USING GEOPHYSICAL TECHNIQUES IN THE CRITICAL ZONE TO DETERMINE THE PRESENCE OF PERMAFROST

Electric Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) are geophysical techniques utilized worldwide to study the evolution of alpine permafrost and ice lenses. Combining these techniques maximizes the accuracy of each method while reducing their inherent ambiguities and limitations. The alpine Green Lakes and 4^th of July Valleys within the Colorado Front Range offer ideal locations to verify the existence of ice masses within rock glaciers, where they are predicted by many models. On nearby Niwot Ridge, geomorphological, hydrological, and GIS techniques have been used to calculate probabilities of permafrost at depth, but predictions have not previously been verified in the field. Permafrost and ice lenses beneath active solifluction lobes were also documented in several studies during the 1970’s along Niwot Ridge. This study examines: (1) the results of 15 ERT lines totaling 765 meters within the Critical Zone, sited to test permafrost predictions in alpine zones; (2) soil temperature profiles and morphology in several pits excavated to saprolite along ERT lines; and (3) measurements of the 4^th of July Rock Glacier’s retreat during the past century including its implications for nearby permafrost. Analysis of seven ERT lines from elevations of 3500 to 3900 meters on Niwot Ridge demonstrates that these locations lack permanent ice lenses (resistivity of approximately 200-1000 kΩm) beneath a surface layer of coarse, blocky debris (resistivity of ~ 20 kΩm). Solifluction lobes, as well as nearby snow field areas may retain semi-permanent ice lenses that were considered permafrost features, but ice often melts out completely by late summer. The rock glaciers have experienced decades of retreat due to high-elevation warming and decreases in moisture supply (i.e. droughts during the winters of 2002-2003 and 2011-2012), resulting in partial melting of subsurface ice and deposition of a layer of rocks previously supported by ice. Better understanding of the present distribution of permafrost and active periglacial features may help predict changes to alpine landscapes as permafrost disappears as well as implications for the quantity of runoff from these areas in the future.