Abstract : The study of the vulnerability of water to pollution is a concept in perpetual change. The DRASTIC method [5], by the number of parameters taken into consideration, demonstrated that it was easy to work on the vulnerability of water to pollution because the information taken into consideration is the operator's responsibility, where its reliability as to the results obtained. To compensate for this weakness, many researchers have associated other parameters with the method DRASTIC. The objective of this work is to study the physico-chemical quality of natural waters in El Kala Park and to measure the risks and assess the vulnerability to groundwater pollution of the Tonga Lake watershed in order to better manage the risks. The results obtained show that the ground waters, influenced by the topography, flow from the south and east of the study area towards the interior, resulting in the supply of ground waters and the water ponds. The physico-chemical analyzes show that the waters are sparsely acidic and mineralized. The salinity of waters ranges from 0.1 to 0.5 g / l, while high levels are recorded east of the Tonga Lake watershed. The dominant chemical facies, by graphical representation of Piper is chlorinated sodium. The areas most likely to be polluted are those in and South-Est. At the level of the others the risk is present.