US EPA

1344308 

Journal Article 

Characterization of arsenic-bearing sediments in the gangetic delta of West Bengal, India 

Chakraborti, D; Basu, GK; Biswas, LK; Chowdhury, UK; Rahman, MM; Paul, K; Chowdhury, TR; Chanda, CR; Lodh, D; Ray, SL 

2001 

27-52 

WOS:000177816200003 

In the Ganges-Meghna-Brahmaputra (GMB) delta of West
Bengal, India, and neighboring Bangladesh ingestion of arsenic-contaminated groundwater has
caused serious and widespread human health effect. In West Bengal about 55%, and 34% of 90,000
hand tubewells we had so far analyzed from affected districts by FI-HG-AAS showed arsenic values
above 10 mug/l and 50 mug/l, respectively, while in Bangladesh the values were 73% and 59%,
respectively, out of a total of 27,000 samples we had analyzed so far jointly with the Dhaka
Community Hospital. Altogether over 100 million people are potentially at risk in these two
countries. The source of arsenic is geologic. Examination of the sediments (it = 2235) from 112
boreholes showed that 85 samples contained arsenic 10-196 mg/kg and opaque particles separated
from the parent sediments contained arsenic up to 2778 mg/kg. Mineralogical studies from several
laboratories clearly show that both pyrite and iron oxide contain very high concentrations of
arsenic. If oxidation of pyrite is the source of arsenic in groundwater then there are two
questions to be answered: (a) whether arsenic-rich pyrite is sufficient to account for the mass
of arsenic mobilized in groundwater; and (b) whether increased pyrite oxidation has led to
increased arsenic and sulfate concentrations. Ore microscopic examination using reflected light
indicates abundant highly reflective opaque particles, suggesting the presence of sulfide
minerals. Electron microprobe analysis of the pyrite particles revealed arsenic contents ranging
from 0.07 to 1.36 wt%,,. Gypsum may be a product of oxidation of sulfur in pyrite to sulfate
followed by dissolution to calcite common to borehole samples studied. We have also identified
framboidal and other secondary pyrite; this might partially explain the low sulfate
concentrations in groundwater. Interviews of thousands of old people in affected area lead LIS to
conclude that arsenical skin lesions were not present in those areas before the early eighties.
Also, we have found that initially low-arsenic groundwater from some tubewells now contains
unsafe arsenic concentrations. We believe that a single mechanism is not responsible for the
release of arsenic from groundwater aquifer sediments in this region. 

groundwater arsenic contamination in GMB-delta; regional setting; borehole sediment analysis using XRD; EPMA; SEM; LAMMA; arsenic-rich pyrite; probable mechanism of arsenic leaching from source to aquifer