Water supply study at Manama, southern Zimbabwe

This paper describes a groundwater prospecting project, including two parts, in the rural area in southern Zimbabwe. The first part deals with a gelogical survey and locating of waterbearing fracture zones with the electromagnetic methods Slingram and VLF, and a resistivity instrument. The field study consisted of geological mapping and geophysical profiling in an area of 5 km by 5 km. The results from the profiles are commented and interpretated together with the geological observations. The second part of the project is about groundwater extraction by wellpoints from a river sand dam. A survey was made to find a better location of the wellpoint system, and a study in order to find a better technical solution was also included. The methods used were mechanical sounding in cross-sections, leveling of the groundwater table and collecting of soil samples in an area of 250 m by 1100 m. Recommendations are made for four borehole sitings for water supply, and concerning the sand dam to change the extraction system to one with a lower intake level.Summary: Manama Mission is a Lutheran mission station in the Gwanda District, Zimbabwe, that suffers from a shortage in water supply. Its only source of water at present is a sand dam in the nearby Tuli river. During June, July and August 1990 a ground water study was carried out at the mission. This was done by Patrich Holmström, Per Möller and Mats Svensson, students at University of Lund, Sweden. The aim was to investigate if improvements can be made concerning the use of the sand dam, and also to investigate the possibilities of drilling boreholes for groundwater supply in the area. A secondary purpose was to compare the different geophysical instruments that were used. The methods that were used in the study were : -Prestudies of areal photos. -Geological mapping. -Geophysical profiling with Slingram, VLF and resistivity instrument. -Leveling the basement rock, groundwater level and the surface of the sand dam. The areal photos gave a clear main fracture direction striking around 290°. During the geological mapping this direction was confirmed. The geological mapping also gave us the geological setting of the area. It mainly consists of gneiss with frequent intrusive dykes of dolerite following the main fracture direction. The geophysical investigation indicated no waterbearing fractures in between the dolerite dykes and the gneiss. It also clearly indicated that areas of interest for borehole siting always were close to streams or the Tuli river. We found two areas that we believe are interesting for groundwater prospecting. In these areas four spots has been marked for borehole sitings. The investigations performed on the sand dam gave results that indicated that the present location of the extraction system, a wellpoint system, could not be better. However, the results also indicated big losses of water in the dam due to leakage in the weir and in the river bed rock. Another problem with the sand dam is that the wellpoint system through its design doesn't allow maximum outtake from the sand dam. The comparison between the instruments showed that the VLF-instrument, a ABEM WADI, is the most efficient instrument of the three, but that receiving conditions makes it unsuitable in Zimbabwe. The ABEM Slingram 3600 and the resistivity instrument, an ABEM SAS Terrameter 300, showed to be slower, but more reliable. As recommendations we suggest Manama Mission to drill boreholes according to the instructions in chapter 8 as the first step to improve the water situation at the mission. If this does not give sufficient results, an exchange of extraction system according to suggestions in the chapter 7 and 8 should be carried out. Concerning the comparison of the instruments we recommend the use of Slingram and resistivity instrument for this kind of studies in Zimbabwe in the future