GROUNDWATER OCCURRENCE, AGE, AND CONTRIBUTION TO STREAMFLOW IN AN ALPINE BASIN IN THE COLORADO ROCKY MOUNTAINS

Groundwater occurrence, age, and its contribution to streamflow were investigated in Loch Vale, an alpine catchment in the Colorado Rocky Mountains.  Hydro-geomorphologic mapping, seismic refraction measurements, and porosity estimates indicate that talus slopes are the primary groundwater reservoir, with a storage capacity of 5 to 17 x 10⁶ m³.  Ice stored in permafrost is the second largest groundwater reservoir in Loch Vale, accounting for 2 to 4 x 10⁶ m³ of water.  The estimated storage capacity of bedrock fractures is comparatively small (0.007 to 0.013 x 10⁶ m³).   

Although snowmelt provides the majority of annual water flux from the basin, tracer tests and gaging along a stream transect indicate that groundwater flowing from talus can account for greater than 75% of streamflow at certain times.  Talus springs respond quickly to storms, then slowly recede, reflecting rapid transmittal of water through coarse debris at the talus surface and slower release of water from finer-grained sediments at depth.  CFC and tritium (³H) data permit setting bounds on mean residence times and on the relative contributions of "new" (< 1year old) and "older" (45 -- 50 years old) water under two different simplifying scenarios.  The first scenario assumes piston subsurface flow, and the second assumes water is a mixture of "new" and "older" water.   

CFC samples collected from two springs in Loch Vale during August 2003 had piston-flow model ages of 17 to 22 years, based on CFC-12 and CFC-113 results.  Under the old water -- new water mixture scenario, 55 to 65% of the water was "new".  Monthly samples collected from one of the springs and a nearby creek had ³H concentrations ranging from 9.6 (near current precipitation values) to 17.1 TU.  Minimum values occurred during snowmelt and peak values occurred during winter baseflow; this pattern indicates a large new water influence during snowmelt, with a noticeable component of older water during winter.  Piston-flow model ³H ages are poorly constrained due to potentially large variations in input values; estimates range from 0 to 30 years.  Mixture-model ³H ages are even more uncertain; results do indicate that most water is modern, but there is an "older" water component.   

These results indicate surprisingly large contributions from old water in Loch Vale surface and groundwaters at certain times of the year.