Paul F. Schuster Carol Kendall Micaela B. Reddy Michael M. Reddy 2006 <p>¹⁸<span>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.&nbsp;</span>¹⁸<span>O variations (&delta;</span>¹⁸<span>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 &delta;</span>¹⁸<span>O seasonal variations similar to those of groundwaters, and seasonal &delta;</span>¹⁸<span>O variations plotted versus time fit seasonal sine functions. These seasonal &delta;</span>¹⁸<span>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</span>²<span>&nbsp;from the basin boundary to the most downgradient well. Estimated MRT error (&plusmn;0&middot;3 to &plusmn;0&middot;7 years) is small for short MRTs and is much larger (&plusmn;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 &delta;</span>¹⁸<span>O amplitude approach appear to be consistent within the limits of each method. The results from this study suggest that use of the &delta;</span>¹⁸<span>O seasonal variation method to determine MRTs can help assess groundwater recharge areas in small headwaters catchments.</span></p> application/pdf 10.1002/hyp.5953 en Wiley Characterization of surface and ground water δ18O seasonal variation and its use for estimating groundwater residence times article