NATURALLY-OCCURRING ARSENIC IN GROUNDWATERS OF THE MIDWESTERN UNITED-STATES

High concentrations of naturally occurring arsenic are present in alluvial groundwater systems in the mid-western United States. These occurrences tend to be sporadic because the arsenic is mobilized only under a narrow range of redox conditions. The reducing conditions must be sufficient to reduce and dissolve iron and manganese but not to produce sulfide. Typically, the affected aquifers are relatively high in clay content and of relatively low yield. For that reason, many of these arsenic occurrences are in aquifers supplying single families. The mechanism by which the arsenic is mobilized begins with the deposition of iron oxides during streamflow while the alluvium is being deposited. The oxides have a strong affinity for dissolved arsenic and adsorb it from the streamwater. As the alluvium is buried, it eventually becomes subject to slow groundwater movement. As conditions become more reducing, ferric oxides are reduced to soluble ferrous oxides, resulting in mobilization of the adsorbed arsenic.