”Rotting”
Algae
Obviously, at other measuring plants only a decrease in the
oxygen concentation was shown. Upon closer inspection there was
a small peak to be seen in the measurement of clearly observable
substances . A similar increase before was due to the herbicide
terbutryn. Further analysis confirmed that in Eijsden
there was indeed a concentration of 0.7 µg/l terbutryn
found. This was also reproducible in the Algae Toximeter
by adding 0.7 µg/l terbutryn to the water to show
exactly the known increase in fluorescence.
This also explains the decrease in the oxygen concentration
in the river. Since algae growth was inhibited, oxygen
production decreased. In addition, some of the algae died. This
“rotting process” needs oxygen, which is why the oxygen
concentration in the river continued to decrease.
On 20th April 2007, the Daphnia Toximeter at Eijsden emitted
a signal that the waterfleas were having difficulties.
Additional waterfleas were added. On Sunday morning, 22nd April,
they were dead. Even in this example the automatic chemical
measuring device did not indicate any peculiarities.
For verifcation, the results of the Daphnia Toximeter were
compared with chemically anaylsed samples taken directly at the
river. A calculation was made where the water was at that moment
from Friday and Saturday and samples were taken. The laboratory
in Lelystad measured amounts of herbicides at increased
concentrations and subsequently exceeded the alarm threshold for
drinking water (1 ug/l): chloropyriphos.
In both cases, the biomonitors warned the authorities of
substances which at first glance had not been detected by
chemical monitoring. Thus it was possible to stop the drinking
water uptake in time in order to avoid subsequent damage.
Without the biomonitors this would not have been detected.
