Groundwater nitrate pollution dynamics in Souss-Massa (Morocco): spatial trends and contribution of nitrate stable isotopes

This study aims to determine the distribution of nitrate contamination across Souss-Massa aquifers in Morocco to identify their key sources using isotopic tools (δ15N-NO₃−, δ18O-NO₃−). The analysis of 146 water samples shows a clear upstream-to-downstream variability of nitrate concentrations. At the valley of Tifnoute, where rainfall and snow are relatively abundant, nitrate concentrations are not significant. In contrast, the downstream section of the plain appears to be the most affected by nitrate contamination, with values exceeding 130 mg/L. The correlations between NO3− and other major ions suggest an anthropogenic impact. Nitrate isotopes vary between 8.1‰ and 12.2‰ in Souss and between 3.4‰ and 27.4‰ in Massa. The values confirm that the leaching of wastewater and manure runoff are the primary sources of nitrate contamination, in addition to the excessive use of fertilizers.