Colloid-Facilitated Transport of Plutonium at the Nevada Test Site, NV, USA

Abstract

Increasing observations of subsurface actinide transport has reinforced the need for a basic geochemical and hydrologic understanding of how these contaminants are transported in groundwater. At the Nevada Test Site, NV, USA, low-levels of plutonium have been shown to migrate on the scale of kilometers. At all but one sampling location the Pu is associated with the colloidal fraction (<1 μm) consisting predominantly of clays and zeolites. The majority of the Pu (∼70%) is associated with the smallest nanoparticle (e.g., colloid) size fraction of 10–100 nm and in one case was identified as associated with a clay colloid. At one unique sampling site, Pu is predominantly associated with dissolved organic matter. Monitoring at over 20 contaminated groundwater sites at the NTS suggests that low-levels of Pu can be mobilized either via colloid-facilitated transport in fractured rock or as a result of aqueous Pu stabilization by anthropogenic levels of dissolved organic matter. Nevertheless, activities above the EPA’s Maximum Contaminant Level (MCL) for alpha-emitting radionuclides (15 pCi/L, 0.56 Bq/L) have been measured at only two locations at the NTS.