GEOCHEMICAL CHANGES AFFECTING ARSENIC IN THE SEDIMENT OF THE GANGES—BRAHMAPUTRA—MEGHNA RIVER SYSTEM

Arsenic (As) concentrations in ground water (up to 2 mg/L) extracted from young alluvial sediment in the plains of the Ganges-Brahmaputra-Meghna (GBM) drainage basin are responsible for the greatest mass poisoning in human history. The GBM drainage basin is 1.6 x10 ⁶ km ² and its rivers annually transport 2 x 10 ⁹ tons of sediment containing an estimated 4000 tons of labile As. The USGS South Asia Arsenic Project is investigating the processes that affect arsenic mobility in the GBM in Nepal and Bangladesh.

Cycling of As between water and sediment and among phases within the sediment begins in the source areas of the Himalayan Mountains and extends to the Bay of Bengal. GBM sediment is largely derived from lithified sediments and crystalline rocks of the Himalayan Ranges. At least 50% of the sediment eroded is deposited on the flood and delta plains where it is subjected to intense chemical weathering. Variations in redox conditions and mineral dissolution/precipitation redistribute As among the solid phases. A steep redox gradient between aerated and water saturated sediment is responsible for formation of secondary ferric oxyhydroxides containing up to 800 ppm As(V). Immediately beneath the water table, ferric oxyhydroxides are reductively dissolved and As(V) becomes As(III) bound in a poorly defined site. Re-exposure of shallow reduced sediment to oxygen, as may occur during erosion by a moving channel, results in oxidation of As(III) back to As(V). Downstream near the Bay of Bengal seawater sulfate is reduced to form pyrite, which is the principal residence of As in buried coastal and estuarine sediment.

Effects of anthropogenic change within the GBM on As cycling remain uncertain but major changes in land-use have undoubtedly modified shallow sediment geochemistry. Jungle, covering much of the land 200 years ago, has been removed to promote intensive agriculture that relies heavily on chemical fertilizers. Multiple rice crops per year require sustained irrigation, in many areas with water containing >100 µg/L arsenic. Accumulation of As from the irrigation water into soils may result in substantial decreases in crop production. Sustaining irrigation has also resulted in construction of large dams that have changed the pattern of annual water and sediment distribution across the basin.