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Accueil du site → Master → Afrique du Sud → 2019 → An investigation of the impacts of Acacia Mearnsii plantations on secondary aquifer systems within the Two Streams catchment, KwaZulu-Natal, South Africa

University of KwaZulu-Natal (2019)

An investigation of the impacts of Acacia Mearnsii plantations on secondary aquifer systems within the Two Streams catchment, KwaZulu-Natal, South Africa

Ngubo, Caiphus Zimise.

Titre : An investigation of the impacts of Acacia Mearnsii plantations on secondary aquifer systems within the Two Streams catchment, KwaZulu-Natal, South Africa

Auteur : Ngubo, Caiphus Zimise.

Université de soutenance : University of KwaZulu-Natal

Grade : Master of Science : Hydrology 2019

Résumé
The Two-Streams catchment, located in the Seven Oaks District in the KwaZulu-Natal midlands of South Africa has been used as an experimental catchment over the past decade to investigate the impacts of Acacia mearnsii stands on hydrological processes. As part of the ongoing hydrological study, the hydrogeology of the Two - Streams catchment covering an area of about 0.74 km2 is investigated and characterized to understand the impacts of Acacia Mearnsii plantations on groundwater. A combined hydrological, hydrogeological, hydrochemical and environmental isotope methods were employed in characterizing the hydrogeology of the study catchment. Geologically, the study area is underlain by three units, namely ; top weathering profile, mainly of clay, which is underlain by weathered shale. The shale is in turn underlain by granite basement rock. Two hydrostratigraphic units are identified : a water table or unconfined aquifer occurring along the weathered shale and the underlying regional weathered and fractured semi confined basement granitic aquifer. The regional weathered and fractured granitic aquifer is characterised by a transmissivity that ranges from 0.15 to 0.48 m2 /day and a hydraulic conductivity of 0.04 m/day. The TwoStreams catchment receives a mean annual rainfall of 778 mm, average annual evapotranspiration of 802 mm, average annual stream discharge of 20387 m3 and average annual recharge of 31.9 mm or 4.1 % of MAP. Hydrochemical data indicates both groundwater and stream samples are characterised by mean specific electrical conductivity (EC) of 28.5 mS/m and Ca-HCO3 and Ca-Cl dominant hydrochemical facies. All samples have δD and δ 18O isotopic values that plot on or above the local and global meteoric water lines, indicating recharge from rainfall with insignificant evaporation during or prior to recharge. Seasonal stream isotope data analysis indicates that groundwater is the main contributor of streamflow during dry season. Furthermore, the impacts of Acacia mearnsii trees on groundwater through direct groundwater uptake by roots and impacts of trees on recharge, groundwater levels and baseflow were investigated. The results show that direct groundwater uptake by tree roots from the saturated zone at Two-Streams would not be possible due to limiting root depth. Thus, in instances where the regional groundwater table is not available for direct abstraction by tree roots, Acacia mearnsii trees can have a large impact on groundwater through extracting water from within the unsaturated zone, reducing the proportion of rainfall that eventually contributes recharge to the aquifers and baseflow, without necessarily having direct access to the groundwater proper.

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