THE "TEFLON BASIN" MYTH: A SURFACE WATER HYDROLOGIST'S PERSPECTIVE ON THE GROUNDWATER SYSTEM OF MOUNTAIN CATCHMENTS IN THE WESTERN US

In much of western North America, snow and snow melt provides the primary means for storage of winter precipitation, effectively transferring water from the relatively wet winter season to the typically dry summers. A common assumption is that high-elevation catchments in the western United States behave like "Teflon basins" and that water released from seasonal storage in snow packs flows directly into streams with little or no interaction with geologic or biologic materials. Here we show that streams and lakes are a vital window into the groundwater system and understanding one requires understanding the other.

The Precambrian rock of the Rocky Mountain Front Range immediately west of the Denver Basin is composed of crystalline rocks that generally have low permeability and storage capacity. However, discrete fault and/or fracture zones do occur and may be able to store and transmit sizeable volumes of ground water. We determined source waters and flowpaths in the Green Lakes valley of the Front Range using isotopic, geochemical, and hydrometric data in 2- and 3- component hydrograph separations along with end-member mixing analysis (EMMA). EMMA results showed that much of the water released from the seasonal snowpack infiltrated into subsurface reservoirs and that old groundwater contributed almost 50% of flow on the rising limb of the hydrograph and 80% on the recession limb. Measurements of stable and radiogenic isotopes of snow, streams, springs, and groundwater near Leadville, CO show that groundwater was dominated by infiltrating snowmelt with a residence time of years to decades. These results suggest that much of the water released from seasonal storage in snow recharges groundwater reservoirs and may contribute to mountain-block recharge of downslope aquifers.