Mineralogical study of arsenic-enriched aquifier sediments at Santiago del Estero, Northwest Argentina

Abstract

Ashallowaquifer in an alluvial fan at Santiago del Estero, northwestern Argentina, is enriched in arsenic (As). Sediments from sites with high As concentration were collected by hand auger and studied by X-ray diffraction and by electron microprobe. X-ray diffraction confirmed the presence of quartz and albite. Concentration of total organic carbon (TOC) in soil is low, but concentration of total inorganic carbon (TIC) may be significant. The electron microprobe investigation found abundant glass particles with fluidal structure. The grains show significant weathering features such as voids and dissolution pits. Biotite grains present in sediments are also weathered. Iron oxyhydroxides occur in isolated spots on the surface of silicate minerals, but not as continuous coatings. Heavy minerals were represented by altered ilmenite, monazite, zircon, and garnet with predominant almandine component. The primary source of As could not be determined unequivocally, but volcanic glass and biotite are potential candidates. Ferric oxyhydroxides, which are important adsorbents of As, seem to have formed by precipitation of iron released from minerals like titano-magnetite, and ilmenite. However, the amount of precipitated ferric oxides and hydroxides is low and, furthermore, their As adsorption capacity depends on factors like pH and ionic strength of groundwater and on concentrations of species competing for adsorption sites