gpa_east_africa_case.. - GRID Africa GeoPortal - UNEP

Bacteria content of naturally occurring water is found to be important in the riversBOD and SS: The water from an unpolluted river have values ranging from 3 to 5 mg/l BODand 30 mg/l for suspended solids. The river water analysis was found to be of very poorquality in terms of BOD.River area PO3 Mg/l MO Mg/l1 Northolme 0.02 2.72 Upper St Louis 0 2.13 Lower St Louis 0.03 3.74 Mare Anglaise 0.05 2.55 Danziles 0 06 Upper Niol 0 1.37 Lower Niol 0.01 2.48 Sullivan 0.03 2.6Source: SOGREAH, 1996Freshwater analysis from different rivers sources confirm that these sources are polluted to avariable degree, mainly attributed to human activities.Marine watersBacterial pollution of seawater concerns essentially bathing water and shellfish cultivationwater. Bacterial contamination is a major factor for urban sewage being disposed into the sea.Appropriate European regulation define the threshold levels to be respected in bathing waterand in the flesh of shellfish destined for human consumption.Organic pollution materials are well received in the marine environment on condition that theyare practically all biodegradable. Organic pollutants may have harmful effects only ifinsufficiently dissolved with oxygen or if they are disposed off in stagnant water areas.The coastal areas close to the two river outlets, namely the Mare Anglaise and the SullivanRiver have the highest level of faecal contamination at least for 1.3 months in each year anddeemed unbatheable according to European standards for bathing water. The unsuitablelevels are expected to occur after periods of heavy rains. In only 2 months on average peryear are the levels of faecal coliform at its lowest level. The best area for swimming is locatedin front of Coral Strand Hotel. (Payet, 1996)Indicators of sea water quality in terms of faecal coliform values around the Beau Vallon coastshowed 60% of values unsatisfactory for recreational use by WHO standards (Payet, 1996).The level of faecal coliform in the water is depended upon rainfall and the inputs dischargedby hotels. The observable trend of faecal coliform along the Sullivan river shows that there isa contribution of faecal coliform from the hotels. There is also a relatively rapid decay offaecal coliform close of the coast, probably as a result of the wetland area present at the riveroutlet. The relative contribution of point source pollution (e.g. from hotels) in terms of faecalcoliform to the overall quality of the coastal waters is estimated to be about 43%. The poorturn-over of water in the bay induces a decrease in seawater sodium chloride concentration,itself highly detrimental to coral life, and therefore to all marine organisms associated with thecoral reef ecosystem and trapping of nutrients and polluting substances.Coral ReefsThe coral reefs of Beau Vallon are poor with little diversity. The special conditions in the bayhave led to the formation of poorly developed structures forming two embryonic fringing reefs.The coral reefs consist of an inner reef flat, a reef front and an outer slope. Under presentconditions good coral populations are located only in the southern part of the bay. Other coralreefs are also present at depths of 7 to 20 m beyond the outer slope. Unfortunately, soft coral,which thrives on organic matter enrichments, are thought to be competing with existing coralspecies. The coral reefs are poorly developed in the bay due to the desalinisation of theseawater from inland fresh water. There is also an extensive seaweed bed between thebeach and the reef flat. Its role is very important, especially in the Beau Vallon bay. TheseFinal Draft Report – Cost Benefit Case StudiesGPA Strategic Action Plan on Sewage, October, 200093