The influence of dairy wastes stored in an unlined lagoon on the total nitrogen concentration of shallow groundwater was investigated in western Hokkaido. The lagoon was constructed in a meadow consisting of an Andosol with a high water table. Increase of the total nitrogen concentration in groundwater was observed at a distance of 75 m from the lagoon within 4 days after a low concentration dairy slurry was loaded into the lagoon. At a distance of 15 m from the lagoon, higher total nitrogen concentration was still observed 85 days after slurry loading. These facts clearly indicate that groundwater contamination by cattle slurry stored in the unlined lagoon occurs very quickly and persists for a very long period of time.