Hydrodynamic characterization of a Sahelian coastal aquifer using the ocean tide effect (Dridrate Aquifer, Morocco)

Abstract

In wells tapping coastal aquifers, piezometric fluctuations can be observed in response to the ocean tide. Simultaneous recordings of the ocean tide and of the piezometric variations may provide a basis for characterizing the hydrodynamics of the aquifer. This approach was attempted to characterize the Dridrate aquifer, located on the Atlantic coast of Morocco. This aquifer accounts for most of the regional drinking water resources. However, its hydrodynamic characteristics are very poorly known. The study compares observed and simulated piezometric fluctuations, under various assumptions (confined, semi-confined aquifer). The model, which best explains the hydrodynamic behaviour of this aquifer is a semi-confined and strongly heterogeneous aquifer model (calculated hydraulic diffusivity values vary over several orders of magnitude). This result is new and rather surprising, since to date this aquifer was considered confined in view of its geological setting. Consequently, new questions are raised regarding the protection and management of the groundwater resources of this aquifer.

Hydraulic Parameters of Coastal Aquifer Systems by Direct Methods and an Extended Tide–Aquifer Interaction Technique

“…Two tide-affected sites I-2 (well depth=25 m and screen length=1 to 25 m) and H-5 (well depth=15 m and screen length=1.12 to 15 m) were selected from the Konan aquifer located in Kochi Prefecture, Japan (Jha et al 1999) and three sites 1272/34, 1525/34, and 235/26 were selected from the Dridrate aquifer located in Qualidia Sahel, Morocco (Fakir and Razack 2003). Sites I-2 and H-5 are located at 350 and 500 m from the Pacific Ocean coast, respectively (Fig.…”

“…These red clays constitute the basement of the Plioquaternary sediments composed of calcareous sandstones. The details about this aquifer system can be found in Fakir and Razack (2003).…”

“…Thus, adequate and reliable aquifer parameters are key to the sustainable management of vital groundwater resources. There are several methods for determining the hydraulic parameters of aquifer systems such as pumping-test or slug-test data analysis (e.g., Freeze and Cherry 1979;Todd 1980;Batu 1998;Butler 1998;Fetter 2000), numerical modeling (e.g., Yeh 1986;Pandit et al 1991;Sun 1994), floodwaveresponse technique (e.g., Pinder et al 1969;Reynolds 1987;Jha et al 2004), and tidal response technique (e.g., Jacob 1950;Ferris 1951;Carr and van Der Kamp 1969;Nielsen 1990;Erskine 1991;Pandit et al 1991;Millham and Howes 1995;Townley 1995;Trefry and Johnston 1998;Shih 1999;Fakir and Razack 2003;Li and Jiao 2003a, b;Teo et al 2003;Trefry and Bekele 2004;Chuang and Yeh 2007;Li et al 2006;Guo et al 2007;Li et al 2007a, b;Ren et al 2007;Xia et al 2007), among others. Out of these methods, the pumping-test data analysis is very popular and is deemed as the standard method to date.…”

“…Nevertheless, this technique has received less attention and very limited applied studies are reported in the literature. In past studies on coastal aquifer parameter estimation (i.e., inverse modeling) using tide-aquifer interaction technique (Erskine 1991;Pandit et al 1991;Millham and Howes 1995;Jiao and Tang 1999;Trefry and Johnston 1998;Fakir and Razack 2003), except for Shih (1999), Wang and Tsay (2001), Jha et al (2003), Trefry and Bekele (2004) and Li et al ( , 2006Li et al ( , 2007a, it was assumed that the tidal data as such represent sinusoidal waves. In reality, however, the observed tide level consists of several simple harmonic or sinusoidal components (called 'tidal components/constituents') that include the effects of the sun, moon and earth, etc.…”

Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory andin situmonitoring

“…Then, aquitard formations are equipped for long-term pressure monitoring, and the data are used to compute hydrodynamical parameters (specific storage coefficient, hydraulic conductivity) from periodic models deduced from temporal analysis. The study of sinusoidal pressure propagation in an aquifer is a well-known approach to estimate the diffusivity of the media (ratio of the transmissivity to the storage coefficient) (Fakir and Razack 2003), but the methodology here is adapted to a downwards propagation of a pressure signal through the semi-permeable formation.…”