2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 102-15
Presentation Time: 11:30 AM


MCKENZIE, Jeffrey M.1, GORDON, Ryan P.2, BARAER, Michel3, LAUTZ, Laura K.2, MARK, Bryan G.4, CHAVEZ, Daniel1 and AUBRY-WAKE, Caroline5, (1)Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 0E7, Canada, (2)Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, (3)Ecole de Technology Superieure, University of Quebec, 1100 Notre-Dame Ouest, Montreal, QC H3C 1K3, Canada, (4)Department of Geography and Byrd Polar Research Center, The Ohio State University, 1036 Derby Hall, 154 North Oval Mall, Columbus, OH 43210, (5)Earth and Planetary Sciences, McGill University, Montreal, QC H3A 0E7, Canada, jeffrey.mckenzie@mcgill.ca

Although more that 370 million people live in watersheds where glaciers provide a minimum of 10% of runoff, questions still remain concerning the processes controlling the storage and baseflow contributions of groundwater in high mountain watersheds. We present research from the Cordillera Blanca, Peru, an area with the highest density of glaciers in the tropics. While glacier meltwater buffers stream discharge throughout the range, we have shown that groundwater is a major component of dry season runoff, contributing up to 50-70% of outflow in some tributaries. In order to assess the potential for groundwater to maintain streamflow and offset future hydrologic changes, we need to understand groundwater dynamics in these high elevation watersheds, including recharge mechanisms, subsurface pathways, storage, and net fluxes to rivers. We present results from research in the Cordillera Blanca using numerous methods, including drilling/piezometers, geophysics, and artificial and natural tracers.

Our research focuses on low-relief ‘pampa’ valleys which have a total area of ~65 km2 and are comprised of unconsolidated glacial, talus, lacustrine and wetland (bofedales) deposits. The valleys commonly have buried talus aquifers (hydraulic conductivity of 10-5 m/s) that are overlain by low permeability, glaciolacustrine deposits (hydraulic conductivity of ≤ 10-7 m/s). Glaciofluvial outwash deposits also act as aquifers (hydraulic conductivity of 10-4 m/s), and are enhanced when cross-cut by streams. A ubiquitous feature of the pampa valleys are springs located at the base of talus deposits, which comprise a large component of the hydrologic systems, based on hydrochemical mixing analysis. Groundwater discharge and recharge to streams is spatially heterogeneous. For example, for a headwater basin of the Llanganuco Valley, almost all of the stream water exiting the catchment spent some time in the subsurface, with approximately half of the surface water originating as groundwater. The residence time of water stored in these systems is generally less than 3-4 years and the maximum observed dry season groundwater velocity is 60 cm/day (based on average linear velocity calculations). The results are synthesized with conceptual models of pampa formation and hydrogeology.