Abstract |
The determination of hydraulic parameters is central to aquifer studies including groundwater flow models, sustainable yield assessment and groundwater residence times. It is essential that hydraulic conductivities and effective porosities are known in their stratigraphic context. In the herein discussed study the hydraulic conductivity has been determined and evaluated for the aquifers in the Ohangwena Region based on the fully cored borehole WW203302. The borehole penetrated the Upper Kalahari Sequence to a depth to 400 meters and encountered perched and deeper confined aquifers. This study focuses on the upper 150 meters of the core. The first 8.7 meters are composed of unconsolidated sandy sediment, however the core of this uppermost interval could not be retrieved. It is known that this top interval hosts a shallow perched aquifer.
Samples have been collected in 10 meter intervals with a total of 18 samples collected. The falling head method was used to determine the hydraulic conductivity of 18 samples. With the obtained hydraulic conductivities the presence of an aquiclude with a thickness of 61.8 meters and deeper aquifer (KOH-1), just below the aquiclude, has been delineated. A borehole litholog shows that the aquifer is made of sandy material with minor silt and clay, while the fine grained fraction is significantly higher for the overlying aquitard.
Calculation of vertical groundwater movement from the shallow perched aquifer through the aquitard down to the confined aquifer KOH-1 reveals a travel time of 1096 years at a distance velocity of 5.5E-2 m/year. Such a travel time indicates that KOH-1 can receive limited recharge from the shallow perched aquifer above. The general hydrological setting allows for additional significant lateral recharge deriving from a northern source. |
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