TOPOGRAPHIC CONTROLS ON GROUNDWATER IN TWO SMALL ALPINE WATERSHEDS, FRONT RANGE, COLORADO

Surface-groundwater interactions are rarely studied in alpine systems due to the shortage of wells. Chemical and isotopic analyses of wells recently installed by the Niwot Ridge LTER program, coupled with hydrometric measurements, reveal the importance of groundwater contributions to streamflow in alpine systems where snowmelt has traditionally been viewed as the primary source.

Fourteen paired groundwater wells were installed at two small alpine watersheds underlain by granodiorite bedrock with differing topography and hydrologic conditions in October 2005. We obtained water table elevations and water samples weekly through the summer and biweekly-to-monthly through the winter. Well water, surface water from nearby streams, and snowmelt were analyzed for major solutes and stable water isotopes.

The Martinelli snowpatch is a steep 8 ha catchment with a mean slope of 18°, located at an elevation of 3410 meters. A long-lasting snowfield occupies almost half of the catchment area which does not support vegetation. Water table measurements indicate artesian flow from most wells during peak runoff in response to snowmelt. δ¹⁸O values were not significantly different among streamwater, groundwater, and snowmelt δ¹⁸O reservoirs indicating a uniform snowmelt source contributing to both Martinelli stream and local groundwater. Base cation chemistry of groundwater and streamflow shows a strong dilution signal during high flows and supports the isotopic results.

In contrast to the Martinelli watershed, the nearby Saddle site has a ridge-top topographic setting characterized by gentle slopes, low rounded hills, and shallow saddles with patchy snow cover. Deep (6-8 m) and shallow (1.5 m) wells adjacent to an intermittent stream respond immediately to snowmelt whereas one pair of wells down gradient responds with a lag time of about 30 days. Ca²⁺ concentrations reached values of 6500 µEq L^-1 at the well furthest from the stream indicating heterogeneity in the groundwater system. Median groundwater δ¹⁸O (-15.9 ‰) is more enriched than both streamwater (-17.9 ‰) and snowmelt (-19.8 ‰). δ¹⁸O from both shallow groundwater wells and zero-tension soil lysimeters indicate that upwelling groundwater and not new snowmelt appears to be the dominant source of streamflow and soil moisture during the period of snowmelt runoff.