Reconstructing the formation of a costal aquifer in Nampula province, Mozambique, from ERT and IP methods for water prospection

Abstract

In continental margin basins, the hydrogeological setting is complex due to transgression/regression events that removed old sediments in the basin and formed new geologic units. Due to the geological complexity, the use of vertical electrical sounding has proven to be insufficient for groundwater explorations. The lack of understanding the geological underground has resulted in many boreholes with low yield or poor water quality. By performing electrical resistivity tomography (ERT) and induced polarization (IP) measurements in 11 villages in Mongicual district, three different layers covering the basement were identified: a weathered autochthon layer, a weathered allochthon layer (paleo-coastal dune) and eolian white sand layer. The drilling at successful boreholes penetrates formations where the resistivity value is between 220 and 770 Ωm, whereas at unsuccessful boreholes the lower parts of the drilled range have resistivity values higher than 770 Ωm. Also, the thickness ratio of the weathered and semi-weathered layer in the unsuccessful boreholes is less than 1/3, whereas in all successful boreholes the ration is higher than 1/2. The difference between autochthon and allochthon layers was detected by heavy minerals content in the red eolian sand layer (Tupuito formation) that increased the chargeability value. The groundwater with a conductivity higher than 2000 µS/cm is linked to the white eolian sand. The surface extension of white eolian sand layer is small to be mapped; therefore, by mapping the eolian white sand formation and the use of ERT and enhanced with IP method would lower the failure rate.