Environmental Engineering Reference
In-Depth Information
The algal test is based on growth inhibition of the algae due to toxic substances.
A specific volume of an algae suspension is added to a dilution series of a sample.
Triplicates were made for each dilution. The growth rate was hereafter estimated
by measuring the optical density (OD) at 670 nm after an exposure time of 0, 24,
48 and 72 h. The growth rate of the algae is inhibited by toxic substances and a rela-
tive inhibition to a control can be calculated. In this study the toxicity test kit
Algaltoxkit FTM, MicroBioTests Inc., was applied and the tests were carried out
using 10 cm cuvettes as incubation chambers. An UVmini-1,240 spectrophotome-
ter from Shimadzu was used to measure the OD670. Approximate 10,000 algae
cells/mL were added to each cuvette at the beginning of each test, which due to the
high number of individuals minimized the statistical fluctuations. The test is based
on the international standard DS/EN ISO 8692.
The last test in this study applied the Crustacea Daphnia magna as test organism.
The ability of the daphnia to swim is affected by toxic compounds and is therefore
used as an expression of the toxic effect. The daphnia were exposed to dilutions of
a sample and immobile daphnia were counted after 24 and 48 h. Twenty daphnia
were exposed to each dilution of the sample, which was a much lower number of
individuals than for the two other tests, resulting in larger statistical fluctuations.
For each test, a control was made to ensure that the immobilization was caused by
the toxic effect and not by any physical stress. In this study the toxicity test kit
Daphtoxkit FTM, MicroBioTests Inc., was applied, based on the international
standard DS/EN ISO 6341.
Results and Discussion
Data obtained from the four samples and the three toxicity tests were analyzed
according to the respective standards. Dose-response curves were obtained and
EC50 and EC10 values were estimated when possible. It was a general conclusion
for the samples tested that the bacteria test was less sensitive than both the
Daphtoxkit FTM and Algaltoxkit FTM. This is consistent with previous reported
observations
[2, 9]
. The stormwater runoff had no observable effect on the lumines-
cence readings (data not shown), indicating that the samples were non-toxic.
The daphnia were only inhibited by undiluted aliquots, that is 100% stormwater
runoff. Estimations of EC50 values were therefore not possible. The inhibition by
100% stormwater runoff samples was in the magnitude of 20-50% after 48 h of
exposure, indicating a low toxic effect. The algal test was on the other hand much more
sensitive to the toxic compounds in the runoff. Hence, calculation of EC50 values
was possible, giving EC50 values in two groups: sample I and II had a low EC50 about
10% dilution (after 72 h of exposure) and sample III and IV had a relative high EC50
about 85% dilution (after 72 h of exposure). The significant difference in toxic effect
may be due to a strong natural algal blossom in the wet detention pond, begin-
ning by the end of March. This algal blossom may cause removal of toxic
compounds from the water phase due to uptake by the algae and sorption at their surfaces.
