Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 656

Fig. 14.10 Different
contaminants will exhibit
different shapes and positions
for their SSDs, as shown
schematically. An increase of
concentrations in the
environment (

Δ y

Δ y

X) implies
different increases in PAF
(

Δ x

Δ x

Y)

Concentration

are that the former has an improved ecological interpretation. While an Index (or
a summed Index over compounds) has no upper boundary, the upper limit of the
PAF (and net PAF for mixtures) is 100% of species possibly affected. Furthermore,
PAF correctly addresses the increase of risk ( X) when slopes of

the concentration-effect relationship differ between contaminants, and doesn't suf-
fer from the problems of different (hidden) uncertainty factors. Finally, it has been
subject to validation studies (see Section 14.5 ).
When the regulatory context asks for follow-up actions in the case of exceedance
of soil quality standards, practitioners must simply execute the prescribed activities,
such as determining the urgency of remediation, given the local soil use. When
an exceedance does not lead to prescribed action, expected impacts can better be
quantified and ranked by estimating the local PAF.

Y for a given

14.10.2 Dealing with Natural Background Concentrations

It has sometimes been observed that protective soil quality standards are lower than
the natural background concentration, as a consequence of neglecting Exposure
Assessment details. In this case, implementing the risk-based soil quality stan-
dard would imply that such sites are considered “unacceptably contaminated”, in
the sense of “there is a man-made cause, we have a problem”. Care should be
taken to look at the variability of natural background concentrations in an area,
region or country, before a risk-based protective soil quality standard is set and
used for defining the presence of man-made environmental risks and a need for sub-
sequent Risk Management. Soil concentrations may result from anthropogenic or
natural sources (e.g., surface metal ores), or from both. In this case, SSD-based
Risk Assessments must be combined with an appropriate Exposure Assessment
to address this phenomenon. In the case of naturally elevated concentrations such
as heavy metals in soils from near-surface ores, it may be wise to manage risks
by preventing sensitive and valued organisms to be exposed, while not consider-
ing remediation. In the specific case of surface metal ores, remediation would be
called “mining”! And remediation would lead to the local extinction of protected
“metal flora”. When, however, elevated metal concentrations result from human
actions (or even an environmental “criminal act”) Risk Management approaches,
and eventually remediation, could be an option.