From on high: Geochemistry of alpine springs, Niwot Ridge, Colorado Front Range, USA

Abstract

Snowmelt‐fed springs and small (0.5 km2) upland catchments in alpine areas of the western United States contribute significantly to the quantity and inorganic chemistry of water delivered to downstream basins but have not been studied extensively. Mineral weathering, transit time, and hydrologic mixing control the solute chemistry of waters that drain the upland zone of Niwot Ridge, Colorado Front Range, and adjacent areas in the granitic core of the Southern Rocky Mountains. Water in 37 springs sampled in this study flows in generally short steep paths (~0.3 km) through shallow regolith with mean transit times (MTT) of weeks to months, producing solutions dominated by Si, Ca2+, Na+, and HCO3−, locally SO42−. Rock type is a significant control on spring, surface, and shallow groundwater chemistry, and plagioclase (oligoclase) is the major source of dissolved Na+ and Si. Concentrations of Ca2+ exceed stoichiometric predictions of oligoclase weathering by ~3.5×; excess Ca2+ likely represents weathering of aeolian material, vein calcite, or trace minerals. Concentrations of base cations and Si increase slowly with estimated MTT of 0.2 years for Niwot Ridge spring waters, and several years for shallow groundwater sampled by wells. Chemical weathering of silicate minerals is slow with estimated rates of ~2.0 and 0.2 pmol·m−2·s−1 for oligoclase and microcline, respectively; the most mineralized spring waters are saturated only with respect to kaolinite and montmorillonite. More than 50% of the dissolved base cations + Si measured in Boulder Creek at Orodell (~25 km downstream) accumulate before water emerges from alpine springs on Niwot Ridge. Warming global temperatures are shifting more high‐elevation precipitation to rain, potentially changing run‐off patterns, transit time, and solute loads. Acquisition of solutes by alpine waters thus has implications far beyond small upland catchments.

Depth of Solute Generation Is a Dominant Control on Concentration‐Discharge Relations

“…If the concentration decreases with increasing discharge (i.e., b < 0), it shows a diluting behavior (Godsey et al, 2009). Recent studies have explored possible drivers of the C‐Q relations, including hydroclimatic forcing (Aguilera & Melack, 2018; Blaen et al, 2017; Bouchez et al, 2017; Duncan et al, 2017; Godsey et al, 2019; Ibarra et al, 2016; Musolff et al, 2017) and catchment characteristics such as land‐cover, catchment size, and lithology (Bouchez et al, 2017; Diamond & Cohen, 2018; Fields & Dethier, 2019; Ford et al, 2017; Hoagland et al, 2017; Minaudo et al, 2019). The tendency is toward identifying generalizations on the solute behavior in the C‐Q space with some emerging pattern, such as the prevalent diluting behavior of geogenic solutes (Botter et al, 2019; Godsey et al, 2009; Kim et al, 2017; Musolff et al, 2017; Thompson et al, 2011).…”

Sources of variability in springwater chemistry in Fool Creek, a high‐elevation catchment of the Rocky Mountains, Colorado, USA

“…The chemical composition of surface water draining headwater areas reflects their underlying lithology (Fields & Dethier, 2019; Miller, 2002). The Fraser Experimental Forest is located north of the Colorado Mineral Belt, and the Ca 2+ and ANC‐dominated chemistry of Fool Creek spring and streamwater is consistent with weathering of intermediate to mafic mineral groups in the gneiss and schist bedrock common throughout the area (Wanty et al, 2009).…”

“…Similarly, high alkalinity and SC emerged from springs in the Scottish Highlands adjacent to small calcareous outcrops (Scheliga et al, 2017). At other Colorado Front Range sites, divergence between measured springwater Ca 2+ concentrations and stoichiometric predictions based on bedrock weathering were attributed to release from unmapped calcite veins and aeolian dust inputs (Clow et al, 2016; Fields & Dethier, 2019). These unmapped factors are also known to influence soil, snowpack and emergent groundwater chemistry (Bergstrom et al, 2018; LaPerriere Nelson et al, 2011; Retzer, 1962; Rhoades et al, 2010) in numerous catchments at and around FEF.…”

“…Water that percolates through soils or fractured bedrock recharges groundwater sources and sustains late‐season streamflow (Golden et al, 2017). Interactions within the soil profile and underlying saprolite influence the chemistry and temperature of water routed along near‐surface pathways (Constantz, 1998; Fields & Dethier, 2019; Scheliga et al, 2017). Characteristics of emergent groundwater may be useful for identifying source water, differentiating shallow from deep flowpaths and confirming linkages between subsurface and surface water (Barbera & Andreo, 2017; Soulsby et al, 1999; US EPA, 2015).…”

“…The chemistry of emergent groundwater responds to subtle differences in bedrock composition, degree of alteration and distinct mixtures of source water (Fields & Dethier, 2019; Horton et al, 1999; Valett et al, 1996). For example, an evaluation of groundwater‐fed Colorado wetlands demonstrated that the chemical composition of wetland porewater and adjacent streams were sensitive to relatively minor differences in bedrock type (LaPerriere Nelson et al, 2011).…”