Taking into account the high toxic index
determined for soils near the mine, especially for their As content
(Table 8), a higher toxicity in S2 to plants and in S3 to earthworms is
expected. In S4, a TI higher than 1 was measured for As to earthworms
and plants. However, earthworms were unaffected in this soil. In these
cases, risk analysis based on geochemical data alone would overestimate
the toxicity of the samples, because the observed toxicity was
less than what would be expected from their soil concentration (TI).
A possible explanation is that the L(E)C50 values described in the literature
are based on laboratory toxicity tests, which are performed on
soils freshly spiked with element salts. However, in the field, element
availability decreases with time, mainly due to weathering processes
(Ma et al., 2005; Song et al., 2006).