Geophysical investigations comprising electrical resistivity and electromagnetic conductivities methods were deployed in a 350 m2 sector, strewn with 11 wells. Within the framework of an environmental study on a small scale in the south of Benin, the water conductivity of these wells was measured to determine in a direct way mineralization of the coastal water table in the littoral zone. This environmental study aimed to prospect by the geophysical methods the space extension of the water table mineralization obtained by direct measurements of water conductivity in the well and the depth of the fresh water/salted water interface in the coastal aquifer. Electromagnetic measurements of conductivities made it possible to chart a gradient of mineralization in the northwest direction. The logs of vertical electric soundings showed a deepening of the fresh water/salted water interface in the southern part and its rupture in the northern part. The electrical resistivities of the interface are sensitive to the degree of its mineralization. It has been observed that the geophysical methods in electrical and electromagnetic prospection are a great contribution to the environmental study of the water table mineralization in the littoral zone for a sustainable management of the water resource.
The zone in the south of the town of Cotonou (economic capital of Benin), lying between the Atlantic Ocean in the south and the most populated city of the country in the north, has vocation to develop and know an economic advancement on the tourist level. This development, thus, will imply an increase in the exploitation of the subsoil water resource a priori of the coastal groundwater aquifer. The knowledge of the extension of the saline water intrusion and depth of the fresh water/salted water interface contributes to the decision making for the safeguard of the environment for the users of the water resource. The saline water intrusion in the aquifers of several coastal zones had like consequences of the acute environmental problems [
Within the last decade, classic hydrogeological information has been increasingly complemented with subsurface geophysical information that allows obtaining more accurate images of aquifer systems [
The littoral environment of Benin is a zone of interface between the tableland at the north and the Atlantic Ocean at the south. The littoral zone in the south of Cotonou is characterized by a subequatorial climate. Pluviometry in this sector is on average 1200 mm per annum [
Study site map.
From the geomorphologic point of view, the littoral is as a whole made up of three sandy bars intersected with muddy levels. These sandy bars consist of accumulations of marine granular sediments, current or inherited of last quaternary transgressions [
Map of sand bars in littoral plain.
In the littoral zone, two aquifers with relatively less potentialities are distinguished: the aquifer of Quaternary and Terminal Continental and that of Paleocene [
(a) Hydrogeological section of littoral plain. (b) Hydrogeological section of upper aquifer.
The PU2 drilling data well in the study zone between the Atlantic Ocean and the coastal lagoon show that the upper sand aquifer thickness does not exceed 30 m (Figure
Two principal types of geophysical surveys were carried out on the Beninian littoral sector with an aim of charting the salt water intrusion in the surface aquifer. It is the electromagnetic method to measure the side variation of electric conductivity in the sandy aquifer and the method of the vertical electric soundings to estimate in certain points of high conductivity the depth of the fresh water/salted water interface. It was shown that DC resistivity and loop-loop electromagnetic data interpreted together can also help overcome model ambiguities [
Another method of direct measurement of the conductivity of water in the wells was used to come in support to the geophysical methods. In our study, the results of geophysical surveys are compared with direct measurement.
The direct measurements campaign has been done on mid-May 2011 when the great dry season ended and the great rain season begun. Direct measurements of the conductivity of the wells were taken in situ on 11 wells distributed in a random way on the surface of the studied sector (Figure
Chart of wells points distribution.
There is a huge scientific literature on near-surface electromagnetic geophysics [
Chart of electromagnetic profiles.
The device used for the vertical electric soundings (VESs) is of the Wenner type. This method makes it possible to obtain, according to the depth, the apparent resistivity of a certain volume of ground [
The southnorth section passing by these vertical electric soundings made it possible to build the fresh water/salted water interface level (Figure
The values of electric conductivities of water in the wells vary between 271 and 826
Chart of water conductivity.
The chart of conductivities of mineralization of water in the wells shows that the water table presents significant variations of mineralization. Electric conductivity is generally high in the south and oscillates between 600 conductivity < 100 100 < conductivity < 200 200 < conductivity < 333 333 < conductivity < 666 666 < conductivity < 1000 conductivity > 1000
In all the littoral zone of the studied sector, the mineralization of the water table thus varies from significant value to a medium value passing by an accentuated mineralization value. Water with high mineralization is in southern seashore in contact with the ocean. In the south of the studied sector, any well does not have listed because of the high mineralization of water. The lack of wells in the southeastern part of the studied zone is marked by the break off data interpolation on the chart of mineralization conductivities. The chart of mineralization conductivities presents in consequence a gradient of mineralization with south-east-north-west direction. This gradient of mineralization shows a deepening of the brackish water which marks the fresh water/salted water interface as one moves away from the shore.
The chart of electromagnetic conductivities distribution shows a strong variation as well in the eastwest direction as in the southnorth direction. This rate of variation is estimated at approximately
Chart of electromagnetic conductivity.
VES localization on chart of electromagnetic conductivity.
The vertical electrical soundings were located in the zones of different electromagnetic conductivities (Figure
The log of vertical electric sounding VES1 presents its conducting ground of resistivity 5,04 Ωm at a depth of 1,02 m (Figure
(a) VES1 log and model layers. (b) VES2 log and model layers. (c) VES3 log and model layers. (d) VES4 log and model layers. (e) VES5 log and model layers.
Geoelectrical section on the fresh water/salted water interface.
Range of rocks resistivity.
The log of vertical electric sounding VES2 presents its conducting ground of 5,04 Ωm resistivity at a depth of 4,75 m (Figure
The log of vertical electric sounding VES3 presents a ground of 5,83 Ωm resistivity at a depth of 11,6 m (Figure
The log of vertical electric sounding VES4 presents a ground of 5,95 Ωm resistivity at a depth of 9,79 m (Figure
The northwestern Southeastern vertical section, taking into account the logs of the four vertical electrical soundings, delimits the deepening slope of the fresh water/salted water interface according to the distance from the shore (Figure
Comparing the electromagnetic conductivity chart with that of mineralization, it is possible to note that the orientation of electromagnetic conductivity gradient is in general in conformity with that of mineralization. Nevertheless, a lack of conformity is noted about a small anomaly and especially in the southeastern part of the studied zone. For 10 m spacing between the EM34 loops, the depth of investigation is lower than 10 m. Thus, the zones, in which the depth of the fresh water/salted water interface borders exceeds the 10 m depth, are badly charted by this device. That can explain the anomaly in the center of the studied zone where the interface plunges to more than 10 m of depth. This device is also sensitive to the surface and very near surface disturbances which could be the cause of the anomaly in the south-east of the studied zone. All things considered, the electromagnetic prospection appears as a tool able to chart the mineralization of the ground water to low depth in the zones where the electromagnetic disturbances are negligible.
Comparing the deepening slope of the fresh water/salted water interface with the gradient of mineralization, it should be noted that in the south where the interface is close to surface, mineralization is high. Moreover, the weakest variations of the gradient of mineralization in the northern part of the studied zone shows the tendency towards the stability of mineralization when the slope of the interface ceases dipping. The deepening of the interface thus implies a reduction of the mineralization. Moreover, in the southwest where the resistivity of the interface is equal to 5,04 Ωm for vertical electric sounding VES5, mineralization has a conductivity of 680
Comparing the deepening slope of the fresh water/salted water interface with the gradient of electromagnetic conductivity, it can be noted that the slope and the gradient have a strong variation in the south of studied zone and a soft variation in north. Moreover, in the northern part of the zone where the change of incline is carried out, conductivities remain rather stable. Strong electromagnetic conductivities in the south, where the interface is close to surface, decrease gradually with the deepening of the interface in north. Nevertheless, in the southwestern part where the fresh water/salted water interface is still close to surface, electromagnetic conductivities remain high. This electromagnetic anomaly is partly due to the nature of the surface grounds located at the top of the interface. Indeed, at southwest, the log of vertical electrical sounding VES5 presents at the top of the fresh water/salted water interface, the most resistant ground (15784 Ωm) of the studied zone (Figure
The coupling of the geophysical methods of electromagnetic conductivities and vertical electrical soundings made it possible to chart the mineralization of ground water in the coastal aquifer on the one hand and to detect the depth and the dip of the fresh water/salted water interface in the studied zone on the other hand. The vertical electrical soundings showed that the dip of the fresh water/salted water interface is in the order of 5% in the southern part of the studied zone, and in the northern part, a change of incline takes place. The variations of the resistivities of the interface are sensitive to the variations of its mineralization. Nevertheless the electromagnetic method is able to define the gradient of mineralization but remains sensitive to the environmental disturbances and to the resistant grounds on the surface.
The geophysical methods in electrical prospection are thus adequate for the cartography of the mineralization of the ground water in coastal zone. On the environmental level, they can be used in the zones where drilling of the wells is difficult, even impossible or useless, in order to delimit the zones where the water resource is sustainability exploitable.
The authors thank, on one hand, the Laboratory of Applied Hydrology (LHA) of the University of Abomey-Calavi in Benin for the facilitation during chemical analysis of the water samples and, on the other hand, the Institute of Research and Development (IRD) of France for the provision of the geophysical material of prospection.