Background

Here, we outline the case study of 2-D electrical resistivity and induced polarization imaging reported in the 2010 paper by Slater and others, which focused on improving understanding of the hydrogeological framework regulating exchange of groundwater with surface water of the Columbia River at the United States Department of Energy Hanford 300 Facility, Richland, Washington, USA. The basic hydrogeological setting consists of a coarse-grained aquifer (the Hanford Formation) underlain by a lower permeability, fine-grained confining unit (the Ringold Formation). A legacy of nuclear waste processing and disposal at the site extending through the Cold War era resulted in significant potential for radionuclide-contaminated groundwater to discharge into the Columbia River. The existence of relict paleochannels incised into the Ringold Formation had previously been proposed to provide preferential flow paths promoting rapid transport of contaminants from the aquifer into the river. The risks of radionuclide contamination led to a high cost of drilling at this site, encouraging the use of geophysical surveys to understand the structure of the region of interaction between surface water and groundwater. Waterborne surveys have been successfully used to investigate coastal processes and groundwater-surface water exchange in other systems (e.g., Day-Lewis et al., 2006).

Data Collection

Two-dimensional resistivity and induced polarization imaging surveys were performed to improve estimates of the spatial variability in the depth to the contact between the Pleistocene Hanford formation and the Pleistocene Ringold Formation. The surveys were designed to explore for evidence of incisions into the Ringold Formation that might represent the location of high-permeability paleochannels. The rationale for the application of IP was a suspected strong contrast in polarizability between the coarse-grained, Hanford sediments (low polarizability) and the fine-grained Ringold sediments (high polarizability). The acquisition of ER measurements alone would have been less informative because of the expected influence of variations in the groundwater electrical conductivity due to variable surface water-groundwater interaction on the electrical images.

To rapidly image a long reach of the river corridor, measurements were acquired using a floating array of 13 graphite electrodes spaced at 5 m intervals pulled behind a boat. In this study, ER and IP measurements were performed on approximately 30 km of 2-D line profiles in water depths varying from 2 m to 18 m. Data were collected in July 2008 from a Gregor aluminum-hull jet boat, using a 10-channel time-domain ER/IP instrument (Syscal Pro, Iris Instruments, France) as shown in Figure 12. This time-domain instrument records the apparent integral chargeability (Equation 4) determined from the decay curve after current shutoff. Measurements were recorded every 0.5 to 3.0 m depending on survey speed, resulting in more than 65,000 measurements over the 30 km of line.

Figure 12 – Photographs of deployment of waterborne resistivity and induced polarization data acquisition at the Hanford site. a) Gregor jetboat used for data acquisition, b) deployed floating array concept (yellow symbols depict 8 of 13 electrodes, remaining 5 are closer to boat).

Data Processing

The waterborne resistivity measurements were inverted for an estimated subsurface distribution of electrical conductivity and chargeability using the commercially available RES2DINV package (Loke et al., 2003). The variable-thickness water layer was constrained to a uniform conductivity and zero chargeability (water is non-polarizable at low frequencies). The dataset was treated as a series of near-parallel 2D lines for individual inversions. Electrode locations were calculated from a GPS located on the boat and knowing the length of the electrode takeouts on the cable pulled behind the boat. Each line was then inverted using the conventional smooth regularization constraint except for the fixed/known surface water layer.

Data Interpretation

Figure 13 shows the 2-D inversion of one line (Line 20 approximately 20 m from the shore). The inverted electrical resistivity distribution (lower panel) and the inverted normalized chargeability (Mn = M/ρ) (upper panel) are shown. This normalization of the chargeability by the resistivity provides a direct measure of the polarizability that is unaffected by variations in pore fluid conductivity of the groundwater and therefore is exclusively related to the physical properties of the sediments (Slater and Lesmes, 2002). The riverbed is shown as a black line.

Figure 13 – 2D resistivity and induced polarization image acquired along a 3 km section of the Columbia River, WA, USA corridor. Black solid line is base of water layer and white dashed line is interpreted contact between aquifer and underlying confining unit. Crosshairs denote suspected paleochannels incised below the contact of the aquifer with the confining unit. Modified from Slater and others (2010).

Beneath the riverbed, the resistivity image is primarily composed of a higher resistivity layer underlain by a low resistivity layer that appears to come into contact with the riverbed between 1100-1500 m along the line. The upper resistive layer was interpreted as the coarse-grained, Hanford formation sediments with the lower, more conductive layer representing the finer-grained Ringold Formation sediments. The IP image (normalized chargeability) provides a much clearer picture of this hydrogeological structure. The water layer and the coarse Hanford formation sediments are both low polarizability whereas the fine-grained Ringold sediments are highly polarizable. From this IP image, the variation in the depth to the Hanford-Ringold interface along this portion of the river corridor is highlighted (white dashed line). The IP image reveals strong evidence for the location of at least two coarse-grained paleochannels incised into the Ringold sediments (black crosshairs). One of these paleochannels was subsequently mapped with ER and IP measurements performed inland (Mwakanyamale et al., 2012). Overall, this 2-D ER and IP survey notably improved understanding of the hydrogeological framework along this important river corridor.