Nitrate contamination of groundwater in irrigated perimeters under arid climate (the case of Souss-Massa aquifer, Morocco).

Abstract

The objective of this study was to clarify, the current status of alluvial aquifer in the Souss-Massa basin, where, the nitric pollution of groundwater is being increasing along the last decades. A multi-approach methodology using hydrogeology, nitrate concentrations, irrigation mode and Oxygen-18 and Deuterium isotopes data, was carried out to identify the sources of this pollution. According to the spatial distribution of nitrate contents, nitric pollution occurs mainly in the Chtouka-Massa plain. More than 36% of the sampled wells exceed the value of 50 mg L-1 which, constitutes the threshold value of nitrate concentrations for drinking water Moroccan standards. The groundwater in Souss plain is less polluted comparing to Chtouka-Massa. Only 7% of wells exceed the permitted level. The widespread distribution of high nitrate contents agrees with the distribution of irrigated areas, which can explain the major origin from agricultural fertilizers. High nitrate levels are associated with high δ18O values, clearly indicating that significant quantities of evaporated (isotopically enriched) irrigation water infiltrate along with fertilizer nitrate to the groundwater system. Different δ18O-NO3- trends suggest isotopically distinct, non-point source origins, which vary spatially and temporally, due to different degrees of evaporation/recharge and amounts of fertilizer applied.