Sustainable abstraction due to unconfined conditions in multi-layered aquifers: Examples from northwest Bangladesh

Abstract

A survey in 1982 by the United Nations of the groundwater resources of Bangladesh suggested that for the Barind Tract in the northwest the aquifers could only meet domestic demands. Despite this negative finding, numerous tubewells have been constructed; two or three crops are currently grown each year. This has resulted in a significant fall in most water levels. The fall in water levels has prompted several publications which use a variety of techniques to anticipate whether the aquifers are over-exploited; these approaches are reviewed critically. A field study of an individual deep tubewell is described, data have been collected for more than seven years. The aquifer is overlain by an aquitard which has a significant impact on the passage of water. Quantified conceptual models are presented for near surface conditions, for the movement of water through the aquitard and for the flow of groundwater through the underlying aquifer to the abstraction well. Of special importance is the development of unconfined conditions beneath the overlying aquitard. This results in a transfer of water through the aquitard to the main aquifer which varies little throughout the year; consequently water levels become stable. The establishment of unconfined conditions beneath an aquitard is a situation which occurs in other heavily exploited multi-layered aquifers.