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162 Chapter Five

Application of the chloride budget method to the Quaternary sand BO X
aquifer of Senegal, West Africa 5.1

The water balance of the coastal Quaternary sand aquifer in the negligible surface runoff in the sandy terrain and a chloride source
West African sahel region of Senegal is sensitive to short- and from atmospheric sources only, the direct recharge, R , is estimated
d
long-term climatic change, with groundwater resources in many from:
areas dependent on recharge during former wet periods (Edmunds
=
& Gaye 1994). Hence, it is important to be able to determine the R P C p eq. 1
recharge amount in this region in order to quantify the available d C s
groundwater resource. For this purpose, the chloride budget
method was employed and included collection of well waters where P is the mean annual precipitation amount, C is the mean
p
and porewaters from the unsaturated zone recovered from hand- chloride concentration in rainfall and C is the mean chloride
s
augered material down to depths of 35 m (Fig. 1). From analysis concentration in the well water or interstitial porewater. The ratio
of the well water and extracted porewater, and assuming both C /C represents the evaporative concentration of chloride in
p s

Fig. 1 Chemical and isotopic profiles of water in the unsaturated zone of a coastal Quaternary sand aquifer from a site west of Louga
in Senegal, West Africa, interpreted by the chloride budget method to provide a 60-year recharge chronology. The symbols C , C ,
p s
E, ET, P and R represent mean chloride concentration in rainfall, mean chloride concentration in well water or interstitial porewater,
evaporation, evapotranspiration, precipitation and recharge, respectively. After Edmunds (1991).