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Zeliff, Morgan M. ¹ ; Williams, Mark ²

¹ University of Colorado
² University of Colorado

Stable isotopes and hydrochemical constituents of water were used to evaluate the role of groundwater in two high-elevation, snowmelt dominated areas of the Colorado Front Range: The 8-ha Martinelli catchment at 3,440 m and the nearby Saddle site at 3,530 m. The Martinelli site has three shallow (1.5 m) and 3 deep (> 6 m) piezometers, while the Saddle site has 4 sets of deep and shallow piezometers. All wells were screened for their bottom 1.5 m. All shallow piezometers had water only during snowmelt. In contrast, all deep piezometers had water year-round, with water depths generally deeper than 4 m during the winter and water rising to the surface during snowmelt, followed by a long recession limb. Starting in 2006, water samples were collected weekly to monthly from all piezometers, and for precipitation, snow, and surface water and analyzed for stable isotopes and major solute chemistry. The snowpack had a mean δ18O value of -20.43 ‰ (n = 52), ranging from -22 ‰ to -18 ‰ during snowmelt. The 4 per mil difference in δ18O during snowmelt was from fractionation of meltwater as it infiltrated through the snowpack. The mean δ18O value of -19.14 ‰ (n =299) for the piezometers at the Martinelli site did not differ significantly between shallow and deep piezometers (p = 0.1684), nor between the piezometers and either the snowpack or stream. In contrast, shallow and deep piezometers at the Saddle site were significantly more enriched than Martinelli with a mean δ18O value of -16.95 ‰ (p < 0.001, n = 417). A plot of the δ18O versus δD values from all wells along with the Global Meteoric Water Line (GMWL) provides evidence of depletion of these isotopes in the well waters. Mean calcium concentrations at the Martinelli piezometers (89 μeq L-1) were more than an order of magnitude more dilute when compared to mean values from the Saddle piezometers (1684 μeq L-1). Mean silicon concentrations are also significantly (p<0.001) more dilute at the Martinelli piezometers (62 μeq L-1) than mean concentrations from the Saddle piezometers (200 μeq L-1). These results, combined with hydrogeological information, suggest that the Martinelli catchment is an unconfined aquifer with direct hydrologic connection between surface waters and groundwaters and has a groundwater system dominated by direct snowmelt recharge. In contrast, groundwater at the Saddle site appears to be in a confined aquifer with a relatively long residence time and with no direct connection between piezometers and overlying snowmelt.