A long-term field and modeling study of hexavalent uranium (U(VI)) migration in an unconfined alluvial aquifer has been conducted to identify key hydrogeochemical processes that control U(VI) mobility. The field study was conducted at the site of a former uranium mill near Naturita Colorado and is adjacent to the San Miguel River. Uranium was extracted from ores at this site by salt roasting and carbonate extraction and residues were deposited on site. Residual U(VI) then infiltrated into the aquifer over a period of several decades resulting in a 0.7km long plume. Flow and transport in the aquifer was therefore characterized using observed U(VI), alkalinity and age dating results. The U(VI) concentrations at the site were spatially variable with a U(VI) concentration that ranged from background values of 0.01µM up to 10µM. Alkalinity values ranged from 5 to 18 meq/L and had a nearly constant pH equal to 7.1. Groundwater samples collected over a 30 year period by DOE and USGS generally showed a gradual decrease in U(VI) concentrations from a maximum concentration of 10µM in 1986 to 2.5µM at many wells by 2014 suggesting a slow desorption of U and a long residence time in the aquifer. This long residence time was likely controlled by both slow advection and slow mass transfer rates. Tritium helium age dating results ranged from 25 to 40 years at the 1km long site providing estimates of advection rates. Reactive transport simulations using a multirate mass transfer model coupled to the U(VI) surface complexation model reproduced the long- term slow decrease in U(VI) concentrations that resulted from leaching the U(VI) from the sediments. Simulations are underway to quantify the impact of parametric uncertainty on U(VI) transport simulations. In addition to the original monitoring well data, we now include the results from nonreactive and reactive tracer test results, local and site-wide geophysical observations and seasonally variable flow fields in the model formulation. The combination of the expanded well sampling results and new types of observations will be used to better quantify the performance of the reactive transport model.

Post audit of a reactive transport model of hexavalent uranium migration in a shallow alluvial aquifer

255th National Meeting (2018)

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