Environmental impact assessment of Taza city’s wastewater on the watershed of Oued Larbaa, Morocco

To fully understand the environmental issues that are damaging the quality of water resources in the Oued Larbaa watershed, a GIS was used to represent the spatio-temporal evolution of pollution indicator parameters. The Oued Larbaa is severely affected from the negative impacts of wastewater from the city of Taza, with an estimated daily volume of wastewater of 16,534 m3/day in 2020, which is expected to reach 20,056 m3/day by 2030. This wastewater is discharged via an open drain into the tributaries of the Oued Larbaa without any prior treatment and flows directly into the Oued Inaouène during periods of drought. In order to determine and better understand the impact of these discharges on the Oued Larbaa and its tributaries crossing the city, sampling was carried out at ten selected stations during the high water period (January 2025) and the low water period (July 2025). The physico-chemical characterisation of the wastewater from the tributaries/effluents revealed that these discharges in terms of TSS (O4 = 963 mg/L), COD (O4 = 1491 mg O2/L) and BOD5 (O4 = 718 mg O2/L) are higher than the usual range for Moroccan urban wastewater, whereas the TSS/BOD5 and COD/BOD5 ratios are within the usual range for Moroccan urban wastewater.