Uranium and Strontium Isotopic Study of the Hydrology of the Alluvial Aquifer at the Rifle Former U Mine Tailings Site, Colorado

Abstract:

The Rifle Site consists of a floodplain along the Colorado River that was remediated through the removal of surface material underlying former uranium-vanadium mill tailings. The semi-arid (precip. = ~30 cm/year) catchment for the site has an area of ~1km². The Rifle Site provides an excellent field laboratory for the study of the fluxes of water and carbon from the vadose zone to groundwater (LBNL SFA2.0, http://esd.lbl.gov/research/projects/sssfa2/). A network of monitoring wells, particularly a set instrumented in the vadose zone, provide the opportunity to closely sample groundwater and vadose zone porewater both in space and time. In order to better understand the spatial and temporal variation of vadose zone interaction with groundwater within the Rifle floodplain and provide constraints for a Rifle hydrological model, we have analyzed the Sr isotopic compositions, ²³⁴U/²³⁸U activity ratios, and d²³⁸U of groundwater, vadose zone porewater (sampled through depth-distributed lysimeters) and surface water including the Colorado River. Significant contrasts in ⁸⁷Sr/⁸⁶Sr and ²³⁴U/²³⁸U allow the identification of different sources contributing to Rifle groundwater, while d²³⁸U provides an additional tracer and insights into redox processes.

Vadose zone porewater is characterized by high ⁸⁷Sr/⁸⁶Sr and Sr concentrations and falls at one end of a mixing line with Rifle groundwater, while upgradient groundwater with lower ⁸⁷Sr/⁸⁶Sr and Sr concentrations falls at the other end. A mixing model using vadose zone porewater and upgradient groundwater as endmembers suggests that the contribution of vertical recharge through the floodplain increases to ~20% systematically across the floodplain towards the Colorado River. An exception to this pattern is a well located 150m from the river with recent high U concentrations (>300 ppb) and U and Sr isotopic compositions consistent with a 38% vadose zone contribution. U and Sr isotopes show that an irrigation-return ditch that cuts the site near its eastern edge has significant hydraulic connectivity with the aquifer affecting nearby wells and constraining the boundary conditions for the Rifle hydrologic model. Vadose zone porewater has d²³⁵U ranging from 0.03 to 0.6‰ reflecting local smear-zone redox processes or the history of ore sources processed at the Rifle Mill.