AN EVOLVING CONCEPTUAL MODEL OF GROUNDWATER-SURFACE WATER INTERACTIONS IN ALPINE BASINS IN THE WESTERN U.S

Alpine catchments in the western United States are characterized by high relief, an abundance of exposed bedrock and talus, and a scarcity of well-developed soils and vegetation. Their hydrology is driven by seasonal accumulation and subsequent melting of deep snowpacks; typically over 75% of annual precipitation occurs as snow. The combination of rapid snowmelt inputs, high hydraulic gradients, and lack of obvious storage capacity suggests that alpine catchments may behave like “Teflon basins,” implying that terrestrial landscapes have little ability to absorb atmospherically-deposited pollutants.

Recent work, however, indicates that even in alpine basins, subsurface biogeochemical reactions can strongly modify the chemistry of infiltrating water, suggesting the “Teflon basin” paradigm is too simplistic. Hydrograph separation analyses on alpine streams using d¹⁸O and silica indicate that “old” water is rapidly flushed from subsurface reservoirs, and that snowmelt rapidly acquires silica through fast geochemical reactions in shallow alpine soils.

Talus groundwater plays a particularly important role in controlling the chemistry of alpine streams, especially during storm events and winter. Dry and wet deposition and N mineralization/nitrification at the talus surface contribute solutes that infiltrate talus debris and combine with solutes derived from subsurface weathering reactions. These solutes are rapidly flushed from talus into streams during snowmelt and storms, and continue to be released slowly through the winter season.

Other alpine environments with potentially important groundwater-surface water interactions include wetlands and riparian areas, which tend to occur geographically between talus and surface water bodies. Spring water emanating from the base of talus slopes typically flows over wetland and riparian soil with little alteration. However, water that flows from talus into wetland and riparian areas through subsurface flowpaths undergoes a geochemical evolution as groundwater moves from an oxygen-rich to an oxygen-poor environment, where sulfate reduction and denitrification are important. Many alpine basins lack extensive wetland and riparian landscapes; in those basins surface water typically carries the chemical signature of talus groundwater.