In situ thermal soil and aquifer remediation technologies (e.g., electrical resistance heating [ERH], conductive heating, steam-based heating) have undergone rapid development and application in recent years. These technologies offer the promise of more rapid and thorough treatment of non-aqueous phase liquid (NAPL) source zones; however, their field-scale application has not been well-documented in the technical literature. The objective of this project was to provide a performance assessment of thermal remediation technologies for dense non-aqueous phase liquid (DNAPL) source zone remediation.
This project did not involve the development or demonstration of a technology. Rather, the performance of thermal technologies designed and applied by others for DNAPL source zone remediation was assessed, with emphasis on post-treatment groundwater quality and mass discharge (sometimes referred to as mass flux). This independent evaluation involved an empirical analysis of available design and operating information and performance results from pilot- and full-scale applications. This was supplemented with post-treatment field sampling at selected sites to fill data gaps. This project was complementary to and made use of knowledge gained from other SERDP and ESTCP projects that were looking at relationships between DNAPL architecture, treatment effectiveness, and groundwater mass discharge (flux).
Documents from 182 applications were collected and reviewed. These applications included 87 ERH, 46 steam-based heating, 26 conductive heating, and 23 other heating technology applications conducted between 1988 and 2007, approximately 90% of which were implemented after 1995 and about half since 2000. Document reviews identified the geologic settings in which these technologies were applied, chemicals treated, design parameters, operating conditions, and performance metrics. Particular emphasis was placed on gaining a better understanding of settings in which thermal technologies have been applied, the design and operating conditions that were used, and the performance of the systems.
Additional data were collected by performing post-treatment groundwater sampling at sites where full-scale thermal applications were applied consistent with recent practice. This involved high spatial density groundwater sampling and hydraulic conductivity characterization along transects oriented perpendicular to groundwater flow at the downgradient edge of the treatment zones at five thermal treatment sites. The data were then used to calculate post-treatment contaminant mass discharge for each site.
The data collected in this study are captured in tables that relate site characteristics, thermal technology choice, design specifics, operating conditions, and performance. These tables are integrated with technology descriptions in the State-of-the-Practice Overview of the Use of In Situ Thermal Technologies for NAPL Source Zone Cleanup, which is intended to be a useful tool and primer for program managers considering the use of thermal technologies at their sites.
Some key conclusions from this study are:
- A significant number of applications have occurred, and this reflects the acceptance of in situ thermal technologies as viable source zone treatment options.
- It is apparent that the spatial extents of many source zones are likely ill-defined prior to treatment. This results in undersized target treatment zones, untreated source zone areas, and minimal beneficial impact to groundwater quality and mass discharge.
- Approximately half of the 182 applications have been implemented since 2000, and more than half of those were ERH systems. ERH applications outnumber all other applications since 2000 by about a factor of three. There also seems to be a recent trend in the increasing use of conductive heating and decreasing use of steam-based heating.
- There seems to be a convergence towards relatively closely spaced energy delivery points in the design of ERH and conductive heating systems. Spacing for most ERH and conductive energy delivery points was less than 20 ft (6 m), while steam application well spacing was usually greater than 20 ft (6 m).
- To date, most applications have been applied to relatively small treatment zones; 117 of 121 treated areas were <4 × 104 ft2 (<4000 m2 or an acre) and two-thirds of those were <104 ft2 (<1000 m2 or one-quarter acre).
- The effect of geologic setting on performance is difficult to discern in this data set because most treatment systems were installed in layered settings, characterized as either primarily fine-grained materials with higher permeability lenses or primarily permeable materials with finer-grained lenses. Thus, understanding of system design parameters and operating conditions is limited to those scenarios.
- Most applications (independent of specific technology) lasted less than 6 months; there was little documentation as to the criteria or rationale used to determine the duration of operation. There was little indication that the duration of operation was linked to mass removal-, groundwater quality-, or soil concentration-based criteria.
This project summarized knowledge on the performance of in situ heating technologies. The approach was to identify sites where thermal technologies have been applied and collect and synthesize as much of the available data/documentation for those sites, thus allowing for knowledge on how often each individual technology was being applied. The most challenging issue was a lack of sufficient documentation for most of the 182 applications identified.