Hydrochemistry, residence time and nutrient cycling of groundwater in two, climate-sensitive, high-elevation catchments, Colorado Front Range.
MA: University of Colorado Boulder, 2013.
Groundwater is recognized as an important component of the hydrologic cycle of high-elevation catchments because it contributes significant proportions to surface water flows, exerting important controls on biogeochemical fluxes and reactions at the hillslope to catchment scale. The impacts of climate change are already causing earlier onset of snowmelt and changes to stream discharge quantity and quality in these catchments along the Colorado Front Range. Questions still remain as to how these changes will impact groundwater recharge, residence time and linked biogeochemical cycling. Using six years of hydrometric, hydrochemical and isotopic measurements from 14 piezometers in two adjacent headwater catchments in the alpine tundra of the Colorado Front Range, I explore the answers to these questions.
In general, cation concentrations and residence times were higher in the bedrock aquifer than in the colluvial aquifer. Temporal variation in groundwater solute concentrations tended to show a dilution effect from incoming snowmelt near the time when water tables were at their highest points, which was just after the stream hydrograph peaked. Groundwater residence time in the bedrock aquifer averaged 2.1-4.2 years. The bedrock aquifer had a higher proportion of groundwater flow than the streamflow in the catchment with the colluvial aquifer. These findings suggest that as climate change alters the timing and magnitude of hydrologic inputs to these high-elevation catchments, initial changes in surface water flow will occur where dependence on snowmelt is higher, such as in the colluvial aquifer of the Martinelli catchment, but over time, these changes will impact groundwater in bedrock aquifers as well.
Groundwater flow estimations were made to assess the magnitude of biogeochemical cycling of nitrate (NO3-), dissolved organic nitrogen (DON), and dissolved organic carbon (DOC) in the bedrock aquifer. Results suggest that relative to the total export in the stream flow the nutrient flux through the bedrock aquifer may be significant. These findings highlight the importance of the groundwater system to alpine nutrient cycling and suggest that further work needs to be done to evaluate the impact of climate change on this aspect of the hydrologic and biologic cycles of high-elevation catchments.