Hydrogeologic Synthesis Report

• Hydrogeologic Studies of the Pajarito Plateau (1998 - 2004)

The Hydrogeologic Synthesis Report provides a comprehensive description of the hydrogeologic setting beneath the Pajarito Plateau and Los Alamos National Laboratory (LANL).

• Hydrogeologic and geochemical data collected from 1998 through December 2004
• 25 regional aquifer wells for hydrogeologic characterization
• Six intermediate-zone wells have been completed for hydrogeologic characterization
• Fulfilled regulatory requirements for characterization and monitoring prior to signing the 2005 Order on Consent

The Site

Los Alamos National Laboratory (LANL) is situated on the Pajarito Plateau, located within the Española Basin section of the Rio Grande Rift. The Española Basin, as well as the Pajarito Plateau on its western edge, is filled with Miocene and Pliocene-age sediments and volcanic rocks. The topographic plateau is formed by Pleistocene Bandelier Formation ash-flow tuffs from the Jemez volcanic field, which cover the basin-fill sediments.

Groundwater Setting

Groundwater occurs in three settings beneath the Pajarito Plateau: alluvial groundwater, intermediate-perched saturated zones, and the regional aquifer.

Alluvial Groundwater

Alluvial groundwater occurs to a limited and variable extent in the alluvium lining canyon bottoms. Alluvial groundwater provides pathways for LANL-derived contamination introduced into canyons to migrate to significant lateral distances and infiltrate to greater depths.

Vadose Zone

Flow and transport of water in the vadose zone varies by rock type. Most of the plateau is covered with Tshirege and Otowi Member ash-flow tuffs of the Bandelier Tuff. Unsaturated flow and transport through these tuffs occurs predominantly through the porous matrix. On the western edge of the plateau, both fracture and matrix-dominated flow can occur, depending on the degree of welding (or matrix conductivity) of the tuff. In contrast to the flow behavior in the Bandelier Tuff units, much of the vadose zone flow through the basalt units is fracture dominated. Beneath the Pajarito Plateau, perched water bodies in the vadose zone may be important components of subsurface pathways that facilitate movement of contaminated fluids from the ground surface to the water table of the regional aquifer.

Perched Water

Perched water is most often found in Puye fanglomerates, the Cerros del Rio basalt, and in units of the Bandelier Tuff.

Regional Aquifer

The regional aquifer beneath the Pajarito Plateau is part of an aquifer which extends throughout the Española Basin (an area roughly 6000 km2). This aquifer is the primary source of water for the Laboratory, the communities of Santa Fe, Española, Los Alamos, and numerous pueblos. The sources of recharge to the regional aquifer are diffuse recharge in the Sierra de los Valles and focused recharge from wet canyons on the Pajarito Plateau. Natural discharge from the regional aquifer is primarily into the Rio Grande directly or to springs that flow into the Rio Grande. The aquifer is under water-table conditions across much of the Plateau, but exhibits more confined aquifer behavior near the Rio.

Determining Future Sites for Monitoring Wells

The regional aquifer model has been applied to predict fate and transport of contaminants in the regional aquifer, in order to optimally place monitoring wells and inform risk assessment studies; and to provide guidance in prioritization of data collection activities. Armed with the understanding gained from the Hydrogeologic Workplan activities, it is now possible to develop improved groundwater monitoring strategies or conduct more cost-effective detailed studies of individual canyons where initial studies have suggested that groundwater risk may exist.