This study aims to assess the quality of groundwater in Grand-Lahou department in southwestern Côte d’Ivoire and to determine the origin of their mineralization. To do this, a sampling campaign conducted in september 2021 focused on twenty boreholes and nine wells. The methodology is based first on the physico-chemical characterization of these waters through elemental statistics and the determination of their chemical facies using the Piper diagram. Then the numerical index WQI method was applied. The results show that the waters are acidic with pH values ranged from 4.6 to 7.8 with an average of 5.75. Apart from iron, the parameters of the groundwater respect the WHO standards and they are fit for consumption. The groundwater has three chemical facies: the calcic and magnesian bicarbonate facies (31.03%), the sodi-potassium bicarbonate facies (17.24%) and the sodi-potassium chloride facies (51.72%). WQI results confirmed those of elementary statistics. They showed that 96.55% of waterworks belong to excellent quality class and one waterwork to good quality class. These results constitute a database for the management of the department’s groundwater resources.