Application of Borehole Radar for Monitoring Steam-Enhanced Remediation of a Contaminated Site in Fractured Limestone, Maine, USA

Abstract.

Steam-enhanced remediation (SER) has been successfully tested in porous media to remove DNAPL and LNAPL contaminants. This paper focuses on the use of borehole radar for geophysical monitoring of a SER pilot test in fractured limestone at the former Loring Air Force Base (Maine, USA). Theoretical calculations indicate that changes of dielectric and electric properties are expected due to the increase in rock matrix temperature and groundwater infilled fractures temperature during the SER test. Borehole radar reflection and tomography data were collected in two wells before the start of the steam injection, at the beginning of the injection, and near the end of the injection. Temperature logs were collected in several wells on the same days as the radar measurements. The frequency content analysis of the radar reflectors suggests that steam had not replaced water in the fractures. A significant attenuation of the signal was observed around the well where an increase of fluid temperature was observed at the end of the steam injection. After calibration of the tomography data, the inversion of the differences in travel-times indicates a small decrease of velocity, resulting from fractured rock heating. Based on this pilot test, it is concluded that borehole radar has potential to be used to monitor decrease of electromagnetic wave velocity and increase in attenuation in the fractured limestone due to steam injection.