Environmental Engineering Reference
In-Depth Information
-
Elemental
iron
can
immobilize
As,
Ni
and
Mo
during
oxidation
and
co-precipitation;
-
Silicates present in or added to the soil are able to capture metals into a stable
mineral structure.
7 CONCLUSIONS ABOUTTHE SITE-SPECIFIC RISK ASSESSMENT
AND MANAGEMENT FOR POINT AND DIFFUSE SOURCES
The same approach, but different tools are used for the environmental risk manage-
ment of point and diffuse sources. The risk management approach includes hazard
identification, risk assessment, risk control and reduction, if necessary.
Point sources can be managed at local scale. Prevention and remediation in the
source or along the transport pathway is generally possible.
Diffuse contamination can be managed at watershed, regional or global scale.
Restriction is the most feasible risk reduction alternative.
Risk assessment of point source contamination can be done in several steps, based
on measured data. The transport models are analytical in the screening step, but gener-
ally numerical in the detailed assessment step. Validation of the model and the results
of the management option are plausible, measurable in the source, on the transport
pathway or in the vicinity of the receptors.
Risk assessment of diffuse contamination uses the combination of numerical and
analytical transport and fate models. Known parameters such as those of hydrologi-
cal transport are modeled numerically, but the fate and behavior of the contaminants
in a large, heterogeneous watershed area—the black box—is generally estimated by
analytical models, based on known or simulated input and output data. If the con-
taminated area is a watershed, the water system and the flow accumulation models
support the assessment of water-transported pollution and erosion models support the
assessment of solid transport. The management option is limited to the restriction of
the production and use of the chemical substances, products or disposed wastes as well
as to the technologies applied at the diffusely contaminated site. Validation of the risk
reduction measure is only possible at the output of the watershed or the black box
model for the diffusely contaminated area.
The transport and fate model can be applied in an inverse (backwards) mode both
for point and for diffuse source contaminated land.
Point and diffuse sources require different risk reduction methods for contami-
nated sites: point sources can be removed, isolated, treated by physico-chemical or
biological remediation techniques, whereas diffuse sources generally cannot be delin-
eated, and therefore cannot be treated in the source. The transport pathway from the
source to the receptor can be identified in the case of point sources and as a con-
sequence, intervention is possible at any point along the transport pathway (runoff
treatment, subsurface water treatment by passive or active barriers or treatment facili-
ties). The transport pathway from a diffuse contamination source is usually the runoff
from rain. Runoff flux varies in time according to the rain conditions and topography,
therefore decontamination/treatment is used but only in the main collectors if a feasible
technology is available. The treatment of subsurface waters along the transport path-
way is in most of the cases not feasible. Treatment in the vicinity of remote receptors is
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