Geoelectrical exploration for groundwater in Al Maha Forest, Ain Jouhra, Morocco

Abstract

AbstractAn area of about 30 km2 located in Ain Jouhra, south of Rabat, Morocco, was the subject of a geoelectric resistivity investigation. The main goal of the investigation was the assessment of the groundwater potential of the uppermost aquifer. The aquifer conditions such as depth, thickness and boundaries were also investigated. The obtained apparent resistivity curves were first analysed qualitatively and classified using simple curve shapes. Thereafter, the data were converted to resistivity and thickness pairs semi‐quantitatively by means of master curves and then quantitatively by computer modelling using ATO and Winsev software (Zohdy, 1989; Zohdy and Bisdrof, 1989). Lithological control from the available single well with a stratigraphic log aided in the correlation of the resistivity values to different rock units. Three different AB‐spacing iso‐resistivity maps, an isopach map of the main groundwater‐bearing horizon, the depth to the aquifer substratum map and five geoelectric cross‐sections were constructed.The interpretation of these soundings indicates the presence of an unconfined to semi‐confined sandy aquifer with relatively important extent and varying thickness. The maximal thickness of the aquifer is recorded in the central part of the investigated area and is thinning southwards to pinch out farther to the south. Geophysical as well as field data indicate a hydraulic connection between the upper and deeper aquifers. Indeed, the two aquifers are separated from each other by a marly substratum that is indicated throughout the area by the lowest values of the interpreted true resistivity. The value of this resistivity varies laterally, most likely due to the lateral variation in the shale‐to‐sand ratio. The altitude of the substratum decreases towards the north, and increases southwards.Regarding the availability of the groundwater in the study area, zones with high potential are theoretically expected to occur in the central part where the transversal resistance is greatest. However, sufficient water supply and high flow rates from wells intended to produce restrictively from the most upper aquifer are not likely to exist. This conclusion, which seems to be very pessimistic, is evidenced from two real field and experimental observations. The first is the rapid fall of the level of Gharnoug lake, despite the ongoing feeding by three wells. Hence, the amount of water level drop cannot be accounted for by the evaporation alone. That means that the deeper aquifer is continuously draining the upper aquifer at a high flow rate. Very low rates are recorded in all the wells that penetrated only the upper aquifer, the exception being the well that reached deeper into the lower aquifer. The flow rate in this lower aquifer measured 18 litre s−1. Copyright © 2007 John Wiley & Sons, Ltd.