INFLUENCE OF LITHOLOGY ON STRUCTURAL CONFIGURATION AND WATER CHEMISTRY OF ALA DRAINAGE SYSTEM, AKURE, SOUTHWESTERN NIGERIA

Abstract

Drainage morphology relies on several factors such as climate, vegetation, precipitation, soil, rock type, and susceptibility of the underlying rocks to weathering. Geological conditions have been a major control on stream processes due to their influence on the nature of flow, sediment transport, and chemistry of the water. In this study, some of such geological conditions were investigated for their influence on structural configuration and water chemistry of Ala drainage system in Akure. Ala River emerges from deep-seated fractures on granite and charnockite outcrops at the North and North-western region of Akure, consequently flowing through the quartzites and the granite gneiss. Landsat imagery of the study area was acquired and processed for extraction of lineament and production of lineament map. The stream channels in the drainage system were classified into orders and the stream orientation analysis were carried out using rose diagram. Channel width, depth and flow velocity measurements were carried out at the thirty seven established study points within the system to obtain the channel geometries. Water samples were collected at each of the established study points and subjected to chemical analyses for dissolved major cations (Na+, k+, Ca2+, Fe2+, Mg2+) and anions (NO3 -, SO4 2-, PO4 3-, HCO3 -, Cl-). Physico-chemical parameters (pH, Temperature, Total Dissolved Solids and Electrical Conductivity) of the water samples were measured on the field. Results indicate that the predominant direction of lineaments in the study area is ENE which lies parallel with the second order streams. The general trend in the Ala River is the geological control with decreasing stream order. The indices of downstream geometries are 0.346 for width, 0.526 for depth, and 0.178 for velocity while the coefficients are 7.18, 0.85 and 0.29 for width, depth and flow velocity respectively. The imperfect nature of the relationships showed that Ala River does not have a normal hydrologic regime and is not well adjusted to the channel morphologic variables of width, depth and flow velocity. This may be due to the prevailing anthropogenic vii effects on the drainage system. The pH of the sampled waters varied between 7.16 and 7.75 with an average of 7.38, electrical conductivity (EC) ranged from 123 (μs/cm) to 520 (μs/cm) with mean of 259.82(μs/cm), Total Dissolved Solid (TDS) showed variation between 62mg/l CaCO3to 260mg/l CaCO3 with a mean value of 130.62mg/l CaCO3 while the temperature had an average value of 26.65oC. Results show that relative concentration of cations and anions were in the order of Ca2+ > Mg2+ > Na+> K+ and HCO3 - > SO4 2- > Cl- > NO3 -. Mean values of dissolved Na+, K+, Ca2+, Mg2+, Fe2+, Cl-, HCO3-, SO4 2-, NO3 -, and PO4 2- were 34.028mg/l, 8.872mg/l 34.175mg/l, 18.289mg/l, 0.428mg/l, 16.543mg/l, 56.41mg/l, 12.503mg/l, 0.495mg/l, and 2.029 mg/l respectively. Two hydochemical facies namely Calcium-Bicarbonate facies and mixed Calciu-Magnesium-Chloride facies were found to dominate the chemistry of the water. CaHCO3 facies reflect the contribution of geogenic processes while the mixed CaMgCl type reflects domestically-induced anthropogenic activities influencing chemistry of the water