Theme: 01. Remediation Technology Innovations

Topic Area: 01q. Electrical Resistance Heating: Best Practices and Lessons Learned

Formatted Title:

Design and Installation of Electrical Resistance Heating System Using Horizontal Directional Drilling

Background/Objectives:

Background/Objectives. Tetrachloroethene (PCE) detections above maximum contaminant levels (MCLs) in private wells at the PCE Southeast Site in York, Nebraska prompted the EPA to designate it a Superfund site and allocate funds to remove contaminant sources in soil and address site-wide groundwater contamination. The soil source area was targeted for an expedited response since residents of the community were at direct risk from exposure to contaminants in groundwater as well as potential vapor intrusion to interior air. In situ thermal remediation (ISTR), specifically electrical resistance heating (ERH) using ET-DSP™ technology, was selected as the remedy for one of two soil operable units. ISTR provides rapid cleanup of sites with volatile contaminants, as heating soil to high temperatures quickly volatilizes the contaminants to accelerate their removal through vapor extraction. Since the contamination posed direct health threats to the community, the higher initial cost of ISTR balanced against the immediate need to remedy the site.

Approach/Activities:

Approach/Activities. The operable unit for ISTR was occupied by a restaurant that needed to remain operational during treatment, which prevented the placement of ET-DSP™ electrodes under the building via conventional vertical or angled drilling. Horizontal directional drilling became the preferred alternative for these installations. The design comprised a 3x3 matrix of electrode assemblies, with rows of assemblies at three depths, three assemblies spaced equally per row. Each assembly comprised three, 10-foot steel electrodes, separated by HDPE spacers, for a total of 27 installed electrodes. Additional components in each assembly included power cables and water injection lines connected to each individual electrode. Additional installations included multiple vapor recovery wells, quench wells and temperature sensors under the building to monitor temperatures, and prevent vapor intrusion.

The wells were installed with a compact directional drill rig with approximately 30,000 lb. thrust and pullback. Despite the relatively large diameter of the electrodes (six inches), all assemblies were installed as single ended, without the need to extend the bores back to the surface to facilitate a double-ended pullback installation method. The electrode bores were backfilled with sand after the electrodes were installed to assure conductivity between the electrodes and surrounding formation. All bores were sealed with grout at their entry points. The system was installed as part of a larger project that included vertical and angled electrodes. Drilling and installation were completed in June through July, 2023, with operation of the system beginning in October 2023.

Results/Lessons Learned:

Results/Lessons Learned. Horizontal directional drilling proved to be a viable method to install even complex ISTR systems in areas that are impossible to directly access from above. Such installations require space to prepare the electrode strings and close coordination between the consultant, thermal contractor, and drilling contractor to assure that all facets of the program are addressed and that the work can proceed smoothly.

Although the system has not yet been activated, early results are anticipated to be available to include in the presentation for the conference in early June 2024.

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Thomas Lackman