Judith Robinson

This methods brief introduces FLARE (Flow‐Log Analysis with Radial Exchange), a spreadsheet‐based inverse modeling tool for reconstruction of the vertical concentration profile (VCP) around long‐screened wells or open boreholes affected by borehole flow. In such wells, vertical flow and radial exchange can mix water from multiple aquifer intervals, causing sampled concentrations to differ from true aquifer conditions. FLARE uses flow logs and sampled concentrations to construct a forward flow‐and‐mixing model and formulate a regularized inverse problem to reconstruct the VCP in the aquifer surrounding the well. Implemented entirely in Excel without macros, the tool provides accessible calculations, traceable formulas, and diagnostic outputs that quantify sensitivity, model resolution, and estimation uncertainty (i.e., posterior covariance). These diagnostics help distinguish well‐constrained intervals from those where limited information or unfavorable flow conditions reduce interpretive confidence. An example application demonstrates how flow‐controlled mixing can cause sampled concentrations to misrepresent local aquifer conditions and documents the use and interpretation of FLARE's inversion and diagnostic results. FLARE enables practitioners to evaluate borehole‐flow impacts, extract more information from existing wells, and better target additional depth‐discrete characterization or focus remediation efforts where needed.

Soil organic matter (SOM) interactions with minerals, such as ferrihydrite, are fundamental to processes that control carbon sequestration, nutrient cycling, and maintaining soil health. This study investigates the sensitivity of geoelectrical methods, such as induced polarization (IP), to the formation of organo-mineral complexes, focusing on ferrihydrite (a common mineral in soils), and pentaglycine (a component of the SOM). Multiscale experiments (millifluidic system and column) using ferrihydrite-coated sand, ceramic beads, and a polydimethylsiloxane (PDMS) pore network show a large increase in imaginary conductivity (an IP parameter), following the initial pentaglycine injection, which may be attributed to surface complexation reactions. Subsequent decreases in the imaginary conductivity suggest surface saturation or reduced adsorption site availability. Complementary scanning electron microscopy (SEM) imaging revealed morphological changes on the ferrihydrite aggregate surface, supporting IP measurements. Notably, a consistent strong IP response is observed across both the millifluidic and column systems, validating the reproducibility of these measurements under controlled conditions. Overall, these findings demonstrate the potential of geoelectrical methods such as IP as a noninvasive tool for monitoring complex interactions at the mineral-fluid interface.

Surface electrical resistivity tomography (ERT) was used at a waste site to monitor vadose zone changes in electrical properties as a proxy for contaminant flux over a span of 17 years. The BC Cribs and Trenches (BCCT) site at the Hanford site contains 20 disposal trenches and six disposal cribs. Wastes include a large inventory of technetium‐99 and large masses of nitrate and uranium‐238. ERT data were collected along 41 profiles in 2005 to characterize regions of elevated bulk electrical conductivity (BEC) associated with past liquid waste discharges. Previous analyses performed on samples from four boreholes showed a high correlation between nitrate concentration and BEC. In 2022, ERT data were re‐collected along the same profiles and six additional profiles in an area not previously surveyed. Compared to background uncontaminated areas, BEC was higher in contaminated areas at the waste sites. Given the correlation between nitrate concentration and BEC previously found at this site, ERT images show the spatial distribution and relative ionic concentration of vadose zone contaminants at BCCT. Between 2005 and 2022, ERT difference images showed a decrease in BEC surrounding most waste sites, with exceptions where there were known anthropogenic surface changes. An evaluation of recharge‐driven nitrate migration using synthetic flow and transport simulations showed that downward migration causes a decrease in BEC from the decrease in ionic strength at the trailing end of the plume where contaminants migrated downward. From this, we interpret ERT difference images as showing the predominant regions of downward ion flux.

Co-precipitation of contaminants within the crystalline structure of calcite is a promising natural attenuation or remedial technology being considered at contaminated sites. We explore the sensitivity of the spectral induced polarization (SIP) method to induced calcite precipitation in natural sediments as a path forward to non-invasively monitor these sites. We performed time-lapse column experiments using phased (I–IV) injections over 40 d on natural sediments from the Hanford Site (WA, USA). In the phased injections, abiotic calcite precipitation was induced and confirmed to have occurred. Previous work on glass beads and homogeneous sand was limited to high frequency detection of calcite, however in this work we observed the development of two polarization mechanisms, one at high frequency (>100 Hz) and one at low frequency (L) were computed where the adsorption mode of Na⁺ versus Ca²⁺ was compared by using diffusion coefficients corresponding to Na⁺ versus an arithmetically averaged value for Na⁺ and Ca²⁺. Using the diffusion coefficient of Na⁺, the high frequency L was found to correlate well with the size of the calcite crystals. The low frequency L correlated well with the individual natural sediment grain sizes within the columns. During late experimental times (day 36 and 40), the characteristic low frequency in two of the experimental columns shifted to lower frequencies (<0.001 Hz) which may signify SIP sensitivity of the formed calcite with the sediment grains. In field applications, the development of a low frequency polarization length scale to monitor calcite precipitation is promising for field monitoring applications, however further laboratory work needs to be performed to examine the SIP sensitivity of calcite formation in the presence of natural sediments.

Hydrogeophysics, Inc. (HGI) conducted an electrical resistivity tomography (ERT) dataset at the BC Cribs and Trenches site, located in the Central Plateau of the Hanford Site. The 20 trenches and 6 cribs received large volumes of liquid inorganic waste in the 1950s, resulting in a large inventory of contaminants in the vadose zone. The objective of the ERT survey was to map plume extents resulting from the legacy discharges. The HGI interpretation of the resistivity data was performed using geometric inversion to interpolate 2D lines into a 3D image. To demonstrate a newly developed geophysical code capability (E4D), the resistivity data were re-processed to fit a full 3D model of the bulk electrical conductivity. This proof-of-concept model inversion was executed in calendar year 2011, as the large dataset was well-suited for the use of high-performance computing. The 3D re-processing of the BC Cribs and Trenches ERT data conducted in 2011 resolved the true bulk electrical conductivity. This means that all of the resistivity data were fit to a single model of the bulk electrical conductivity, with true horizontal and vertical dimensions. This differed from the HGI data interpretation approach that used geometric inversion to process 2D lines independently, which weremore » then interpolated into a 3D image. Both the full 3D re-processing and the 2D interpolation to a 3D image demonstrated a higher electrical conductivity observed immediately beneath the trenches and cribs. The electrical conductivity is strongly correlated with nitrate concentrations, indicating the presence of nitrate and other co-located contaminants. « less

Long‐term monitoring solutions at contaminated sites are necessary to track plume migration and evaluate the performance of remediation efforts. Electrical resistivity imaging (ERI) can potentially provide information about plume dynamics; however, the feasibility and likelihood of success are seldom evaluated before conducting a field study. Coupling flow and transport models with geoelectrical models provide a powerful way to assess the potential effectiveness of an actual ERI field campaign. We present a coupled approach for evaluating the feasibility of monitoring nitrate migration and remediation using 4D time‐lapse ERI at a legacy nuclear waste facility. This kilometer‐scale study focuses on depths below the water table (∼70 m). A flow and transport model is developed to perform simulations of nitrate migration and removal via a hypothetical pump‐and‐treat system. A tracer injection is also simulated at the leading edge of the nitrate plume to enhance the conductivity contrast between the native subsurface and the groundwater fluids. Images of absolute bulk conductivity provide limited information concerning plume migration while time‐lapse difference images, which remove the static effects of geology, provide more useful information concerning plume dynamics over time. A spatial moment analysis performed on flow and transport and ERI models matches well during the tracer injection; however, inversion regularization smoothing otherwise limits the value in terms of locating the center of mass. We find that the addition of a tracer enables ERI to characterize plume dynamics during pump‐and‐treat operations, and late‐time ERI monitoring provides a conservative estimate of nitrate plume boundaries in this synthetic study.