Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 335

chemical substance and the water flux of the river, giving the dilution rate and the initial
concentration. The constant velocity of the river determines the transport velocity of
the contaminated water, which moves downstream as a plug (plume). The procedure
is the same in subsurface waters, when the contaminant has reached the water table.
Because this water flows into the pore volume of the saturated soil, the water and the
dissolved contaminants interact continuously with the solid phase, which results in the
partition of the contaminant between solid and liquid phases, depending on the char-
acteristics of the contaminant and the soil. In addition to the partition among phases,
other transport and fate processes can be taken into consideration such as degradation,
accumulation (including biological) and chemical transformations.
The transport of a chemical substance between the source (with too high con-
centration of “ C '') and the compliance point (with an eligible “ c '' concentration) in
Figure 10.4 can be described by a linear concentration decrease if the medium does
not experience contaminant retention and degradation. If these mitigating interactions
take place, the concentration curve between C and c will be exponential.

Case study 1

Unknown contaminant mixture of probably petroleum origin was found in the ground-
water. The creek is at 30 m distance from a well where the highest concentration
measured was 50 mg/L. The question is whether or not the contaminant mixture poses
unacceptable risk to the aquatic ecosystem.

Risk assessment concept for case study 1

1 Determining the attenuation rate: having measured the total concentrations -

TPH total petroleum hydrocarbon without identifying the components - along

the supposed plume (50 mg/L at 30 m, 20 mg/L at 20 m and 12 mg/L at 10 m from
the creek), the current attenuation rate can be determined.

=

2 Calculating the dilution rate: the contaminant content in the plume will be further

diluted after flowing into the creek.

3 Modeling attenuation and dilution by the same rate of dilution prepared from

the groundwater sample taken from the source. The adverse effect of the diluted
source sample on aquatic organisms is tested.

4 Comparison of contaminant attenuation rate with the “no-effect'' dilution.

5 Characterization of uncertainties.

6 If the result says that the contaminant cannot reach the “no effect'' level due to

attenuation and dilution, further assessment and risk reduction may be necessary.

Calculations for case study 1

1 Attenuation rate from the measured concentration gradient: C 0 /C creek inflow =

4.4-fold.

2 Dilution in the creek is calculated as the ratio of the creek to the plume flow rate

by the following equation:

Q SW
Q di =

Q SW
k ∗ i ∗ d mix ∗ L SW ≈

DF SW = 10 . 6