Although abiotic/biotic technologies have shown some success in initially degrading/mineralizing energetics, in many cases degradation rates are not sustained over time, due to partial oxidation of the reduced zone by other oxidants present (i.e., dissolved O2, co-contaminants). Prediction of the long-term reactivity requires characterization of the spatial distribution (and temporal changes) of reduced phases, which is challenging at field scale. Time-lapse electrical resistivity tomography (ERT) has been used successfully for decades to track high ionic strength injections, but little work has been done to use ERT/induced polarization (IP) for monitoring the long-term performance of abiotic reactive systems. Overall, the linkage necessary to relate changes in electrical properties to changes in geochemical properties for a given subsurface abiotic reactor is not well established.
The objective of this project is to determine the relationships between abiotic reactivity of reducing systems and ERT/IP imaging during reductant injection and subsequent long-term oxidation. Specific technical objectives include determining: (1) if geochemical changes due to abiotic/bioreaction of mineral are quantifiable by changes in complex electrical resistivity; (2) how sensitive ERT/IP is to detecting geochemical phase changes for four reduction technologies that involve different iron phase mass, minerology, and spatial distribution; (3) how sensitive ERT/IP is to evaluating the longevity of the reduced zone reactivity during reduced sediment oxidation; and (4) how significant the spatial averaging of geochemical reactions and ERT/IP is at particle scale and discontinuous iron oxide zone scale. This research will provide the information necessary to quantify how useful ERT/IP imaging is for predicting the long-term spatial distribution of contaminant degradation.
In this project, redox reactivity changes and relationships to changes in complex conductivity with four in situ reduction technologies will be evaluated under five tasks: (1) during single mineral phases at a small scale, (2) during reactant injection in sediments with particle scale geochemical heterogeneities at a 1-D column scale of 30 to 100 cm, (3) during long-term barrier oxidation at a 1-D column scale of 30 to 100 cm, (4) during injection and oxidation in a 2-D flow system with inclusions/layers, and (5) long-term performance at field scale. Because most (except nanoscale zero-valent iron [nZVI]) technologies rely on some aspect of the sediment mineralogy, the project team will also correlate sediment properties (iron oxide mineralogy and concentration) to resulting ferrous iron precipitates. The significance of spatial averaging of contaminant reduction and also ERT/IP in systems with sub-particle scale mineral phase distributions and larger scale chemical heterogeneities will be evaluated in modeling studies for geochemical spatial averaging and through the use of different electrode spacing for ERT/IP.
Quantified relationships between geochemical changes caused by reductant technologies (i.e., FeIII mineral dissolution, FeII phase increase) and corresponding changes in complex electrical conductivity can be used at field sites to quantify the spatial distribution of a reduced zone and to predict longevity. In addition, investigation of spatial averaging ERT/IP images will show how important spatial averaging of the geochemical/geophysical properties is and aid in determining ERT/IP resolution requirements and electrode spacing (cross borehole or surface arrays) at field sites. (Anticipated Project Completion - 2019)