The losing ground of Environment.
Kharagpur, WB, June 16 -- In the parts of the West Bengal (WB) lying east of River Bhagirathi-Hoogly, more than 40 million residents use groundwater from shallow, alluvial aquifers, in which high dissolved arsenic concentrations has been previously detected. The general idea is that arsenic contamination is largely restricted to less than 100 m depth and deeper aquifers are safe. However, our recently published, regional study from Murshidabad district to South 24 Parganas district shows that this idea is not accurate. Dissolve arsenic concentrations, much higher than World Health Organization prescribed safe limit of 10 microgram/liter for drinking water, have been observed up to 250 m depth in WB, in ~60% of the sampled 125 deep well sites (~80% north of the latitude 22.75 degN [southern Nadia]). The reason is probably a combination of geologic structure of subsurface, sediment and water chemistry and land-use such as irrigational pumping.
North of latitude ~24 degN in WB (northern and central Murshidabad), potable groundwater is only available to a depth of ~80 to 90 m; below this, there exist a regional-scale, thick clay layer. From 24.5 degN-22.75 degN (Murshidabad to southern Nadia), a regional-scale, continuous, sand aquifer (that thickens southward) provides vertical water flow connectivity from near land surface to a depth of 300 m. This is in contrast to previous studies, which advocated existence of intermediate-depth, thick clay layers between arsenic-rich, shallow aquifers and safe, deep aquifers. Hence, deep groundwater in the aquifer north of 22.75 degN is naturally vulnerable to arsenic pollution. Further, water chemistry deduced from the sampled deep wells and sediment chemistry (analyzed up to 232 m depth) indicates that conditions conducive to arsenic mobilization occur at greater than 200 m depth.
Simulation of regional groundwater flow models that incorporates observed shallow and deep irrigation pumping, shows that arsenic-laden groundwater could have moved to greater than 150 m depth in the continuous aquifer since the early 1970s (beginning of the extensive irrigation pumping in WB). This is substantiated by depth-profiles of environmental tracers (including oxygen and hydrogen isotopes, naturally occurring chemicals) at a local-scale study site in Nadia. Deep pumping for irrigation and public supply could have attracted arsenic polluted water from shallow depth (e.g. 50 m or below) to depth of 150 m in less than 40 years, where as in natural conditions (without pumping), the travel time for same depth would take more than 250 years. It is noteworthy that most of the deep irrigational pumping started in the area only about 40 years ago. About 70% of the surveyed wells, which were installed in recent times, were found to have arsenic concentrations increased within ten years of the time of their installation.
Hence, it can be concluded that deep irrigational pumping would lead to further contamination of deep aquifer in areas between Murshidabad and southern Nadia, if current deep pumping scenario continues in future.
Notes to Editor
Dr. Abhijit Mukherjee is Assistant Professor in the Dept. of Geology and Geophysics, Indian Institute of Technology (IIT) - Kharagpur. He has done Ph.D. (2006) from University of Kentucky, USA. Hewas a Postdoctoral Fellow (2006-08), University of Texas at Austin, USA and a Hydrogeologist (2008-2010) in Alberta Geological Survey, Canada. Presently Dr. Mukherjee is leading several arsenic related projects worldwide (in collaboration with researchers from USA, Canada, Sweden, Bangladesh, China etc.). He is also acting as the Chair for sessions on groundwater arsenic in Geological Society of America and American Geophysical Union (since 2005 to present) and Chair for project on drilling of arsenic affected areas in India, funded by the National Science Foundation (NSF) of USA. He serves as the Associate Editor for the Elsevier press published Journal of Hydrology (4th most highest ranked journal in water studies). In the past he has served as Guest Editor for special volumes on arsenic research in Elsevier press published Journal of Contaminant Hydrology (v. 99) and Applied Geochemistry (v. 26, no. 4). Dr. Mukherjee can be contacted at firstname.lastname@example.org
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|Date:||Jun 16, 2011|
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