18O 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. 18O variations (??18O), 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 ?? 18O seasonal variations similar to those of groundwaters, and seasonal ??18O variations plotted versus time fit seasonal sine functions. These seasonal ?? 18O 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 km2 from the basin boundary to the most downgradient well. Estimated MRT error (??0.3 to ??0.7 years) is small for short MRTs and is much larger (??10 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 ??18O amplitude approach appear to be consistent within the limits of each method. The results from this study suggest that use of the ?? 18O seasonal variation method to determine MRTs can help assess groundwater recharge areas in small headwaters catchments. Copyright ?? 2005 John Wiley & Sons, Ltd.
Additional Publication Details
Characterization of surface and ground water ??18O seasonal variation and its use for estimating groundwater residence times