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To see if you could have access to groundwater as a water supply in your area you need to conduct ‘groundwater prospecting’.
In brief, groundwater can be more or less easily exploited and can be found in: major aquifers, fracture zones, weathered zones, alluvial deposits and scree and/or river valleys.
During your groundwater prospecting, you are looking for signs and clues of structural characteristics for groundwater aquifers and you are trying to answer some basic questions such as: What are the main geological formations in this surveyed area? What kind of aquifer systems are presents? Where is the surface water infiltrating? Where is it not? Are there any traces of fractures? …
A preliminary study would allow you to accept or reject the groundwater as supply for the targeted population and if accepted, you’ll identify the most suitable technical solution. Then, the construction study would confirm you or not the utility of the selected aquifer and allow you to implement precisely the siting of the borehole or well. Finally, the construction itself, where you will assess in deep the resource and the borehole or well drilled or dug, equipped and tested.
Desk study: you need to collect and analyse the existing information (from national institutions, universities, geological centres, NGOs….) on the type of geological formations and aquifer systems, the climatic data (rainfall and temperature), the groundwater resources and technical solutions already tested and implemented locally (boreholes or wells; construction techniques, depth, diameters and yields of existing installations) and the costs and available resources (equipment, skilled personal, access, logistics). This first step is crucial and the assistance of local professionals will help you to understand the local area and its groundwater. Don’t stand alone!
Cartography: combined to the desk study and with the help of local professionals, topographic and geologic maps of the surveyed area would allow you to confirm/identify the structural features such as the geological formations, the vegetation cover, the relief, the hydrographic network (i.e. draining and infiltration zones), and so, the groundwater feeding and releasing zones.
Aerial/satellite images: assisted by professionals, you can appreciate on these images the vegetation covers (i.e. dry and humid zones), the structural features (i.e. fracture zones) and geomorphological (i.e. draining and infiltrating zones) features.
Field visit Following the preliminary study, the field visit would confirm and refine the collected and analysed data. It’s time to leave your office and discuss with people!
Preliminary visit: discussions with population and local authorities regarding the structural features (i.e. vegetation covers, relief, draining and flooded areas, geology), the water resources and their behaviour (i.e. wells, boreholes, springs, rivers and ponds; as well as abandoned and dry wells and boreholes; dry and wet seasons; water quality; history of these water resources). Then, you need to visit some sites, some specificities identified during the preliminary study and the discussions (i.e. depth and diameter of wells and boreholes, water quality, fractures, flooded areas); a GPS would be useful to transfer these information on maps.
Technical meetings: you need to get local and updated information from professionals of local authorities and ministries, private companies, others organizations… any relevant actors and informant of the sector, in order to confirm and refine collected and analysed information (i.e. geology, hydrology, yield of wells and boreholes, water quality).
Hydro-geophysical methodologies These methodologies don’t replace the steps described above. Even in some cases, the information collected through mapping and discussions with local professionals is enough to locate properly the sitting for the borehole or the well.
Electrical resistivity method: a direct current flow is fed into the ground and the resistivity of the formations is calculated from the measured potential differences. The variations of the resistivity allow you to describe the nature and structure of the aquifers. Wide range of applications.
Electromagnetic method: uses electromagnetic waves through time-varying currents to measure the conductivity of the formations. Easier to use than the electrical resistivity.
Magnetic resonance sounding: it gives direct information on the presence of groundwater, measuring spin and magnetic moment of the hydrogen nucleus. Difficult to use.
These different steps for the borehole prospecting are more detailed in the Action Against Hunger book: ACTION CONTRE LA FAIM, 2005. Water, sanitation and hygiene for populations at risk. 2nd edition. Paris : Hermann. Also available at https://www.ircwash.org/sites/default/files/acf-2005-water.pdf