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.
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.