Many sites at which remediation using thermal methods is being considered contain thick clay layers in the subsurface. The difficulty with treating clay is that it is not very permeable, meaning liquids will not move through it rapidly. Chemicals often get stuck within the clay matrix and cannot be removed easily unless one heats the clay to boiling temperatures. But how will the chemicals move out of such a tight material? If the clay consists of tiny particles and minerals, too small for water to move more than a few meters in a year, how can we get the chemicals to move out, and how can we be sure that they are extracted?
When the clay is heated by injection of either heat (Thermal Conduction Heating [TCH]) or electrical current (Electrical Resistance Heating [ERH]), the chemicals and the pore water boil, turning into vapor and steam. The steam has a volume about 1,600 times larger than that of liquids. When a liquid is converted into steam, the increase in fluid volume creates local pressure. The steam pushes outward, producing small cracks or openings within the clay. By boiling less than 30% of the pore fluids, one can mobilize more than five hundred pore volumes of steam to move through the clay over a period of a few months. The chemicals partition into the injected and mobilized steam.
- Next, the steam must be extracted. A good design includes careful consideration of where the steam will flow and an effective capture system. Sometimes one can rely on the steam moving up and a permeable layer above the clay may be enough to ensure its capture. This, for instance, was the case at Point Richmond in California, where we extracted steam from a 5-ft thick sand layer above clayey Bay Mud. The Bay Mud at that site extended to a much greater depth than the bottom of the tetrachloroethylene (PCE) source, so there was no risk of the PCE vapors moving down (Heron et al. 2013; the mechanism is illustrated in Figure 1 below). At another site in Reerslev, Denmark, a similar PCE source was distributed across the entire depth interval of a clay layer, above the water table, and the steam could move either up or down (Griepke et al. 2012). At this site, it was necessary to extract vapors both above the clay (from a fill layer placed on top) and below the clay (from the top of a sandy zone). Each site is different, but these examples illustrate some of the options and how they vary.
It is critical to ensure pneumatic control and capture of the volatilized chemicals. Here are the key parameters and design issues that must be addressed:
- Identification of the vapor flow paths likely to control the flow (up, down, to the sides?)
- Placement of vacuum extraction wells in the permeable zones to which the steam and chemicals will flow. If a permeable surface layer does not exist, a fill layer may need to be added at the ground surface and then covered with a surface seal to prevent fugitive emissions.
- Provisions for heating these zones such that the steam and chemicals cannot simply condense in cool zones and be left untreated.
- Use of a treatment system with the capacity to extract and treat the full amount of steam and chemical vapors, such that a vacuum can be maintained at all times. At some large sites, more than half of the energy injected may come out as steam, so the cooling system must be sufficient to handle it.
- A monitoring system which will provide an early warning if something is not going as planned. This could be a combination of temperature sensors, pressure monitoring points, and screening level data on Volatile Organic Compound (VOC) presence.
For some sites, where we are treating only a portion of a tight clayey zone, combined TCH and vacuum extraction wells are used. This is done to optimize the vapor removal and to eliminate the need for placement of permeable material, which may be impractical. Since the heaters create a hot, dry zone around each heater boring/well, this facilitates pneumatic control and can eliminate some of the requirements listed above. However, this mechanism has only been proven effective for TCH (Heron et al. 2009) and not the other thermal methods where a more even distribution of energy can create boiling in places where there is no preferential path back towards an extraction well.
Signs of poor thermal design and insufficient pneumatic control include:
- Lack of a vapor cover.
- A shallow zone above a clay layer that is not heated to near 100oC (condensation risk).
- Steam seeping out of the ground.
- VOC odors in the wellfield.
- Chemical detection in surrounding soils during or after treatment – could indicate that the vapors migrated and condensed outside the heated zone.
TerraTherm builds these mechanisms and lessons into each one of our thermal projects.
Griepke, N., S., P.J. Jensen, G. Heron, J. LaChance, J. Galligan, N. Plough and P. Johansen. 2012. “Soil Sampling During and After Thermal Remediation: How and When?” Paper 920, In: Remediation of Chlorinated and Recalcitrant Compounds – 2012. Eighth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2012). Battelle Memorial Institute, Columbus, OH.
Heron, G., K. Parker, J. Galligan and T.C. Holmes. 2009. “Thermal Treatment of 8 CVOC Source Areas to Near Non-Detect Concentrations.” Ground Water Monitoring and Remediation, 29(3): 56-65.
Heron, G., J. LaChance, and R. Baker. 2013. Removal of PCE DNAPL from Tight Clays using In Situ Thermal Desorption. Ground Water Monitoring and Remediation, 33(4): 31-43.