Monitoring environmental remediation with reductive iron amendments via complex electrical conductivity

Abstract:

Geophysical methods can be used during active biogeochemical remediation as indicators of amendment delivery, reactivity, and secondary transformations. Direct current electrical resistivity tomography (ERT) methods can be employed to measure spatiotemporal changes in the bulk electrical conductivity during injection. This geophysical technique is commonly used to correlate changes in the bulk electrical conductivity with reactions occurring in the subsurface. However, electrolytic conduction pathways sampled with ERT cannot be correlated directly with specific surface alterations or reactions induced by an amendment. However, alternating current (AC) measurements via spectral induced polarization (SIP) collects frequency-specific electrical impedance and AC current phase shifts which is sensitive to both electrolytic conduction pathways and surface polarization. SIP can be used to distinguish between aqueous properties (e.g., ionic strength) observed in the real conductivity term and surface reactions (e.g., adsorption, precipitation, biofilm growth) that modify surface properties (e.g., surface area, grain size, polarizability) observed in both the real and imaginary conductivity terms.

In this research, we evaluated iron amendment injection, delivery, and subsequent reactivity at the laboratory scale. Three phases of experiments were conducted in saturated columns with monitoring of major geochemical parameters (e.g., solution conductivity, pH, oxidation-reduction potential, and dissolved oxygen) to identify the (1) sensitivity for detection of amendments in static (no-flow) columns packed with targeted amendments, (2) detectability of amendments during in situ amendment injection, and (3) change in SIP signal over time following amendment delivery to quantify reactivity. The greatest sensitivity was observed for conductive iron materials, including zero valent iron, sulfur-modified zero valent iron, and iron sulfide, with detection limits as low as 0.1 wt.%. In addition, changes in imaginary conductivity and phase shifts over time were correlated with geochemical changes (e.g., Fe⁺² adsorption and precipitation). These results highlight the potential for using field-scale SIP to monitor remediation activities.