Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 608

significantly more efficient for organisms that were exposed to higher concentrations
for a longer period, probably due to genetic differences. Sokhn et al. ( 2001 ) showed,
in spite of a reduced microbial activity and an incomplete mineralization of phenan-
threne, the presence of phenanthrene-degrading, copper-resistant and/or -tolerant
microorganisms, at copper levels in between 700 and 7000 mg/kg dw , indicating the
presence of phenanthrene-degrading, copper-resistant and/or -tolerant microbes. In
other words, adaptation does take place, but the system might become less efficient:
Adaptation has a cost. This cost is related to energy requirements to cope with con-
taminants, for example, production of extra stress proteins, or energy to pump out a
contaminant. Other microbiologist made the same observations earlier, for example,
Doelman et al. ( 1994 ).
Adaptation is usually incorporated in generic Risk Assessment through the
derivation of ecological generic Soil and Groundwater Quality Standards. This is
done by following the added risk approach (Struijs et al. 1997 ), that is, assum-
ing that the natural background concentration does not impose risks, since the
ecosystem is adapted to these long prevailing conditions.
For site-specific Ecological Risk Assessments, risk assessors must account for
possible adaptation. Biological measurements implicitly include the influence of
adaptation. But critical soil concentrations, even when normalized for their actual
soil properties, do often not apply for any adaptation.
13.7.3 Land Use
Unlike Human Health Risk Assessment, land use does not always significantly
impact exposure of the soil ecosystem. However, one should not overlook the impact
of subsurface building activities, infrastructure, agriculture and industrial areas on
Ecological Health. The main difference with Human Health Risk Assessment is that
human behaviour and residence time strongly influence the exposure, while the soil
ecosystem is more or less constantly exposed to contaminants in soil, independent of
the land use. One exception is the land use 'Agriculture', for which the contaminant
inputs and, hence, exposure, are variable.
On the other hand, there are good arguments for using variably acceptable eco-
logical risks levels for different land uses, certainly with regard to Risk Management
purposes (ecological soil recovery).
The soil ecosystem is an important entity for above-ground processes and above-
regional (or even global) processes. The presence of plants, bushes and trees is
crucial at most sites. The global significance of the soil ecosystem, related to nutrient
and carbon cycling and the fixation of greenhouse gases, is important everywhere,
independent of land use. Also, the cleaning function with regard to groundwater
quality is crucial at most sites (and especially at agricultural and industrial sites,
since generally the 'supply' of contaminants is highest at these sites). Without this
cleaning function, the use of pesticides would even be impossible in agricultural
management practices, without compromising the groundwater quality. From the
perspective of the cleaning function, the inclusion of a 'basic ecological protection
level', at every site independent of land use, is politically defensible. The possible