Synthetic and field-based electrical imaging of a zerovalent iron barrier: Implications for monitoring long-term barrier performance

We performed a study of electrical imaging sensitivity to geochemical alteration of a zerovalent iron permeable reactive barrier (PRB) over time. Complex-resistivity measurements of laboratory cores from an operational PRB defined the electrical properties of both unreacted and geochemically altered (reacted) iron, as well as the growth rate of the reacted front on the up gradient edge of the barrier. Laboratory results were used to generate models of the electrical structure of the PRB at 0, 15, and 30 years of operation. Synthetic cross-borehole resistivity and induced-polarization data were generated and perturbed with errors representative of noise at the site. To generate reliable images of the engineered structure, a complex-resistivity inversion was employed with a disconnect in the regularization between the part of the finite-element mesh (FEM) representing the internal structure of the barrier and the remainder of the FEM mesh.Synthetic results show that although the internal structure of inverted images at 15 and 30 years does not accurately reflect the width of the reacted front, modeled along the up-gradient edge of the barrier, perturbations to the internal structure of the imaged PRB are diagnostic of the growth of the reacted front. Cross-borehole electrical data, obtained at the field site during a 15-month period, demonstrate that the complex-resistivity algorithm can resolve reliably the PRB target using the engineering design specifications to define the correct shape of the regularization disconnect. Both resistivity and induced-polarization reciprocal errors are low, and the induced-polarization data are highly repeatable over this period. Changes in the electrical properties of the PRB over time were small but consistent with growth of a reacted front, based on the synthetic study. Significantly, resistivity imaging alone may be sufficient for long-term monitoring of precipitation, leading to reduced PRB performance.