Thermal Remediation - Combined Remedies

Complex sites often require a combination of technologies for complete remediation. At heavily contaminated sites, in situ thermal treatment may be used to treat the source, while other technologies are used in the transition and plume zones. The three major in situ thermal remediation (ISTR) technologies are Steam Enhanced Extraction (SEE), Electrical Resistance Heating (ERH), and Thermal Conduction Heating (TCH). These technologies can be combined with in situ chemical oxidation, in situ chemical reduction, bioremediation, permeable reactive barriers, and monitored natural attenuation to provide complete site solutions. They may be enhanced using hydraulic or pneumatic fracturing and emplacement of permeable materials.

Related Article(s):

• Thermal Remediation
• Steam Enhanced Extraction (SEE)
• Electrical Resistance Heating (ERH)
• Thermal Conduction Heating (TCH)

CONTRIBUTOR(S): Dr. Gorm Heron

Key Resource(s):

• Thermal DNAPL Source Zone Treatment Impact on a CVOC Plume.

Introduction

Figure 1. Conceptual model of a complex site with multiple zones with different levels of contamination.

Most complex sites can be divided into three zones (Figure 1):

1. A source zone where higher concentrations exist – typically in the form of non-aqueous phase liquids (NAPLs) or strongly sorbed contaminants. Note that source zones often have a component above the water table (in the vadose zone) and below the water table (in the aquifer).
2. A transition zone where some NAPL may exist in small stringers, ganglia or droplets, but where the mass is smaller and concentrations lower. In some instances this is referred to as a dilute source or the plume core.
3. A dissolved plume where contaminants of concern (COCs) exist in dissolved or adsorbed phases. Often, the plume is affected by lower permeability layers, where the COCs have diffused in over time and can become a long-term source as the chemicals diffuse back out because concentration gradients are reversed during remediation.

For successful remediation, it is crucial that every site is well understood and that a good site conceptual model (CSM) is developed. A good CSM is the basis for selecting remedial technologies that are effective in each of the zones. In source zones, technologies must often be effective both above and below the water table. The specific physical characteristics of the contaminants of concern and of the soils must also be considered in selecting the optimal remedy. Several commonly applied technologies for the three different zones are shown in Table 1.

Table 1. Technologies applied in the different zones of a complex contaminated site.

These technologies must be combined based on site-specific characteristics, remedial objectives, and the capabilities of each technology. One example of a combined remedy is when thermal treatment is used to reduce the source concentration/mass, and a set of permeable reactive barriers are used to control and remediate the plume in the years following thermal treatment (Figure 2). Another example is thermal treatment of the source zone combined with bioremediation of the transition zone and dissolved plume (Figure 3).

Figure 2. Example combined remedy: Thermal treatment of the source zone combined with permeable reactive barriers for treating a long dissolved plume. Thermal source treatment is implemented in the first year, followed by plume treatment.

Figure 3. Example combined remedy: Thermal treatment of the source zone combined with bioremediation for treating the transition zone and dissolved plume. Red dots = heaters, green dots = bioremediation injection or extraction wells.

Case Studies

Recent examples of combined remedies with a thermal component include:

1. Endicott, NY: A PCE source zone was removed using Thermal Conduction Heating (TCH), reducing the source mass discharge by three orders of magnitude. Pump and treat and monitored natural attenuation were used for the plume. Five years after thermal treatment, the plume had shrunk enough that the pump and treat system was shut down.
2. Memphis Depot, TN: Thermal treatment removed eight source zones from the upper 30 ft of a silty loess formation, while soil vapor extraction was used to remediate the less concentrated zones in between and below the source zones.
3. Fort Lewis, WA: Thermal treatment of three source zones followed by low temperature and energy ERH with in situ bioremediation in the transition and plume zones.

Summary

Thermal technologies can be coupled with less intensive solutions to provide integrated solutions for complex sites. By treating the source zones thermally, most of the chemical mass may be removed. This facilitates treating the transition zone and the plume effectively. Thermal treatment is most often combined with soil vapor extraction and enhanced bioremediation, but site-specific analyses must be used to determine the best options for each site.