Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4g. Evaluating Surface Water/Groundwater Interactions: Innovative Monitoring Approaches and Modeling Applications

Formatted Title:

Intertidal Geophysics to Improve Characterization of Groundwater to Surface Water Contaminant Transport

Background/Objectives:

At Naval Base Kitsap Keyport in Washington State, a landfill constructed in the 1930s within an estuarine wetland is a source of chlorinated solvents transported in groundwater to surface water and sediment in the adjacent unfilled portions of the remaining wetland and a marine embayment. Post-ROD investigations over the last five years have substantially updated the conceptual site model, showing contaminant transport deeper in the aquifer and along preferential flow pathways within paleochannels in the upper 80 feet of the geologic section. Numeric groundwater modeling implied that contaminants in terrestrial freshwater could daylight into the surrounding wetland and marine embayment. However, the modeling results could not account for some of the observed contaminant concentrations in wells located at distal locations far away from the predicted discharge locations. The project team sought a more detailed understanding of how freshwater and saltwater interacted within the complex site stratigraphy to address these findings.

Approach/Activities:

Temporal intertidal geophysics was used to assess conductivity changes in the subsurface during extreme low-low tides in January 2021. The geophysical investigation included the collection of six high-resolution, multi-electrode electrical resistivity tomography (ERT) traverses using an Advanced Geosciences, Inc. (AGI) SuperSting R8 resistivity meter with either an 84-electrode marine cable or a 56-electrode marine cable. The investigation also included collection of four high-resolution, multi-electrode time-lapse electrical resistivity tomography (TRP) traverses: using an AGI SuperSting R8 resistivity meter and a 56-electrode marine cable. Time-lapse data were collected over 22-hour period as the extreme tide cycled. The ERT and TRP data were processed, corrected for terrain (relative elevation) variations, and were transposed onto the Mean Lower Low Water Datum and analyzed using EarthImager 2-D™ V2.1.7, a two-dimensional resistivity inversion software. The inversion results were then presented in color gradient apparent resistivity models that illustrate the electrical resistivity contrasts in the subsurface materials.

Results/Lessons Learned:

The time-lapse images clearly showed progressive conductivity changes in the subsurface correlated to the tidal cycle, with the subsurface becoming more conductive as seawater displaced freshwater during the flood stage tide, and the subsurface becoming less conductive during the tidal ebb. Most revealing were the spatial variations in the strength of the conductivity changes observed, showing apparent geologic paleochannels in the subsurface in which the conductivity changes over a tidal cycle were more pronounced. For an example traverse, resistivity changed from a low of 5 Ohm-meters (Ohm-m) during high tide to a high of 17.6 Ohm-m during low tide, while much of the remaining survey area remained at 5 Ohm-m. These zones of stronger conductivity change, interpreted as paleochannels, are hypothesized to provide potential preferential flow pathways for movement of contaminated freshwater further into the marine embayment than implied by numerical modeling based on a homogenous and isotropic subsurface.

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Andy Baird