Formatted Title
Playing the Long Game: Predicting Heat Dissipation following In Situ Thermal Remediation to Enhance Degradation
Background/Objectives
Heat-enhanced degradation processes can be leveraged during in situ thermal remediation (ISTR) as temperatures rise peripherally and downgradient of the heated volume. Many studies have investigated the effects of subsurface heating on soil properties, phase partitioning, and rates of hydrolysis, reductive dechlorination, and cometabolic biodegradation reactions. In practice, however, relatively few practitioners attempt to predict specific zones of temperature rise that can be leveraged both during and after active thermal treatment. The ability to predict long-term temperature distributions is of great benefit to the practitioner, as it helps to design plume management approaches more strategically, utilize energy more efficiently, and improve the overall sustainability profile of the remedy. The objective of this study was to quantify how subsurface temperatures evolve after thermal treatment is completed in the source zone.
Approach/Activities
Both simulated and field-measured data are presented to provide insights to practitioners on how cooldown patterns evolve in a variety of hydrogeologic settings. A thermal reservoir simulator was used to quantify transient cooldown patterns at the field scale in increasingly heterogeneous permeability fields, both in a single silt layer and in a silt layer with an underlying sand aquifer. The following process was developed to reliably predict temperatures over the life cycle of a thermal remedy: (i) initializing the water table, the capillary fringe, and recharge rates; (ii) setting boundary conditions to drive groundwater flow through the domain; (iii) simulating active heating operations for 180 days; and (iv) simulating passive heat dissipation for 5 years. Initial temperatures were set to 15°C. Sensitivity analysis was performed to illustrate how cooldown patterns were affected by groundwater flow velocities of 0 to 0.2 ft/day, applied energy densities of 200 to 300 kWh/cy, thermal conductivities of 1.5 to 3 W/m/K, and recharge rates of 0 to 100 mm/yr. Gas phase streamlines and groundwater quiver plots were generated to confirm that pneumatic and hydraulic control were maintained during operations, as well as to explore long-term fate and transport effects. Cooldown patterns were also assessed with a 50-gpm pump and treat system operating downgradient in an underlying aquifer. Finally, depth-discrete temperatures measured in the field following ISTR operations are exhibited for comparison to the modeled estimates.
Results/Lessons Learned
Subsurface temperatures diffused laterally following the end of active heating, returning to ambient at a lateral distance of approximately 15 m from the heated volume. Peak temperatures in the center of the heated volume were approximately (i) 40 to 60°C after 6 months, (ii) 20 to 40°C after 1 year, and (iii) 15 to 20°C after 3 years. This suggests that residual heat can be leveraged to enhance degradation processes for a 3-year period following active thermal treatment. Cooldown patterns were influenced most by subsurface thermal conductivities and groundwater flow velocities, and least influenced by recharge rates. Cooldown patterns in the single silt unit were relatively insensitive to heterogeneities in permeability; however, in a layered system where there was a sand aquifer underlying the silt unit, groundwater fluxes carried heat further downgradient and along preferential pathways. Interestingly, collapse of the gas phase was generally observed within 2 weeks of cooldown and caused inward gradients to develop as groundwater resaturated the ISTR zone. Following this, groundwater gradients flipped to align with the principal direction of flow, though buoyant flow patterns (i.e., rising groundwater) developed within the ISTR zone. Both the method and results presented herein can be used as a basis for practitioners to assess zones to target with heat-enhanced remediation processes and optimize remediation designs.