A Vadose Zone Flow and Transport Model for Los Alamos Canyon, Los Alamos, New Mexico

To gain an integrated understanding of flow and transport in the vadose zone and its role in transmitting conA vadose zone flow and transport model for Los Alamos Canyon tamination to the regional aquifer, conceptual flow and is presented that demonstrates that a comprehensive understanding transport models must be combined with site-specific of vadose zone hydrologic processes can be obtained by integrating data from geologic, hydrologic, and site characterization sources. The information on hydrostratigraphy, hydrologic condicomplex hydrostratigraphy of the canyon is captured using geologic tions, and spatially distributed field measurements. In characterization of extensive deep-well drilling samples, along with our study, we developed a large-scale numerical model surface mapping. A water budget study for the surface and shallow of vadose zone flow and transport in Los Alamos Cansubsurface hydrology is used to estimate spatially varying infiltration yon with the objective of advancing our understanding rates along the canyon. Three-dimensional flow model results show of vadose zone processes and behavior. In addition to that mesa-top infiltration rates on the order of 1 mm yr 1, and canyonelaborating on the site-specific details of flow and transbottom rates several orders of magnitude higher, capture the water port at this site, the present study provides insights on content profiles from available wells within the model domain. Howthe role of large-scale numerical simulation in the hydroever, a range of a factor of three higher and lower than these mean logic behavior of vadose zones in semiarid systems. values is also consistent with the data. Transient flow simulations show Los Alamos Canyon, shown in Fig. 1, is one of the most that episodic infiltration events can be effectively averaged in steadycomplex sites at the Laboratory. A number of LANL state flow modeling, but transients that last on the order of a decade or facilities have been or are currently located in or adjamore, such as climate related or anthropogenically induced transients, significantly change the predicted water content. Modeling of tritium cent to the canyon, resulting in multiple effluent and transport through the vadose zone indicates that even for a nonsorbcontaminant release locations along the canyon. Because ing contaminant, most, but not all, of the contaminant released since the canyon serves as a collector of a wide range of conin the past 40 yr should still be present in the vadose zone. The mass taminants and consists of a wide range of hydrologic conpredicted to reach the water table is primarily in locations in the ditions, it was decided that a numerical model at the scale canyon where the Bandelier Tuff is not present. Available groundof the canyon, rather than at the smaller scales typical of water surveillance data for regional aquifer water are consistent with an instrumented field site, is appropriate. Through a synthis result. thesis of available data and the development of a numerical model we examined fluid flow and contaminant transport in the vadose zone beneath Los Alamos CanL Alamos National Laboratory (LANL or the yon. Although we primarily restricted our attention to Laboratory) is performing groundwater investiflow issues, tritium transport in the vadose zone was gations as part of its Environmental Restoration and also modeled. Tritium, in the form of tritiated water, is Groundwater Protection Programs to assess the impact of an excellent tracer for groundwater, and hence it is inpast contaminant discharges on the underlying groundcluded in this modeling study as a constraint on the water system. One of the most important pathways from flow model. the points of discharge to the groundwater receptors is This paper is organized as follows. First, the model the vadose zone. In the semiarid environment in which hydrostratigraphy and development of numerical grids the Laboratory exists, a deep vadose zone separates surare described, followed by hydrologic model assumpface and alluvial groundwater from the regional aquifer tions and sources of information for property values and used to supply the local population with domestic water. boundary conditions such as the infiltration rate. Then, Birdsell et al. (2005) describe the elements of the connumerical simulation results of water content measureceptual model developed for vadose zone flow and transments are presented, followed by sensitivity studies examining the degree to which uncertain parameters can port at the site, and Robinson et al. (2005) and Stauffer be varied within the constraints provided by the data. Fiand Stone (2005) further elaborate on the flow and transnally, tritium transport results are compared with availport mechanisms that are consistent with the observed able data on the distribution of tritium and other conserfield data. vative contaminants to evaluate the ability of the model to describe the observed contaminant distributions. B.A. Robinson,* G. Cole, J. W. Carey, M. Witkowski, C.W. Gable, Z. Lu, Earth and Environmental Sciences Division Los Alamos National Laboratory, Los Alamos, NM 87545; R. Gray, Daniel B. Stephens, MODEL DEVELOPMENT Inc., Albuquerque, NM 87109. Received 30 Nov. 2004. *Corresponding author ([email protected]). Hydrostratigraphy Accurate modeling of groundwater flow and transPublished in Vadose Zone Journal 4:729–743 (2005). Special Section: Los Alamos National Laboratory port in Los Alamos Canyon requires the integration of doi:10.2136/vzj2004.0169 © Soil Science Society of America Abbreviations: FEHM, Finite Element Heat and Mass; LANL, Los Alamos National Laboratory. 677 S. Segoe Rd., Madison, WI 53711 USA 729 Published online August 16, 2005