FROM ON HIGH: THE GEOCHEMISTRY OF ALPINE SPRINGS, NIWOT RIDGE, COLORADO FRONT RANGE

Snowmelt-fed springs and small (0.5 km²) upland catchments in alpine areas of the western USA contribute significantly to the quantity and chemistry of water delivered to downstream basins. Understanding these springs is important as warming global temperatures cause more precipitation to fall as rain at high elevations, affecting long-term runoff and storage patterns. In years of low snowpack, shallow groundwater from alpine and subalpine zones is an additional water source. We report the geochemistry of snowmelt springs that drain the alpine zone of Niwot Ridge, Colorado Front Range and examine how rock type, contact time and hydrologic mixing control their chemistry and that of surface and groundwater from this area. Dilute calcium bicarbonate waters (mean total dissolved load of 450 ± 170 μmol L^-1) are typical of springs and small streams draining Niwot Ridge and adjacent areas in the granitic core of the southern Rocky Mountains and are a mixture of soil water, deep groundwater, and snowmelt runoff. The water in 48 sampled springs takes generally short flow paths (~0.3 km) through shallow regolith, resulting in short contact time with rock materials. However, rock type is an important control on spring chemistry. Chemical weathering of feldspars is the primary contributor of solutes; oligoclase is the major source of Na and SiO₂. Concentrations of Ca in the springs and shallow groundwater exceed stoichiometric predictions of oligoclase weathering by ~3.5x, indicating that Ca has other sources, such as eolian material, interstitial calcite, or trace minerals. Solute concentrations in springs and shallow groundwater increase with contact time; silica provides the best measure of transit time for waters draining from Niwot Ridge. Using silica concentrations and estimated groundwater flow paths, we estimate an average residence of 0.6 yr for Niwot spring waters, and ~3 yr for shallow groundwater sampled by wells. Alpine waters acquire solutes rapidly despite cold temperatures, and concentrations change slowly downstream. Over 50% of the dissolved load of waters exiting the Boulder Creek watershed >20 km downstream accumulated before the waters emerged from alpine springs. Understanding how alpine waters acquire solutes thus has implications beyond small upland catchments.