Environmental Engineering Reference
In-Depth Information
where:
- k
=
hydraulic conductivity of the creekbed wall (0.014 cm/s);
-
horizontal hydraulic gradient (0.018);
- L sw
i
=
=
creek-bed length, where the plume flows in (250 cm);
- d mix
=
groudwater-creek-water mixing zone (150 cm);
creek water flow rate (100 cm 3 /s);
- Q sw
=
- DF sw
=
dilution rate of the groundwater with creek water.
Attenuation and dilution rate together is 47-times the original concentration.
3
Preparation of dilution series for ecotoxicity testing and measuring acute ecotox-
icity of the groundwater. Aquatic ecosystem is represented by fish, daphnia and
algae, organisms belonging to three different trophic levels. A dilution series of 2x,
5x, 10x, 50x and 100x was prepared from the groundwater sample taken from the
source (the highest contaminant concentration). The results in Table 10.1 indicate
that 20-times and greater dilution of the most contaminated groundwater show
neither fish, nor daphnia and alga toxicity.
4
Comparison of contaminant attenuation rate with the “no-effect'' dilution: 47-
fold natural attenuation
10-fold dilution, which is the highest concentration
showing any effect on one species (algae) and higher than the next 20x dilution,
which has not shown any adverse effect, i.e. a 47-fold dilution is sufficient for
clearing adverse effects. Thus the 47-fold natural attenuation satisfies the “no
effect criterion''.
5
Characterization of uncertainties:
a.
The “no effect'' answer of this initial assessment applies for the momentary
situation and for short term. The history of the contamination, placing the
actual concentrations and effects on a larger time-related and spatial scale is
necessary to judge the risk correctly.
b.
We supposed a steady-state situation with constant contaminant resupply:
both milder and worse situation may occur with no resupply or extreme
recharge.
c.
The composition of the contaminant plume is not known in this case;
d.
Components' attenuation rate may differ. The composition of the mixture
that flows into the creek might be more disadvantageous than the initial one;
e.
Chronic toxicity cannot be estimated due to short available time and
unknown components.
f.
The fluctuation in the small creeks' flow rate may be significant causing
differences in dilution rates.
Table 10.1 Acute ecotoxicity of the dilution series from the most contaminated groundwater.
Contaminated sample dilution
Background sample dilutions
Ecotoxicity test
-
2x
5x
10x
20x
50x
-
2x
5x
10x
20x
50x
Fish lethality (%)
10
20
2
0
0
0
0
0
0
0
0
0
Daphnia immobilization (%)
0
0
0
0
0
0
0
0
0
0
0
0
Algae inhibition (%)
70
40
5
2
0
0
6
0
0
0
0
0
 
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