Combined electrical resistivity tomography and magnetic resonance sounding investigation of the surface-water/groundwater interaction in the Urema Graben, Mozambique

This study focusses on the hydrogeology of Urema Graben, especially possible interactions between surface water and groundwater around Lake Urema, in Gorongosa National Park (GNP). Lake Urema is the only permanent water source for wildlife inside GNP, and there are concerns that it will disappear due to interferences in surface-water/groundwater interactions as a result of changes in the hydraulic environment. As the lake is the only permanent water source, thiswould be a disaster for the ecosystem of the park. The subsurface geology in Urema Graben was investigated by 20 km of electrical resistivity tomography (ERT) and three magnetic resonance sounding (MRS) surveys. The average depth penetration was 60 and 100 m, respectively. The location of the ERT lines was decided based on general rift morphology and therefore orientated perpendicular to Urema Graben, from the transitional areas of the margins of the Barue platform in the west to the Cheringoma plateau escarpments in the east. ERT andMRS both indicate a second aquifer, where Urema Lake is a window of the first upper semi-confined aquifer, while the lower aquifer is confined by a clay layer 30–40 m thick. The location and depth of this aquifer suggest that it is probably linked to the Pungwe River which could be a main source of recharge during the dry season. If a dam or any other infrastructure is constructed in Pungwe River upstream ofGNP, the groundwater level will decrease which could lead to drying out of Urema Lake

On the applicability of geophysics for groundwater investigations in areas with lack of hydrogeological information. Examples from Mozambique

The work presented in this thesis has aimed to improve the knowledge about the groundwater occurrence in different geological conditions in Mozambique. The main focus in this study has been on the understanding of hydrogeological conditions in area with little background information by use of geophysical investigation. The lack of background information is a problem in many countries because the existing information is not well organized, old reports are missing in the archives and data is not published in scientific journals. In such conditions, the approach of combining different geophysical methods measuring different physical properties of geological material can improve the interpretation.Four geophysical methods have been assessed regarding their applicability in solving real hydrogeological problems in Sofala and Nampula provinces in Mozambique. The implemented methods are electrical resistivity tomography (ERT), magnetic resonance sounding (MRS), induced polarization (IP) and geophysical borehole logging (BL). Also other methods such slug tests, dilution tests and grain size analyses were included in this study.The ERT method was used throughout the investigation however the interpretation of the results were often not conclusive due to lack of reference data. The Interpretation of ERT results compared to groundwater extraction rates from new boreholes (in Nampula provincne) showed that areas with a well-developed weathered layer have better potential for groundwater extraction. The MRS method work well in unconsolidated sediments in an attempt to understand the interaction between surface water and groundwater in the Urema floodplain, Sofala province. However, when MRS was used to identify the main reason for low possible groundwater extraction rates in some of the wells, MRS did not work well due to the low signal caused by low water content in combination with different types of noise, both natural and anthropogenic. IP results, presented as normalized chargeability, give a good contrast between clay and sand layers and between clay layers and unweathered rock in Mongicual district, Nampula province. The high value of normalized chargeability found in sand layers in this study are attributed to a content of heavy metal minerals. The electrical conductivity of groundwater increases when these minerals are dissolved. Borehole loggings together with ERT results have shown that the lithological descriptions in the drilling reports are sometimes inconsistent. The inconsistence may be caused by the mix of site information or copy and paste of the description from different reports. As a consequence of this inconsistency, it is expected that even some VES presented in the borehole drilling report are presumably wrong.Combining hydrogeophysical methods and other non-geophysical methods has proven to be useful when no other information is available. This effort has increased the knowledge of hydrogeological situation in different parts of Mozambique. Therefore, it is recommended to continue this but with more control (to reduce the uncertainties) on how they can be interpreted together and thereby overcome the lack of background data