Observations and Modeling of Deep Perched Water Beneath the Pajarito Plateau

Abstract

est because (i) they represent natural groundwater resources that are protected under State law; (ii) their Geologic characterization of the Pajarito Plateau, in support of chemical and isotopic characteristics help constrain environmental investigations of potential groundwater contamination at Los Alamos National Laboratory, has provided the opportunity to groundwater transport rates through the vadose zone; examine the nature and extent of deep perched groundwater in this (iii) perching horizons may divert, slow, or stop the semiarid setting. Deep perched groundwater occurs at widely dispersed vertical migration of groundwater through the vadose locations across the Pajarito Plateau. A total of 33 perched-zone occurzone, or they may indicate the presence of a fast subrences were identified in 29 wells. The saturated thickness of perched surface pathway, depending on the characteristics of the zones is highly variable, ranging from about 1 to Ͼ122 m (Ͼ400 ft).perched zone; and (iv) they can be used as vadose zone Observations are consistent with a conceptual model of low-permemonitoring points that provide early warning of conability horizons on which infiltrating water sits. Deep perched groundtaminants approaching the regional aquifer. water is most often found beneath wet canyons, suggesting that in The vadose zone beneath the Pajarito Plateau ranges addition to perching horizons, locally high percolation rates are rein thickness from 180 m (590 ft) to more than 365 m quired to yield saturated conditions. Two conceptual models of perching systems are considered, one relatively stagnant and one more dy-(1200 ft), and it consists of Pliocene alluvial fan deposits namic. To simulate deep perched groundwater in vadose zone flow covered by thick Pleistocene ash-flow tuffs. The Pliomodels, a new method is developed that considers the interfaces becene sediments interfinger laterally with lavas from the tween hydrogeologic units to be the horizons where the saturated Jemez volcanic field to the west and the Cerros del Rio permeability is lower than either of the units above or below the intervolcanic field to the east. An in-depth description of face. A constant multiplier called the permeability reduction factor is the geology of the Pajarito Plateau is presented by Broxapplied at the interface between two hydrostratigraphic units to simuton and Vaniman (2005), who explain and illustrate the late the perching horizon. We demonstrate the method with two-dimenstratigraphic units discussed here. Perched groundwater sional numerical simulations performed for Los Alamos Canyon, repliis found within canyon-bottom alluvium and within bedcating perched saturation as observed and showing how contaminant rock units of the vadose zone. Characterization of these dispersal may be enhanced in certain perched systems compared with dispersion in the underlying zone of regional saturation.

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

“…According to Robinson et al (2005), assessment of the impact of perched water on subsurface flow is one of the most difficult aspects of vadose zone characterization and modeling because perched water interferes with vertical flow and may lead to lateral diversion of infiltrating water. Perched water may represent an important subsurface pathway for facilitating contaminant transport from the ground surface to the regional aquifer and complicating contamination problems in several ways.…”

“…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.…”

“…Perched-water conditions occur beneath the Pajarito Plateau at 33 dispersed locations in the subsurface of the LANL (Robinson et al 2005). 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).…”

Radiogenic and Stable Isotope and Hydrogeochemical Investigation of Groundwater, Pajarito Plateau and Surrounding Areas, New Mexico

“…Results of calculations for recharge elevation for the White Rock Canyon springs are consistent with the Pajarito Plateau providing a source of infiltration occurring along wet canyon bottoms (Kwicklis et al 2005, 090069;Birdsell et al 2005, 092048;Robinson et al 2005, 091682). Contaminated alluvial groundwater provides an important "line source of recharge" to perched intermediate zones and the regional aquifer beneath the Pajarito Plateau.…”

“…Figure 3-1 shows generalized expected trends in groundwater age for a conceptual model for groundwater flow within the Sierra de los Valles and beneath the Pajarito Plateau with discharge occurring at the White Rock Canyon springs. This conceptual model is based in part on previous hydrologic conceptual models presented in LANL (2005, 092028), Birdsell et al (2005, 092048), Keating et al (2005, 090039), Robinson et al (2005, 091682), and Kwicklis et al (2005, 090069).…”