Surface Water–Groundwater Connection at the Los Alamos Canyon Weir Site: Part 1. Monitoring Site Installation and Tracer Tests

and canyon sediments in several canyon reaches are contaminated with low-levels of radionuclides from hisFollowing the May 2000 Cerro Grande fire at Los Alamos, NM, toric LANL operations (Reneau et al., 1998). To mitisurface water control structures were constructed near Los Alamos to mitigate the transport of contaminant-bearing sediment toward the gate potential off-site migration of these sediments, the Rio Grande river due to increased runoff caused by the removal of U.S. Army Corps of Engineers constructed a low-head vegetation by the fire. A low-head weir was constructed in Los Alamos weir (rock-and-mesh gabion) in Los Alamos Canyon Canyon, 5 km to the east of Los Alamos, to capture contaminant-bearnear the eastern boundary of LANL (Fig. 1). ing sediments and to allow runoff to pass downstream without signifiThe low-head weir was designed to restrict off-site cant ponding behind the weir. During construction of the weir, channel movement of sediments and limit ponding behind the alluvium was removed and the underlying fractured basalt was exposed. weir. However, during construction of the weir, the base To monitor any downward transport of contaminants into fractured baof Los Alamos Canyon at the construction site was salt, and potentially downward to the regional groundwater, three borestripped of sediments and alluvium, exposing highly fracholes (one vertical, and two angled) were installed for environmental tured basalt bedrock in the floor of the canyon. After monitoring. An innovative monitoring system was installed using FLUTe (Santa Fe, NM) liners for both vadose zone and perched groundwater the weir was constructed, it was hypothesized that any zones. The vertical borehole intersects several perched water zones, ponding would result in downward movement of water, and groundwater can be sampled from four ports. One angled boreand potentially contaminants, through the fractured bahole has an inflatable liner with sensors to measure relative water consalt into underlying groundwater systems. Refer to Fig. 2 tent. The second angled borehole was abandoned. Tracer tests were inifor a photograph of the weir immediately following a tiated in April 2002 and June 2003 with the application of solutions flood and ponding event in April 2001. of potassium bromide and potassium iodide, respectively, onto the In 2001, the LANL Cerro Grande Restoration Project basin floor above the weir. The hydrogeologic characterization from initiated an investigation of the potential impact of the drilling the boreholes, in conjunction with groundwater elevation and Los Alamos Canyon low-head weir on groundwater flow vadose zone moisture monitoring, and results from the tracer tests within Los Alamos Canyon. The investigation has inshow that the subsurface hydrogeology is complex, and surface water and perched groundwater systems are in apparent close communicacluded the installation of one groundwater monitoring tion. Infiltration is rapid, and movement from the surface to the deepwell, two angled vadose zone monitoring boreholes, an est perched zone (at a depth of 78 m) occurs within 8 to 14 d. Downautomated vadose and saturated zone monitoring sysward flow occurs predominantly via fracture flow. tem, subsequent groundwater and surface water monitoring, and two tracer tests. Here we focus on the monitoring site installation and the tracer tests. Refer to the I May 2000 the Cerro Grande Fire (CGF) swept companion paper (Stauffer and Stone, 2005) for results through the Sierra de los Valles and western Pajarito of numerical modeling of the first tracer test. Additional Plateau near Los Alamos, NM (Joseph, 2001). The CGF details of the monitoring system installation and monidamaged Los Alamos National Laboratory (LANL) toring results can be found in Stone and Newell (2002) property, destroyed many structures on the western and and Stone et al. (2004). northern flanks of the town of Los Alamos, and burned much of the forested watersheds on the west side of Site Description town. In the wake of the CGF, concerns grew regarding The Los Alamos Canyon low-head weir is located on the potential for greatly increased runoff from damaged the eastern flank of the Pajarito Plateau, near the LANL drainage basins resulting in high canyon flows and poeastern boundary, within Los Alamos Canyon, near the tential transport of contaminant-laden sediments off of junction of NM SR 4 and NM 502 (Fig. 1), approximately LANL property. The Los Alamos Canyon drainage sys6 km east of Los Alamos, NM. To the east of the weir, tem was one of the most severely damaged watersheds, Los Alamos Canyon passes through land belonging to San Ildefonso Pueblo before entering the Rio Grande D.G. Levitt and D.S. Wykoff, Energy and Environmental Engineering near the Otowi Bridge. Division, Apogen Technologies, Inc., 1350 Central Avenue, 3rd Floor, The Pajarito Plateau is constructed of volcanic tuffs Los Alamos, NM 87544; D.L. Newell, Department of Earth and Planof the Bandelier Tuff, erupted from the Valles Caldera etary Sciences, University of New Mexico, Albuquerque, NM 87131; to the west between 1.6 and 1.2 million years ago. Underand W.J. Stone, stoneWATER, 1090 Ranchero Rd., Bosque Farms, lying the Bandelier Tuff and the Pajarito Plateau are NM 87068. Received 29 Nov. 2004. *Corresponding author (daniel. [email protected]). the alluvial fan deposits of the Puye Formation. Interfingered with the Puye Formation on the east and southPublished in Vadose Zone Journal 4:708–717 (2005). Special Section: Los Alamos National Laboratory doi:10.2136/vzj2004.0167 Abbreviations: asl, above sea level; bgs, below ground surface; bwf, below the weir floor; CGF, Cerro Grande Fire; LANL, Los Alamos © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA National Laboratory; PVC, polyvinyl chloride. 708 Published online August 16, 2005