Abstract
In order to document the source of arsenic of the arsenic contaminated groundwater in the Ganges-Bramaptra Plains, detrital biotite and chlorite were analyzed by microbeam XRF in SPring-8. Although the biotite rarely included detectable arsenic, the chlorite included arsenic ubiquitously in the crystal. The arsenic concentration of chlorite could be quantified, a few tens to hundreds ppm was estimated from the X-ray intensities in comparison with arsenic bearing minerals such as realgar and arsenocpyrite. The chlorite included submicron-order size goethite, assured by X-ray diffraction using Gandorfi camera and TEM observation. However, the goethite was not a product of chemical weathering in situ, judged from the well crystallized nature. Unlike the widely accepted hypothesis, in which the adsorbed arsenic onto Fe-hydroxides via chemical weathering of arsenic bearing sulfide minerals in Himalayas was released into groundwater in association with microbial reduction-dissolution of the Fe-hydroxides, the arsenic was released from the chlorite in the aquifer.
