Sedimentary Architecture Modeling to Predict the Spread of Arsenic Contamination in Holocene Ganges River Deposits, Bihar, India

Arsenic contamination of shallow aquifers in Holocene Ganges River sediments in the State of Bihar (India) was first detected in 2002. To date an estimated 24% of the 80 million population of Bihar is exposed on a daily basis to arsenic-contaminated drinking and irrigation water. Arsenic concentrations in Bihar reach 1800 μg/L, whereas WHO guidelines for safe drinking water are 10 μg/L. The arsenic contamination has a geogenic origin and is associated with hydrated iron-oxide coatings on quartz and clay minerals. Microbial respiration in groundwater and surface water triggers the reductive dissolution of solid iron and arsenic coatings in a redox-controlled environment rich in organic carbon. The ongoing inventory of water wells in Bihar shows a large spatial variability in arsenic concentration in the upper 50 m of Holocene fluvial sediments in the Ganges channel belt. The aim of this paper is to link the spatial variability of arsenic-contamination to fluvial lithofacies heterogeneity and inherent permeability distribution.

Core and well-log data analysis in combination with a time-domain electro-magnetic survey and Google Earth mapping of a contaminated area in the floodplain along the south bank of the Ganges River reveals the heterogeneous nature of Holocene succession. Highly-permeable alluvial fan and braided river conglomerate are the dominant lithofacies type below 28 m depth. Overlying are stacked, up to 12-m-thick heterolithic point bar sand and clay deposits. Arsenic measurements suggest an elevated concentration at the boundary between the two lithofacies types. The point bars are associated with abandoned channel deposits, which consist of laminated clay and silt rich in organic material. The high organic carbon concentration suggest that the abandoned channels were the locus of microbial activity and thus responsible for the arsenic release in adjacent point bar sand. Surface mapping shows that: (a) abandoned channel fills commonly surround point bars on all sides and, (b) the point bars comprise well-developed ridge-and-swale topography, which reflects the grain size heterogeneity. A 3D sedimentary architecture model of the area shows that the abandoned-channel fill in combination with the heterolithic point bar top creates an effective stratigraphic traps for poorly-flushed arsenic-enriched groundwater. Awareness of the lithofacies conditioning will help predict the spatial propagation of arsenic in the entire Ganges channel belt.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California