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

Abstract

field observations. To best develop and test conceptual models, supporting data should be derived using a num-The Pajarito Plateau in northern New Mexico, on which the Los ber of observational techniques and include a variety Alamos National Laboratory is situated, is characterized by a thick of data types. vadose zone overlying the regional aquifer of the western Espanola Basin. In this study, conceptual models of vadose zone flow and Los Alamos National Laboratory (the Laboratory or transport processes are presented and then supported through the LANL; Fig. 1) has performed research and development interpretation of field data, including synthesis with numerical models. in nuclear weapons technologies and other national de-The conceptual models differentiate the rate of percolation by their fense activities for more than 60 yr, beginning with the location and surface hydrologic setting, including wet and dry canyons, Manhattan Project in the 1940s. During this time, Laboand wet, dry, and disturbed mesas. Net infiltration beneath wet canratory operations have been accompanied by both disyons is the highest, with rates on the order of a meter per year posal of and intentional or accidental releases of chemi-(100-1000 mm yr Ϫ1 ). Transport to the regional aquifer beneath the cal contaminants into the environment at a variety of wettest canyons is likely on the order of several years to several sites. Contaminants with possible negative impacts to decades, depending on the thicknesses of the various hydrostratigroundwater include high explosives, radionuclides, chemgraphic layers. Perched water is sometimes found beneath wetter canyons and is associated with near-surface alluvial systems and at ical solvents, and metals. Today, the Laboratory is reintermediate depths along low-permeability interfaces such as buried sponsible for ensuring that none of its past contaminant soils or unfractured regions of basalt flows. Percolation through the releases pose a threat to human health now or in the volcanic tuffs is generally considered to be via matrix-dominated flow, future, and to carry out remediation activities to clean whereas fracture flow may play a key role in contaminant transport up contaminated sites. One of the key potential risks is through densely welded tuffs or basalt units beneath wet canyons.groundwater contamination, possibly affecting drinking Infiltration beneath dry canyons and dry mesas is much slower (10 water quality in municipal or private wells. Contaminants mm yr Ϫ1 or less), yielding transport times to the aquifer of hundreds must travel through a thick vadose zone to reach the to several thousands of years. However, long-term surface disturregional aquifer. Therefore, a well-developed conceptual bances at mesa-top locations may alter infiltration rates such that model describing vadose zone flow and transport beneath at a local scale, the infiltration rates temporarily approach those of wetter canyons.

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

“…Perched-water zones at much greater depths affect flow and contaminant transport at a large number of arid U.S. Department of Energy (DOE) sites, such as Yucca Mountain (Wu et al 1999(Wu et al , 2004, the Idaho National Engineering and Environmental Laboratory (INEEL; Nimmo et al 2002Nimmo et al , 2004Duke et al 2004;Wang et al 2010), the Pantex Plant in Amarillo, Texas (Adam et al 2004), and the Los Alamos National Laboratory (LANL; Birdsell et al 2004;Robinson et al 2005Robinson et al , 2011.…”

“…An extensive set of observations made in deep wells makes this plateau one of the most studied vadose zones among research efforts targeted at understanding mechanisms that give rise to perched water and the hydraulics of perched-water zones (e.g., Birdsell et al 2004;Kwicklis et al 2005;Robinson et al 2011). A large number of perched-water zones at LANL are located below wet canyons, suggesting that in addition to perching horizons, locally high percolation rates with values up to 1 m/yr are required to yield saturated conditions (Kwicklis et al 2005).…”

Estimating travel time of recharge water through a deep vadose zone using a transfer function model

“…In undulating landscapes, however, where the size of the vadose zone will generally be much larger, higher travel time is expected. In arid environments, for instance, the vadose zone can be hundreds of meters thick, and infiltration fluxes very low [10], resulting in the residence time of water ranging from several hundreds to thousands of years [5]. However, also in such a deep vadose zone, flow can be extremely fast.…”

Numerical Evaluation of Unsaturated‐Zone Flow and Transport Pathways at Rainier Mesa, Nevada

“…To our knowledge, detailed large‐scale, multidimensional models of variably saturated systems with perched water are relatively rare because of the computational challenges involved. Prominent examples of large‐scale, multi‐dimensional variably saturated models include the three‐dimensional (3D), site‐scale model of the unsaturated zone at Yucca Mountain, which was under consideration as a high‐level radioactive waste repository (Liu et al, 2003; Wu et al, 2006), and models of moisture and contaminant movement beneath wet canyons at the Los Alamos site in New Mexico (Birdsell et al, 2005; Robinson et al, 2005). In both instances, modeling demonstrated that significant lateral redistribution of infiltration in the unsaturated zone occurs along low‐permeability perching layers.…”

“…The analysis of flow and transport pathways near and through perched water at Rainier Mesa can help clarify flow and transport behavior at other existing and potential DOE waste‐storage sites in the arid west where the presence of deep perched water complicates the understanding and monitoring of radionuclide transport. These sites include (i) the Radioactive Waste Management Complex at the Idaho National Laboratory, where lateral flow has been observed in the sedimentary interbeds between lava flows (Duke et al, 2007); (ii) Yucca Mountain, a proposed site for high‐level nuclear waste disposal in volcanic tuffs in southern Nevada, where the stratigraphic and structural setting promotes perching, lateral flow, and possible fault drainage beneath the proposed repository (Bagtzoglou, 2003a, 2003b; Flint et al, 2001, 2002); and (iii) the Los Alamos National Laboratory (LANL) site in northern New Mexico, where perched water above low‐permeability tuff layers beneath wet canyons also creates the potential for lateral flow and complex vadose zone transport paths (Robinson et al, 2005; Birdsell et al, 2005).…”