Determination of Base Flow Residence times within the Piedmont Province of Georgia Using Long-term Tritium ( H)variation

An algorithm utilizing measured temporal tritium ( H) variation in selected Piedmont basins is 3 developed for the computation of subsurface residence times of shallow ground water comprising stream base flow. This algorithm accounts for the effects of radioactive decay, the mixing of shallow subsurface water with precipitation, and ground-water flux to streams (base flow). The input data required include at least two H measurements made at least 3 a decade apart in base flow for a given stream and the longterm H input in regional precipitation. The resulting 3 residence times (which are the inverse of the ground-water flux to stream base flow) are sensitive to small changes in measured H concentrations. The accuracy of modeled 3 residence times is limited by analytical uncertainties in the measurement H to -±5 years. H concentrations within the 3 3 Upper Oconee and Ocmulgee Basins in the Georgia Piedmont Province varied between 20-30 T.U. during the early 1990s. These concentrations have declined to -5-10 T.U. in the mid2000s. H input to these basins from precipitation has been 3 generally < 6 T.U. during the interval between sampling periods. Model results indicate that the best fitting residence times of stream base flow in these Piedmont basins is between 14-18 years. These values are consistent with ground-water residence times determined by tritium-helium dating.