Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 357

concentration of RBSC carc adult = 3 µ g/L in the compliance point will be the crucial

value.

4. Inverse transport modeling and creation of the target concentration

The transport model describing the transport and fate of chloroform from the source
to the compliance point will be applied in an inverse mode to calculate the permissible
groundwater concentration in the source. The result for the maximum permissible
concentration in the source is 25

g/L. This concentration is less than most of the
intervention values for chloroform (30-100

µ

g/L); the explanation may be the small
distance between the source and the residential area. An overestimation may also
play a role: the result of the inverse transport model should be refined in a second
assessment step.

µ

5. Uncertainties

Applying the maximum exposures for the calculations may cause overestimation in
the risk values and in the protection levels.
The absorption rate is considered as 1, since no high-quality data on the actual
absorption rates from different tissues is available. After screening the literature, more
realistic values may be found under 1.
The compliance point is close to the source so that the target value in the source
is rather small.
Smallest values of chloroform partition among soil solid and liquid phases and
biodegradation rate were selected from published data for transport modeling.
100% of the animal and 25% of the consumed plant products were assumed as
contaminated from the water, which values can be assessed more precisely.
Exposures due to multiple water uses could also be considered by interpreting
the tolerable risk as an aggregated value from different exposures. This means in the
inverse calculation that the RCR

= 1 threshold should be shared between the risks via

all exposure routes.

4.3.2 Setting the target emission from diffuse sources and planning
risk reduction at watershed scale

The concept of calculating the targeted emission, or the maximum permitted emission
from diffuse sources is similar to point sources, but the inverse transport modeling is
performed at watershed scale. The scheme in Figure 10.11 shows the concept of setting
the target emission with the help of a watershed-scale combined model, using jointly
actual input parameters and a black box. Establishing the site-specific target value at
watershed scale includes the following steps:

- First step : selection of the compliance point(s) and the relevant receptors.

The primary target(s) or point(s) of compliance are one or more points or locations
downstream of the source and can be the outflow point of a watershed, a reservoir,
a lake, a wetland or a floodplain, where the groundwater/surface water must fulfill
the quality criteria.