Arsenic exposure through groundwater in the middle Ganga plain in the Varanasi environs,India: A future threat |
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| Authors: | N Janardhana Raju |
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| Institution: | 1.School of Environmental Sciences,Jawaharlal Nehru University,New Delhi,India |
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| Abstract: | The study area covers an about 100 km2 of the middle Ganga plain in Uttar Pradesh, experiencing intensive groundwater extraction. In order to recognize the arsenic
contamination zones of the Varanasi environs, sixty eight groundwater samples have been collected and analyzed for major ions,
iron and arsenic. Twenty one sediment samples in the four boreholes were also collected to deduce the source of arsenic in
the groundwater. The preliminary survey reports for the first time indicates that part of rural and urban population of Varanasi
environs are drinking and using for irrigation arsenic contaminated water mostly from hand tube wells (<70 m). The study area
is a part of middle Ganga plain which comprises of Quaternary alluvium consists of an alternating succession of clay, clayey
silt and sand deposits. The high arsenic content in groundwater samples of the study area indicates that 14% of the samples
are exceeding the 10 μg/l and 5% of the samples are exceeding 50 μg/l. The high arsenic concentration is found in the villages
such as Bahadurpur, Madhiya, Bhojpur, Ratanpur, Semra, Jalilpur, Kateswar, Bhakhara and Kodupur (eastern side of Ganga River
in Varanasi), situated within the newer alluvium deposited during middle Holocene to Recent. The older alluvial aquifers situated
in the western side of the Ganga River are arsenic safe (maximum As concentration of 9 μg/l) though the borehole sediments
shows high arsenic (mean 5.2 mg/kg) and iron content (529 mg/kg) in shallow and medium depths. This may be due to lack of
reducing conditions (i.e organic content) for releasing arsenic into the groundwater. Rainfall infiltration, organic matter
from recently accumulated biomass from flood prone belt in the newer alluvium plays a critical role in releasing arsenic and
iron present in sediments. The main mechanism for the release of As into groundwater in the Holocene sandy aquifer sediments
of Varanasi environs may be due to the reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well
as altered mica content. The high societal problems of this study will help to mitigate the severity of arsenic contamination
by providing alternate drinking water resources to the people in middle Ganga plain and to arrange permanent arsenic safe
drinking water source by the authorities. |
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