and carcinogenic to animals and humans [Pickering, 1999]. The assessment of the hazards posed by PAH-con- taminated soils ordinarily focuses on a restricted set of priority PAHs, generally 16, that were originally selected by the US Environmental Protection Agency [Keith and Telliard, 1979; US EPA, 2000]. However, a growing body of research now shows that there are many more hazardous and carcinogenic PAHs, and some regulatory agencies (e.g., California EPA and the International Agency for Research on Cancer) now highlight additional compounds (e.g., dibenzo[a,l]pyrene and 5-methylchry- sene) as probable and possible human carcinogens [IARC, 2008; State of California EPA, 2008]. Moreover, these agencies assert that these compounds should be consid- ered in hazard/risk assessments. The method commonly employed to monitor the sta- tus of a contaminated site (e.g., a PAH contaminated site) that is slated for remediation and redevelopment generally involves chemical-speciﬁc surveys for priority chemicals, human health risk assessment for an identi- ﬁed receptor (e.g., construction worker), and reduction of target chemicals to a designated safe level. For example, in Canada, the Canadian Council of Ministers of the Environment (CCME) recommends monitoring a subset of the aforementioned 16 priority PAHs, and cal- culating the cumulative excess cancer risk associated with the most hazardous PAHs (e.g., benzo[a]pyrene and dibenz[a,h]anthracene) [CCME, 2008]. However, many PAH-contaminated sites contain complex mixtures of pollutants, and the chemical proﬁle of all of the toxic compounds in the contaminated soil is rarely, if ever, available. Therefore, it is not known whether the levels of all hazardous compounds at the site decrease concomitantly with the priority PAHs that are monitored during site clean-up. The removal of PAH-related hazard at contaminated sites is most commonly accomplished via containment (e.g., capping) or incineration. However, these options are costly and do not always produce a site that is ideal for redevelopment. For example, incineration can cost twice as much as bioremediation, and often requires extensive excavation and transportation of contaminated media [Pellerin, 1994]. Bioremediation, a treatment strat- egy that employs microorganisms, fungi, or plants to convert hazardous organic pollutants into harmless degra- dation products, is an appealing alternative for the cleanup of PAH-contaminated sites [Wilson and Jones, 1993]. Moreover, bioremediation can leave the site in a state that is suitable for redevelopment and plant cultiva- tion [Wilson and Jones, 1993; Donnelly et al., 2005]. Although bioremediation has been recommended for use at PAH-contaminated sites, its use is not widespread and some researchers have advised caution since the transfor- mation products of PAH degradation that are formed during bioremediation can exhibit increased toxicity, Environmental and Molecular Mutagenesis. DOI 10.1002/em Mutagenicity of Soil During Bioremediation 405 mobility, and bioavailability relative to the parent com- pounds [Lundstedt et al., 2007]. The transformation of PACs in contaminated soil, through processes such as microbial and fungal degrada- tion, can yield by-products such as hydroxylated and oxygenated PAHs (e.g., oxy-PAHs), and aromatic amines [Cerniglia, 1992; Eriksson et al., 2000; Saponaro et al., 2002]. Several studies have suggested that some products of PAH degradation (particularly oxy-PAHs) may be toxic and/or genotoxic [Donnelly et al., 2005; Lundstedt et al., 2007]. These compounds will therefore contribute to the total risk associated with the contaminated site; however, since their identities are rarely known, their presence, or changes in their concentrations cannot be monitored. Soil genotoxicity assessment using bioassays offers an effective alternative to chemical analyses and can adequately monitor hazard before, during, and after remediation. Toxicity can be used as a measure to assess the efﬁcacy of remediation, and signal the presence of hitherto unidentiﬁed compounds that may be of concern. It is therefore important to monitor not only changes in the concentration of noteworthy chemicals (i.e., priority PAHs) at a contaminated site undergoing bioremediation but also changes in toxicological activity. In an effort to determine the degradation rates of var- ious PAHs and PACs in a heavily contaminated soil from a former gasworks facility, Lundstedt et al. [2003] monitored the levels of many PACs during a pilot-scale bioslurry remediation. They tracked the degradation of over 100 compounds in the soil, including a range of N-, O-, and S-heterocyclics and oxy-PAHs, and showed that the levels of some oxy-PAHs may in fact be increasing over the course of the bioslurry treatment. The authors suggested that these compounds may con- tribute to toxicological hazard and that measurement of oxy-PAH levels during bioremediation may be war- ranted. However, reliable assessment of actual changes in the toxicological activity of the complex samples investigated by Lundstedt et al. [2003] requires the use of a bioassay. The Salmonella reverse mutation assay is a logical choice for monitoring changes in the geno- toxic activity of PAH-contaminated soils, such as those at former gasworks sites, which are undergoing reme- diation. Several additional strains are now available (e.g., YG1021, YG1024, YG1041), and these strains provide enhanced sensitivity to several of the aforemen- tioned chemical classes [Watanabe et al., 1989; Hagi- wara et al., 1993]. The present study employed the Sal- monella reverse mutation assay to monitor changes in the mutagenic activity of the samples examined in the Lundstedt et al. [2003] study. Changes in the mutagenic activity for the nonpolar neutral (i.e., PAHs and alkyl- PAHs) and semipolar aromatic fractions (i.e., N-hetero- cyclics and oxy-PAHs) were tracked throughout the course of the bioremediation.