Combating Eutrophication to Restore Ponds: A Case Study of an Urban Pond in Casablanca, Morocco

Urban ponds are facing multiple threats which could limit their functions. El Oulfa pond, located in a residential area of the largest city in Morocco, presents a degradation that is increasing year after year manifested by the proliferation of algae and multiple nuisances that affect the quality of life of local residents. Its restoration then becomes an emergency. The objective of this work is to contribute to the characterization and monitoring of the surface water quality of El Oulfa pond to help make appropriate decisions for its restoration. To do this, a diagnosis of the physicochemical and bacteriological quality of surface water is carried out in September 2018 on 12 points well-distributed on the pond. The results showed that water quality of El Oulfa pond does not comply with Moroccan standards relating to surface water quality. The total phosphorus concentrations, which vary from 2.88 mg/l to 4.13 mg/l, attest to the hypertrophic state of the pond. To curb this eutrophication, a biological treatment coupled with mechanical aeration was implemented by the site managers between March and August 2020. The monitoring of the water quality of the pond, carried out between September 2020 and February 2021 showed in the first month, after the beginning of the biological treatment, a fall of the total phosphorus of 47% in the centre of the pond, compared to the levels of 2018. However, in subsequent months, analyses performed on the majority of samples showed increasing phosphorus levels. This situation could be due to direct effluents discharge into the pond or to the release of phosphorus from the sediment which would act as an internal source of this nutrient.