Investigating the potential for long-term permeable reactive barrier (PRB) monitoring from the electrical signatures associated with the reduction in reactive iron performance
Research Objective The objective of this project is to quantify the ability of the electrical induced polarization (IP) method to noninvasively monitor the reduction in reactive iron performance that is known to reduce the effectiveness of the permeable reactive barrier (PRB) with time. The primary scientific goals include: A] fundamental laboratory studies to evaluate the sensitivity of the IP method to: Fe 0 total surface area Fe 0 surface chemistry physical/chemical changes to the Fe 0
... e resulting from oxidation and precipitation B] monitoring of the electrical tomographic response of the Kansas City PRB over a three-year period and assessment, via correlation with aqueous geochemical data and extracted iron cores, of whether electrical signatures associated with reduced PRB performance are resolvable in field studies C] optimization of a three-dimensional tomographic imaging algorithm for application to highly conductive, high electrical contrast environments as represented by a PRB IP theory and empirical data resulting from the original development of the method for mineral exploration suggests that the method is highly relevant in the study of reactive iron barriers. Laboratory and field IP studies on mineral deposits illustrate the sensitivity of IP parameters to metal concentration, particle size and metal surface chemistry. IP theory, based on electrical (Warburg) impedance associated with diffusive ion transfer to/from the electrolyte to electron exchange sites on the metal surface, provides a framework for interpreting IP signatures of PRBs as a function of redox chemistry. Research progress and implications This report summarizes laboratory, numerical and field work completed after seventeen months of this thirty six month project. Laboratory accomplishments: Postdoctoral research scientist Jaeyoung Choi and PI Slater are responsible for this laboratory research We have conducted a wide range of fundamental studies aimed at determining the low-frequency electrical properties of Fe 0 -sand systems. Our objectives are two-fold [a] determine the sensitivity of IP to reactive Fe 0 surface area and its dependence on electrolyte chemistry [b] monitor the changes in electrical properties that result from precipitation of a range of compounds on the reactive Fe 0 surface. At the time of writing, we have largely completed objective [a] and initiated four parallel column experiments (currently running for 6 weeks) to achieve objective [b]. We find that all our measurements show strong polarization relaxation peaks within our measured frequency range (0.1-1000 Hz). We fit these spectra to relaxation models that provide three important parameters (i) the magnitude of the polarization (ii) the magnitude of the conductivity (iii) the characteristic time of the relaxation.