Water resources detection at Santiago Island, Cape Verde using electromagnetic methods

Abstract

Abstract Freshwater availability is a growing problem all over the world. This is especially true in drier regions with marked population growth. That is the case at the Cape Verde archipelago off the coast of West Africa. The problem of water availability has been part of the archipelago’s history since the 15th century, when these uninhabited islands were discovered by Portuguese navigators. The inconstancy and shortage of drinking water has always been related to low rainfall and prolonged drought. Most of the islands do not have high elevations, which does not favour orographic precipitation. Population and agricultural activity have always been limited by the use of existing water; either stored from the rainy season or from wells excavated in the valley’s riverbanks. To overcame this condition many boreholes have been drilled since the 1950s and 1960s at the known aquifiers. This was done under the scope of geologic and hydrogeological data and methods. The present study is an electromagnetic geophysical survey at Santiago Island, the biggest and most populated of the archipelago. Beginning in 2005, there was at least one campaign a year until 2009. The surveyed regions are the most important valleys and agricultural areas, where the water needs are bigger. The problem of groundwater is not just its amount and availability but also its quality. The problem of sea or salty water intrusion into the near sea groundwater aquifers was detected at many boreholes and by the wells’ hydrochemistry data. The aim of this EM survey was to image the groundwater condition; location and quality throughout the studied areas. To fulfill this, transient electroMagnetic (TEM) soundings have been done. In addition some very low frequency (VLF) profiles and magnetoteluric (MT) soundings have been carried on to confirm and cross-check TEM data.