TIDAL CHARACTERISTICS AND THE CHEMICAL WATER QUALITY OF BONNY ESTUARY IN THE NIGER DELTA BASIN OF NIGERIA
Hydrobiology Section (Laboratory of Limnology), Department of Zoology, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.
Accepted in January 2004.
This study focused on the horizontal changes in some chemical variables of water quality along Bonny Estuary, the estuary with the second largest tidal area and the highest discharge of all the estuaries in the Niger Delta of Nigeria. Surface water samples were collected from seventeen sampling stations over the dry and the rainy seasons of one annual cycle and analysed for pH, electrolytic conductivity, major anions and cations, organic matter, plant micronutrients (NO–3, PO3-4 and SiO2), dissolved oxygen, BOD5 and some heavy metals, using standard analytical methods mostly within 1-10% precision levels. With regard to the horizontal variation along the major axes of the estuary, the investigated chemical variables fell into two sharply defined groups, the abiotic and the biogenic variables. The former showed steady decrease in concentration from the brackish waters through tidal fresh to fresh waters while the latter varied inversely. The waters of the basin were all strongly influenced by the sea and, in view of the basin’s low lying configuration and the extensive penetration of polyhaline water into it, the estuary can be classified among the coastal plain or drowned valley estuaries of the world. The estuary is influenced (at the Bonny bar area) by a tide regime with a maximum possible range of 2.5 m, mean range of spring tides of 1.66 m, mean range of neap tides of 1.31 m and an overall mean tide level of 1.39 + 0.79 m above reference datum of the bar. The fresh waters were slightly acidic in pH (range = 5.7 – 6.3; mean + s.d = 6.0 + 0.3), of low oxygen concentration (1.0 – 2.5; 1.3 + 1.1 mgl-1) low oxygen saturation (generally less 50%) and very poor in electrolytes. On the other hand, they were rich in organic matter, silicate, and nitrate, compared to the brackish waters. In this respect and in their characteristic dark brown colouration, they are similar to temperate bogs and humid tropical swamps, especially the blackwaters of Amazonia, and suggestive of relatively high content of organic matter decay products. The brackish waters varied from near neutral to alkaline in pH (6.7 – 8.3; 8.0 + 0.4), of relatively high oxygen concentration (6.0 – 8.1 mgO21-1; 6.3 + 1.2 mgO2-1) and very similar to sea water both in ionic order of dominance and in their probable salt composition. However, their levels of heavy metals (chromium, cobalt, copper, and nickel) were high compared to the average concentrations in the open sea. The relatively high levels of biogenic variables in waters in the residential areas of the basin is suggestive of organic pollution (most probably of domestic origin) while the relatively high levels of heavy metals at the estuary’s mouth was related to the heavy water traffic and the many activities of the petroleum industry in that area of the basin. Variation in tidal hydrology had a more pronounced effect on the water quality of the estuary than seasonal variation.