An integrated approach to monitoring a field test of in situ contaminant destruction

The development of in situ thermal remediation techniques requires parallel development of techniques capable of monitoring the physical and chemical changes for purposes of process control. Recent research indicates that many common contaminants can be destroyed in situ by hydrous pyrolysis/oxidation (HPO), eliminating the need for costly surface treatment and disposal. Steam injection, combined with supplemental air, can create the conditions in which HP0 occurs. Field testing of this process, conducted in the summer of 1997, indicates rapid destruction of polycyclic aromatic hydrocarbons (PAHs). Previous work established a suite of underground geophysical imaging techniques capable of providing sufficient knowledge of the physical changes in the subsurface during thermal treatment at sufficient frequencies to be used to monitor and guide the heating and extraction processes. In this field test, electrical resistance tomography (ERT) and temperature measurements provided the primary information regarding the temporal and spatial distribution of the heated zones. Verifying the in situ chemical destruction posed new challenges. We developed field methods for sampling and analyzing hot water for contaminants, oxygen, intermediates and products of reaction. Since the addition of air or oxygen to the contaminated region is a critical aspect of HPO, noble gas tracers were used to identify fluids from different sources. The combination of physical monitoring with noble gas identification of the native and injected fluids and accurate fluid sampling resulted in an excellent temporal and spatial evaluation of the subsurface processes, from which the amount of in situ destruction occurring in the treated region could be quantified. The experimental field results constrain the destruction rates throughout the site, and enable site management to make accurate estimates of total in situ destruction based on the recovered carbon. As of October, 1998, over 400,000 kg (900,000 lb) of contaminant have been removed from the site; about 18% of this has been destroyed in situ.