Earth Tide Effect in Karstic and Non-karstic Aquifers in the Guinea Gulf

Abstract

During long-term pumping tests in Gabon and Benin, we were surprised to observe large-amplitude tidal signals in boreholes located more than 20 km from the sea. This article describes these tidal signals (maximum amplitude, variation in time and space, phase shift, etc.) and attempts to interpret them. The signal amplitude varies considerably from one aquifer to another (from 2 mm to 110 cm) but is uniform within an aquifer and constitutes a signature. We are therefore seeking to use this signature to characterize the transmissive or capacitive properties of each aquifer. The use of this tool is limited by the difficulty of isolating the piezometric tidal signal among other phenomena that can mask it (pumping, rain, seasonal drying, etc.). Once the tidal signal is properly isolated, it can be used as an indicator of the risk of seawater intrusion. This concern is particularly acute if the aquifer consists of karstified rocks, as the intrusion is likely to extend several kilometres inland. It is therefore essential to be able to distinguish three situations with different levels of risk. Two of these situations have been relatively well documented: earth tides that do not raise a risk of seawater intrusion and ocean tides, which induce a very high risk of karst aquifers in direct contact with the sea. A third case should be added: that of ocean tides that induce periodic pressure variations in captive aquifers. The risk of seawater intrusion is then moderate, even when this tidal signal is very spectacular, as in some confined karst aquifers in Benin.