- Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants produced mainly from the incomplete combustion and pyrolysis of organic matter. PAHs are among the major contaminants at over 800 Superfund sites, nationwide. Thermal remediation is used as a method to remediate soils contaminated with PAHs through the use of heat to mobilize and recover pollutants. Steam enhanced extraction (SEE) is an in-situ thermal remediation technique which relies on the injection of steam (heat) to soil in order to increase the removal efficiency and recovery of volatile and semi-volatile contaminants, including PAHs. However, limited studies have been conducted on the formation of PAH derivatives during and after thermal remediation of unsubstituted PAHs. This is due to an inability to predict the structures of transformation products, historical lack of chemical standards, and challenges in the analysis of PAHs and their derivatives in complex environmental mixtures, such as soil. PAH derivatives are more mobile in the environment than unsubstituted PAHs, and some are more toxic to humans, animals, and plants, compared to the corresponding unsubstituted PAHs. There is an urgent need to develop analytical methods that can accurately quantify these PAH derivatives in complex environmental matrices, to improve our understanding of what chemistry occurs during and after SEE, and how this interferes with the remediation of Superfund sites.
In this study, we modified a previously published method for the extraction of oxygenated PAHs from soil samples, and optimized a solid phase extraction method for the extraction of polar PAHs from aqueous samples. A gas chromatography/mass spectrometry (GC/MS) method was modified for the analysis of hydroxy-PAHs (OHPAHs). Soil samples from the Wyckoff/Eagle Harbor Superfund Site were thermally remediated with a laboratory scale SEE setup. Soil and aqueous samples were collected and analyzed with GC/MS for unsubstituted PAHs, OHPAHs, oxy-PAHs, nitro-PAHs and MW302-PAHs. A majority of the 120 PAHs (unsubstituted and derivatives) were identified and quantified in creosote contaminated soil before and after SEE, in effluent samples collected throughout SEE, and in pre- and post-leachate samples (which mimic rain runoff and/or groundwater in the site).
Unsubstituted PAHs decreased in mean concentration post-SEE soil, while oxygenated PAHs increased in mean concentration post-SEE soil. We calculated the mass balance ratio of different PAHs by taking into account the mass of PAHs removed through the SEE system. Some PAH mass balances were above 100%, indicating the potential formation/transformation of PAH derivatives during (and after) SEE.
A quantitative human health risk assessment was done, where B[a]Peq concentrations and human estimated lifetime cancer risk (ELCR) estimates were calculated. The ELCR estimates were calculated for an ingestion human exposure pathway in 3 different scenarios. The B[a]Peq concentrations increased by 14.7% for soil pre-SEE and 61.5% for soil post-SEE when MW302-PAH concentrations were included in the risk assessment. The B[a]Peq concentrations decreased in soil post-SEE, compared to soil pre-SEE, by 69.4% with MW302-PAHs included and by 78.2% without MW302-PAHs. The ELCR estimates decreased overall, but were higher when accounting for MW302-PAHs. These results suggest that risk assessments could be underestimated by omitting MW302-PAHs concentrations.
Overall, this project determined that some lower molecular weight unsubstituted PAHs partially degrade and transform to PAH derivatives in soil during and after SEE. It also suggests that future SEE pilot and treatability studies should include PAH derivatives in risk assessments to assess the full effectiveness of SEE.