T88 Uranium in Groundwater: Biogeochemical Controls on Mobility and Persistence

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 211AB

QUANTIFYING THE IN SITU FLUX OF WATER, URANIUM, AND MICROBIAL BIOMASS

Paper number: T88-5
Program: Technical Sessions
Time: 8:55 AM – 9:10 AM

Authors

Abstract

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. Measurable uranium fluxes are essentially the product of concentration and groundwater flux or specific discharge. The sensor uses two sorbents and tracers to measure uranium flux and specific discharge directly; however, sensor principles and design should apply to fluxes of other radionuclides. Flux measurements will assist DOE 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. The methodology for generating flux measurements from BMLS data and independent measures of water flux was validated against independent direct flux measures of sulfate and uranium fluxes using the PFM in wells located in La Quinta and Super 8 galleries. Expected microbial discharge for Eubacteria in the La Quinta gallery was estimated to be 1.7 x 1012 cells per day. 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.