Physico-chemical characterisation and spatio-temporal variation of water quality in the Sô and Djonou rivers tributary to Lake Nokoué in West Africa

Abstract

The aim of this study is to quantify nutrient inputs from the collection of water quality data at twelve (12) sites over a twelve-month period (January 2023 to December 2023). A total of twenty-one water pollution indicator parameters were monitored. An inventory of anthropogenic pressures on the two tributaries (Sô and Djonou) was used to define the points at which water samples would be collected in the field. Physico-chemical parameters were analysed using molecular absorption spectrometry (HACH DR 5000). TMEs were detected and quantified in the water using atomic absorption spectrometry. The results of the laboratory analyses indicate non-compliance with Benin's wastewater quality standards and France's Environmental Quality Directives (NQE), resulting in physico-chemical pollution of the water in the two rivers. Nitrogen pollution was observed, with total nitrogen levels ranging from 0.18 to 8.25 mg/L (Djonou) and from 0.03 to 10.89 mg/L (Sô). Then there is phosphorus pollution, with total phosphorus levels ranging from 0 to 2.14 mg/L (Djonou) and from 0 to 2.45 mg/L (Sô). These two types of pollution observed resulted in algal pollution with chlorophyll a varying from 0 to 4.01 mg/L (Djonou) and from 0 to 3.36 mg/L (Sô).  Cadmium (Cd) varied from 0.01 to 0.24 mg/L (Djonou) and from 0 to 0.10 mg/L (Sô) and lead (Pb) varied from 0.01 to 0.35 mg/L (Djonou) and from 0 to 0.04 mg/L (Sô). The correlation matrix and the PCA carried out on the mean annual values of the parameters reveal that in the dry season the water in the Sô and Djonou rivers is rich in Biochemical Oxygen Demand after Five Days (BOD₅), Chemical Oxygen Demand (COD) , Cadmium (Cd), Conductivity, Nitrites (NO₂^-) and Salinity, in contrast to the water in the rainy season, which is rich in Total phosphorus, Phosphates( PO₄^2-), Nitrogen Total (NT), Ammonium (NH₄⁺) and Nitrates (NO₃^-).