Ecological Risk Assessment of Diffuse and Local Soil Contamination Using Species Sensitivity Distributions - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 676

land was evaluated in scenario studies. The database was used to explore not only
which fraction of sediments could be (cost) effectively spread on land using the
“old” standards-based assessment (based on five classes of sediment contamina-
tion), but also to devise a “new” assessment system, based on a systems-analysis
in combination with toxic pressure assessment. This means that, in addition to con-
taminant toxicity (as in the chemical regulations), the behavior of the contaminants
in the soil-groundwater system was also studied. That is: contaminants that are
deposited on land might be subject to aerobic breakdown and leaching (as disappear-
ance terms), and this type of changes can be taken into account in land-deposition
Risk Assessments.
A complete conceptual model was made, to identify the relevant pathways of
contaminant behavior in the system, with or without deposition of sediments on land
(Fig. 14.16 , top). Implementing the model with relevant parameter values yielded
systems-based Predicted Environmental Concentrations, given regular deposition
scenarios, with repeated removal of sediments, for example every five years. These
PECs were determined for all typical deposition scenarios, and the results (in terms
of predicted accumulation of contaminants in soils, and in terms of toxic pressures)
were compared to various regulatory soil protection targets.
14.14.3.3 Conventional Risk Assessment Results
Figure 14.16 (bottom) presents the variability in the change of net mixture risk for
all the sediment loads in the inventory. The graph represents the change in toxic
pressures in the terrestrial soil. That is, the contamination level of the terrestrial soil
prior to sediment deposition was taken into account according to data inventories
like those shown in Fig. 14.15 , and the delta-toxic pressure was calculated, showing
the increase or decrease in net risks after sediment deposition. It presents the toxic
pressure change after a long time of regular sediment deposition on land (Y-axis)
versus the assignment of contamination classes in the older classification system. In
the figure, the implications of using the “old” classification system versus the “new”
systems-based approach are explored, using the expected toxic pressure increase per
site as a benchmark for expected risk levels in terrestrial soils.
Apparently, “clean” sediments of the former class-0 do not induce increases in
toxic pressure in terrestrial soils; a slight decrease of net risks was predicted. Further,
the higher the “old” class, the higher on average the increase in net toxic pressure
of mixtures in terrestrial soils. However, in each of the old-system Classes, the vari-
ation in expected toxic pressures between volumes of sediment in the inventory list
was shown to be large. For example, some moderately contaminated “old” Class
3-sediments appeared less hazardous than various “old” Class-2 sediments. Upon
inspection, these Class-3 sediments were contaminated with only a few organic con-
taminants with a high breakdown potential (causing the low expected toxic pressure)
and at concentrations near the lower Class-3 limit, while the “worst” Class-2 sedi-
ments could contain a multitude of compounds that exhibit slow breakdown and/or
at concentrations near the high Class-2 limit.