Evaluation of the adequacy of a rehabilitation programme, implemented in two Red Sea coastal lagoons, using the hydrological characteristics of surface water

Abstract EN

This study has been undertaken to examine the results of the rehabilitation programme of Al’Shabab and Al’Arbaeen lagoons (Eastern Red Sea), that the municipality of Jeddah (Saudi Arabia) has implemented more than 8 years ago.

The strategy of the study is based on the analysis of a number of physical and chemical characteristics of the surface water of the lagoons and to compare them to available results produced before the implementation of the programme.

Field observations and results of chemical analysis indicate that sewage discharge is continuing.

This is particularly indicated by a significant salinity lowering (up to 11).

The presence of very elevated concentrations of nitrogen and phosphorus, and the presence of ammonium nitrogen as a dominant nitrogen inorganic species is also a proof of sewage dumping.

A daily flux of 82-149 kg of total phosphorus and 1330-2711 kg of total nitrogen is injected into Al Shabab and Al Arbaeen respectively the presence of nutrients in excessive quantities has turned the trophic status of the two lagoons to eutrophic to hyper eutrophic which resulted in organic matter build up in the two basins.

When applying the phosphate/total inorganic nitrogen ratio at the source (zero salinity) or the plot of phosphate and total inorganic nitrogen against salinity, phosphate appears as the limiting nutrient at least until salinity 33.

Comparison of the present data with available data collected before the implementation of the rehabilitation programme has shown that, excepting the hyper aeration of the surface layer as a result of continual air pumping and enhanced photosynthetic activity, all the measured variables showed levels comparable to those obtained before rehabilitation.

Moreover, the bottom layer is still low in oxygen (2 mg l-1 in some cases) due to the stability of the water column and the buildup of organic matter.

In order to achieve a real rehabilitation of these two sensitive lagoons, it is imperative to stop all types of waste disposal ; air pumping alone will never be sufficient because of the enhancement of primary productivity and its inability to remove pollutants.