Geophysical and hydrogeological survey in a part of the Nhandugue River valley, Gorongosa National Park, Mozambique : area 2 and 3

Sammanfattning: Gorongosa National Park in central Mozambique is situated at the southernmost extension of the East African Rift System (EARS), the Urema Rift. The park is protecting a vast ecosystem of floodplains, grasslands and woody coverage. From 20 years of civil war the large-mammal population in the park had in year 1994 been reduced by 90 % or more, and in 2004 a restoration project was started in order to rebuild and rehabilitate the park. For an ecosystem in balance, water resources are indispensable and Lake Urema in the central part of the park is vital. In order to maintain a sound management and decision-making about water resources in the Gorongosa region park authorities want to predict the hydrological effects due to changes in different environmental factors. For this, new and additional information is needed about the hydrogeological conditions of the park. Focus in this paper is to provide hydrogeological information about the western margin of the Urema Rift, since it has been anticipated that this area is a potential major groundwater recharge zone to the deeper Rift valley floor sediments. The methods used in the study are resistivity and induced polarisation (IP) measurements, together with geological ground truthing and discharge measurements. Data were collected from three areas (Area 1-3) along the Nhandugue River at the north-western border of the park. In this paper the results from Area 2 and 3 are presented, whereas the data from Area 1 is presented in a paper by Stenberg (2010). Three layers of Quaternary alluvial sediments and one bottommost layer of sandstone can be identified in the resistivity models. Soil samples show variations in grain sizes and mineralogical composition depending on where in the river valley the material was deposited. A topmost unconfined aquifer is identified, below which there is a partly permeable aquiclude in turn covering two different semi-confined aquifers. A downstream water loss in the Nhandugue River suggests that surface water infiltrates to the deeper aquifers and is transported as groundwater in an east/southeast-ward direction. The hydrogeological conditions confirm that the rift margin is a likely groundwater recharge zone and the east/southeast-ward hydraulic groundwater gradient indicate that groundwater is flowing towards, and feeds, Lake Urema.