INTERLINKING STREAM MORPHOLOGY, WATER QUALITY, AND RE-AERATION RATES: A SEMI-EMPIRICAL MODELING STUDY OF EKULU STREAM, NIGERIA

Chidozie Nnaji; Andrew Ajah

doi:10.4314/njt.v44i3.12

Authors

C. C. Nnaji University of Nigeria, Nsukka https://orcid.org/0000-0001-7575-6978
A. Ajah University of Nigeria, Nsukka

DOI:

https://doi.org/10.4314/njt.v44i3.12

Keywords:

Stream, Pollution, Re-aeration, Dissolved oxygen, Self-purification, Flow

Abstract

Global population explosions coupled with increase in industrial activities have resulted in a corresponding rise in effluent discharge which end up in streams and rivers. Re-aeration rate plays a vital role in the self-purification of these rivers and streams. This study was aimed at investigating the re-aeration of Ekulu Stream in Enugu metropolis and the effects of hydraulic factors and water quality parameters on its re-aeration coefficient (k₂). Water and sediment samples were obtained from a 2.56 km stretch of the Stream and subjected to physico-chemical characterization. Water samples were tested for temperature, pH, total dissolved solids, total solids, electrical conductivity, dissolved oxygen and biochemical oxygen demand. Hydraulic parameters of the channel namely, slope, velocity, flow depth and width were also measured and then used to estimate the value of k₂ at each sampling point based on ten selected empirical re-aeration models designated as E1 to E10. Dissolved oxygen values ranged from 2.4 to 5.4 mg/L indicating pronounced oxygen sag. The re-aeration coefficient of Ekulu Stream as computed using the various models ranged from 1.42 to 57.11 day^-1. The models of Churchill (E4), Owens et al (E8 & E9) and Bennett and Rathbun (E1 & E2) gave the highest k₂ values ranging 57.11 to 18.90, 49.46 to 16.56, 47.95 to 15.40, 45.96 to 23.56 and 41.54 to 14.74 day^-1 respectively. Apart from E1 and E3, all the models used in this study produced k₂ values that have positive relationship with pH of water. Between 55 and 91% of the variation in the value of K₂ can be explained by a linear combination of TDS and the interaction of electrical conductivity and temperature (EC*Temp). Re-aeration models incorporating only velocity and flow depth (E2, E4, E7, E8 & E9) exhibited the best linear relationship with water quality parameters with p-values of 0.012, 0.001, 0.000, 0.005 and 0.014 and R² values of 0.72, 0.88, 0.9, 0.78 and 0.71 respectively. Water quality parameters affect the re-aeration rates of Ekulu Stream. This finding will open a new research frontier with respect to stream re-aeration and is expected to help improve re-aeration models by integrating water quality parameters or an index thereof.

Author Biographies

C. C. Nnaji, University of Nigeria, Nsukka

Department of Civil Engineering, University of Nigeria Nsukka, Enugu, Nigeria

Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa
A. Ajah, University of Nigeria, Nsukka

Department of Civil Engineering, University of Nigeria Nsukka, Enugu, Nigeria

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