A laboratory study of the effectiveness of thermal remediation on creosote impacted soil

Abstract

Groundwater can be impacted by many types of contaminants, some of which are difficult to remediate, such as dense non-aqueous phase liquids (DNAPLs). DNAPLs can exist for long periods of time in the subsurface and may result in decreased groundwater quality, decreased land value and an increased risk of indoor air quality impacts to nearby infrastructure. In situ thermal remediation has been proposed and implemented at sites impacted by volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), though additional information is required to elucidate fundamental mechanisms and establish performance expectations. Preliminary laboratory studies were conducted to determine the effectiveness of heating at moderate temperatures on soil impacted to various degrees with creosote, a multi-component SVOC DNAPL. Co-boiling temperatures observed during heating were near the boiling point of water due to the low volatility of the creosote. Though VOC removal from soil occurred quickly during co-boiling, polycyclic aromatic hydrocarbon (PAH) concentrations in soil appeared unchanged, indicating that higher temperatures or longer heating durations are necessary to achieve typical regulatory standards. Subsequent laboratory tests were conducted to determine the effectiveness of heating at moderate, elevated and highly elevated temperatures on soil impacted with creosote. PAH concentrations in soil decreased the most during co-boiling for all heating tests, and continued to decrease during extended heating periods of up to 10 days at highly elevated temperatures (320 °C). However, PAH concentrations in soil remained above Ontario’s regulatory standards after completion of all tests. Dissolution tests designed to simulate groundwater flow through treated sites post-heating revealed continuing PAH dissolution into water. The results from these two studies indicate that in situ thermal remediation of multi-component SVOC DNAPLs, at least to the temperatures tested in this study, does not result in soil PAH concentrations below applicable regulatory limits. Furthermore, remaining PAHs in treated soil continue to partition into the aqueous phase. Additional research into higher temperature thermal applications and the effects of pumping during heating are recommended to fully determine whether in situ thermal remediation is a viable option for SVOC DNAPL impacted sites.