Hydro-ecological relations in the Delta Waters

tracefore possible to convert the absolute measlured concentrations to
an equivalent concentration that would be expected in a standard river
bed with 550 ~f the grain fraction less than 16 um.
The separation of the Eastmrn Scbeldt and Lake Erevelingen from the
water flowing in the rivers Rhine ad Meuse has resulted in remaining
extremely low on cent rations in these areas. The values found in the
Eastern ScheldC are thought to approximate to natural hackground
levels. The concentration in Lake Orevelingen is known ta be aamedhat
higher. Unlike the Eastern Scheldt, this lake becm stagnant after its
closnre. .As a consequence, there is less interaction between the water
in the lake a d the bed deposits (resuspenaioe) in Lake CreveUngen
than in the Eastern Scheldt. This explains why the concentration of
contaminants in this lake still approaches the pre-1965 values.
The lake beds and river beds in the s,e4imentatlon areas in the northern
part of the Delta are hewily polluted. In the eastern part (Nieuwe
MerwBde/&met) high contamination levels have been detected, affectiqg a
layer at least 2 metres thick. Zhis material was deposited between 1970
and 1975. As an equilibrium has now developed between the hydraulic and
morphalugtcal precesses, it is unlikely tKat this la~rer will be coveted
by new se&ments in the future.
The central section of the Delta (Hollands Diep) hag a thick layer of
less polluted river sediment that was deposited in the years aftek
1975. However, more contaminated material from the 1970-1975 period is
contaised below this layer. Underneath these layers can be fonnd
estuarine sediments from *he period before 1970. l'he different layers
referred to above can be qlearly dlvting=ished fro= core samples taken
from the area (Fig. 6). It is reesgnized that the quality of the tap
layer of sediment will improve as the quality of the surface water
iaproves.