Monitoring in Situ Bioremediation at the Rifle, Colorado IFRC Site with Nuclear Magnetic Resonance and Magnetic Susceptibility Measurements

Abstract

Nuclear magnetic resonance (NMR) and magnetic susceptibility (MS) borehole logging measurements were collected at the Rifle Integrated Field Research Challenge (IFRC) site. The Rifle IFRC site is located at a former uranium ore‐processing facility in Rifle, Colorado. Although removed from the site by 1996, leachate from spent mill tailings has resulted in residual uranium contamination of both groundwater and sediments within the local aquifer. Since 2002, research at the site has primarily focused on quantifying uranium mobility associated with stimulated biogeochemical processes. Ongoing studies at the site include an acetate amendment strategy, in which stimulation of native microbial populations by introduction of a carbon source serves to alter local redox conditions, and immobilization of uranium in insoluble forms. NMR and MS logging measurements were taken before, during, and after acetate amendment. Changes in these signals are expected to correlate with changes in redox conditions and iron speciation thus potentially allowing their use as a non‐invasive tool for monitoring bioremediation. Experimental data was collected from two wells upstream of the acetate amendment, used as controls, and from three downstream wells. The MS measurements revealed vertically stratified magnetic mineralization, likely the result of a detrital magnetic fraction within the bulk alluvium. Data were highly replicable over the monitoring interval, with little to no change observed in the MS measurements, suggesting negligible production of magnetic phases (e.g. magnetite, pyrrhotite) as a result of enhanced microbial activity. NMR measurements had high levels of noise contamination requiring significant signal processing, and ongoing analysis suggests that any changes due to enhanced microbial activity may be difficult to differentiate from simultaneous changes in water content.