Authors:
Thabang Mmutle
and
Saheed Adeyinka
Affiliation:
Department of Civil Engineering, Central University of Technology, President Brand, Bloemfontein, South Africa
Keyword(s):
River Catchment, Self-purification, Dissolved Oxygen, Biochemical Oxygen Demand, Oxygen Deficit, De-oxygenation, Re-oxygenation, Eutrophication, Water Quality Modelling.
Abstract:
Dissolved oxygen is the most essential element in natural water bodies for one of the most important reasons, namely aquatic life. This content is usually affected by the type and amount of pollution introduced in natural water bodies. The dissolved oxygen level is usually lowered at any point where a natural water body such as a river is contaminated (deoxygenation); however, using natural purification forces, rivers work hard to gain back the amount of oxygen lost in the water due to pollution (re-oxygenation). This study articulated the self-purification capacity of the Mooi River catchment as a function of the rate of change of the amount of dissolved oxygen in flowing water to illustrate the purification strength of a river flow segment between sampling points. This is to subsequently present the impact of inflowing pollution from different types of adjacent sources and tributary rivers. This was achieved by conducting measurements of dissolved oxygen and temperature directly fr
om the river, using an electrolyte dissolved oxygen meter. Respective samples (three-litre samples) were also collected at every sampling point for a biochemical oxygen demand laboratory analysis taken over five days. Using the biochemical oxygen demand and oxygen deficit analysis, deoxygenation and re-oxygenation factors or constants were determined for every flow segment. The mathematical ratio between the two constants were then used to calculate the self-purification capacity of every segment. Because the hydraulic dynamics of the river also influence the strength of the river to purify itself, a re-oxygenation model of hydraulic properties, such as flow velocity hydraulic depth and radius, was developed and presented by means of a regression analysis. The findings have proven that the river has the capacity to purify itself along its existing length for both dry and wet seasons. The purification fluctuations were high during the wet seasons due to the increase in hydraulic flow depth and pollution by run-off. Oxygen deficiency was very low before the Mooi River confluences with the Vaal River; therefore, it did not significantly affect the oxygen content of the Vaal River.
(More)