Factors controlling the distribution and behaviour of organic carbon and trace elements in a heavily sewage polluted coastal envoironment

الملخص EN

The coastal area south of Jeddah, Saudi Arabia, eastern Red Sea, receives daily more than 100,000 m3 of domestic sewage.

The solid material of the dumped wastes was analyzed for its organic carbon, Al, Fe, Mn, Cu, Zn and Cr contents and their mass emission was calculated.

To trace the impact on the coastal area, the same elements were analyzed in sediments and suspended matter in the receiving basin and along a coastal strip extending for about 10 km southward the discharge point.

It was found that the impact of the effluent was limited to the area in the proximity of the effluent discharge point where concentrations in both sediments and suspended matter were several times higher than in the other parts of the area.

Away from the effluent, concentrations were comparable to those measured in uncontaminated carbonate sediments.

In the dilution basin, the distribution of the elements correlated with the dilution pattern of the effluent water.

Normalization to aluminium revealed however, that only Fe, Cu and Zn were enriched in the sediments (EF 1.7, 6.3 and 6.8 respectively), while Mn and Cr were depleted.

This behaviour was attributed to the difference in the oxidation-reduction kinetics of the redox sensitive elements (Fe, Mn, Cr) and to interaction with the organic matter and formation of insoluble sulphides (Cu and Zn).

In the suspended matter all the elements showed a positive deviation from linearity at intermediate salinity.

The excess was accounted for by the resuspension of bottom sediments (Fe, Cu and Zn) and adsorption of mobilized elements (Mn).