Estimation of hydraulic parameters of the shallow, perched Ohangwena aquifer (KOH-0) and determination of its interaction with the deeper regional Ohangwena aquifer (KOH-1), Ohangwena region, Namibia

A research thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science

The Ohangwena Multi-Layered Aquifer (KOH) is one of the six Aquifer Systems found in the Cuvelai-Etosha Basin, Namibia. The aquifer system is multi layered, consisting of a shallow perched Ohangwena 0 aquifer (KOH-0), semi-confined Ohangwena I aquifer (KOH-1), and confined Ohangwena II aquifer (KOH-2). Currently, a large portion of the population in the study area rely on water from KOH-0 by means of hand-dug wells. The occurrence and potential of this perched aquifer is not fully understood. This fact motivates the characterization of the aquifer properties, to enable more thoughtful management of the resource. The study evaluates the hydraulic properties of KOH-0 and determine its connection to KOH-1. The methods used in this study included soil sampling, analysis for grain size distribution and review of existing borehole lithology logs. Examination of existing borehole lithology logs indicate that the perched KOH-0 is contained in the Kalahari sand layer from the surface up to 30 m depths, underlain by a multi-layered semi-confining layer with alternating silty sands, clays and calcrete. Based on the documented textures and standard hydraulic conductivity tables, KOH-0 has hydraulic conductivities from 1.20E-04 to 2.83E-04 m/sec and the multi-layered aquitard below, from 3.17E-9 to 1.90E-4 m/sec. Grain size analyses of the aquifer material show that the material consist of fine to course grained sand indicating a hydraulic conductivity that spans between 10-6 and 10-4 m/sec, a porosity between 0.49 and 16.25% and a coefficient of uniformity between 0.369 and 25. Potentially an amount between 2.01E-03 and 5.02E-03 m3/day, may flow from the perched KOH-0 into the unconfined KOH-1 per square meter of aquitard. The estimated amount of horizontal flow through KOH-1 is much higher than this, ranging relatively between 690 m3/day (at minimum hydraulic gradient) and 6700 m3/day (maximum hydraulic gradient). The results have shown that KOH-0 is hydraulically connected to KOH-1, especially in the northern part of the study area with higher hydraulic gradients where most hand dug wells occur and groundwater is younger. This will aid future groundwater development and utilization in the study area.