How to Implement ISTR in Complex Geologic Settings

Having a good understanding of geology, groundwater flow, and contaminant distribution is essential to designing an effective thermal remedy. Having an accurate conceptual site model (CSM) is especially true at sites with complex geologies consisting of multiple layers of different types of unconsolidated sediments and/or bedrock units with complex fracture patterns, where groundwater flow patterns can be complex and rates high. Zones with high groundwater flow can result in excess cooling and prevent the thermal remedy from attaining the necessary temperature for efficient and effective removal of contaminants of concern (COC) and achieving the remedial performance objectives. High groundwater flux zones, if not properly identified and controlled, can also allow COCs mobilized during in situ thermal remediation (ISTR) to leave the target treatment zone (TTZ), spreading the COC mass and increasing concentrations outside the source zone.

For ISTR to be effective, the technology (or technologies) and design have to be properly matched with the site conditions and remedial goals. In this blog post, we’ll cover what you need to consider during the planning and design stages of ISTR projects in complex geologic conditions to ensure that you select the right approach for your site.

If you’d like a deeper dive, watch the recorded webinar ISTR in Complex Geologic Settings with Highly Variable Permeabilities and High Groundwater Flux Zones.

How Much Groundwater is Too Much?

For sites with volatile organic compounds (VOCs) such as chlorinated volatile organic compounds (CVOCs) and low remedial goals, the target temperature for treatment is typically 100^◦C or the boiling point of water. The boiling point of water is selected because this generates 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 (e.g., the boiling point at 20 ft below the water table is 113.5^◦C.

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 TTZ. This will result in cooler temperatures in the permeable zones—possibly well below the boiling point of water—and below the temperatures of the surrounding 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 100^◦C/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 detailed CSM of site geology, including groundwater gradients, permeabilities, and flux rates, is available for a site, there are some good options to ensure that all portions of the TTZ 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 thermal conduction heating (TCH) or electrical resistance heating (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 towards 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 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 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 can be 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 all of the fractured bedrock being targeted. Combining SEE with TCH has several significant advantages:

• Effective heating of both the rock matrix and fracture zones is achieved by limiting groundwater flux through permeable fractures.
• Sufficient heating of the rock matrix ensures the COC mass diffused into the matrix is removed, resulting in effective removal of COC mass from the bedrock system and long-term achievement of remedial goals with no rebound.
• Active injection of steam into the fracture zones and capture by multi-phase extraction (MPE) wells aids in the removal of COCs from the bedrock system and shortens the remedial timeframe.

For more information on how to determine the right treatment for complex geological project sites, register for the upcoming webinar ISTR in Complex Geologic Settings with Highly Variable Permeabilities and High Groundwater Flux Zones. Thermal expert John LaChance discusses the type and nature of high permeability and complex geologic settings, how those factors affect thermal treatment, and examples of effective ISTR designs in these conditions.