Nuclear Energy & Waste Program
The Yucca Mountain Project: Ambient Testing: Systematic Hydrological Characterization of the Topopah Spring Lower Lithophysal Unit
The objective of these tests was to investigate the hydrological properties important to repository performance:
- Fracture permeability
- Effective porosity
- Characteristics of seepage into drift
The lower lithophysal unit has many fractures that are less than 1 meter in length, and lithophysal cavities ranging in size from 15 to 100 cm. Both the fracture and lithophysal cavity size and spacing vary significantly along the 5-meter drift walls over an 800-meter stretch within the same unit. Therefore the tests were conducted in 20-m-long boreholes at regularly spaced locations along the drift irrespective of (and unbiased to) specific features from fractures and lithophysal cavities.
Systematic Approach
A systematic approach was taken in order to gain an understanding of the "average" and the "statistical distribution" of the hydrological characteristics for the spatially heterogeneous unit.
Equipment System
Two criteria governed the design of the equipment system: automation and mobility. Because the field-scale measurements involving liquid flow in unsaturated rocks require continuous testing for periods of weeks to months, the control of test equipment was fully automated to allow the equipment system to be operated remotely with no human presence at the field site. The second criterion of mobility was achieved by designing all equipment needed for the systematic characterization as a complete unit to fit on flatbed rail carts. This design allowed the equipment system to be efficiently transported from one test station to another along the tunnel for testing in the regularly spaced low-angle boreholes.
Test Results
Hydrologic characterization of the lower lithophysal zone of the Topopah Spring welded tuff zone (Tptpll) was performed in the ECRB Cross-Drift, using the systematic approach of testing at regular intervals. Analyses of data from several sets of tests were performed in 10 zones, using four low-angle boreholes. The results indicated that:
- Small fractures (less than 1 m in length) were well connected, giving rise to air-permeability values on the order of 10−11 m2. The connected fractures probably constituted the main contribution to fast paths for liquid flow.
- About one-quarter to one-half of the existing lithophysal cavity porosity of 0.125 participates in liquid storage and fracture porosity was found to be 0.013 based on the establishment of fast paths during the tests.
- For low rate injection, some water imbibed into the rock matrix, and some water seeped into the lithophysal cavities. At higher rates, water flow primarily occured in the fractures, with little participation from the matrix or lithophysal cavities.
- Under steady-state conditions, water introduced from one to several meters above the drift flowed down toward the drift in preferential paths, not in a plume. A fraction of the water missed the drift because of nonuniform flow from fracture heterogeneity, and a fraction of the water was diverted around the drift because of capillary effects.
The systematic-testing data set was used for calibration of drift seepage models.
Reference
- Cook, P.J.; Salve, R.; Freifeld, B.M.; and Tsang, Y.T. 2003. “Measurement System for Systematic Hydrological Characterization of Unsaturated Fractured Welded Tuff in a Mined Underground Tunnel.” Ground Water, 41, (4), 449-457. Westerville, Ohio: National Ground Water Association. TIC: 211670. 165424