Formatted Title
Fractured Rock: Treatment of Igneous and Metamorphic Bedrocks Using Thermal Conductive Heating
Background/Objectives
Remediation of contaminated fractured systems can be extremely challenging due to the difference in hydraulic conductivities in the bedrock systems. Especially igneous and metamorphic bedrocks like granite and gneiss with very little matrix porosity can be extremely challenging. The contamination is typically found, often as DNAPL, in the fracture porosity which consist of large fracture planes, smaller fractures and “microfractures”. Remedial attempts of these with remedies relying on distribution in the fracture system are likely to bypass major contaminant mass due to the variety in fracture sizes.
Utilizing thermal technologies in igneous and metamorphic bedrocks is limited to the thermal conductive gheating (TCH) technology, because the rock itself is too electrical resistant for electrical resistance heating (ERH) to be effective and utilizing steam enhanced extraction (SEE) will suffer the same challenges as other hydraulic governed technologies.
Approach/Activities
TCH has over the past two decades been used to address particularly chlorinated volatile organic compound (CVOC) source zones on more than a dozen of bedrock sites both in Europe and the US. The success of TCH in granite, for example, is as result of the fact that the thermal conductivity of granite varies in a very narrow range of approximately 1.7 - 4.0 W/mK which results in the possibility to transfer heat through the rock and across the different fractures in a very effective and predictable way, leading to volatilization and mobilization of the CVOCs. In this way it is possible to reach, heat and remediate all of the fracture systems without knowing the exact location of the trapped CVOCs.
From 2017 to 2020 three fractured bedrock sites were treated in Sweden totalizing more than 100.000 cy (77,000 m3) of bedrock material. Two of the sites were situated in completely saturated settings and the last site represented a combination of unsaturated and saturated material. The treatment depth at the sites varied from 66 to 164 ft (20 to 50 meters [m]) below ground surface. While the heating approach was equal on the three sites, then the extraction approach differed substantially from a “passive” extraction on two of the sites to extraction from extraction points co-located to the TCH heaters on the last site, which actually shows in the off gas concentration profile, that were measured continuously on all three sites. All three sites were heated successfully and remedial targets were met. Upstream and downstream groundwater concentrations have been reduced significantly while heating is ongoing. This we attribute to the fact that the amount of groundwater in the fractures is low and thereby the vaporization of water serves as hydraulic containment. One of the sites has been monitored now for 4 years since the thermal remedy to document post thermal groundwater concentrations and monitoring results will be presented at the conference.
A fourth site in Sweden is under closure at the moment (November 2023). At the site the bedrock is overlain by 20 ft (6 m) overburden of soil, with the groundwater level located 7 ft (2 m) bgs while part of the site is treated 30 ft (9 m) down into the bedrock. The site has heated up to target temperatures very successfully and sampling is currently ongoing and will be available in the conference presentation.
Results/Lessons Learned
It is proven, by numerous projects, that TCH is an efficient and reliable technology for remediation of CVOCs in fractured bedrock systems both above and below the groundwater table. This presentation will cover state of the art and the latest results in treatment of igneous and metamorphic bedrocks using thermal technologies.