Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 352

4.3 Site-specific target value for point and diffuse sources
To determine the targeted emission locally or at watershed scale with the aim to achieve
an acceptable environmental quality in the compliance point, we have to apply the
transport models in an inverse way. The compliance point will be selected according
to the aim of the risk management, taking into account the conceptual model of the
contaminated land. The target quality criteria will be set according to the requirements
of the intended land use in the compliance point. According to Hjelmar et al. (2001) and
Gruiz et al. (2009), the quality criteria imposed on the groundwater/surface water at the
target/inflow points may, by inverse modeling, be “back-calculated'' to corresponding
site-specific limit values in the source material. This gives the site-specific target values
for risk reduction measures/remediation of the source (Gruiz et al., 2007, 2009). In
this subchapter, we define the basis of target value setting and outline the differences
between the tools used in the case of point and diffuse sources.

4.3.1 Setting the target value and its application to point sources

The site-specific target value is the maximal allowable contaminant concentration,
which does not pose unacceptably high risk to the users of the environment that may
be humans or ecosystems. The screening level relevant to the acceptable risk can be
expressed as a ratio characterizing the risk (RCR).
RCR PEC/PNEC , where PEC is the predicted environmental concentration and

PNEC is the predicted concentration of chemical contaminants with no effect on
the ecosystem. A similar risk characterization ratio is used for human health risk:
RCR

=

D/ADI , the ratio of average daily intake ( D ) (inhaled, dermal or oral uptake)
to the acceptable daily intake ( ADI ) of a chemical substance by humans. These cri-
teria mean that the contaminant concentration in the environment should not exceed
the “no effect'' concentration that is low enough not to have adverse effect on the
ecosystem or humans.
The management approach based on the PEC/PNEC or D/ADI ratios (see also
Volume 3) aims to keep or reset the environmental contaminant concentration to
or under the acceptable threshold. The targeted risk characterization ratio of point
and nonpoint contaminant sources should satisfy the PEC/PNEC

=

1
requirements at both local and watershed scale in compliance points where human or
ecological receptors are targeted to be protected.
For the purpose of setting risk-based target concentrations for remediation goals,
a site-specific quantitative risk assessment is used that aims to address public safety
and environmental quality. In such an approach, decisions are made by comparing
concentrations of pollutants in the environment, for example in soil and groundwater
measured at the site with criteria related to the actual or intended use of the soil and
groundwater.
The steps for calculating the target concentration for a point source are shown in
Figure 10.9 and include the following:
-

≤ 1or D/ADI ≤

Calculation of the PEC in the compliance point (in a concrete time point) from an
estimated emitted amount using the site-specific transport model;

- Comparison of the calculated PEC with the required environmental quality

(PNEC), which can be a default one (stipulated by regulations) or a site-specific
local environmental criterion;