Many groundwater contaminants such as chlorinated solvents can be effectively treated with a variety of existing technologies, but the pace of remediation, which is influenced by the remedial processes, system design, and geologic complexity, has significant control on overall costs. Heat transfer properties of a subsurface soil matrix do not vary considerably and are relatively independent of grain size, unlike hydraulic properties that can vary many orders of magnitude. Consequently, thermal remediation can readily address contaminant mass present in both high- and low-permeability aquifer settings, including fractured rock. Traditional high-temperature thermal options, which are designed to remove contaminants primarily through volatilization, are readily available but require significant infrastructure, capital costs, operational and maintenance costs, and presents scale-up challenges. However, low-temperature heating (in the mesophilic range of ~15 to 40°C), which is much easier and less costly to implement, can significantly increase the rates for multiple biotic and abiotic treatment processes.
This project will demonstrate the low-temperature solar-powered thermal technology known as Thermal In Situ Sustainable Remediation (TISR) for accelerating treatment by enhancing biotic and abiotic contaminant degradation rates by modest increases in subsurface temperatures. Objectives include:
- Demonstrate subsurface heating efficiency and treatment enhancement at a chlorinated solvent source zone undergoing active enhanced in situ bioremediation (EISB)
- Confirm and quantify dominant treatment mechanisms (biodegradation, multi-phase mass partitioning, chemical hydrolysis, etc.)
- Collect high-resolution temperature data to support detailed thermal and contaminant transport numerical modeling and the development of a practical guidance document and design tool.
TISR is an inexpensive and practical method for modest heating temperature of target contaminant treatment zones in the subsurface by solar collection and closed loop heating via low-maintenance borehole heat exchangers (BHEs). Thermal conduction and some advection results in the heating of a subsurface target zone by approximately 20°C. This elevated temperature results in the enhancement of biological, chemical, and physical processes that attenuate, degrade, and remove contaminants. TISR is most appropriate for dissolved phased mass located in low-permeable zones, although it may also offer potential for high concentrated source zones and large dissolved plumes in some cases (e.g., using TISR transects). Furthermore, because it is complimentary to many other remedial technologies (e.g., air sparging/soil vapor extraction [AS/SVE], chemical oxidation, and reductive dechlorination), it can be readily incorporated into existing remediation systems for a moderate additional cost. TISR is particularly well suited for accelerating treatment rates for existing in situ remediation systems designed to promote bioremediation of chlorinated solvents.
TISR enhances both biotic and abiotic treatment processes, is applicable in a broad range of hydrogeologic settings, requires low capital and operational costs, and has a minimal carbon footprint. Its application will shorten remedial timeframes and lower lifecycle costs. The approach holds much potential to significantly benefit hundreds of current United States Department of Defense (DoD) sites where either active remediation or natural attenuation is taking place. In addition to the sustainable nature of operation through the use of solar power, the enhanced degradation rates can help DoD save significant costs being “add-on” technology that is compatible with many other remediation technologies and can accelerate remediation timeframes. Capital costs associated with TISR are low and the return on investment is expected to be one to three years for most sites, with the potential for significant overall remediation life-cycle cost reductions. (Anticipated Project Completion - 2023)