THE USE OF LOW FIELD PROTON NUCLEAR MAGNETIC RESONANCE TO MONITOR MICROBIAL AND IRON MINERALIZATION PROCESSES IN SOILS: LABORATORY AND FIELD EXAMPLES

Effective contaminant remediation is often limited by our ability to monitor in situ microbial and geochemical processes in soils and sediments. As such, in recent years non- and minimally-invasive geophysical methods such as spectral induced polarization (SIP) have been successfully applied to characterize contaminant plumes and monitor contaminant remediation. The success of such methods stems from their sensitivity to processes associated with contaminant remediation, including microbial activity and mineral precipitation. However, geophysical datasets generated from a single method can be difficult to interpret, as they are sensitive to multiple processes and thus are not uniquely sensitive to the process of interest. The geophysical method, nuclear magnetic resonance (NMR) is sensitive to the molecular-scale physical and chemical environment of pore-water in geological materials. In our research we are interested in assessing the use of NMR, both alone and in combination with other geophysical methods, specifically SIP and magnetic susceptibility (MS), to monitor biogeochemical processes associated with contaminant remediation.

Using laboratory measurements on model microbial organisms and synthetic sediments, we determined the sensitivity of NMR to the presence of microbes, microbial growth, and iron mineralization. Following this analysis, we evaluated the use of NMR to monitor natural and stimulated redox processes at the Rifle Integrated Field Research Challenge (IFRC) site. The Rifle IFRC site is located at a former uranium ore processing facility in Rifle, CO and efforts are underway to remediate and immobilize residual uranium contamination using natural and stimulated biogeochemical processes. A 15 month continuous NMR borehole logging data set collected in an acetate-amended well showed measureable changes in the NMR relaxation response associated with spring run off, likely due to the oxidation of previously amended sediments. A final set of laboratory NMR, SIP, and MS measurements collected on stimulated and non-stimulated columns packed with Rifle sediments confirmed the results of the field study and allowed us to evaluate the efficacy of these geophysical methods for monitoring the central biogeochemical processes.