Passive methods for quantifying the In Situ Flux of Water, Uranium, and Microbial Biomass

The goal of this project was to develop a novel sensor that incorporates field-tested concepts of the passive flux meter (PFM) to provide direct in situ measures of uranium and groundwater fluxes. The sensor uses two sorbents and tracers to measure uranium flux and specific discharge directly-sensor principles and design will apply to fluxes of other radionuclides, metals, and co-contaminants. Flux measurements will assist in 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 fluxes of uranium, groundwater and salient electron donor/acceptors are coupled to spatial variations in measured microbial biomass/community composition, effective field-scale uranium mass balances, attenuation, and stability. Field tests in the La Quinta and Super 8 galleries at the Rifle IFRC site were conducted to assess ambient groundwater, uranium, and microbial biomass fluxes. The latter were determined using a newly designed Baffled Multilevel Sampling (BMLS) device installed in typical screened monitoring wells to provide aqueous concentrations of dissolved or suspended constituents over multiple isolated vertical sections of the well. Biomass mass fluxes were calculated from the product of BMLS data for microbial cell counts from PCR analyses and PFM water fluxes collected from coincident well sections. Expected microbial discharge for Eubacteria in the La Quinta gallery was estimated to be 1.7 x 10¹² cells per day. The biomass discharges for Geobacter, Methanogens, and Anaeromyxobacter remain to be determined. Expected uranium discharges predicted from stochastic simulations using PFM measures of flux over the La Quinta gallery transect and the injection-well transect of the Super 8 gallery were 26 mg/d and 183 mg/d respectively. The uranium flux measured by the PFM sensor and Biomass fluxes estimated from BMLS data and PFM water fluxes will be used to characterize microbial community and active biomass at synonymous wells in order to quantify spatial changes in uranium flux and field-scale rates of uranium attenuation (ambient and stimulated).