GROUND WATER QUALITY PROBLEM IN VARIED HYDROGEOLOGIC REGIONS OF WEST BENGAL – AN OVERVIEW

Ground water quality of the State of West Bengal is assessed for the last three decades by various institutions including State Water Investigation Directorate. The parameters determined include pH, specific conductance, dissolved major ions (pass through 0.45 mm pore size membrane filter) like Ca⁺⁺, Mg⁺⁺, Na⁺, HCO3^-, CO3^{- -}, Cl^-, SO4^{- -} . Minor ions like iron, arsenic and fluoride are also included to ascertain the quality of ground water for drinking purpose. Partial analysis of water quality parameter reveals that the quality of groundwater in the State can be grouped into three classes (i) low chloride and high bicarbonate type, (ii) chloride – bicarbonate type, (iii) sodium or magnesium – calcium – chloride type with variable bicarbonate. Since historical past the State of West Bengal is considered as one of richest states in respect of ground water storage . The quality problem in ground water, extracted from shallow depth aquifers (< 100 m. bgl.) has been high level of total iron content in major part of riverine plains. Besides, the coastal aquifers of prograding Ganga – Brahmaputra delta front are infested with both connate and cyclic salinity. These two problems are of perpetual nature. While the first problem i.e. high iron content is usually mitigated by individual capacity, the issue of development and management of the second one i.e. salinity in ground water is under concern of the Government. But after eighties the Government Departments and researchers dealing with ground water are challenged with new facets of problems due to toxicity in groundwater by contamination of arsenic and fluoride.  Out of 341 blocks of the State, ground water in almost all the Blocks are suitable for agriculture except 59 Blocks of coastal region. But for safe drinking water purposes the scenario is complex as a consequence of toxic ion contamination by mainly Arsenic and Fluoride. The problems of Arsenic toxicity in ground water have been detected, mostly in the top and middle aquifers, from 81 blocks of the State. Presence of fluoride in ground water has been detected in the hard rock (basalt and granite) terrain of Birbhum district in 4 blocks only.  Arsenic contamination has also been found in scattered locations of South Kolkata. In the State of West Bengal severe effects of Arsenic toxicity are found in 26 blocks and that for fluoride is 1. Causes of the high level of Arsenic and Fluoride contamination are thought to be geogenic. However, the detailed geochemistry of such occurrences is under study. The Chromium contamination (~3mg/lit) is detected at Raghabpur in Purulia –I block. The source of chromium contamination is under investigation. Groundwater in and around industrial areas of Howrah and Burdwan are prone to contamination by industrial effluents.

 The multidimensional facets of groundwater problems prevailing in the state made it open for SWID to extend its scope of work in all the major user sectors. Significant database in respect of groundwater quality is generated and periodical updating of the same is a continuous process for the SWID within its available resources. Investigations are in progress throughout the state for updating the arsenic and fluoride spread maps. Such a centrally assisted Project for study of arsenic spread was taken up under Rajib Gandhi National Drinking Water Mission in the year 2003. Central assistance may help to take up new projects with the said objective. The Ministry of Water Resources, GOI, has provided financial assistance to SWID to study dilution of Arsenic in ground water by artificial recharge. Another centrally funded R&D project of special nature has been completed in collaboration with B.C.K.V.V., Kalyani and NBSS & LUP, wherein the effect of use of arsenical water for irrigation on selected agro-produces has been studied in parts of North 24 Parganas. Central assistance is needed to continue the study in other areas as well. The mechanism of salinisation in parts of the coastal tracts by isotope techniques was studied jointly with BARC. The study may be continued if fund is made available. Detail research on tube wells sealing methodology is essential for development of groundwater in coastal saline region. Intensive study on aquifer geometry of coastal area may be envisaged by other methodologies like resitivity survey, tracer study, fresh-sea water interface geochemistry and exploratory drilling.