Geophysical and hydrogeological survey in a part of the Nhandugue River valley, Gorongosa National Park, Mozambique : area 1 and 2

Abstract: Gorongosa National Park in central Mozambique is situated at the southernmost extension of the East African Rift System (EARS), the Urema Graben. The Nhandugue River flows over the western margin of the Urema Graben, and marks the northwest border of Gorongosa National Park, Mozambique. The park is protecting a vast ecosystem of floodplains, grasslands and woody coverage. The study is focusedd on the western rift margin, using resistivity measurements, shallow auger drilling for soil sampling, and discharge measurements to give more information about the hydrological conditions. The resistivity results suggest that the area is heavily faulted and most likely constitutes a major groundwater recharge zone. Moving from the rift margin and into the rift valley, the resistivity models indicate that solid gneiss is replaced by fractured and weathered gneiss, sandstone and alluvial sediments. The top 10-15 m of this sequence measure high resistivity, interpreted as alluvial sand. It extends back to the rift margin thus also covering the gneiss. The sandstone outcrops a few kilometres from the rift margin and dips towards the southeast. Further into the rift valley, the alluvial sand is underlain by low resistivity lenses of silt/clay, followed by a medium resistivity layer of sand. The topmost layer of alluvial sand constitutes an unconfined aquifer under which the solid gneiss forms a hydraulic boundary. The adjacent fractured gneiss, however, has been interpreted as an unconfined fractured aquifer and a zone of infiltration. The sandstone is also considered to be unconfined, while the lenses of silt/clay constitutes an aquiclude, underlain by the sand, and representing a semi-confined aquifer. The discharge measurements show a large downstream decrease in runoff, suggesting that the surface water is infiltrating and recharging the aquifers, and is then transported as ground water in a southeast direction.