Master of Engineering Science
Civil and Environmental Engineering
Power, Christopher A.
Clean-up of sites polluted with dense non-aqueous phase liquids (DNAPLs) remain a highly challenging problem. Numerous technologies are available for remediating DNAPL-contaminated sites, but their performance relies on accurate characterization and monitoring of the subsurface. Electrical resistivity tomography (ERT) is a well-established and widely used geophysical method that has been used effectively for mapping subsurface features and processes of interest. ERT can gather large volumes of continuous subsurface information in a non-destructive, cost-effective, and time-efficient manner, and exhibits highly desirable characteristics for application to DNAPL sites. However, the most traditional configuration for employing ERT is from the surface, and this suffers from poorer imaging quality with increasing depth from the surface. To overcome this issue, which is particularly problematic at DNAPL sites, ERT may take advantage of horizontal borehole technology to enhance image quality at depth.
The objective of this thesis is to evaluate the performance of novel three-dimensional ERT that utilizes both the ground surface and horizontal boreholes to improve characterization and monitoring capabilities of ERT at DNAPL sites. A range of numerical and laboratory tank experiments were conducted on various DNAPL targets in different environments (e.g., water and plastic, sand and NAPL). Results demonstrate the high potential of 3D S2HB ERT for characterizing DNAPL targets, especially when compared to surface ERT. Furthermore, implementation of a single borehole with a 2D surface array (i.e., S2HB-1BH) provided adequate resolving ability compared to a S2HB configuration that utilized a 2D horizontal borehole array matching the surface array (e.g., 11 surface lines and 11 borehole lines) (i.e., S2HB-FULL). This enhanced our understanding of ideal borehole electrode implementation. A full 2D array of boreholes would be highly impractical at DNAPL sites, and the adequate performance by a more practical single borehole is highly encouraging.
Summary for Lay Audience
The release of pollutants from industrial applications can be very damaging to the environment. Many of these pollutants leach into the ground and can cause unsafe groundwater conditions. If not managed in a timely manner, these pollutants can be a long-term source of groundwater contamination. There are many methods for cleaning up pollutants from the ground, but for them to be effective, the location and volume of these pollutants needs to be determined. This is a major challenge because traditional methods for determining properties of the ground, such as drilling or digging, can be destructive and gather information over a very limited space, thereby being ineffective for this task. A new method of locating these pollutants was therefore desired. This was accomplished using a geophysical technique that sends electrical current into the ground and monitors where, and how fast, the current moves to produce an image much like an X-ray. It also has the advantage of not being destructive and can gather data over very large volumes with little effort compared to traditional methods. In this study, a brand-new configuration is proposed that uses horizontally drilled holes in the ground to use more sensors and get better images. A range of numerical models and laboratory experiments in a plastic tank were used to understand this new configuration and show that it has the potential to be used to help clean up pollutants in the ground.
Servos, Maxwell L., "Improved Imaging Of Dense Non-Aqueous Phase Liquids With Three-Dimensional Surface-To-Horizontal Borehole Electrical Resistivity Tomography (S2HB ERT)" (2022). Electronic Thesis and Dissertation Repository. 8833.