Browsing by Author "Mengistu, Haile"

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    Assessing hydrogeological characteristics to establish influence of aquifer-river interaction in non-perennial river systems, Heuningnes catchment
    (University of the Western Cape, 2019) Banda, Vincent Santos Dzulani; Mengistu, Haile; Kanyerere, Thokozani
    Over half of total flows in the global river network are composed of non-perennial rivers. This indicates the importance of non-perennial river systems in supporting the biodiversity. It has been established that groundwater is one of the elements that control the flow regimes and classification (whether perennial or not) of a river system. However, the use of hydrogeological characteristics to establish the influence of groundwater on non-perennial river systems remain to be widely unpublished. This study, therefore, intends to conceptualize and explain the role of hydrogeological characteristics in non-perennial rivers, using the Heuningnes catchment in the Western Cape Province of South Africa as a case study. The study has argued that thorough characterization of aquifers is essential in order to adequately establish the extent of aquifer-river connectivity and how groundwater influences flows and chemical loading in non-perennial river systems. The study has three objectives namely: (i) to determine the aquifer characteristics (ii) to characterise the aquifer-river interaction and (iii) to conceptualize the groundwater flow system. Records review, field, analytical and laboratory-based methods were used to collect and interpret geological, groundwater level, pumping test, hydro-chemical and environmental stable isotopic data in order to characterise groundwater occurrence, flow system and its interaction with the rivers of the study area. Water samples were taken from groundwater, surface water and rainfall during both dry and wet periods. Results show that the study area has a topography-controlled water table with shallow depth to groundwater levels ranging on average from 3 - 10 m, which result into largely a local groundwater flow system. Transmissivity values determined from constant rate pumping test range between 0.17 and 1.74 m2/day. Results exhibit that the low transmissivity values are associated with the weathered nature of the Table Mountain sandstone and the unfractured Bokkeveld shale formations. Hydrochemical data results indicate that both groundwater and river samples in the upstream part of the study area are characterised as fresh water with TDS values of less than 1000 mg/l while the downstream part has saline waters with TDS ranging from 2000 – 35000 mg/l. Results also show that Na-Cl is the dominant water composition for both groundwater and river water. The order of major ion dominance is similar for the two water sources, with concentration ranges from high to low in the order of Na+>Mg2+>Ca2+>K+ and Cl->SO42->HCO3- for cations and anions respectively. The similar patterns and trends in salinity and major ion data suggest the connectivity between the aquifer and the river. Environmental stable isotope data indicate river samples in upstream areas having depleted δ18O (-4.3 to -5.12‰) and δ2H (-22.9 to -19.3‰) signatures similar to groundwater indicating a stable and continuous groundwater contribution to the river flows. Meanwhile, high evaporative enrichment of δ18O (1.13 to 7.08‰) and δ2H (38.8 to 7.5‰) is conceived in river samples from downstream areas. Ionic ratios and isotope-salinity relationships suggest that groundwater chemistry is derived from sea sprays, evaporation and dissolution of Bokkeveld shale host rock. Geological, hydrogeological, hydrochemical and environmental stable isotope data were used to develop a conceptual hydrogeological model which explains the role of groundwater in non-perennial river systems. Results indicate that the North East – South West fault on the north-eastern part of the study area seem to act as a conduit to groundwater flow thereby supplying water to the upstream rivers while the East -West fault in the northern part seem to act as a barrier to groundwater flow resulting into a hydraulic discontinuity between upstream and downstream areas. Meanwhile, the relatively low conductive formation in the downstream areas coupled with a relatively low hydraulic gradient (0.000843) suggests there is slow Darcian groundwater flows resulting in less flushing and high salinization of groundwater. Eventually, in the downstream part of the study area there is slow and minimal groundwater discharge to the rivers resulting into groundwater failing to maintain the river flows and pools. In general, rivers of the study area largely gain water from groundwater although the amount of groundwater discharge varies from one river segment to another in both upstream and downstream parts. The conceptual model has led to the development of a proposed optimum management of non-perennial rivers including the effects of groundwater abstraction on the river flows.
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    Assessment of catchment scale groundwater-surface water interaction in a non-perennial river system, heuningnes catchment, South Africa
    (Scientific African, 2023) Banda, Vincent Dzulani; Mengistu, Haile; Kanyerere, Thokozani
    A significant proportion of the world's river networks are non-perennial rivers that are characterized by segments of dry, standing, and flowing water. However, the role of groundwater and the controlling elements governing the flow processes in these rivers is not widely documented. In this study, aquifer-river interaction was assessed using a combo of geological, hydrological, environmental stable isotope, and hydrochemical data in the Heuningnes catchment, South Africa. Results showed the depth to groundwater levels ranging from 3 to 10 m below ground level and aquifer transmissivity values of 0.17 to 1.74 m2/day. The analytical data indicated that Na-Cl type water dominates most groundwater and river water samples. Environmental stable isotope data of river samples in upstream areas showed depleted δ18O (-4.3 to -5.12 ‰) and δ2H (-22.9 to -19.3 ‰) signatures similar to the groundwater data, indicating a continuous influx of groundwater into the river water. Conversely, high evaporative enrichment of δ18O (1.13 to 7.08 ‰) and δ2H (38.8 to 7.5 ‰) were evident in downstream river samples. It is evident from the local geological structures that the fault in the north-eastern part of the study area passing Boskloof most likely acts as a conduit to groundwater flow in the NE-SW direction thereby supplying water to upstream river flow, while the Bredasdorpberge fault likely impedes groundwater flow resulting in hydraulic discontinuity between upstream and downstream areas. Relatively low conductive formation coupled with an average hydraulic gradient of 8.4 × 10−4 suggests a slow flow rate resulting in less flushing and high salinization of groundwater in downstream areas. The results underscore the significance of using various data sets in understanding groundwater-river interaction thereby providing a relevant water management platform for managing non-perennial river systems in water-stressed regions. Overall, the study provides important insights into the need for maintaining moderately high groundwater levels in shallow and local groundwater systems for sustaining the ecological integrity of non-perennial rivers.
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    GIS-based modelling of climate variability impacts on groundwater quality: Cape Flats aquifer, Cape Town, South Africa
    (Elsevier, 2021) Gintamo, Tesfaye Tessema; Mengistu, Haile; Kanyerere, Thokozani
    The need to improve groundwater security remains critical, especially in urban areas where demand for groundwater as an alternative source of water supply is increasing following unprecedented population growth. Climate change continues to threaten groundwater resources in such areas. This study assessed and analysed data from a variety of sources that required holistic analytical tools to demonstrate the impacts of climate change on groundwater quality at the local level. We evaluated how climate conditions affect groundwater quality using a hydrological model (WaterWorld model) in a GIS context. The Cape Flats Aquifer in the city of Cape Town in South Africa was chosen as a case study. The WaterWorld model was used to calculate hydrologic scenarios based on climate change factors and groundwater quality parameters for the period 1950–2000. Mean annual precipitation and temperature were simulated using the multi-model mean and Representative Concentration Pathway 8.5 for the years 2041–2060.
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    Zambezi river basin aquifer systems: opportunities and challenges in using freely available data sources and groundwater flow modelling for spatial exploratory analysis
    (Elsevier B.V., 2025) Mengistu, Haile; Banda, Kawawa; Crestaz, Ezio
    A groundwater flow model was implemented over the Zambezi River Basin using the state-of-the-art DHI-WASY finite element code Feflow. The analysis was based upon different freely available datasets that include a hydrologically consistent digital elevation model from HYDROSHEDS, the BGS (British Geological Survey) quantitative hydrogeological maps, and the regional hydrogeological SADC-GMI database. The model implementation was aimed at: (i) to identify and analyse challenges and limitations (data scarcity, accuracy of regional datasets, impact of geological, tectonic and hydrogeological complexity on model reliability) in applying groundwater flow modelling at basin scale; (ii) to perform an exploratory spatial analysis with reference to the magnitude and spatial distribution of effective recharge, aquifers’ properties and interlinks between surface water and aquifer systems (surface water – groundwater interactions). High uncertainty is generally associated with the estimation of hydrological and hydrogeological parameters, whose high spatial variability is not necessarily captured by the regional data products. This study evaluates how integrating freely available datasets (such as the DEM, BGS maps) influences model accuracy and uncertainty, particularly in terms of parameter estimation. The findings illustrates that, despite the limitations, freely available datasets can still effectively be used to develop a groundwater model that captures regional piezometric trends and provides insights into spatial variability. This demonstrates the potential for using such models in similar data-scarce regions. The modeling approach is expected to provide valuable quantitative insights into groundwater trends and variability, helping to identify key areas of uncertainty and guiding future data collection and model refinement efforts.