T84 Recent Advances in Studies of Dissolved Arsenic and Other Metals in Global Hydrologic Systems I

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room M100FG

SPATIAL VARIABILITY IN SEDIMENT ARSENIC CONCENTRATIONS, MURSHIDABAD DISTRICT, WEST BENGAL, INDIA

Paper number: T84-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

Multiple studies in the Bengal basin have shown that elevated As in groundwater coincides with reduced, relatively dark sediments of Himalayan provenance. In West Bengal state (India), As concentrations > 10 μg/L tend to occur east of the River Bhagirathi-Hoogly, the main distributary of the Ganges. We examined associations among sediment chemistry and mineralogy for four cores from either side of the Bhagirathi-Hoogly (cores 1 and 2 to the east, 3 and 4 to the west) in Murshidabad district. Ten sediment samples were collected from each boring at various depths to a maximum of 38 to 43 m. Particle-size distributions were determined by sieve and hydrometer analyses for cores 1–3. Median grain size for 23 of 30 samples was medium to fine sand. Total As was extracted from sand-sized samples (29 of 40 overall) in cores 1–4 by HNO3-HF digestion in a Parr bomb, then analyzed by ICP-OES. Total carbon was measured for 20 sediment samples by CNS analyzer. TIC was determined gravimetrically by HCl digestion and TOC was calculated by difference. Mineralogy of the same 20 samples was identified using thin-section petrography. Compositions of selected grains were confirmed and relative As concentrations were determined by EDS on an electron microprobe.

Arsenic was detected in all sediment samples analyzed (range 1.99–17.94 mg/kg) but was > 5 mg/kg in only three samples. Concentrations decreased with depth in core 4, which was collected ~ 1 km west of the Bhagirathi-Hoogly. No vertical trend was evident in As concentrations for other cores. Total C was mostly inorganic, with concentrations of 0.3–1.7% in cores 1, 2, and 4. The highest As, TIC, and TOC concentrations occurred in the shallowest sample analyzed from core 4 (20 m depth). Total C was < 0.1% in core 3, which was collected ~ 5 km west of the river and appeared oxidized. Cores 1, 2, and 4 were dominated by quartz, but all cores contained rock and paleosol fragments. Other phases included kaolinite, 2:1 phyllosilicates (e.g., micas), Fe oxides, K-feldspar, calcite, rutile, and, in cores 1 and 2, pyroxenes. All phyllosilicate and pyroxene grains probed contained As, as did 70% of paleosol fragments. Factors other than sediment As concentrations (e.g., a 20-m surficial clay layer) appear to limit As in groundwater in the vicinity of boring 4.