An Integrated Approach To Monitoring A Field Test Of In Situ Contaminant Destruction

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