The hydrochemical characterization of the Mgobezeleni catchment in Sodwana Bay
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Date
2019
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Publisher
University of Zululand
Abstract
The Maputaland region comprises one of the world's most remarkable areas of biodiversity and aquatic systems. It also has the largest primary aquifers in South Africa, constituting 25% of all coastal plain aquifers found in the country.
The increasing number of rural households’ development along the Maputaland coastal plain pose significant eutrophication risks on the hydrological system, owing to the use of informal sewage management facilities. To understand the anthropogenic impacts on the hydrological system the Mgobezeleni catchment was selected in this study as it represents a microcosm of the Maputaland region. The Mgobezeleni Estuary is connected to the two lakes (Lake Mgobezeleni and Lake Shazibe), and large swamp forest, and mouth is affected by tidal influence.
The hydrological system in the catchment is directly controlled by the groundwater system. The overlying paleo-dune sands are permeable permitting high percolation of the annual rainfall. The catchment is dominated by three aquifers namely, the uppermost KwaMbonambi Formation, Kosi Bay Formation and Uloa Formation. The water table is dynamic, affecting the dimension of the unsaturated zones, the hydraulic gradient and the groundwater flow in the aquifers. The groundwater recharge only becomes visible as base flow in surface water resources after several years. Contaminants that have accumulated over decades are therefore still present in the aquifers, being restricted mainly in the upper KwaMbonambi Formation. Therefore, leaching of nutrients from the informal sanitation systems pose adverse impacts on the groundwater chemistry, which in turn can result in eutrophication of surface water resources.
The growth in population in the Mgobezeleni catchment has therefore increased the nutrient load in the groundwater system due to the presence of confining layer that restricts nutrients to the shallow aquifer. Nutrients are relatively elevated in groundwater close to highly populated areas such as Mbazwana. Moreover, the discharge of groundwater as a base flow component in surface water resources (lakes, estuary and wetland) causes the eutrophication (growth of dense macrophytes and microalgae, including toxic cyanobacteria species) due to high nutrient composition. The dense macrophytes species together with the cultivated peat, are decomposing to produce high levels of dissolved OM with humic substances (humic acid and fulvic acid) causing the blackwater in the surface water resources and also increasing the population of microalgal species.
The water in the catchment is generally characterized by a sodium-chloride ions signature with relatively low Ca, Mg, K, and SO4 concentrations. It is generally high in Al, Fe and Mn concentrations.
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The high levels of nutrients and planktonic algae in hydrological system threaten the ecological system in the Mgobezeleni catchment. The presence of the bacteria Microcystic aeruginosa can affect the domestic and wildlife including the water supply to Ezemvelo KZN Wildlife’s residential areas. In addition, the cultivation of peat in the Mgobezeleni catchment causes the peat to decompose, thus also increases the N and P concentrations and OM content in the wetlands which are also influencing the water quality in the lakes and estuary.
Description
A dissertation submitted to the Faculty of Science and Agriculture in partial fulfilment of academic requirement for the Degree of Doctor of Philosophy in the Department of Hydrology at the University of Zululand, 2019.
Keywords
Hydrochemical, Groundwater quality