Geologic Framework of a Groundwater System on the Margin of a Rift Basin, Pajarito Plateau, North‐Central New Mexico

Abstract

that includes the LANL and the adjacent communities of Los Alamos and White Rock, San Ildefonso Pueblo The Pajarito Plateau is an important source of abundant potable land west of the Rio Grande, and a northern outlying groundwater for Los Alamos National Laboratory (LANL) and the part of Bandelier National Monument (Fig. 3). Areas communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau-wide hydrogeological investiga-of the southern Pajarito Plateau in Bandelier National tion to develop conceptual models of the groundwater system as a Monument are not discussed because there are no hyframework for numerical simulations of groundwater flow. The Pajadrogeologic data from wells in this area. The northern rito Plateau is located in the western part of the Españ ola basin where plateau is not discussed because hydrogeologic data for rocks of the Jemez and Cerros del Rio volcanic fields overlie and the Santa Clara Pueblo is not available to the public. interfinger with Neogene basin-fill sedimentary rocks. The vadose Groundwater drawn from beneath the plateau is the zone is about 200 m (600 ft) thick beneath mesas on the east side of primary source of municipal and industrial water used the plateau and more than 375 m (1245 ft) thick on the west side. by LANL and by adjacent communities. Water supply Groundwater occurs as shallow groundwater in canyon-floor alluvium, wells on the plateau are 600 to 950 m (2000-3110 ft) moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone.deep (Purtymun, 1995;Koch and Rogers, 2003), andThe most productive rocks of the regional aquifer occur in a westward-they tap the western-central part of the Españ ola basin thickening wedge of coarse-grained, Miocene and Pliocene volcaregional aquifer in Miocene and Pliocene sedimentary niclastic rocks derived from the Jemez volcanic field. Eastern aquifer and volcanic rocks. Although numerous wells penetrate rocks consist of fine-grained, somewhat less productive Miocene sedithe upper part of the regional saturated zone, none fully mentary deposits derived from highland sources to the east and north.penetrate the aquifer. Intermediate and mafic lavas interbedded with the Miocene and Plio-Water quality is typically good, but the effects of cene sedimentary deposits are components of the regional aquifer LANL operations can be detected in parts of the locally. The hydrogeology of the Pajarito Plateau is probably typical groundwater system. Locally, shallow and intermediate of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their perched groundwater systems of the vadose zone conrecharge from adjacent mountainous areas.

Analyses of the PM-2 Aquifer Test Using Multiple Observation Wells

“…Hence, some question marks along the trough perimeter are shown in Figure 1 because the presence of these units in the subsurface has not been fully verified by drilling. This trough has also been described by numerous authors (e.g., Broxton and Vaniman 2005;Broxton and Reneau 1996;Purtymun and Stoker 1988). Figure 1 also shows the location of geologic cross-section A-A' that runs west-to-east across Pajarito Plateau.…”

“…This condition is also implied by the interpretative cross-section of Broxton and Vaniman (2005) in Figure 2. In addition, screen 1 in well R-20 and screens 1 and 2 in well R-32 are located above the horizontal elevation of the top of the PM-2 screen (i.e., above 5,711 ft above mean sea level).…”

“…Figure 1 also shows the location of geologic cross-section A-A' that runs west-to-east across Pajarito Plateau. This geologic section is shown in Figure 2 and is based on recent work by Broxton and Vaniman (2005). This cross-section is important because it shows a complex 5 Figure 2.…”

Mountain ecohydrology: quantifying the role of vegetation in the water balance of montane catchments

“…The geological and geomorphic history of the basin, which is underlain by rhyolitic parent material ranging in age from 1Ð13 to 0Ð13 Ma, is well characterized (e.g. Goff and Gardner, 1994;Formento-Trigilio and Pazzaglia, 1998;Broxton and Vaniman, 2005).…”

Fumarolic pipes in the Tshirege Member of the Bandelier Tuff on the Pajarito Plateau, Jemez Mountains, New Mexico

“…In the definition (Smith 1960) the Tshirege Member constitutes one cooling unit, with densely welded tuff making up the entire ash-flow sequence in proximal areas and in caldera-fill facies (e.g., Hulen and Nielson 1984;Gardner et al 1989). Building on the work of Smith and Bailey (1966), the Tshirege Member is subdivided into four principal subunits, based on reversals in welding trends in vertical sections, apparent in the sequence of outflow sheets on the Pajarito Plateau at medial to distal sites (in the sense used by Smith and Bailey 1966) from the caldera source (Broxton and Reneau 1995;Gardner et al 1999;Broxton and Vaniman 2005).The welding trend reversals indicate different rates of welding and variations in emplacement temperatures for individual ash-flows or packages of ash-flows (Smith and Bailey 1966). As these subunits have been mapped increasingly closer to the source, contacts have been refined to rely less on porosity and welding characteristics and more on pumice content, phenocrysts, and lithic fragment characteristics (e.g., Gardner et al 1999;Lewis et al 2002;Broxton and Vaniman 2005) because physical characteristics of the tuff (including the degree of welding, thickness of subunits, and post-depositional mineralization) vary with distance from the caldera source.…”