Dynamics of waste assimilation in a tropical coastal water: A case study of the Okrika Creek in south-eastern Nigeria

Using the Streeter-Phelps method as modified for tidal regimes, the pollutant dispersal, re-aeration and waste stabilization characteristics of the Okrika Creek in the coastal zone of Nigeria were evaluated as a basis for obtaining estimates of waste loading necessary to maintain ecologically acceptable levels of dissolved oxygen. The dispersion and deoxygenation coefficients varied from 0.4 km2&middotday-1 to 1.2 km2&middotday-1 and 0.3 day-1 to 0.25 day-1, respectively, during the ebb and flood stages. Reaeration coefficient ranged from 0.64 day-1 to 0.46 day-1 during the corresponding flow stages. The dissolved oxygen profiles showed higher concentration levels during tidal current in-rush and early full flood stage. Much lower values were recorded during the late full flood and low ebb stages, indicating a dependence of oxygen content on tidal regime. Unionized ammonia, a major pollutant in the Creek associated with the nearby fertilizer production plant effluent, showed profiles having their peak concentrations at progressively further downstream locations over a number of isolated stages of tidal cycles. This suggests a dispersive-advective transport mechanism capable of moving pollutants oceanwards over a number of consecutive tidal cycles. The optimal BOD loading is 28 mg&middotdm-3, which is about 12% above the nationally recommended level. The implied higher tolerance of the Creek may be due to the strong dispersive-advective and reaeration effects. Timing of waste input with the tidal regime characteristics would provide for an improved pollutant dispersal, flushing and attenuation, and hence the sustained protection of the ecosystem.