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introduced over a period equal to the turn-over time 
gives, when divided by the total mass of water in the 
areas, the mean concentration, If this refers to a 
discharge of pollutants, discharge safety margins might 
be established on the basis of a permissible concen- 
tration. 
With respect to this reasoning it is important to note 
that, apart from the condition that the water entering 
from other sea areas (besides that entering with the 
source) should be clean, the condition of stationarity 
and well-mixed conditions should apply. When, as is 
usual in reality, this is not the case, the estimates 
obtained in this way are only to be considered as a 
first guess for the average concentration. Locally higher 
concentrations may be expected and for very variable 
fluxes the most pessimistic (lowest) values might better 
be considered if one wants to be safe. 
Also if one wants to use other, less general methods for 
assessing concentrations, e.g. numerical models, the 
time scale given by the turn-over time is important 
in deciding whether such a method should be based upon 
annual mean conditions, seasonal conditions or whether 
shorter time variability should explicitly be taken in- 
to account. 
The turn-over time is also an important parameter when one 
sonsiders the vulnerability of an area for uncontrolled 
discharges of substances that may turn up to be harmful, 
It is clear that areas with a long mean flushing time may 
remain longer affected than areas with a short mean flushing 
time, all other conditions being equal. As an estimate of 
the mean flushing time the turn-over time is a measure for 
such a vulnerability. 
Finally, the turn-over time may play a role in deciding the 
most efficient monitoring frequency of a sea area.