As climate change continues to present daunting obstacles in the eastern region of Morocco, the reuse of treated wastewater has emerged as a vital solution for combating water scarcity, particularly for irrigation purposes. This investigation delves into the implications of reusing treated wastewater along the seasonal Isly River, with a specific focus on its downstream area, commonly referred to as "Oued Bou Naim". A comprehensive campaign was conducted in 2023 to analyse the physicochemical quality of 12 water samples from Oued Bou Naim and 13 soil samples from nearby agricultural plots. The findings from the water analysis show a marginal increase in temperature downstream from the wastewater treatment plant (WWTP) in Oujda, ranging from 19.0°C to 24.8°C. The pH varied between 7.08 and 8.33 and generally remained within acceptable limits. However, the electrical conductivity levels, ranging from 2154 to 2898 μS·cm⁻¹, exceeded WHO standards, indicating potential risks to the soil‒plant‒health ecosystem. Additionally, the total dissolved solids increased significantly downstream from the WWTP, reaching 1437 mg·l⁻¹. For the nitrites, a level exceeding the limits, especially upstream, had an average of 2.03 and a maximum of 5.48 mg·l⁻¹. Upon studying the soil samples, a substantial increase in orthophosphates, nitrites, and nitrates was observed downstream. Orthophosphate levels increased significantly downstream, with an average of 361.25 mg·kg⁻¹, indicating the contribution of treated wastewater. The nitrite concentration (average of 11.61 mg·kg⁻¹) and nitrate level (average of 60 mg·kg⁻¹) raise concerns regarding contamination through irrigation with treated wastewater. This research highlights the critical need for responsible irrigation practices to safeguard the delicate balance of the "Water‒Soil‒Plant" ecosystem and the well-being of humans. Although wastewater treatment plants play an important role, the presence of high levels of harmful pollutants downstream emphasizes the urgent need to carefully manage this treatment combined with the reasonable use of agrochemical inputs to prevent their leaching into nearby ecosystems. By doing so, we can effectively reduce the risks to both the ecosystem and human health in the study region.