FLOW SOURCES AND PATHWAYS USING ISOTOPIC AND CHEMICAL TRACERS AT THE HIGH-ELEVATION GREEN LAKES VALLEY, COLORADO FRONT RANGE

The high-elevation watersheds of the Rocky Mountains form the physical starting point for most waters that recharge into either surface water reservoirs or infiltrate into groundwater aquifer systems. Identification of flow generation mechanisms is crucial to understanding the interaction between surface water and groundwater in these high-elevation watersheds. In this study, sources and pathways of streamflow were determined using isotopic and geochemical tracers for the snowmelt runoff season in 1996 at two sub-catchments at Green Lakes Valley, Colorado Front Range. A two-component hydrograph separation using d¹⁸O indicates that new water (82± 6%) dominated stream flow at the 8 ha Martinelli catchment. Somewhat surprisingly, old water (64± 2%) dominated stream flow at the 220 ha Green Lake 4 catchment. Complicating interpretation of hydrograph separation models, snowmelt becomes isotopically enriched as snow melting progresses. A significant error would be introduced if temporal variation of d¹⁸O in snowmelt were not characterized properly or if d¹⁸O values in snowpack were used, usually leading to an underestimation of old water. A Monte Carlo procedure was developed to calibrate d¹⁸O values in snowmelt at a point to the catchment scale. Information on flow sources alone cannot explain hydrological controls on stream flow chemistry. Flowpath analysis using single geochemical tracers for two components was relatively successful at the 8 ha Martinelli catchment but not successful at the larger Green Lake 4 catchment. Using d¹⁸O and seven geochemical tracers, end-member mixing analysis (EMMA) was successfully conducted to identify flowpaths at both catchments. The solute and d¹⁸O contents of stream flow were well reproduced using the EMMA results. Surface flow and subsurface flow accounted on average for about 50% at the Martinelli catchment. Surface flow, subsurface flow from riparian zone, and subsurface flow from talus contributed nearly equal fractions at the Green Lake 4 catchment. Subsurface flow is thus recognized to be major control on the stream flow chemistry in the high-elevation watersheds of the Rocky Mountains.