Estimation of suspended sediment balance of a small catchment in Southwestern Nigeria.

Human use index caused by population pressure in Opa catchment Ile Ife, Southwestern Nigeria, has resulted in catchment denudation, accelerated erosion, and sediment delivery into a man-made (1978) reservoir (Opa) in the catchment. This study is aimed at evaluating the suspended sediment balance of Opa reservoir with a view to ascertain the annual siltation rate. River discharges, water levels, suspended sediment concentrations/yield, and flow velocities were monitored, and data collected from all the tributaries of the Opa reservoir and the spillway (outlet) in the period 2017-2018. Water samples collected were subjected to gravimetric analysis, and the results utilized to obtain sediment rating curves and to compute the suspended sediment balance. Suspended sediment inflow from the six sub-catchments ranged from 8.49 to 29.05tons/ha, with a total inflow of 1146.50tons/ha into the reservoir during the hydrological year. The corresponding outflow through the spillway was 615.70tons/ha. 530.80tons/ha was sequestered in the reservoir, equivalent to an estimate of 46% suspended sediment deposited in the reservoir. The high sediment yield in Opa reservoir is due to the dredging and clearing of the river channels in the catchment of any impediment thereby enhancing sediment delivery into the reservoir. Sediment loads were higher in the rainy season suggesting catchment erosion as the main factor responsible for the sediment yield into the reservoir. The study concluded that the positive suspended sediment balance suggests sediment sequestration with an increasing tendency for accelerated elimination of the wetland in the face of poor environmental management and enhanced human activities.

The effect of Oja-titun market effluent on the chemical quality of receiving OPA reservoir in Ile-Ife, Nigeria.

The effect of market effluent from the Oja-titun market in Ile-Ife, Nigeria on the chemical quality of the Opa Reservoir located 3.5 km downstream was investigated between February and November 2000. Water samples were collected in February, May, August and November from 16 sites, four along each of the market drainage channels (MDC), market stream, tributary stream and the Opa River and Reservoir. The peak level of each variable-biochemical oxygen demand, temperature, total alkalinity, Na(+), K(+), Ca(2+), Mg(2+), PO(4)(3+), SO(4)(2+), Cl(-), NO(3)(-), Pb and Zn-occurred at the MDC, and decreased significantly downstream, except pH, conductivity and dissolved oxygen, which increased. Seasonal fluctuation in most variables was pronounced. Generally, there were high values in the early dry and dry seasons and low values in the rainy and early rainy seasons. Comparison of the reservoir water with international limitation standards for drinking water supply showed that the quality of the reservoir water was very low and that treatment required to achieve minimum limitation standards for drinking water would be both intensive and expensive. The study concluded that the stream borne effluent from the market impacts significantly on the chemical quality of the reservoir water although other tributaries within the Reservoir's catchment are other possible sources of pollutants in the reservoir.

A study of an outbreak of schistosomiasis in two resettlement villages near Abeokuta, Ogun State, Nigeria.

Frequent reports of haematuria among resettlers at Ibaro and Abule-titun located close to a newly constructed reservoir, 20 km north-west of Abeokuta, Ogun State, Nigeria, prompted a pre-control survey to assess age and sex distribution of urinary schistosomiasis in the communities. 210 persons were examined at Ibaro and 145 at Abule-titun between April 1988 and February 1989. Overall prevalence was about 80% in both villages, and all age and sex groups examined were affected. The pattern of infection in both villages was similar, prevalence being high in all age and sex groups, and, unlike most endemic communities, the rate of infection was not significantly higher among the 5-14 year age groups. Intensity of infection was also high in all age and sex groups with mean range egg output of between 73 +/- 73 and 2016 +/- 3875 ova/10ml urine at Ibaro and between 231 +/- 122 and 1340 +/- 1786 ova/10ml urine at Abule-titun. This is attributed to an occupation-related high rate of human water contact in both villages.