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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.
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