NATURALLY HIGH NITRATE CONCENTRATIONS IN GROUNDWATERS FROM THE SAHEL

Nitrate concentrations in excess of 50 to 100 mg L-1 NO3-N are found w idely in interstitial waters from unsaturated Quaternary sands in nort hern Senegal. These high concentrations and correspondingly enhanced N O3/Cl ratios are being produced in the soil zone beneath both uncultiv ated and rain-fed cultivated areas. High nitrate (avg. 11 mg L(-)1 NO3 -N) is also found extensively in shallow groundwaters. The NO3-N signa ture is preserved as a result of the aerobic conditions. The unsaturat ed zone contains a history of N inputs over a period of approximately 50 to 400 yr. Variations in the NO3/Cl ratio are mainly related to the growth of natural or introduced leguminous vegetation. There is also evidence that high ratios relate to wetter and low values to more arid climatic periods, These results, taken together with evidence from ot her parts of the world, demonstrate that high nitrate concentrations a re likely to be a widespread and established phenomenon of groundwater s in semiarid regions.