Environmental and Molecular Mutagenesis. DOI 10.1002/em 410 Lemieux et al. presented in Lundstedt et al. [2003]. This is not unex- pected and several authors have noted a distinct lack of correspondence between the results of toxicological analy- ses and matched chemical analyses [e.g., Brack et al., 2007]. The expected lack of correspondence conﬁrms the presence of hitherto unknown mutagens that are likely being formed during the bioremediation treatment. For example, it is plausible that oxygenated aromatic amines are formed during the bioremediation process and are the causative agents of some of the increases in the mutage- nicity that were observed during the treatment. Formation and accumulation of some polar transformation products, such as oxy-PAHs, is of particular concern since such products are more water soluble, and consequently display higher environmental mobility, than their parent PAHs. Lundstedt et al. noted that the increased relative mobility of oxy-PAHs may signiﬁcantly augment ecological hazard [Lundstedt et al., 2007]. Moreover, several oxy-PAHs are known to be toxic and mutagenic [Lundstedt et al., 2007]. We have assessed the mutagenic activity of one oxy-PAH highlighted by Lundstedt et al. (i.e., 4-oxapyrene-5-one); however, it failed to induce a positive mutagenic response in Salmonella strains TA98 and YG1041 (data not shown). Screening of other suspected agents and/or testing of oxy-PAHs and other PAH transformation products with different test systems, such as in vitro mammalian muta- genicity assays, may prove useful. This study is one of very few that has simultaneously tracked changes in both mutagenic activity and chemical proﬁle for a contaminated soil undergoing bioremedia- tion. Similar studies include that of Morelli et al. [2001], who observed a similar pattern of increasing and decreasing Salmonella mutagenicity for sludge-amended soils contaminated with PAHs [Morelli et al., 2001]. Belkin et al. [1994] measured the genotoxicity of PAH- amended soil and PAH-contaminated soil with the Muta- tox assay during a 3-month biodegradation period [Belkin et al., 1994]. They found that direct-acting geno- toxicity was actually enhanced following the degradation of high molecular weight PAHs. In another study, Don- nelly and colleagues [2005] demonstrated an increase in the genotoxicity of a soil amended with benzo[a]pyrene during a 720-day bioremediation, despite the fact that less than 10% of the benzo[a]pyrene remained at the end of the treatment [Donnelly et al., 2005]. The authors attributed the increase to an accumulation of genotoxic metabolites, including oxy-PAHs. Alexander and Alexander [1999] reported a net decrease in mutagenicity for PAH-contaminated soil as the soil PAHs age. How- ever, the reduction in mutagenic activity may be the result of reduced bioavailability of the PAHs or the PAH metabolites, rather than reduced mutagenic activity of the chemical constituents, in the aged soil [Alexander and Alexander, 1999]. OVERALL CONCLUSIONS In the absence of a more thorough, rigorous investiga- tion that includes extended bioslurry treatment and a vari- ety of treatment conditions, as well as simultaneous chemical and toxicological analyses, it is difﬁcult to determine the underlying causes of the temporal changes in mutagenicity observed in this and other studies. More- over, the ultimate risk of adverse health and environmen- tal effects posed by soils undergoing bioremediation will also be determined by alterations in physical-chemical properties that control environmental fate, transport, and exposure. Future studies should employ extended biore- mediation treatments, a variety of treatment conditions (e.g., nutrient augmentation), and continued examination of the chemical and toxicity proﬁles to determine the treatment duration required to reduce toxicity, hazard, and risk to background levels. It should be noted that reduc- tion to background does not imply complete elimination of mutagenic activity, hazard, and risk. Several research- ers [e.g., Jones and Peace, 1989; Donnelly et al., 1995; White and Claxton, 2004] have noted that natural back- ground levels of mutagenic activity can be expected to be less than 0.1 revertants per mg dry soil. Moreover, Jones and Page [1989] note that levels above 1 revertant per mg should be cause for concern. The results obtained in this study, as well as earlier work cited above, indicate that declines in the concentra- tion of noteworthy soil contaminants (e.g., priority PAHs) do not necessarily correspond to declines in the genotoxic activity of a complex pollutant mixture such as that pres- ent in a contaminated soil. Although bioremediation is a viable option for the treatment of PAH contaminated soils, additional research will be required to determine if declines in both the level of PAHs and the level of muta- genic activity can be achieved in a reasonable time frame. The combined results of the Lundstedt et al. [2003] study and the current study show that bioremediation of PAH- contaminated soils is associated with the formation of speciﬁc PAH transformation products (e.g., 4-oxapyrene- 5-one), as well as augmentation of mutagenic activity. At the present time, the identities of the putative mutagens in the treated soils are not known. Although some by-prod- ucts of PAH and PAC metabolism are known mutagens, (e.g., dihydrodiol derivates of benzo[a]pyrene), and increases in the concentration of PAH transformation products were documented by Lundstedt et al. [2003], the mechanistic relationship between the changes in chemical composition and changes in mutagenic activity is unsub- stantiated and poorly understood. Identiﬁcation of the pu- tative mutagens present at different treatment stages, and subsequent identiﬁcation of the transformation pathways will require a considerable effort. Nevertheless, this type of research, which could employ bioassay-directed fractionation techniques such as those highlighted by