Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 673

the net mixture risks”. Outcome Assessments of Risk Management activities could
consist of monitoring the predicted reduction of soil contamination and risks over
time. An ideal case of successful management would lead to a reduction of the fre-
quency of sites where a soil quality standard is exceeded and thus a reduction in the
toxic pressure at a range of sites.
14.14.2.2 Approach
For regional authorities, soil contamination problems can be presented using GIS
(Geographical Information System) maps. This is helpful to find cases of high
contamination, to explore the presence and location of diffuse or point sources of
contamination, and to take preventive or remedial action when relevant diffuse or
point sources are found. Apart from presenting concentration maps or maps showing
exceedances of soil quality standards, the distribution of local risks can be presented
in terms of a map of toxic pressures (PAF per contaminant, or msPAF for mix-
tures). To further summarize a GIS-map in a summary risk diagram, one can use the
Cumulative Profile Plot concept of Fig. 14.12 . A successful risk reduction strategy
would show up as a flattening of the Cumulative Profile Plot (towards the “low”
curve in the figure) over time.
14.14.2.3 Conventional Risk Assessment Results
GIS-maps can present concentration levels, exceedances of quality standards, and
various ways to express toxic pressures. If the total soil concentration can for
example be split into a “natural background concentration” and a “human-induced
enrichment part”, one can map the added risk caused by human activities such as
sources of diffuse emissions. The latter are most relevant in helping to deriving
realistic and (cost) effective emission reduction techniques.
An example of results of this kind is provided in Fig. 14.15 for lead. Lead has
been used in gasoline for decades, and has been used in building materials and other
man-made products. As a consequence, a diffuse contamination pattern exists on
top of many hot spots.
The figure presents maps from samples in rural and nature areas (no hotspots)
with (1) total topsoil concentrations of lead, (2) total subsoil concentrations of lead
(from samples not influenced by human activities, and (3) the enrichments of the
topsoil per site (resulting from using a so-called baseline modeling (Spijker et al.
2008 ). The enrichment data relate closely to the 0.43M HNO 3 -extractable fraction
(Spijker et al. Subm.); this procedure extracts the chemically reactive fraction of lead
from the soil matrix (including the lead in the pore water). This reactive fraction has
in turn been used to quantify the toxic pressure of lead associated to the enrichments
only; this toxic pressure is thus based on the upper estimate of the potential avail-
ability of lead. This toxic pressure map (4), based on NOEC-data, suggests spatial
variability in the chronic toxic pressure of lead on local soil ecosystems across the
Netherlands as a consequence of human-induced enrichment. A vast majority of the
samples (>>90%) is characterized by a chronic added toxic pressure below 10%.