 180 metre bgl. Below this sand zone gravel bed occurs. Tertiary black and sticky clay occurs at the bottom of the sand and gravel zone.
Due to unique geological setting rainfall in Kolkata does not percolate downwards to recharge the groundwater regime. Rainwater is totally wasted and poses a serious problem like water logging and sewage overload.
The demand of water for domestic purpose in Kolkata is increasing day by day. The municipal water supply to some extent and private domestic water supply in some areas also depend on ground water. Since Kolkata is not within the groundwater recharge zone and the withdrawal of groundwater is very high in comparison with the groundwater recharge from remote area groundwater level has already depleted to a great extent. Recent studies have revealed that groundwater level of Kolkata has already gone down to 10 to 13 metre below sea level. .
To combat all the problems related to the water supply and demand, and also for the problems developed due to over withdrawal of groundwater scientists are advising to utilize rainwater in urban areas.
BENEFIT: Rainfall in Kolkata occurs during the monsoon. Maximum rainfall occurs from June to October. The average annual rainy days are 60. Average daily rainfall is about 30 mm considering only the rainy days. The monthly average rainfall is given below.
The volume of total annual rainfall in Kolkata over a 184 sq km area is 1.402X 184= 258 million cubic metres. This amount of water is required for 10 million people for 151 days. In the same way rainwater available on a 100 sq metre roof is 1.402 X 100 cubic metre annually= 140200 litres. This is enough for a family of five for 140 days.
It will never be possible to harness all available rainwater through any type of harvesting system. Moreover the water requirement of Kolkata is much higher than the amount of rainfall falling directly overhead. So Kolkata will remain dependant on the transported water from upstream regions. But rainwater harvesting in Kolkata has three visible benefits.
1) If a large amount of rainwater is used for groundwater recharge it will supplement the natural recharge to some extent.
2) If about 10% of rainwater is harvested properly through some well managed system in specific locations it will significantly reduce water logging during storm seasons.
3) If rainwater is harvested by individuals or by community housing societies the consumption of piped water will be reduced to a great extent. The Loreto Day School in Sealdah is utilizing rainwater for its toilets and significantly reduced municipal water consumption.