Surface water transport and distribution of uranium in contaminated sediments near a nuclear weapons processing facility

Abstract

The extent of remobilization of uranium from contaminated soils adjacent to a nuclear weapons processing facility during episodic rain events was investigated. In addition, information on the solid phase associations of U in floodplain and suspended sediments was assessed by an eight-step sequential extraction procedure to gauge U chemical lability and its propensity for transport. Comparisons were drawn between the easily dispersible, or water dispersible clay fraction (WDC) of the floodplain sediments to the stream suspended sediments transported during storms. Mass flux estimates determined from base flow measurements potentially underestimate the amount of U transported from contaminated terrestrial sources to surface water systems. During the storm events measured, approximately 145 7 to 2 8 3 8 % more U was mobilized to Upper Three Runs Creek (UTRC) relative to base flow calculations. The suspended sediment load transports the bulk of U in labile forms predominantly as acid soluble (specifically adsorbed), MnO2 occluded and organically bound phases. This implies that U may be available to the environment under a range of changing conditions (e.g., Eh and pH). Sequential extractions of the floodplain sediments demonstrated the presence of chemically labile forms, but in different proportions to the suspended sediments. More U was associated with the organically bound phases in the floodplain sediments, while the easily dispersible fraction of floodplain sediments correlated with the suspended sediments. A strong relationship exists between the suspended sediments and the WDC fraction, suggesting that fine particles are eroded from the floodplain and transported in labile forms. This study demonstrates the need to revise current monitoring schemes to include mass transport evaluation during storms. In addition, sequential extraction studies provide knowledge of U chemical lability in contaminated sediments, which may suggest environmentally sound and more cost effective remediation techniques than ones currently in use.