Part of the challenge of remediating chlorinated solvent sites is that few remediation technologies are well suited for addressing DNAPL pools. Of the available techniques, thermal approaches are probably the best because they do not rely on aqueous molecular diffusion, but rather can benefit from thermal diffusion as well as gas-phase flow brought about by volatilization of the water and the solvent. Both thermal diffusion and gas-phase flow are orders of magnitude more rapid than diffusion. “Traditional” thermal methods (e.g., heating to boiling temperatures with resistance heating, thermal diffusion and/or steam) remove contaminants by volatilization and steam distillation; however, such approaches require extensive infrastructure because of geotechnical concerns and the need to manage multiple fluid streams (vapor, water and potentially DNAPL).
In a low-temperature approach, remediation infrastructure can be significantly reduced because heating is targeted at smaller zones with temperature kept below the boiling point of water. As a result, if desired, remediation operation can proceed without any fluids removal. Electrical resistance heating (ERH) is one of the few available technologies that can heat the subsurface without local boiling and without injection of fluids (i.e., other heating approaches involve boiling temperatures – thermal conduction heating and steam injection – or injection of hot water). As a consequence, it both fills a unique niche and couples well with other remediation techniques (including in situ chemical and biological processes).
The primary objective of this project was to provide a comprehensive understanding of the processes that control fluid flow and mass transport during low-temperature electrical resistance heating (LTERH), including: 1) changes in groundwater flow due to buoyancy and viscosity changes of the water; 2) the interaction between heating and aquifer heterogeneity; 3) bubble formation; 4) heterogeneous (i.e., multi-phase) boiling; and 5) the effect of heating on in situ chemical reactions.
Batch, 1-dimensional, 2-dimensional, and physical model (i.e., “tank”) experiments, as well as numerical modeling studies were undertaken to improve our understanding of the processes controlling heat and mass transport associated with the subsurface application of LTERH. Physical model experiments combined detailed temperature measurements and observation of tracer movement to understand how changes in density and viscosity of heated water affected flow. The physical model results were used to “calibrate” the coupled ERH/flow/transport numerical model that was developed as part of this project. The calibrated model was then used to examine a range of scenarios that could not be represented in the physical models. Smaller scale studies also were conducted to visualize bubble formation, to evaluate the relationship between temperature and volatilization, and to evaluate thermal effects on in situ chemical oxidation.
The results of this work indicate that LTERH has significant potential to improve remediation of DNAPL source zones, especially those for which advective groundwater flow is limited. This is particularly the case for pools of DNAPL, which frequently occur at the tops of low-permeability media and which occur in topographic low points on those layers. Mass transfer is also limited in “diffusion zones” which were produced within low-permeability layers as the result of long-term contact with high-concentration sources (e.g., DNAPL pools). The ability to improve treatment in these zones is important because DNAPL source zones continue to control the timelines for site remediation and many Department of Defense sites.
The experiments and modeling demonstrate several beneficial strategies that may prove useful in regard to DNAPL source zone treatment; and four of them are briefly summarized here:
LTERH can increase local groundwater flow and, in particular, can substantially increase buoyancy-driven vertical flow. Taking advantage of buoyant flow is of particular importance in heterogeneous systems containing both high and low-permeability zones. Our results indicate that vertical flow across high/low hydraulic conductivity interfaces can be dramatically increased by heating to temperatures well below the boiling point of water. This can result in order-of-magnitude increases in mass transfer out of the source zone and into zones where groundwater flow rates are much higher.
Targeted heating to azeotropic boiling temperatures can dramatically increase vertical migration into zones of increased groundwater flow without causing off-gas collection issues. The ability to deliver heat at specific depths and, in particular, to deliver heat preferentially to lower-permeability zones (because they typically have higher electrical conductivity and because advective flow does not carry heat away from those zones) allows diffusion sources and DNAPL pools to be targeted.
LTERH can be used to optimize naturally-occurring processes (e.g., bioremediation, dissolution). Because ERH can be used to produce temperatures anywhere from ambient to ~100°C (unlike thermal diffusion and steam that rely on heat sources at >100°C), LTERH systems can be controlled to optimize a range of processes. At the same time, the lower temperatures reduce infrastructure and energy requirements, and as a result, the operation and maintenance costs for these systems can be very low.
Delivery of chemical oxidants/reductants to diffusion-limited source zones can be optimized by targeted LTERH. Diffusion-limited source zones typically require long remediation time frames because, by their nature, times required for diffusion of contaminants out of the source will be at least as long as they have existed as sources. For scenarios in which those timeframes are too long, it is possible to deliver reactants into the source by diffusion in order to degrade contaminants while they are still inside the source. Of course, delivery of reactants also will be controlled by diffusion. Here we examined the potential for a thermally-activated reactant (persulfate) to be delivered to a source zone at a temperature that maximized the depth to which the reactant would be effective. In practice, the optimization process is straightforward, because both the reaction rate and the diffusion process can be accurately modeled. One complicating factor in this regard is the consumption of reactant by the aquifer materials. However, our data suggest that laboratory experiments can be used to understand this process.