The subsurface is complex – it may have layers of sand, silt and clay, and some layers have groundwater while others do not. When chemicals are spilled they move down into the subsurface via the paths of least resistance, contaminating all the zones they pass through. So when the distribution of materials and chemicals into the subsurface is this complex, would you think that one heating technology would fit all sites?
TerraTherm’s answer is “of course not!” One has to first understand the site well, and then determine the best way to heat a site and how to best extract the chemicals that are released/mobilized by the heat.
After more than 14 years of thermal remediation, with the total of field projects approaching 50, TerraTherm has concluded that it is a major benefit to have a steam-based solution for sites with large permeable zones. Steam Enhanced Extraction (SEE) is now a key component of many of our projects. The major reasons for this are:
- Steam can be injected into zones in which water is flowing, thereby reducing the cooling effect and the overall energy demand for heating and treating a site.
- Steam is the cheapest way to inject energy – since it is generated directly onsite from fuel rather than offsite with associated transmission losses.
- For non-aqueous phase liquid (NAPL) sites, steam movement and displacement of NAPL towards extraction wells can increase the fraction of the mass removed – often more than 50% of the contaminant of concern mass is removed with this liquid and less than 50% is removed by vaporization.
- For deep and large sites, SEE can be much more cost-effective than other competing thermal technologies, yielding unit treatment costs well below $80 per cubic yard. One of the keys to this is the fact that wells can be placed 30-100 feet apart, in comparison to most Electrical Resistance Heating (ERH) projects with electrodes placed 15-20 feet apart, or Thermal Conduction Heating (TCH) projects with heater borings placed 15 feet apart. So with SEE the drilling and construction is a fraction of the cost. Simple well-heads are shown below.
These advantages are reflected in TerraTherm winning two major SEE projects in 2013/2014.
At several sites, SEE has been used to complement heating by electrical methods, as was the case in Groveland, MA. TerraTherm successfully treated sites in Santa Fe Springs, CA, and Tullahoma, TN, using SEE combined with TCH. The combination allowed us to treat both highly permeable aquifers and the tight clay and silt layers around them. A single thermal technology alone would have failed.
How do we know when to used SEE? That decision comes from a careful review of the Conceptual Site Model and the remedial goals. We estimate the rate of water movement, our ability to heat all target zones, and the potential benefits of adding steam. Often, this includes modeling the fluid flow one can achieve with steam. An example of two steam zones growing from the injection wells over time is shown below.
Please contact us if you have a site that could benefit from thermal. We will provide you with the best possible fit for the site without a bias towards any single thermal technology.