Hydrogeological investigation of the Naukluft mountains, southwest Namibia

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

The Naukluft Mountains area is also referred to as the Naukluft Nappe Complex (NNC), that consist of northwest dipping carbonates and sandstones aged 750 Ma and form part of the Damara Super Group that has been thrust over the younger sedimentary rocks of the Nama Group, aged 550 Ma. In this study, three aquifers are characterized within the study area. First, is the Naukluft Karst Aquifer which is fractured and karstified, and partly underlies the surface run-off catchments of the Tsauchab and Tsondab ephemeral rivers; second is the Nama Aquifer, largely stratified aquifer with limited fractures to the east of the Naukluft Karst Aquifer and third, is the Namib Aquifer, which is a porous phreatic aquifer to the west and south of the Naukluft Karst Aquifer, confined to the alluvial formations of the Tsondab and Tsauchab ephemeral rivers. The Naukluft Karst Aquifer is a high rainfall and discharge area in terms of rainfall; however, in terms of recharge the volume of water available for groundwater recharge is low, due to high surface water discharge from the mountains. National average annual rainfall is 200 mm/a while the study area average is 170.36 mm/a based on the rainfall data collected from farmers during this study. Using the Chloride Mass Balance Method rainfall available for recharge is estimated to range from of 0.41 to 24.43 mm/a translating to 0.24 to 14.24% of received rainfall for the season of 2008/2009 calculated considering only wet deposition values for chloride. Places in the Naukluft Karst Aquifer area receive more rainfall and slightly higher recharge than calculated. Reconnaissance pumping tests were conducted in a well and boreholes at Leybank, Naukluft Park office, Zais and Solitaire to estimate transmissivity. Results indicate that transmissivity ranges from 4.1 to 23.1 m2/d. storage coefficient was not estimated due to lack of piezometers during test pumping. The main groundwater flow is oriented along surface water drainages to the northwest and southwest, with minor flow to the northeast. Flow is controlled by elevation above sea level and structural geology of the Naukluft Karst Aquifer. A west-east trending groundwater divide is identified separating two distinct groundwater flows.