Formatted Title
Low-Temperature Solar-Heated Thermal Remediation for Accelerated CVOC Treatment: Case Study in Central Coast of California
Background/Objectives
Thermal In-Situ Sustainable Remediation (TISR®) is a low-temperature solar-powered thermal remediation technology. TISR® not only serves as a sustainable alternative to traditional mass recovery methods like excavation or energy-consuming treatment systems but also complements other remedial technologies such as air sparging, biosparging, chemical oxidation, biologic and other degradation processes. This technology was applied to enhance the treatment of chlorinated ethenes at a site on the central coast of California. The project aimed to demonstrate subsurface heating efficiency and treatment enhancement at a chlorinated solvent source zone undergoing active enhanced in situ bioremediation (EISB). The site was chosen due to several technical and practical factors; however, a key criterion was the complex and heterogenous geology which limited the effectiveness of EISB and resulted in persistent high concentrations despite over a decade of active treatment. Because thermal conductivity is independent of grain size and varies much less than hydraulic conductivity, it is much less affected by geologic heterogeneity than injected-fluid based methods. Additionally, the region's average daily solar radiation of approximately 5 kWh/m2/day provided favorable conditions for efficient solar collection.
Approach/Activities
A full-scale TISR® system was installed in May 2022 which included eight borehole heat exchangers (BHEs) connected to an array of solar collectors and eight TISR® monitoring wells. BHEs were installed within the current source area as demonstrated by the existing monitoring well network at the site. The design of each BHE incorporated an assumed radius of influence (ROI) of 10 to 12 feet, informed by heat transport modeling and empirical data from similar technology applications at other sites nationwide. TISR® monitoring wells were arranged to provide adequate temperature monitoring across the BHE wellfield as well as to support push-pull tracer tests to compare in situ carbon consumption with benchtop microcosm studies. TISR® monitoring wells are also used to monitor contaminant concentrations in the treatment area. Fiber Optic Distributed Temperature Sensing (FODTS) technology was used in 12 direct buried temperature monitoring points capable of measuring a continuous temperature through the entire depth profile in the treatment area.
Results/Lessons Learned
The performance monitoring of the TISR® system demonstrated the temperature of the target zone has been elevated and contaminant treatment rates have been significantly enhanced. The TISR® system successfully achieved the model-predicted subsurface heating; in some areas temperatures have neared the ideal mesophilic range of 30°C to 35°C. The FODTS data provided valuable insights for the development of a high-resolution sub-surface heat model, enhancing our understanding of the thermal dynamics within the heated area. This heating has resulted in an increase in density of dehalococcoides and other beneficial microorganisms in the subsurface compared to pre-heating conditions or background locations. As a result, contaminant concentration data show both an overall reduction of all chlorinated ethenes (in some cases by more than three orders of magnitude) and a shift toward biodegradation products. These findings collectively indicate the successful implementation and performance of the TISR® system in promoting beneficial microbial colonies and enhancing treatment rates withing the target zone. The accelerated remediation will lead to an earlier transition to passive remediation, reducing overall lifecycle costs and carbon footprint.