Urbanized areas world-wide are characterized by, among other things, a high production of wastes of different types. More than 90% of urban solid wastes is brought to deposits. If the waste dumps or landfills are not designed, constructed, managed, and finally recultivated, according to the state of the art, dangerous impacts, especially on urban groundwater, can be expected. Percolating water from rainfall or surface overflow is strongly influenced by the components, the construction and the chemical/biological processes in the deposits. The leakage water has to be considered one of the most polluted and extremely unpredictable waste waters. Percolating into the natural groundwater has the effect that this resource is spoiled and no longer suitable for human use or as a component of ecosystems. Old waste dumps must restored and recultivated.