Characterizing In Situ Uranium and Groundwater Flux

The goal of this project is to develop a new sensor that incorporates the field-tested concepts of the passive flux meter to provide direct in situ measures of uranium and groundwater fluxes. The sensor uses two sorbents and resident tracers to measure uranium flux and specific discharge directly; but, sensor principles and design should also apply to fluxes of other radionuclides. Flux measurements will assist with obtaining field-scale quantification of subsurface processes affecting uranium transport (e.g., advection) and transformation (e.g., uranium attenuation) and further advance conceptual and computational models for field scale simulations. Project efforts will expand our current understanding of how field-scale spatial variations in uranium fluxes and those for salient electron donor/acceptors, and groundwater are coupled to spatial variations in measured microbial biomass/community composition, effective field-scale uranium mass balances, attenuation, and stability. The new sensor uses an anion exchange resin to measure uranium fluxes and activated carbon with resident tracers to measure water fluxes. Several anion-exchange resins including Dowex 21K and 21K XLT, Purolite A500, and Lewatit S6328 were tested as sorbents for capturing uranium on the sensor and Lewatit S6328 was determined to be the most effective over the widest pH range. Four branched alcohols proved useful as resident tracers for measuring groundwater flows using activated carbon for both laboratory and field conditions. The flux sensor was redesigned to prevent the discharge of tracers to the environment, and the new design was tested in laboratory box aquifers and the field. Geochemical modeling of equilibrium speciation using Visual Minteq and an up-to-date thermodynamic data base suggested Ca-tricarbonato-uranyl complexes predominate under field conditions, while calculated uranyl ion activities were sensitive to changes in pH, dissolved inorganic carbon (DIC) and alkaline earth metals. Initial field tests at the Rifle IFRC site were conducted to assess ambient groundwater and uranium fluxes, monitor microbial growth on the sensor during field deployment, and further resolve any unforeseen problems evolving from field deployment. Ten flux sensors were deployed in five wells for three weeks from mid-November to early December 2009. Observed water fluxes varied from 1.2 - 5.3 cm/d while uranium fluxes ranged from 0.01 - 2.2 ug/cm2d. Uranium and water flux variations corresponded closely with changes in lithology. Uranium fluxes were typically observed to increase with depth. Stochastic simulations were conducted to estimate the magnitude of uranium discharge over a 10.5 m2 transect. The mean discharge was approximately 52 mg/d with a narrow 90% confidence interval of ± 11%.