Tritium (HTO) concentrations were studied for an entire year in a floodplain forest adjacent to a low-level radioactive solid waste storage area (SWSA No.5) at Oak Ridge National Laboratory (ORNL) near Oak Ridge, Tennessee, USA. Tritium in soil was the principal source of HTO to the deciduous forest. Evaporation from the surface soil along with transpiration from tree leaves both contributed to HTO in the forest atmosphere. During the growing season, transpiration was the principal contributor of HTO to the forest atmosphere, while during the dormant season, the main source of atmospheric HTO was evaporation from the surface soil. Seasonal changes and the characteristics of vegetation will influence die relative importance of evaporation and transpiration as sources of atmospheric HTO near the ground in temperate deciduous forests.