Estimated Residence Time of Two Snowmelt Dominated Catchments, Boulder Creek Watershed, Colorado

Delta-O-18 and 3-H (tritium) measurements of precipitation and stream waters from 2002 to 2009 were used as isotopic tracers to investigate residence times in two headwater catchments within the Boulder Creek Watershed in the Front Range of Colorado. The 225--ha Green Lake 4 alpine catchment and the adjacent 664--ha Como Creek subalpine catchment are established research sites for both the Niwot Ridge Long Term Ecological Research Network and Boulder Creek Critical Zone Observatory Network. The Green Lake 4 catchment had a mean residence time of 1.12 years while the Como Creek catchment had a mean residence time of 1.8 years. Tritium levels in Green Lake 4 were consistently similar to that of incoming precipitation indicating little exponential decay and indicative of shallow flowpaths and short residence times. Como Creek tritium levels were also close to incoming precipitation during spring and summer snowmelt with elevated levels appearing during baseflow indicating some contribution of bomb Spike waters likely being delivered from deeper flowpaths with longer residence times. These results indicate that headwater catchments within Boulder Creek Watershed have relatively short mean groundwater residence times with the potential for older groundwater contributions during low flow conditions in the sub-alpine.