3 Things You Need to Consider When Implementing ISTR in Complex Geologic Settings with Highly Variable Permeabilities and High Groundwater Flux Zones
By: John LaChanceIdentifying zones where high groundwater flux may occur is essential to an effective thermal remedy. At sites with complex geologies consisting of multiple layers of different types of unconsolidated sediments and/or bedrock units with complex fracture patterns, an accurate conceptual site model (CSM) is especially important, because zones with high groundwater flow can result in excess cooling and prevent the thermal remedy from attaining the necessary temperature for efficient and effective COC removal and achieving the remedial performance objectives (e.g., soil cleanup standards). High groundwater flux zones, if not properly identified and controlled, can also allow COCs mobilized during ISTR to leave the target treatment zone, resulting in a spreading of COC mass and an increase in concentrations outside the source zone.
In situ thermal remediation (ISTR) can be an effective source zone removal technology if the technology (or technologies) and design are properly matched with the site conditions and remedial goals. In this blog post, we’ll cover three things you need to consider during the planning and design stages of ISTR projects in complex geologic conditions to ensure you leverage the right technology for your site.
If you’d like a deeper dive, join us for next week’s webinar, ISTR in Complex Geologic Settings with Highly Variable Permeabilities and High Groundwater Flux Zones. If you have questions, be sure to register as I’ll be answering them live during a Q&A at the end.
HOW MUCH GROUNDWATER FLOW IS TOO MUCH?
For sites with volatile organic compounds (VOCs), chlorinated volatile organic compounds (CVOCs) and low remedial goals, the target temperature for treatment is typically 100C or the boiling point of water. The boiling point of water is selected as this will generate a continuous gas phase (steam and COC vapors) in the treatment volume, which is essential for efficient mobilization and transport of the COC mass out of the subsurface. It’s important to note that to achieve the boiling point below the water table, higher temperatures are required due to hydrostatic pressures. At sites with permeable zones (sands or fractures in rock) below the water table, the rate of energy input may not be sufficient to keep up with the rate of cool water flowing into the treatment zone. This will result in cooler temperatures in the permeable zones—possibly well below the boiling point of water—and below the temperatures of the low permeable zones. The rate of energy input for typical thermal conduction heating (TCH) heater and electrical resistance heating (ERH) electrode spacings (15 and 18 ft, respectively) can accommodate groundwater flux rates up to ~1 ft/day. Higher flux rates, and heat losses will be too high to achieve 100C or the boiling point of water and contaminant removal may not be sufficient to achieve the desired remedial goals within the expected timeframe.
WHAT CAN BE DONE AT SITES WITH HIGH GROUNDWATER FLUX?
If a good CSM of site geology, including groundwater gradients, permeabilities and flux rates is available for a site, there are several options to ensure that all portions of the treatment zone are sufficiently heated and treated. First, the spacing between the heaters or electrodes can be decreased. This effectively increases the energy input density into the subsurface and permeable zones, which may be sufficient to overcome the heat loss associated with groundwater flux rates >1 ft/day. However, decreasing the spacing will increase the drilling, installation, and operational costs of the project.
The second option is to use a combination of TCH or ERH and steam enhanced extraction (SEE). SEE involves the injection of steam into the subsurface and creation of an expanding steam front, which radially pushes steam and condensate away from the steam injection well toward centrally located multi-phase extraction wells. The good news for sites with permeable geologies is that SEE is the most cost-efficient way to heat the subsurface. So, if the permeability is too high for TCH or ERH alone, the cost of adding SEE is offset by the efficiency of the heating process and the ability to achieve the desired remedial goals within the targeted schedule.
The third option is to utilize a physical or hydraulic barrier around the perimeter of the target treatment zone (TTZ), to cut off and limit groundwater flow through the TTZ during heating. Sheet pile or slurry walls are examples of physical barriers and a network of pumping wells can be used to establish a hydraulic barrier.
WHAT ABOUT HIGH GROUNDWATER FLUX ZONES IN FRACTURED BEDROCK?
For some sites, the TTZ extends down into a fractured bedrock. Although the porosity and permeability of the rock matrix may be low, groundwater flux rates can be high in fractures or fracture zones. An effective heating strategy for such sites is to use a combination of TCH and SEE. TCH is highly effective at heating rock matrices, and steam injection into the fracture zones if very effective at controlling groundwater flux and heat losses. These technologies can also be combined in a single borehole, thereby reducing the cost of drilling and well installation. It is important to note that although the fracture zones may be permeable enough for high groundwater flow and for steam injection, the spacing between fractures at most sites is large enough (i.e., greater than several feet) that propagation of a steam zone outward from the steam injection well will be severely limited due to heat losses if TCH isn’t used to heat the rock matrix. Steam injection into fractures alone will have a limited ability to heat and treat fractured bedrock. Combining steam injection with TCH has several significant advantages:
- Effective heating of both the rock matrix and fracture zones by limiting groundwater flux through permeable fractures
- Sufficient heating of the rock matrix will ensure the COC mass diffused into the matrix is removed, thereby resulting in effective removal of COC mass from the bedrock system and long-term achievement of remedial goals (no rebound)
- Active injection of steam into the fracture zones and capture by MPE wells will aid in the removal of COCs from the bedrock system and shorten the remedial timeframe
If you have additional questions about determining the right treatment for complex geological project sites, ask them during next week’s webinar, ISTR in Complex Geologic Settings with Highly Variable Permeabilities and High Groundwater Flux Zones. Sign up even if you’re unable to attend—we’ll send you a link to the recording to watch whenever you’d like.
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