ENVIRONMENTAL TRITIUM SYSTEMATICS OF BASEFLOW IN PIEDMONT PROVINCE WATERSHEDS, GEORGIA (USA)

Environmental tritium (H-3) variation during the 1991 water year was investigated in three Piedmont streams within the Upper Ocmulgee basin of Georgia. Baseflow tritium concentrations typically varied between 20 and 30 TU; approximately twice that of weighted annual rainfall for the past 2 decades. Statistical treatment of these data indicated that the mean residence time of groundwater which comprises Piedmont Province baseflow is approximately 15-35 years. Annual tritium trends were similar in the three basins which varied over three orders of magnitude with respect to watershed area. The observed trends were not, however, directly coupled with rainfall and baseflow discharge rates. Tritium concentrations were highest at the beginning of the water year (24-31 TU) and were diluted by 17-22% during the fall, coincident with only a minor amount of rainfall. Concentrations declined by only a few tritium units during the wettest portion of the year. The 200-300% increase in baseflow which occurred as the result of above-normal rainfall was comprised essentially of 'old' water. At no time during the year did baseflow tritium concentrations approach values representative of recent rainfall. These data indicate baseflow discharge should be regarded as a dynamic mixture rather than as a static reservoir of water. The annual variation is, however, systematic which has important implications for future hydrographic and geochemical studies within this region.