Physical and Chemical Characterization of the Water in the Lake of the Kossou Hydroelectric Dam (Ivory Coast)

Abstract

The quality of lake Kossou is compromised by agricultural and mining activities carried out in its watershed. The objective of this study is to characterize the physicochemical parameters of lake Kossou's waters in order to assess its biological potential. Sampling was carried out from May 2017 to February 2018 over three seasons (the heavy rainy season, the flood season, and the heavy dry season). Physical parameters such as temperature, pH, and dissolved oxygen (O2) were measured in situ using the HANNA HI 9828 multi-parameter meter. Chemical parameters, namely ammonium (NH4+), orthophosphate (PO43-) and total phosphorus, were analyzed in the laboratory with a KONTRON UV visible UVIKON 810 spectrophotometer. These analyzes were performed using spectrophotometric methods, in accordance with French AFNOR standards. Statistical data processing was performed using STATISTICA 7.1 software. Temperature, pH, dissolved oxygen, ammonium ions, orthophosphate ions, and total phosphorus, varied from 28.02 to 29.9 °C, 5.75 to 7.21, 4.23 to 7.23 mg/L, 0.04 to 0.56 mg/L, 0.06 to 0.64 mg/L and 0.08 to 8 mg/L. Their respective medians were 29.16 °C, 6.28, 6.24 mg/L, 0.17 mg/L, 0.13 mg/L, and 2.75 mg/L. Seasonal variations in temperature, pH, orthophosphate ions, ammonium ions, and total phosphorus were statistically significant. Nutrient concentrations were generally higher during the dry season. The Water Quality Assessment System for Watercourses (SEQ-Eau) indicates that, with the exception of total phosphorus, the lake's waters are suitable for biological activity. Total phosphorus appears to be the downgrading parameter responsible for the risk of eutrophication of the lake during the rainy and dry seasons. It poses a threat to aquatic biota and reveals that the increased use of phosphate fertilizers in agricultural activities within the watershed threatens the balance of lake Kossou. Measures to reduce this use must be considered in order to protect biological diversity.