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www.esdat.net Esdat Environmental Database Management Software +61 2 8875 7948 3.5 Derivation of GAC GAC were derived using the CLEA model v1.06. The recommended values from the proformas were transposed to the contaminant database in the CLEA model. A copy of the contaminant database used to derive the GAC is provided as a freely downloadable spreadsheet with this report. A summary of the contaminant database is provided in Appendix A. GAC were produced for those contaminants with recommended values for all the required parameters. Note that, with the exception of di-n-butyl phthalate (which has evidence of localised toxicological effects in the upper respiratory tract – see Section 3.7) the critical toxicity of all compounds is expected to be systemic. For this reason, where HCV were available for both the inhalation and oral routes of exposure, the combined GAC have been reported. The derived GAC and exposure contributions for each pathway calculated by CLEA v1.06 are presented in Tables 4.1a to 4.4e. Note that the GAC have been rounded to two significant figures. The pathway exposure contributions have been reported to one decimal place and are intended to aid the risk assessor in determining the appropriateness of the GAC for the site-specific conceptual site model. Table 4.1a presents the GAC for the metals. As discussed in Section 3.4, it was considered beyond the scope of the project to collate and review plant concentration factors for the metals and therefore GAC have only been produced for land-uses that do not include plant uptake, i.e. the residential without homegrown produce and commercial land-uses. Note that the derived GAC for metals are not dependent on SOM. Tables 4.2a, 4.3a and 4.4a present the GAC for 32 organic contaminants assuming 1%, 2.5% and 6% SOM, respectively. These tables also present the estimated soil saturation concentration 3 for each contaminant (rounded to three significant figures). As discussed in the Environment Agency of England and Wales SR2 report, soil concentrations exceeding soil saturation may indicate the presence of non-aqueous phase liquid (NAPL) and this should be considered when assessing the risks to human health. The presence of NAPL could pose additional risks to human health that are not accounted for in the GAC, such as the inhalation of vapours arising from NAPL sources and direct contact with NAPL. Odours arising from NAPL may also be cause for concern. Methodology As discussed in Section 3.2, HCV were not always available for both the oral and inhalation pathways. Where this was the case and where considered appropriate the use of route-to-route extrapolation, whereby the HCV for the oral route is applied to inhalation exposures and vice versa, has been adopted. This has been done in the CLEA model by selecting “Y,Y,Y” for the recommended HCV in the contaminant database such that the estimated average daily exposures (including MDIs) for the oral, dermal and inhalation pathways are compared to this HCV to derive the GAC. 3 The soil saturation concentration is the theoretical concentration at which soil vapour and/or pore water have become fully saturated. This can indicate that NAPL is likely to be present, but for contaminants that are strongly sorbed to soils (i.e. have high K oc ) and that have a low aqueous solubility and vapour pressure, exceedence of the soil saturation concentration does not necessarily imply that NAPL is present. 19
www.esdat.net Esdat Environmental Database Management Software +61 2 8875 7948 3.6 Contaminants For Which GAC Were Not Derived As discussed in Section 3.5, GAC were not produced for the residential or allotment land-uses for the metals. GAC were not produced for any land-uses for 9 of the researched organic contaminants. The reasons for this are presented in Table 3.3 below: Table 3.3: Reasons for not producing GAC Contaminant 1-Methylnaphthalene 2-Methylnaphthalene 1,3,5-Trimethylbenzene Carbazole Dimethyl phthalate Isopropyltoluene n butylbenzene sec butylbenzene tert butylbenzene Reason for not producing GAC Insufficient data were available to agree a HCV for the inhalation route of exposure for these isomers. There is evidence to suggest that these substances are significantly more toxic when inhaled than when ingested. Furthermore, the CLEA model predicts inhalation to be a significant route of exposure for them. For these reasons, route-to-route extrapolation was considered inappropriate and no GAC have been derived. Insufficient data were available to agree HCV and thus GAC were not derived for these substances. Methodology 3.7 Notes on Specific Contaminants There are some specific issues that should be considered with the following contaminants when using the GAC: • Methylphenols (cresols). The HCV for 2-methyphenol, 3-methylphenol and 4-methylphenol are based on the minimum risk level (MRL) set by the US Agency for Toxic Substances and Disease Registry (ATSDR) for total cresols. GAC have been derived for each individual isomer and the lowest of these chosen as the GAC for total cresols; • Di-n-butyl phthalate. The TDI inhal is based on localised toxicological effects in the upper respiratory tract. For this reason, the lowest of the assessment criteria derived using the oral and inhalation HCV have been used as the GAC rather than using the “combined” GAC; and • Phthalates. The potential additivity of phthalates should be considered by assessors when using the GAC for these substances. Guidance on additivity is provided in the Environment Agency of England and Wales SR2 document. 20
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1,3,5-Trimethylbenzene CAS 108-67-8
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