Based on the change in piezometric level from the middle of the 1950s to the end of the 20th century, decrease of pore water pressure in the aquifers and corresponding increase of the effective stresses in different soil strata of Salt Lake City and adjoining areas of Kolkata are calculated. This increase of effective stress results in consolidation of the soil, which has manifested itself as land subsidence. The city of Kolkata region forms a part of the lower deltaic plains of the Ganga River system. However, in the Salt Lake area, the upper layers of soil are composed of filled up materials, (e.g., alluvium dredged from the riverbed of Hooghly). Due to excessive groundwater withdrawal, the consolidation resulting in subsidence is maximum in Ultadanga—Salt Lake area of Kolkata, (10-20 mm/y); this is also becoming prominent in cracks found in some buildings in a few blocks of Salt Lake.

Kolkata (Calcutta) is located in eastern India, in the province of West Bengal, at the lower end of the Ganges plain. The area forms a part of the lower deltaic plains of the Ganga river system. River Hooghly flows past the western part of Kolkata and the South 24 Parganas district forms the southern and southeastern boundary while the North 24 Parganas district forms the eastern and northern limits of the city.

The subsurface lithology consists of sediments of Quaternary Age. Occurrence of groundwater in the area of Kolkata is controlled by the geological set-up. An aquiclude represented by clay and silty clay with an average thickness of 25 m to 40 m occurs at the top of the sedimentary sequence. The aquiclude is underlain by silt and fine to coarse sand, occasionally mixed with gravels. These sand beds are interconnected and behave as a single unconfined aquifer system (Chaterji et al., 1964) and open themselves at much distance from the periphery or fringes of greater Calcutta (Kolkata Metropolitan Development Authority, 2000).

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