Fractured rock sites are different – often it is difficult even to determine how deep the Non-Aqueous Phase Liquid (NAPL) has gone, and to define what we refer to as the “source area”. Some sites have NAPL in the fractures, and some have most of the mass trapped in the matrix, into which it has diffused over decades. This blog post discusses the two main challenges:
- How to decide on the thermal treatment volume without making the problem worse.
- How to design and implement an effective and safe thermal system.
For any aggressive source treatment to be cost-effective, it is important that the right volume be treated. As illustrated below, it may be relatively easy to determine the treatment area in the overburden, but quite a bit more difficult in the deeper bedrock where the fracture pattern can be chaotic. Generally, most of the contaminant mass resides within and above the more highly weathered upper portion of the bedrock, in contrast with the deeper rock where fractures tend to be fewer and more widely spaced. Thus, the target treatment zone for thermal sometimes does not extend below the bottom of the weathered bedrock. At some sites if treatment of a small mass deeper in the fractured bedrock is desired, then the heaters are extended to heat and treat to the desired depth.
Another general challenge in bedrock is this: When you drill, the shaking of the ground may disturb the NAPL, and in some cases may cause the NAPL to move deeper. Therefore, careful drilling, often using diamond coring, must be used to minimize the risk, at considerable expense. This limits the number of boreholes drilled, despite the fact that the complex geology would have otherwise demanded more drilling and sampling. As a result, the source area is often less well defined than for sites with the contaminants of concern (COCs) in overburden deposits.
Once the treatment volume has been set, the next step is to determine the best thermal method for the site. TerraTherm and our current staff have worked at six different bedrock sites. Our lessons learned include that:
- It has proven extremely difficult to heat fractured rock sites with steam alone. When possible, heating durations have been very long, and hydraulic and pneumatic control has been challenging.
- Thermal Conduction Heating (TCH) has proven very effective for heating of the matrix blocks – by simple heat transfer from hot borings.
- It is desirable to extract fluids from each the heater borings – this ensures that the vapors we generate have a place to go – back towards the heaters and out and up through co-located vacuum extraction wells.
- Electrical Resistance Heating (ERH) is applicable in some types of bedrock – typically the ones with electrical resistivity values below 500 Ohm-meters. Porous and wet sandstone, for example, can be heated using either ERH or TCH. In drier rock, rock with modest porosity (on the order of 10% or less), and deep crystalline rock, only TCH can achieve boiling temperatures and ensure proper thermal treatment, because electric current flow will be limited.
- At sites with a lot of groundwater flow in some fractures, the use of steam injection to heat those fracture zones can help optimize the treatment, but one still needs to heat the matrix in order to remove most of the mass. The combination of TCH and steam injection is promising.
At one of our sites, we succeeded in heating the matrix blocks, but not the fractures where water flowed. This meant that the reduction of TCE concentrations in the rock samples was more modest near the fractures, as illustrated in the plot below.
Water flowed rapidly in fractures at depths of 17 and 31 ft (depths where the TCE reduction was modest). We believe that the cold groundwater flow kept the temperature low enough that the TCE could not be boiled and removed from the matrix in those locations. In contrast, TCE concentrations in the matrix blocks away from the fractures were reduced to modest levels.
In conclusion, TerraTherm believes that the most robust and predictable thermal method for fractured rock is TCH, and that DNAPL constituents can be effectively removed, provided that the site is adequately delineated and characterized. One can heat any type of solid matrix because the heat moves under simple thermal gradients. And the hot boreholes allow for vapor control, as the contaminants are extracted from every borehole near the locations where they vaporize. Below the water table, addition of steam in select locations may enhance the process by heating those fractures in which cold water would otherwise move and slow the heating.
In 2009, in collaboration with the U.S. Navy, Queens University, and USGS, TerraTherm designed, constructed and operated a field demonstration project funded by ESTCP to validate TCH for DNAPL remediation in fractured bedrock sites and proved guidance for its application. Click the following link to read the NAVFAC report on the project: “Dense Non Aqueous Phase Liquid (DNAPL) Removal from Fractured Rock Using Thermal Conduction Heating (TCH)”
Click the following link to view a recording of a webinar I presented in 2013: Thermal Conduction Heating for the Remediation of DNAPL in Fractured Bedrock.
Please feel free to contact TerraTherm for more information on treatment of fractured rock, or for an evaluation of a site if you are considering “going hot” as one of your options.