Formatted Title
Using Multiple Lines of Evidence to Show Thermal Remediation Completeness When Soil Sampling Access is Limited
Background/Objectives
A trichloroethylene (TCE) source zone located 60 to 125 feet beneath an active facility was treated using thermal conduction heating (TCH). Manufacturing operations at the facility occupy most of the building area overlying a 23,000‑cubic yard target treatment zone (TTZ) which prohibited using conventional vertical drilling methods to install subsurface heater casings for TCH. As a result, the subsurface heater casings needed to be installed using both angled and vertical drilling techniques from within a 10,000‑square foot interior room, which also was the location for critical electrical and process treatment system infrastructure for TCH operation. With limited space for drilling operations in both the manufacturing area and interior room, the performance of post-treatment confirmation soil sampling was constrained to narrow corridors located within the interior room and manufacturing area. These corridors only allowed room for a limited number of vertical soil borings to be advanced into less than 10% of the TTZ. Due to the inability to collect a representative distribution of post-treatment soil samples from the TTZ, a multiple lines of evidence approach was used as a means for demonstrating that the remedial goal of 1 milligram per kilogram (mg/kg) for TCE was achieved throughout the TTZ.
Approach/Activities
By using multiple lines of evidence to demonstrate thermal remediation completeness, regulatory agency approval could be attained in an expedient manner resulting in timely shutdown and decommissioning of the TCH system. The lines of evidence were: 1) achievement of a sustained target temperature of at least 90°C at 95% or more of the subsurface temperature monitoring sensors located within the TTZ, which in theory will result in sufficient volatilization and partitioning of TCE into the vapor phase for extraction; 2) demonstration that asymptotic TCE vapor concentrations from extraction wells screened in the TTZ are maintained for at least three consecutive rounds of vapor sampling, indicating that toward the end of the heating period, the TCE mass removed should approximate the estimate of TCE mass within the TTZ prior to heating; and 3) measuring the energy input to the heaters over the period of heating and comparing it to the modeled energy input used for design, which confirms the amount of energy required to reach and maintain the target temperature was applied to the TTZ for completion of the thermal remedy. After demonstrating that these lines of evidence were achieved, soil samples were collected for laboratory analysis from a total of six soil borings to confirm the remedial goal of 1 mg/kg for TCE was met.
Results/Lessons Learned
After approximately 283 days of heating from a total of 180 heaters, temperatures exceeded 90°C at more than 95 percent of the subsurface temperature monitoring sensors and TCE concentrations in the recovered vapors showed that the amount of TCE mass recovered over the heating period exceeded design estimates by 147 percent. The amount of energy applied to the subsurface was within 5 percent of the design energy input at the end of the heating period. Confirmatory soil sampling results showed that TCE concentrations were more than an order of magnitude below the remedial goal of 1 mg/kg. Monitoring and reporting of key performance indicators toward meeting the established lines of evidence was automated using Microsoft® Power BI®. This platform was integral to the analysis and presentation of the multiple lines of evidence data altogether in one dashboard, producing consistent automated reports to share with all stakeholders in near real-time, facilitating regulatory acceptance and timely shutdown of the TCH system without delay. Furthermore, the interior room which was occupied by the TCH system infrastructure was able to be placed back into use by the facility ahead of schedule.