Unraveling the impact of climate on groundwater storage, hydrochemistry, and residence time in the high-elevation catchments of the Niwot Ridge LTER site, Colorado

Groundwater storage, hydrochemistry and residence time are all known to vary widely depending on hydrogeologic conditions. In mountainous terrain hydrogeology can vary greatly over short distances, from bedrock aquifers on ridge tops to colluvial and fluvial aquifers in valleys.  Determining how climate alters groundwater in the context of variable hydrogeologic conditions is needed to understand in-stream flows and biogeochemical cycles in these climactically sensitive alpine settings. In 2005 at the Niwot LTER six piezometers were installed in surficial diamicton and colluvium at the base of a semi-permanent snowfield at the Martinelli site (3440 m).  Eight piezometers were also installed at the Saddle on a ridge-top in the alpine tundra (3528 m). In 2010 12 piezometers were installed at the C1 site (3025 m) in the subalpine atop moraine deposits.  Groundwater monitoring for all sites is year-round and is comprised of depth-to-water measurements by hand and pressure transducers for select wells, as well as chemistry samples for major solutes including dissolved organic matter and stable isotopes of water, δ¹⁸O and δD.  Across the Niwot LTER precipitation falls predominately as snow creating a strongly snowmelt-dominated hydrograph. Groundwater response to this seasonality is reflected in both physical and hydrochemical groundwater measurements.  Snowmelt leads to sharp increases in water level in all piezometers including up to 7 m of water table change at the Saddle, up to 3 m of change at Martinelli and up to 5 m of water table change at C1. Minimum water table levels are not always measureable as the water table can drop below the extent of the piezometers, however, at the Saddle there are decreasing trends in annual minimum groundwater level in 3 of the 4 deep piezometers, possibly reflecting a decrease in total aquifer storage. Hydrochemical groundwater response to snowmelt is evident in distinct harmonic trends in major solute and isotope chemistry.  Time series plots of calcium (Ca²⁺), a predominately rock-derived solute, show annual minimum concentrations in early August, approximately two full months beyond final snowpack melt-out dates at the Saddle snowpit site.  Nitrate (NO₃^-) has peak concentrations in June-July during snowmelt and also in winter months (November–March) perhaps due to the cessation of biological uptake of nitrogen. Time series plots of stable isotopes of water show a significant (p ≤ 0.05) isotopic depletion in groundwater at deep piezometers at the Saddle between 2006 and 2011 from a mean δ¹⁸O value of -15.64 ‰ (n = 38) to -16.05 ‰ (n = 74) but not in other locations. This trend may reflect the greater influence of snow over the study period which was characterized by successively increasing snow deposition since a major drought in 2002. These results indicate that climate has a large impact on the water table and water quality of groundwater in high elevation catchments.