Using NMR, SIP, and MS measurements for monitoring subsurface biogeochemical reactions at the Rifle IFRC site

The Rifle Integrated Field Research Challenge (IFRC) site is located on a former uranium ore-processing facility in Rifle, Colorado (USA). Although removal of tailings and contaminated surface materials was completed in 1996, residual uranium contamination of groundwater and subsurface sediments remains. Since 2002, research at the site has primarily focused on quantifying uranium mobility associated with stimulated and natural biogeochemical processes. Uranium mobility at the Rifle IFRC site is typically quantified through direct sampling of groundwater; however, direct sampling does not provide information about the solid phase material outside of the borehole and continuous measurements are not always possible due to multiple constraints. Geophysical methods have been suggested as a minimally invasive alternative approach for long term monitoring of biogeochemical reactions associated with uranium remediation. In this study, nuclear magnetic resonance (NMR), spectral induced polarization (SIP), and magnetic susceptibility (MS) are considered as potential geophysical methods for monitoring the biogeochemical reactions occurring at the Rifle IFRC site. Additionally, a pilot field study using an NMR borehole-logging tool was carried out at the Rifle IFRC site. These methods are sensitive to changes in the chemical and physical subsurface properties that occur as a result of bioremediation efforts; specifically, changes in the redox state and chemical form of iron, production of iron sulfide minerals, production of the magnetic mineral magnetite, and associated changes in the pore geometry. Laboratory experiments consisted of monitoring changes in the NMR, SIP and MS response of an acetate-amended columns packed with sediments from the Rifle IFRC site over the course of two months. The MS values remained relatively stable throughout the course of the experiment suggesting negligible production of magnetic phases (e.g. magnetite, pyrrhotite) as a result of enhanced microbial activity. However, both the SIP and NMR measurements were found to change as the experiment progressed. Changes in NMR and SIP values are attributed to the microbially induced formation of iron sulfide minerals and changes in the pore geometry. Continuous borehole NMR data were collected at the Rifle IFRC site using the Javelin NMR logging tool developed by Vista Clara Inc. Data were collected in two wells, one at a site that had previously been treated with an acetate amendment, and the second at a background (e.g. control) site. Results from the control well suggest very limited variation in NMR values. However, in the acetate amended well we observed a significant decrease in the mean of echoes (proportional to T2 and free water content) as the water table fluctuated from spring runoff. This decrease appeared to be more pronounced at longer echo spacing, suggesting changes in NMR values are potentially the result of alteration in hydrological properties resulting from microbial biomass and/or mineral accumulation in the pore space.