Geochemical evolution of groundwater in the Quaternary aquifer of Calcutta and Howrah, India

Groundwater in the Quaternary aquifer of the twin city - Calcutta and Howrah - occur in a confined condition. Coarse clastic deposits consisting of fine to coarse sand, at places mixed with gravels, form the aquifer: above and below lie clay sequences. The average thickness of the top confining clay bed is 40 m and the aquifer continues down to a depth of at least 296 m below which the bottom clay bed of Pliocene age occurs. During the last five decades, there is a distinct lowering of piezometric head in Calcutta and Howrah. The groundwater flow pattern changed from an open system, which was from north to south in the mid-1950s to a closed one in the mid-1980s with the formation of a groundwater trough. This closed flow system persisted in the 1990s with the increase in the area of the trough. Detailed geochemical investigation of groundwater samples (N = 57) along with statistical analyses (viz. correlation, principal component and mixing analyses) on these chemical data reveal: (i) that the groundwater samples can be categorised into eight hydrochemical facies, which may be assigned to the three broad types - "fresh", "blended" and "brackish" water; (ii) a pronounced and well-defined trend of variation of chemistry of the groundwater (iii) modified connate water, and/or interaction with the aquifer matrix was possibly responsible for the evolution of the "brackish" water; (iv) mixing of "fresh" water and "brackish" water; (in ca 48:52 proportion) isl presently taking place due to heavy groundwater withdrawal and (v) such mixing will cause deterioration in quality, from "fresh" water to the "blended" variety (mean TDS = 724 mg/l). (C) 2001 Elsevier Science Ltd. All rights reserved.