The results of leaching tests are one of the criteria used to evaluate the performance of cementitious materials for stabilisation/solidification of hazardous wastes. The action of water is of special importance due to its effect on the integrity of the cement solid phases. Likewise, the degraded material originated by the water action may facilitate the transport of toxic components from the material to the environment. Therefore, there is an interest in determining the material behaviour under the long-term action of water in representative conditions of a real storage scenario, in particular where radioactive waste storage times of 300 years or more are needed. During this time, the stabilised waste should maintain its properties. In the present paper a new leaching method to test the performance of concretes for long-term behaviour under natural water interaction is presented. An open system and a system of continuous running of natural water flowing at a fixed rate have been used. The system has been applied to test high- and ultra-high performance concretes for over a year. It gives different results to those previously reported using demineralised water or acid solutions. The method was developed to represent real scenarios in contact with granitic groundwater.