CHANGES IN THE BINDING OF ARSENIC TO SEDIMENT IN SOUTHERN BANGLADESH – A RECORD OF MICROBIALLY FACILITATED TRANSFORMATIONS

The USGS with the Geological Survey of Bangladesh and the Bangladesh Water Development Board are investigating why ‘deep’ aquifers (>50 m) in parts of Bangladesh commonly produce water with low dissolved arsenic contents. As part of this effort, sediments from two boreholes drilled to ~400 meters near Kachua and Rajoir in southern Bangladesh were sampled at 10 to 20 meter intervals. Chemical and mineralogical analyses of the sediment detected three zones of authigenic iron minerals that affect dissolved arsenic concentrations. Goethite is the prominent authigenic iron phase in the shallow, unsaturated sediment (<3 m). From the top of the saturated zone to about 60 m, ferrous iron containing carbonates and phosphates are present. This interval also tends to produce water with high concentrations of dissolved arsenic. Below 60 m, pyrite is a significant component of the sediment and the water contains little dissolved arsenic. Preliminary results from Rajoir are similar.

The sequence of iron minerals is a product of microbes consuming organic matter and reducing available electron acceptors (O2, FeOOH, and sulfate). Total arsenic content of the sediment ranges from <2 to 280 ppm with highest contents measured at 2 and 326 meters. Arsenic in the unsaturated zone (1 to 5 meters depth) occurs primarily as As(V) bound goethite and other ferric oxyhydroxides. Formation of the arsenic-rich FeOOH in local accumulations is attributed to an abrupt redox transition at the top of the saturated zone. From the top of the saturated zone to approximately 60 meters, the sediment contains arsenic as As(III) and abundant 0.5N HCl extractable Fe2+. Arsenic was detected at levels up to 2000 ppm in weathered detrital grains of biotite and chlorite from this interval. Arsenic in these grains is tentatively attributed to unidentified secondary phases formed within the weathered grains. Below 60 meters arsenic is strongly correlated with sulfur. Microbeam analyses detected <0.01 to 1.2 wt.% arsenic within framboidal to massive authigenic pyrite. The low amount of dissolved arsenic beneath 60 meters is attributed to partitioning of arsenic into pyrite. Although low in dissolved arsenic, water in the deeper zone tends to contain high amounts of iron, manganese and TDS.