Threat of land subsidence in and around Kolkata City and East Kolkata Wetlands – A paper in Journal of Earth System Science

This paper in Journal of Earth System Science attempts to estimate the possible rate of land subsidence of Kolkata City including Salt Lake City and the adjoining East Kolkata Wetlands located at the lower part of the deltaic alluvial plain of South Bengal basin. Land subsidence is an environmental hazard which is caused by overdraft of groundwater or oil extraction and results in gradual settling or sudden sinking of the earth’s surface owing to subsurface movement of the materials of the earth. 

Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments. The sediment compacts because the water is partly responsible for holding up the ground. Decline of water table or piezometric surface results in vertical compression of the subsurface materials. Along with vertical compression, lateral compression may also take place due to initiation or acceleration of lateral flow of groundwater. This lateral movement also results in subsidence of the land surface. Any flow or overdraft of groundwater in unconsolidated material should produce some movement of the land surface.

This movement is generally small, but may become very significant where subsurface materials are thick and/or compressible and the groundwater level declines appreciably. Land subsidence may not be noticeable because it can occur over large areas rather than in a small spot. 

Land subsidence is either individual or a combined effect of inelastic compression of the confining bed and/or elastic compression of the solid matrix of the aquifers. Land subsidence due to groundwater overdraft is essentially irreversible in case of inelastic compression of the overlying confining clay bed. It can be stopped only by halting the decline of groundwater level. However, rebound of the land surface is generally insignificant even if the groundwater levels are restored to the height, prior to subsidence.

Demand of groundwater for drinking, agricultural and industrial purposes has increased due to rapid urbanization. The subsurface geology consists of Quaternary sediments comprising a succession of clay, silty clay and sand of various grades. Groundwater occurs mostly under confined condition except in those places where the top aquitard has been obliterated due to the scouring action of past channels.

Currently, the piezometric head shows a falling trend and it may be accelerated due to further over-withdrawal of groundwater resulting in land subsidence. The estimated mean land subsidence rate is 13.53 mm/year and for 1 m drop in the piezometric head, the mean subsidence is 3.28 cm.

The surface expression of the estimated land subsidence is however, cryptic because of a time lag between the settlement of the thick low-permeable aquitard at the top and its surface expression. Therefore, groundwater of the cities and wetland areas should be developed cautiously based on the groundwater potential to minimize the threat of land subsidence.

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