Field Evaluation of Arsenic Speciation in Sediments at the Ground Water/Surface Water Interface.

The speciation and mineralogy of sediments contaminated with arsenic at the ground water/surface water interface of the Ft. Devens Super Fund Site in Ft. Devens, MA were determined using X-ray absorption fine structure and X-ray diffraction spectroscopy.  Speciation and mineralogical results indicate arsenic is predominately present in the reduced form of arsenite.  Spatial patterns in the distribution and concentration of arsenic in the study location combined with major element ground water chemistry indicate discharge of contaminated ground water as the primary source for arsenic.  Adsorption of arsenic onto ferrihydrite derived from the oxidation and precipitation of ferrous iron from groundwater, by dissolved oxygen the surface water column, appears to be the primary mechanism by which arsenic is sequestered.  Reducing environments capable of iron reduction at the ground water/surface water interface indicate a potential instability of arsenic contaminated sediments over and extended time periods.