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Because of the availability of multiple toxicity data for various species and processes, the statistical extrapolation method is used to evaluate the toxicity data, calculating the HC5-50 of the best-fitting curve. The generic HC5-50 in the total risk approach, is 86 mg Pb/kg dw. This HC5 -50 is based on 60 NOEC and EC10 values covering 12 different plant species, 4 invertebrate species and 5 microbial functional processes. The NOEC and EC10 values of the 3 groups (higher plants, invertebrates and microbial processes) overlap in the frequency distribution, suggesting that the sensitivity range of these organisms is overlapping.

Accounting for differences in Pb toxicity between spiked soils and field contaminated soils, using a leaching/ageing factor of 4.2, results in an aged HC5 -50 of 294 mg Pb/kg dw.

Concluding, the data provide sufficient diversity of species and soil types, contains several unbounded data of Pb suggesting little toxicity within the relevant toxicity range and field data were unable to demonstrate toxicity at concentrations below the HC5-50. We note that the European terrestrial risk assessments of metals have used an Assessment Factor (AF) 1-2 to convert the HC5-50 into a PNEC. In the Zn RAR, an AF=2 was used because of a restricted number of invertebrate species. The Ni risk assessment has used AF=2, mainly because of lack of field data. The Cd risk assessment has used AF=1-2, the motivation being the consistency with the Zn risk assessment. The Cu risk assessment used AF=1, the motivation being the data richness and the extensive field validation. In balance, we note that data richness is sufficiently large (soil/species) but that the field data of Pb are limited: there are only 2 studies and no studies providing a LOEC. Therefore we propose an AF=2 to derive the PNEC soil from the HC5-50. The HC5-50 is obtained using the statistical extrapolation technique as suggested in the EU workshop on statistical extrapolation (17-18 January, 2001) resulting in a PNEC soil of (HC5 -50/2 or = 294 mg/kg /2)147 mg Pb/kg dw. This value was obtained following the total risk approach accounting for differences in Pb toxicity between spiked soils and field contaminated soils.