2C) 
If we for the moment neglect the turbulent exchange in 
estimating the turn-over time, this means that the values 
that are based upon the advective flux estimates only are 
estimates of an upper value of 15° As such they are con- 
sidered still useful, because using such upper values 
gives some additional safety in assessing permissible 
discharges in the way outlined at the beginning. 
However, turbulent exchange in the long run brings back 
substances from the sea area "downstream" when there has 
been a gradual build-up of the concentration in that area, 
and it transports substances "upstream", so annihilating 
the condition that the water entering should be clean. It 
is clear that for a better understanding of the concen- 
tration resulting from a source in one sea area the whole 
system of interconnecting sea areas has to be considered. 
An example of this is given in a publication by the National 
Radiological Protection Board and Commissariat ä l'Energie 
Atomique (1979). It should be mentioned that the data used 
in that publication for the North Sea are not easy to 
evaluate in the context of this work, especially also be- 
cause exchange may depend on the choice of the sea areas, 
Yet such a box-model might be further developed on the basis 
of the present work. Also for this reason sections 11 and 12 
give data on run-off and evaporation/precipitation estimates. 
The report gives a subdivision in a number of areas on the 
basis of hydrographic considerations (section 4), supported 
by biological considerations (section 5). 
The subdivision is based upon a rectangular grid of 4) X 1° 
latitude-longitude rectangles. This has been done to faci- 
litate comparison with statistical data, and also because 
boundaries following other contours were thought to suggest 
a better definition of the various areas than in reality. 
Figure 2.1 shows this subdivision.