Time-lapse spectral induced polarization imaging of stimulated uranium bioremediation
Tomographic measurements of the spectral induced polarization (SIP) response of aquifer sediments were conducted at the U.S. Department of Energy’s (DOE) Integrated Field Research Challenge site (IFRC) in Rifle, Colorado (USA), where biostimulation research is ongoing with the purpose to immobilize uranium in tailings-contaminated groundwater. The aims of the SIP surveys were to (a) collect data over a sufficiently broad bandwidth so as to determine the characteristic frequency of the spectral response (at which the strongest polarization response takes places), (b) investigate the distribution of spectral parameters (e.g., characteristic time constant) in an imaging framework and (c) evaluate the potential of these images to delineate changes in the hydraulic properties of the aquifer. Careful field procedures provided high-quality SIP data from 0.060–256 Hz for three different periods during the remediation experiment. Data quality was evaluated by means of analysis of the discrepancy between normal and reciprocal measurements. A Cole-Cole model was fitted to pixel values extracted from the inverted images in order to assess changes in the SIP response – particularly in time constant (τ) and chargeability (m) – due to processes accompanying the stimulation of subsurface microbial activity. A significant increase in both τ and m was observed after halting acetate injection, consistent with the accumulation of semi-conductive minerals (e.g., FeS) during biostimulation and the post-injection rebound in aqueous Fe(II). The reliability of the imaged spectral parameters was assessed by means of a numerical study.