Formatted Title

Groundwater Pathway Investigations of Columbia River Basalts in the Yakima Fold and Thrust Belt via Resistivity/Seismic Imaging and Borehole Geophysics

Background/Objectives

The key objective of the environmental contaminant and groundwater investigation activities was to identify and test the groundwater migrating towards the outer boundary of the project site (Site) located in south-central Washington State.  Assessing the groundwater quality using a drilling-only approach with evenly spaced wells would be suboptimal due to several elements of geologic complexity.  Review of past investigation results indicated that the distribution of the contaminants (per- and polyfluoroalkyl substances [PFAS]) was highly variable laterally and vertically and found in multiple aquifer types including permeable portions of basalt flows that are within the Columbia River Basalt Group (CRBG) of the Miocene Epoch (16.7 Ma to 5.5 Ma) and clastic sedimentary materials deposited as interbeds between basalt flow events. Hydraulic characteristics can vary greatly within and between the individual basalt flows, with the tops and bases of basalt flows generally being the most horizontally hydraulically conductive and the flow interiors the least.

The folded and faulted environment of the Site introduces additional uncertainty to the nature of the groundwater flow pathways.  Fault zones, depending on the type of rock that is faulted, can be either a hydraulically conductive or impermeable feature, often nearly vertical and difficult to locate with drilling alone. Folds in rock can create complex networks of fractures, e.g., along anticlinal axes. In the area of interest at the Site, soil and sediment cover conceal the possible faults and folds making it difficult to select the locations of wells.  Due to the various complexities noted above, the team determined that the use of geophysical investigation methods would potentially provide imaging of the large-scale structures and inform the decision-making for selection of well locations.

Approach/Activities

The importance of understanding the distribution of lithologies and their respective hydraulic properties is essential for developing a conceptual site model (CSM) that accurately describes groundwater flow and contaminant transport.  Surface geophysical measurements can reveal details of the macroscale features that, particularly with the absence of outcrops, can provide significant insight in guiding well placement, while rock cores and borehole geophysics add mesoscale detail. The areal coverage and visualizations of geologic features offered by surface geophysics complement and interconnect the information obtained with drilling and installation of a wells. The goals of the surface geophysics were to 1) determine lithology (i.e., basalt versus various types of sedimentary rocks), 2) detect relatively permeable zones within basalt flows, 3) identify folds and fault zones, and 4) identify macroscopic sedimentary structures such as paleochannels or unconformities along a 1.5 mile transect and to a depth comparable to local water supply wells (approximately 200 to 300 feet).

The following geophysical methods were implemented:  1) 2-dimensional electrical resistivity tomography (ERT), 2) seismic refraction and multichannel analysis of surface waves (MASW), and 3) frequency-domain electromagnetics (FDEM).  Both ERT and FDEM provided electrical resistivity data, which is in general related to porosity (primary or fracture), water saturation, mineralogy, and the electrical conductivity of groundwater (i.e., total dissolved solids).  Typically, there is a strong electrical resistivity contrast between basalt and sedimentary rocks.  The seismic methods provide measurements of the acoustic properties that can be related to permeability.

Results/Lessons Learned

The ERT and seismic data sets were processed and modeled and then plotted in cross-sectional form.  ERT in particular, provided visualization of the lithologies as well as the possible presence of folds and faults, which led to a robust CSM.  The MASW seismic data provided insight into possible permeable zones within basalts and sedimentary rock.  Eight drilling locations were selected based on the surface geophysics, contaminant distribution, and other considerations, and monitoring wells were drilled using the air rotary method.  Rock chip samples were collected and described during drilling and the open boreholes were logged with borehole geophysics including natural gamma, single point resistance, three-arm caliper and acoustic televiewer prior to well installation.  There was a high degree of correlation between the drilling observations and the ERT images.  Additional geophysical and drilling investigation activities are ongoing, the results of which are anticipated to be included in the presentation during the conference.