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TitleArsenic mobilization from source sediments in an aquifer system of West Bengal, India
AuthorDesbarats, AORCID logo; Koenig, C; Pal, T; Mukherjee, P; Beckie, R
SourceAmerican Geophysical Union, annual fall meeting, abstracts; 2014 p. 1
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20140332
PublisherAmerican Geophysical Union
MeetingAmerican Geophysical Union Fall Annual Meeting; San Francisco; US; December 15-19, 2014
Mediaon-line; digital
File formathtml
AreaWest Bengal; India
Lat/Long WENS 88.3167 88.9667 23.2833 22.8000
Subjectshydrogeology; environmental geology; groundwater; groundwater circulation; groundwater flow; groundwater regimes; arsenic; heavy metals contamination; aquifers
ProgramEnvironmental Geoscience, Tools for environmental impacts and adaptation for metal mining
AbstractGeogenic As contamination of groundwater is an enormous public health problem in West Bengal and neighboring Bangladesh. The source of As and its release mechanisms are investigated at a field site in rural West Bengal where As concentrations up to 531 µg/L are measured in domestic wells. The shallow groundwater system is hosted by sediments deposited in a meandering fluvial environment, a typical setting in the Bengal Delta Plain. A numerical groundwater flow model is developed in order to establish the hydrogeological context for As contamination patterns. Reverse particle tracking is then used to map the flow path from each well screen back to its point of recharge. High-As groundwaters are found to be recharged in ponds marking an abandoned river channel; however, the source of As is located within the underlying channel-fill sediments. Analyses of environmental tracers reveal that As release within these sediments is accompanied by a concomitant release of Br and DOC indicating that these species may be decay products of natural organobromines co-deposited along with As. Mass transfer of As to the dissolved phase and its flushing from source sediments are described using a simplified reactive solute transport model. By fitting this model to observations, a characteristic reaction time for mass transfer is estimated at 6.7 years. With the advent of intensive irrigation pumping, bulk groundwater residence times in the source are estimated to have declined from 16.6 to 6.6 years. The ratio of residence and reaction times, a Damköhler number, has declined correspondingly from 2.49 to 0.99, indicating a shift from transport to reaction rate limited As mobilization. Greater insight into the As problem in Asia may be achieved by switching the focus of field investigations from plumes in aquifers to potential contamination sources in aquitards.
Summary(Plain Language Summary, not published)
Groundwater contamination by arsenic from natural sources is affecting the health of over 100 million people throughout Asia and this is the largest mass poisoning in history according to the World Health Organisation. To further understanding on the source of arsenic and on the processes involved in its release, the Geological Surveys of Canada and of India, and the University of British Columbia, undertook a joint research project in an affected village of West Bengal. This paper describes the combined use of computational groundwater flow modeling and geochemical pathfinders to trace the source of arsenic in well water and to estimate the rate at which it is being released. Sediments deposited in abandoned river channels are found to be the source of both the arsenic and the organic matter that fuels its release. This new understanding of the nature of arsenic source sediments is a prerequisite to the development of sound public health mitigation schemes in India and elsewhere.

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