Perched water in fractured, welded tuff : mechanisms of formation and characteristics of recharge

Abstract

Perched water zones have been identified in the fractured, welded tuff in the semiarid to arid environments of Yucca Mountain, Nevada and near Superior, Arizona. An understanding of the formation of such zones is necessary in order to predict where future perched water might form at Yucca Mountain, the proposed site of a high-level nuclear waste repository. The formation or growth of a perched zone above a repository is one factor of the factors to be considered in the risk assessment of the Yucca Mountain site. The Apache Leap Research Site (ALRS) near Superior, Arizona is a natural analog to the Yucca Mountain site in terms of geology, hydrology, and climate. Perched water has been identified over an area of at least 16 km² in the Apache Leap Tuff, a mid- Miocene fractured, welded ash-flow tuff. A primary goal of this investigation was to characterize the physical and hydrologic properties of the tuff in the region above and including the perched zone, and to evaluate those characteristics to develop a model for a perching mechanism in the tuff. A second goal was to determine what fraction of water entering a watershed reaches the subsurface, to potentially recharge the perched zone. The Apache Leap Tuff has been subject to considerable devitrification and vapor phase crystallization, which dominate the character of the rock. With depth to the perched zone, pumice fragments become increasingly flattened and segregated; the pumice fragments are the primary locations of porosity in the rock, therefore porosity also becomes greatly reduced with depth, to the extent that the rock matrix is virtually impermeable at the perched water zone. Fractures are the primary pathways by which water moves through the rock; fracture hydraulic conductivity values were determined to be nine orders of magnitude greater than measured matrix hydraulic conductivity at the perched zone. An increase in fracture filling by silica mineralization beneath the perched zone reduces the secondary permeability, enhancing the formation of perched water. Thus, the primary mechanisms for the formation of the perched zone include fracture flow bringing water into the subsurface, combined with extremely low matrix hydraulic conductivity at depth, and reduced secondary permeability by filled fractures and lower fracture density. Water budgets were calculated for two years in a 51.4-ha watershed. Direct measurements were made of precipitation and runoff', evapotranspiration was both directly measured, and modeled based on measurement of a number of weather parameters. Infiltration was calculated as the residual of precipitation after runoff and evapotranspiration were removed. Infiltration was determined to be less than 10% of the annual water budget; evapotranspiration removes on the order of 90% of precipitation on an annual basis.