18
O is an ideal tracer for characterizing hydrological processes because it can be reliably measured in several watershed hydrological compartments. Here, we present multiyear isotopic data, i.e. 18 O variations (υ 18 O), for precipitation inputs, surface water and groundwater in the Shingobee River Headwaters Area (SRHA), a well-instrumented research catchment in north-central Minnesota. SRHA surface waters exhibit υ 18 O seasonal variations similar to those of groundwaters, and seasonal υ 18 O variations plotted versus time fit seasonal sine functions. These seasonal υ 18 O variations were interpreted to estimate surface water and groundwater mean residence times (MRTs) at sampling locations near topographically closed-basin lakes. MRT variations of about 1 to 16 years have been estimated over an area covering about 9 km 2 from the basin boundary to the most downgradient well. Estimated MRT error (š0Ð3 to š0Ð7 years) is small for short MRTs and is much larger (š10 years) for a well with an MRT (16 years) near the limit of the method. Groundwater transit time estimates based on Darcy's law, tritium content, and the seasonal υ 18 O amplitude approach appear to be consistent within the limits of each method. The results from this study suggest that use of the υ 18 O seasonal variation method to determine MRTs can help assess groundwater recharge areas in small headwaters catchments.