Background/Objectives. In-situ thermal treatment (ISTR) is a highly effective solution for heavily contaminated source zones and can also be effective in the restoration of downgradient groundwater resources if the treatment zone fully encompasses the source zone.  But because ISTR is a source zone treatment technology, assessment of performance and determining when to shut down an ISTR system can be complicated due to the flux of contaminant mass into the treatment zone from outside areas.  As with all aggressive remedial technologies, when deploying subsurface heating it is important to both set realistic remediation goals and optimize the systems through flexible monitoring and data collection and informed analysis to ensure an effective use of time and resources. Ultimately, expediated, flexible data analysis combined with holistic interpretations accounting for conditions both inside and outside the treatment volume, and well-defined criteria for when to stop heating, results in much more sustainable thermal solutions. 

Approach/Activities. This study will introduce a Multiple Lines of Evidence Approach (MLEA) for evaluating ISTR performance and apply it to recently completed ISTR projects.  The MLEA combines the assessment of site heating with evaluation of treatment performance metrics, including observed groundwater concentrations and statistical trend analysis of mass removal rates.  Heating and performance metrics considered include water and energy balances, in-situ temperatures reached, mass removal rates and total mass removed, interim data on contaminant concentrations, and final samples of soil and groundwater. A simple model is also introduced that predicts the rate of decrease in mass removal rates expected in the absence of mass flux into the treatment volume.  The model assumes a fixed starting mass of contaminant in the treatment volume and incorporates partitioning between solid, liquid, and vapor phases.  Comparison of model predicted mass removal rates with observed mass removal rates can provide supporting evidence that: 1) the ISTR system has achieved the objective of source zone treatment, 2) the persistent low rates of mass removal observed at the end of the ISTR project is the result of mass flux into the treatment volume, and 3) the most sustainable decision from a cost-benefit perspective is to shut down the ISTR system.

Results/Lessons Learned. For many ISTR projects, setting remedial goals and monitoring the site for completeness of remediation and determining when to shut down is a complex task. By following a MLEA that incorporates the most commonly used sequence of performance monitoring, such that the energy balance is maintained to ensure enough energy has been delivered to the subsurface, subsurface temperature data is monitored to identify that temperature targets have been achieved, mass removal rates and trends are assessed for diminishing returns and the possibility of mass flux in from the outside, and interim and final soil and groundwater data show compliance with remedial goals, a clear and compelling case can be made as to when to shut down an ISTR system, resulting in significantly shortened operational durations substantial project savings and reductions in consumption of natural resources and emissions of greenhouse gases.