The problems of urban effluent discharged in wadis represents the thorny question of their impact on the groundwaters in relationship with these water courses. This aspect becomes crucial when several drinking water-catchments are located nearby the wadi banks.
Such a case was observed in the low Seybouse plain (North-Eastern Algeria) where a natural obstruction of the wadi produced a rising of the water level, and concomitant direct infiltrations from the wadi to the aquifer. Two wells (MP1 and MP2) underwent a phosphate contamination from the wadi.
Several water field samplings of the wadi (shallow waters), with a monthly period (Station MO1) and of the groundwater on two test sites (MP1 and MP2), with MP2 being discontinuously pumped (6 h d À1 ), were achieved during the water year, from February to December 1999.
Comparing the chemical composition of the wadi water with that of several tapped groundwater samples, we could observe the existence of two pollutants in the wadi: phosphate (23 mg l À1 ) and ammonium (54 mg l À1 ). On the other hand, in the water catchments, only phosphate could be observed, and it could move only under the same chemical species between the wadi and both wells, where the concentrations reached, respectively, 2.3 and 3.1 mg l À1 .
The lowering of the phosphate concentration between the wadi and the wells is due to the dilution effect of the contents by the aquifer water, the precipitation of phosphate under calcium phosphate species and finally by clay adsorption. But then, the absence of ammonium in the wells is due to the transformation of the nitrogen species into nitrate during the water pathway between both media, under the effect of oxidizing conditions.