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Abstract
Almost 10% of wells in the USA tested through the USGS National Water-Quality Assessment program produce water with uranium (U) in excess of the drinking water Maximum Contaminant Level (MCL) of 30μg/L. Uranium exists primarily in two oxidation states in the environment, as oxidized U(VI), and reduced U(IV). Under natural conditions U(VI) readily complexes with ligands such as calcium and carbonate forming highly mobile aqueous species. Whereas, U(IV) primarily exists as an immobile solid-phase mineral. Biological and abiotic reactions can impact the mobility of naturally occurring U. One such reaction is the oxidation of U(IV) by oxidants such as molecular O2 and nitrate producing U(VI). A strong correlation was identified between nitrate and U in two major USA aquifers, the High Plains and Central Valley aquifers. However, in regions with solid-phase U(VI)-bearing minerals, e.g. schoepite or U(VI) co-precipitated with calcite could account for immobilization of U(VI) in sediment. Desorption alone may not account for concentrations of groundwater U. Uranium(VI) adsorption is traditionally quantified under oxic conditions. In the presence of U(IV) minerals, oxic conditions can stimulate the oxidative dissolution of U(IV), leading to an underestimation of actual U(VI) sediment adsorption. Thus an anoxic method for determining U(VI) sediment adsorption in the presence of U(IV) minerals in suboxic aquifers should be used. Even where U is present in sediment as an oxidized mineral, nitrate reduction reactions still have the potential to drive U mobilization by local acid production (H+) production at mineral surfaces, thus causing mineral dissolution. The mobility of naturally occurring U is dependent on U valence state, sediment adsorption, and the availability of ligands for complexation or precipitation. As an influx of an oxidant such as O2 or nitrate into aquifers occur, the potential for mobilization of naturally occurring U from sediments exist.
