IN-SITU THERMAL TREATMENT OF FRACTURED ROCK

In-situ thermal remediation has been used successfully to treat more than 200 source zones world-wide. Thermal treatment of the source zones (e.g., those containing chlorinated solvents) is typically achieved by vaporizing the contaminants of concern and extracting the vapors under vacuum.

In consolidated media, matrix diffusion can lead to strong binding of contaminants in the matrix, and flow in fractures may be difficult to understand and characterize. Once a site is heated, it may therefore be more challenging to (1) ensure that the matrix and fracture zones receive treatment, and (2) ensure that the vaporized contaminants are extracted.

Steam Enhanced Extraction (SEE) is well suited for heating of permeable zones, such as those containing large aperture fractures below the water table. However, it may be difficult to understand a site well enough to ensure that injection and extraction are sufficient. Thermal Conduction Heating (TCH) in contrast heats the matrix blocks relatively easily, but may be challenged by groundwater flow through the fractures. Electrical Resistive Heating (ERH) is challenged by the high electrical resistivity of most rocks, but may work in porous rock such as sandstone.

The presentation will focus on the mechanisms of thermal remediation, and how the lessons learned from treatment in porous media can at times be transferred to fractured rock, and at other times cannot. The potential fit and challenges for each of the three widely used technologies (SEE, TCH, ERH) will be discussed based on their individual characteristics. Field data from six sites with bedrock treated thermally will be presented, each case with its own unique findings.

Experiences from application of SEE and TCH in granite, gneiss, saprolite, chalk, limestone, siltstone, and mudstone will be presented. The optimal solution for many sites will be a combination of TCH and SEE, with TCH used to heat most of the matrix to boiling, while SEE is used to heat the dominant fractures, in which cool groundwater flow would otherwise make vapor recovery difficult. For sites with modest groundwater flow, TCH can be effective on its own. Porous rock with electrical resistivity of less than 500 Ohm-m may also be heated using ERH. The choice of thermal technology should be made after a careful review of site conditions and objectives.