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TitleDetailed vertical profiles of sediment and groundwater chemistry: comparison between high and low arsenic zones in Gotra, Nadia District, West Bengal, India
AuthorBeckie, R D; Sengupta, S; Pal, T; Mukherjee, P K; Hall, G E M; Rencz, A; Desbarats, A J; Koenig, C E M
SourceGeological Society of America, Abstracts With Programs, 2007 p. 518
Alt SeriesEarth Sciences Sector, Contribution Series 20080237
MeetingGeological Society of America Annual Meeting; Denver, CO; US; October 28-31, 2007
AreaGotra; Nadia District; West Bengal; India
Subjectsgeochemistry; hydrogeology; groundwater geochemistry; sediment geochemistry; heavy metals contamination; arsenic; groundwater pollution; pollution
ProgramEnvironment and Health
AbstractSince 2004, The Geological Survey of India and The Geological Survey of Canada have been jointly investigating naturally occurring arsenic in groundwater at a field site in West Bengal, India. Within the village of Gotra, West Bengal, India, high (> 50 ppb) and low arsenic groundwater zones are separated by a transition zone of less than 30 m. This transition provides the opportunity to investigate conditions in the two zones at a single site and thereby identify and understand release mechanisms. Groundwater samples were collected from 48 domestic and irrigation tube wells in Gotra and surrounding countryside. A push-in tool was also used to collect water samples in two vertical profiles, one each in the high and low arsenic zones. pH, EC, alkalinity and dissolved ammonia were measured in the field and water samples were analyzed by ICP-MS, ICP-OES and ion chromatography. In addition, in each zone a continuous 6 cm diameter core was collected to 40 m depth below ground surface. A total of 133 distinct sub-samples of sediment were taken from both cores at intervals of 0.5 m for a near-total digestion analysis. The groundwater geochemistry showed highly variable arsenic values between sample locations, although there was no clear distinction in major - element chemistry between high-dissolved-arsenic and low-dissolved-arsenic zones. The sediment cores showed arsenic values between 1 ppm As in sandy zones up to 20 ppm As in finer-grained material. Sediment arsenic peaks did not coincide with dissolved-arsenic peaks although the highest dissolved and sediment arsenic was observed near the surface. High dissolved arsenic was correlated with high alkalinity, ammonia but was weakly correlated with dissolved iron and inversely correlated with dissolved sulfur. The sediment geochemistry showed that solid-phase arsenic was strongly correlated with solid-phase iron, manganese and the metals copper, nickel, chromium and to a slightly lesser extent zinc. The correlation was stronger in the low-dissolved-arsenic zone than in the high-dissolved-arsenic zone. There was a poor correlation between solid-phase arsenic and sulfur, although any correlation could have been masked by the high solid-phase sulfur detection limits. The solid-phase data suggest that arsenic is associated with oxide minerals, and thus a reductive or desorption process may be responsible for arsenic release, although there is also evidence that arsenic may be sequestered as sulfides in some zones.