2. Where does Chennai's treated water come from

The treated water supplied to Chennai’s citizens is popularly called metro water. This comes overwhelmingly from the three main reservoirs — The Poondi, Sholavaram and Red Hills lakes (See map). These lakes in turn are fed by the rain water falling over the catchment areas around them during the south west monsoon and the north east monsoon. These reservoirs were constructed many years ago when the city’s population was much less than what it is today. As a result, if either of the monsoons fails or even if the total rainfall from both the monsoons is less than normal, the city has to gear itself up to rationing of treated water supply. The situation is aggravated by the fact that, over the years, these storages have become silted, thereby reducing their effective holding capacities. On the other hand, in years of good monsoons, the capacity of these storages is inadequate to store all the rain water flowing into it from the catchment areas. Substantial quantities of valuable rain water consequently ends up in the sea. Such wasteful flow from the Kortalaiyar River alone (See map) had been estimated to be as much as the city’s annual consumption. Recently, a link canal has been built which is able to direct such water from this river into the Sholavaram reservoir. Evaporation further reduces the water available for supply. The annual evaporation from these three storages has been estimated to average about 44% of the total capacity of the storages. In order to cater to the increased needs of the city, a water bearing aquifer (i.e. a soil strata rich in water) hydro-geologically suitable for extraction of ground water was identified about two decades ago in the Araniyar-Kortalaiyar river basin in the north and six well fields were developed (See map). Out of these, Minjur which is close to the sea, showed indications of sea water intrusion and had to be abandoned. A decade ago, a shallow aquifer belt was identified between Thiruvanmiyur and Muttukadu in the coastal belt south of Chennai. But it's continued availability as a ground water source is doubtful due to (a) high level of tapping by private tankers to meet the city’s needs and (b) the shrinking area available for rain water absorption because of considerable colonisation. While the recently inaugurated Krishna Water Scheme can be expected to augment the supply to the city, its benefits may not reach many sections of the city due to the inadequacy of the existing pumping system to distribute the supply equitably to all areas.