Geology of the Quaternary Aquifers of the Twin City of Calcutta - Howrah

The twin city of Calcutta-Howrah is underlain by a succession of Quaternary sediments consisting of clay, silt, fine to coarse sand and occasional gravel. Below these sediments at depths beyond 296 m, there is a thick sequence of Pliocene clay. The top clay bed of Quaternary sequence contains peaty intercalations indicating bog or marshy condition at depths of 2-5 m and 12-12.6 m. The peat layers were deposited during the period c.3990 years B.P. to c. 7030 years B.P. Grain size analysis of the sandy horizons indicates that the sands were deposited in a fluvial environment. Lithofacies analysis of the Quaternary sediments indicates that the gravelly sand horizons occurring in E-W and NNE-SSW trends in the depth range of 80-120 m are potential aquifer zones. These two trends of deposition of very coarse sediments probably indicate the route of sediment movement where high energy condition prevailed. The sandy sediments at a depth span of 24 to 81 m are yellowish to brownish coloured suggesting oxidising conditions of deposition. The sediments occumng above and below this horizon are grey to light grey in colour indicating reducing conditions of deposition. The over-extraction of groundwater from the aquifer has resulted in the change of the groundwater flow pattern from an "open" system in 1956 to a "closed" one in 1993 with the formation of a groundwater trough in the south-central part of Calcutta. Rased on depth of the aquifers, status of the piezometric surface, lithofacies and grain size ratio, an aquifer development plan has been proposed for a sustainable groundwater development in the twin city.