PHYSICO-CHEMICAL QUALITY AND METAL CONTAMINATION OF WELL WATER IN THE SIXTHNEIGHBORHOOD OF COTONOU, SOUTH BENIN

In this research work, we have studied the physico-chemical characterization and searched for twenty-six metallic elements in the well waters of the sixth neighborhoodofCotonou. Well water was sampled in ten neighborhoods of this neighborhood.Physico-chemical parameters such as pH, temperature, conductivity, total dissolved salts (TDS) and salinity were measured on site while ammonium, phosphate, chloride, nitrite and nitrate ions are determined in the laboratory. Twenty-six metallic elements were measured after acid digestion of the water samples using the inductively coupled plasma source mass spectrometer (ICP-MS). The results showed that groundwater in the sixth neighborhood is in a degradation of its physico-chemical quality. The conductivity values varying between 2.06 and 1270 µS/Cm with an average of 872.41 and a standard deviation of 393.90 show that these waters are highly mineralized (the WHO qualifies waters with a conductivity greater than 200µS/Cm of very highly mineralized water). Ammonium and nitrite ions varying respectively between 0.6 and 7.1 mg/L with an average of 2.21 mg/L and 0.02 and 0.61 mg/L with an average of 0.22 mg/L are at abnormal concentrations in groundwater. The ammonium ion contents are beyond the standard value.As for metallic elements, aluminum, which is a trace element, is strongly present in water with an atypical observation of 5.008 mg/L. This value exceeds more than 25 times the standard defined by the WHO which is 0.2 mg/L. Iron is also present with an abnormal concentration of 2.711 mg/L as maximum value. Even if the values of the trace metals whose toxicities approved are lower than the standards, some atypical values are recorded and significant correlations established between these different chemical elements results from the common origins for the positively correlated metals and different origins for those which are negatively correlated.