The physicochemical quality of groundwater of one of the five aquifers of the coastal region of Pointe-Noire, aquifer AQ1 is the subject of this study. Water of 18 wells was taken during the year 2014. 13 variables were given in the water samples: pH, C.E, T, Ca 2+ , Mg 2+ , Na + , K + , HCO 3 -, Cl -, SO 4 2-, NO 3 -, TDS and TH. The pH lies between 4.26 and 7.10. 88.88% of the wells have pH lower to 6.5, which show the acid character. 83.33% of analysed water are fairly mineral-bearing (C.E <

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... 300 µS cm -1 ). On the other hand 16.67% of these water wells (PU1, PU4 and PU17) present an accentuated mineralization. The multivariate statistical methods have been used in this study. Ascending hierarchical classification gave two groups of water well. Except for the temperature and pH, the other variables discriminate the two groups. The factorial analysis highlighted two factors which account for 80.61% of the total variance. The first factor (62.87%) shows the mineralization and the hardness water. The second factor (17.74%) highlights the dissolution of carbonated minerals of magnesium and the character slightly acid of water. However this quality can be faded when external substances come into contact with the aquifer. Such is the case of the even toxic undesirable substances which make subterranean water unsuitable for various uses in particular for the water use of drink. The groundwater has physicochemical characteristics which are particular for it, such as the temperature, the color, the pH, conductivity and the content of major elements. These characteristics resulting from the natural environment are very variable. They depend on the geological nature of the ground from where they come and also of the reactive substances which they could have met during the flow. Thus the quantitative and qualitative composition of subterranean water out of suspended matter and dissolved, of mineral or organic nature, determines its quality 3 .