A PCE DNAPL source zone was treated using thermal conduction heating combined with multi-phase extraction. Thick peat layers, even when contaminated with PCE DNAPL, were remediated to average soil PCE concentrations of 0.17 mg/kg (99.6% reduction compared to starting levels). This was accomplished in 83 days of heating, by elevating the temperature to the boiling point of water, and by boiling a fraction of the pore water (approximately 27%). Substantial subsidence was observed due to shrinkage of the peat. Therefore, special caution must be used during thermal treatment of sites underlain by organic-rich deposits.
Peat layers are abundant in certain geological settings and constitute a special remedial challenge. The high content of organic matter makes sorption a dominant retardation factor, trapping the organic contaminants in a manner similar to adsorption onto activated charcoal. This poses a challenge for any remedial approach based on physical removal of the contamination. Laboratory results on thermal treatment of such layers have shown great promise – PCE removal was achieved at the boiling point of
If peat layers are remediated, partial desaturation can lead to oxidation and shrinkage, as the organic matter is exposed to oxygen. This potentially can result in acid conditions if pyrite (FeS2) is present and oxidized into sulfuric acid. The oxidation and removal of mass can affect the geotechnical stability of the site, as peat layers may shrink and the surface subside. This may limit the applicability of aggressive source removal technologies such as thermal methods. This paper presents a full-scale thermal remediation application to a site with thick peat zones.
Thermal treatment of thick peat layers – DNAPL removal and shrinkage