LA-UR-00-5923 December 2000 ER2000-XXXX A Department of Energy Environmental Cleanup Program Modeling Transport in Los Alamos Canyon: Effects of Hypothetical Increased Infiltration after the Cerro Grande Fire Los Alamos Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the United States N A T I O N A L Department of Energy under contract W-7405-ENG-36. L A B O R A T O R Y Los Alamos, NM 87545 Produced by EES-5, Geoanalysis Authors: P. Stauffer, B. Robinson, K. Birdsell Illustrators: P. Stauffer, M. Witkowski, FIMAD Grid Generation: C. Gable, M. Witkowski This report was prepared as an account of work sponsored by an agency of the United States Government. 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Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its tech- Table of Contents 1.0 Introduction 2.0 Site Description 2.1 Location 2.2 Stratigraphy 2.3 Contaminants of Concern 2.4 Conceptual model 2.5 Hydrogeologic data 2.6 Transport properties 3.0 Numerical Model: Groundwater flow in Los Alamos Canyon 3.1 FEHM 3.2 Model domain and computational grid 3.3 Boundary and initial conditions 3.4 Numerical formulation used to simulate perched water 3.5 Hypothetical region of contamination: Initial tracer distribution 4.0 Results 4.1 Summary of the Base simulation 4.1.1 Dispersive effects 4.2 Increased infiltration scenarios 4.2.1 Changes to saturation cause by increased infiltration 4.2.2 Transport of a conservative tracer 4.2.3 Transport of a non-conservative tracer 5.0 Conclusions 6.0 Acknowledgements 6.0 References ER2000-XXXX iii DRAFT September 26, 2001 List of Figures 1-1 Cerro Grande Fire intensity map. 1-2 High intensity fire damage displayed in Los Alamos Canyon 2-1 Location of Los Alamos Canyon with respect to the Laboratory and the towns of Los Alamos and White Rock 2-2 Geographical information for Los Alamos Canyon and the surrounding area. 2-3 Geologic framework model for the Los Alamos Canyon model study area 2-4 Simplified site stratigraphy. 2-5 Schematic diagram of the conceptual model for flow and transport in the vadose zone of the Pajarito Plateau. 3-1 Map view of the computational grid. 3-2 Cross-section of model stratigraphy. 3-3 Location of hypothetical tracer input. 4-1 Base simulation saturation profile on two cross-sections. 4-2 Base simulation conservative tracer concentration after 100 years. 4-3 Base simulation conservative tracer concentration as a function of time. 4-4 Base simulation non-conservative tracer concentration as a function of time. 4-5 Effects of dispersion on tracer transport to the water table. 4-6 Location and size of A) The small pond and B) The medium pond 4-7 Total mass flow rate to the water table for infiltration Cases 1-9. 4-8 Saturation as a function of time at 50 m depth for infiltration Cases 1-9. 4-9 Conservative tracer concentration as a function of time at the source, Cases 1-9. 4-10 Conservative tracer concentration as a function of time at 30 m, Cases 1-9. 4-11 Conservative tracer movement to the water table as a function of time, Cases 1-9. 4-12 Non-conservative tracer concentration with time at the surface. 4-13 Non-conservative tracer concentration with time, 30 m below the surface. 4-14 Non-conservative tracer concentration with time, 50 m below the surface. List of Tables 1 Stratigraphic nomenclature of the Pajarito Plateau. 2 Physical parameters used in the site model (permeability and porosity). 3 Physical parameters used in the site model (van Genuchten parameters). 4 Summary of values used in the 12 infiltration scenarios. ER2000-XXXX iv DRAFT September 26, 2001 Post Cerro Grande Transport Modeling in Los Alamos Canyon 1.0 - Introduction The Cerro Grande fire swept through the steep canyons and over the tree covered mesas of Los Alamos county during May 2000 with devastating effects. Large portions of the watersheds above the town of Los Alamos were radically altered by the fire. Figure 1-1 shows fire intensity, N High intensity Medium intensity Low intensity Unburned 5x vertical Figure 1-1. Cerro Grande Fire intensity map. Los Alamos National Laboratory lies within the closed black line. Di- amond Drive is the unclosed black line to the north of the Laboratory boundary. Los Alamos Canyon is highlighted by the light blue based line. a measure based on the size of the fire and the degree of burning in the layer of storage in the forest canopy. The fire intensity was high in the upper reaches of Los Alamos Canyon. Fire severity, a measure of how much heat goes into the ground, was extreme in many locations, causing the soil to vitrify and organic material to disintegrate into a fine, waxy ash (Figure 1-2). The changes to the vegetation and soil have profound implications for both surface water and groundwater. Burned watersheds shed more water faster because the soils can no longer soak up rain. For example, estimates from surface water modeling show hundred-fold increases or more ER2000-xxx 1 DRAFT September 26, 2001 Post Cerro Grande Transport Modeling in Los Alamos Canyon N Figure 1-2. High intensity fire damage on the south side of Los Alamos Canyon. The view is looking west toward the Los Alamos reservoir, seen as the green patch near the center [Photo from BAER Team, 2000]. in maximum canyon flow rates during the summer monsoons [BAER Team, 2000; Reneau, pers. com.]. Measured canyon flow rates during July 2000 thundershowers have matched or exceeded the preliminary surface flow model results, requiring updated estimates for the 50 and 100 year rainfall events [Reneau, pers. com.]. Increased flow of water to the unburned lower portions of the canyons is expected to cause more water to infiltrate into the subsurface and subsequently toward the water table. Such increased infiltration is a concern in Los Alamos county because of the historical releases of laboratory generated contaminants. Some of these contaminants are concentrated in the canyon bottom sediments, while some are found in overbank deposits created in previous flooding events [Reneau et al, 1998]. Increased run-off may cause stream channels to widen and reactivate these overbank deposits. When water is flushed through the sediments, contaminants can be transported with the groundwater toward the water table as dissolved species or bound onto colloids. This is of concern because the regional aquifer is the source for the local ER2000-xxx 2 DRAFT September 26, 2001 Post Cerro Grande Transport Modeling in Los Alamos Canyon municipal water-supply wells, which are relied upon heavily for residential and industrial applications. Another concern is the potential for natural or man-made dams to generate large-scale ponding in the canyons. A good example of this is the Diamond Drive fill near the Pueblo complex, which historically has caused ponding to depths of 10 m or more [Reneau, pers. com.]. A substantial dam (36 m) is being constructed in Pajarito canyon to protect TA-18 from potential flooding, and state law requires that this dam be drained within 4 days [Reneau, pers. com.]. The potential effects of such dams on groundwater flow and transport of contamination are not well understood. This report is designed to explore several scenarios involving increased infiltration and ponding in the canyons of Los Alamos County. We present results from a numeric model of central Los Alamos Canyon. The modeling results should be useful in helping to create sampling strategies to better characterize the true behavior of infiltration during higher surface flows. Finally, we make suggestions on ways to limit the effects of ponding on subsurface transport. 2.0 - SITE DESCRIPTION 2.1 Location Los Alamos county is located in northern New Mexico on the eastern flank of the Jemez mountains. Los Alamos National Laboratory is bounded by Bandelier National Monument, the towns of White Rock (east) and Los Alamos (north), Pueblo lands, and Santa Fe National Forest to the west (Figure 2-1). This study focuses attention on a small subset of the Laboratory, the confluence of Los Alamos and DP canyons (Figure 2-2). This area was specifically chosen because potential releases from the Omega West Reactor could combine with waste from TA-21, resulting in measurable alluvial concentrations of several contaminants of concern (COC’s). This location was also chosen because a pre-existing computational grid (Figure 2-3) designed to model flow and transport in Los Alamos Canyon [Robinson et al., 2000] could be readily adapted for the current study. Furthermore, Los Alamos Canyon is one of four canyons to receive the highest risk rating from the Burned Area Emergency Rehabilitation Team [BAER, 2000]. The risk factors examined by the BAER Team include: potential for waste migration, severity of fire in the upper ER2000-xxx 3 DRAFT September 26, 2001 Post Cerro Grande Transport Modeling in Los Alamos Canyon anyon Pueblo Canyon Los Alamos Canyon Bayo Canyon Los Alamos Canyon Pajarito Canyon Ca Sandia Canyon ñon de Valle Cedro Canyon Mortandad Canyon Cañada del Buey Water Canyon Potrillo Canyon Frijoles Canyon Potrillo Canyon Water Canyon Ancho Canyon Frijoles Canyon Chaquehui Canyon Lummis Canyon Ancho Canyon Alamo Canyon Rio Grande Capulin Canyon Figure 2-1.