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Vadose Zone Journal
Vol. 4, No: xx, 2005; Pages: 620–636


Conceptual Models of Vadose Zone Flow and Transport beneath the Pajarito Plateau, Los Alamos, New Mexico

Kay H. Birdsell,* Brent D. Newman, David E. Broxton, and Bruce A. Robinson

EarthandEvironmentalSciencesDivisionLos Alamos National Laboratory,Los Alamos, NM 8754.

Abstract

The Pajarito Plateau in northern New Mexico, on which the Los Alamos National Laboratory is situated, is characterized by a thickvadose zone overlying the regional aquifer of the western Espanola Basin. In this study, conceptual models of vadose zone flow andtransport processes are presented and then supported through theinterpretation of field data, including synthesis with numerical models.The conceptual models differentiate the rate of percolation by theirlocation and surface hydrologic setting, including wet and dry canyons,and wet, dry, and disturbed mesas. Net infiltration beneath wet canyons is the highest, with rates on the order of a meter per year(100–1000 mm yr-1). Transport to the regional aquifer beneath thewettest canyons is likely on the order of several years to several decades, depending on the thicknesses of the various hydrostratigraphic layers. Perched water is sometimes found beneath wettercanyons and is associated with near-surface alluvial systems and atintermediate depths along low-permeability interfaces such as buriedsoils or unfractured regions of basalt flows. Percolation through thevolcanic tuffs is generally considered to be via matrix-dominated flow,whereas fracture flow may play a key role in contaminant transportthrough densely welded tuffs or basalt units beneath wet canyons.Infiltration beneath dry canyons and dry mesas is much slower (10mm yr-1 or less), yielding transport times to the aquifer of hundredsto several thousands of years. However, long-term surface disturbances at mesa-top locations may alter infiltration rates such thatat a local scale, the infiltration rates temporarily approach those ofwetter canyons.

Keywords:hydrologic setting;Percolation;Infiltration beneath dry canyons;Vadose Zone;hydrostratigraphic layers;radionuclide.


Corresponding author:

E-mail: khb@Lanl.gov

 

 
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