Chlorine-36 data at Yucca Mountain: statistical tests of conceptual models for unsaturated-zone flow.
An extensive set of chlorine-36 (36Cl) data has been collected in the Exploratory Studies Facility (ESF), an 8-km-long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport in the unsaturated zone (UZ) at this site. At several locations, the measured values of 36Cl/Cl ratios for salts leached from rock samples are high enough to provide strong evidence that at least a small component of bomb-pulse 36Cl, fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, was measured, implying that some fraction of the water traveled from the ground surface through 200-300 m of unsaturated rock to the level of the ESF during the last 50 years. These data are analyzed here using a formal statistical approach based on log-linear models to evaluate alternative conceptual models for the distribution of such fast flow paths. The most significant determinant of the presence of bomb-pulse 36Cl in a sample from the welded Topopah Spring unit (TSw) is the structural setting from which the sample was collected. Our analysis generally supports the conceptual model that a fault that cuts through the nonwelded Paintbrush tuff unit (PTn) that overlies the TSw is required in order for bomb-pulse 36Cl to be transmitted to the sample depth in less than 50 years. Away from PTn-cutting faults, the ages of water samples at the ESF appear to be a strong function of the thickness of the nonwelded tuff between the ground surface and the ESF, due to slow matrix flow in that unit.
Pore Connectivity , Episodic Flow , and Unsaturated Diffusion in Fractured Tuff
We use an integrated approach consisting of experiments and complementary pore-scale network modeling to investigate the occurrence of sparsely connected pore spaces in rock matrices at Yucca Mountain, Nevada, and their implications for matrix diffusion. Imbibition results indicate that pore spaces in devitrified tuff are not well-connected, and that this lack of connectivity is further compounded by episodic flow in fractured devitrified tuff with low matrix permeability. A rigorous methodology for investigating chemical transport in fractured rock under episodic conditions, employing a suite of both sorbing and non-sorbing tracers (including radionuclides U-235, Np-237, and Pu-242), has been developed and implemented. In addition, gas diffusion and synchrotron microtomography techniques have been under development to examine the scaling issues of diffusion and pore connectivity. Preliminary results from experiments and modeling work are presented in this paper, confirming the need to reexamine our understanding of matrix diffusion and to evaluate the impact on diffusive radionuclide retardation of episodic fracture flow and low pore connectivity.
ESTIMATES OF GROUND-WATER RECHARGE RATES FOR TWO SMALL BASINS IN CENTRAL NEVADA
Estimates of ground-water recharge rates developed from hydrologic modeling studies are presented for 3-Springs and East Stewart basins, two small basins (analog sites) located in central Nevada. The analog-site studies were conducted to aid in the estimation of recharge to the paleohydrologic regime associated with ground water in the vicinity of Yucca Mountain under wetter climatic conditions. The two analog sites are located to the north and at higher elevations than Yucca Mountain, and the prevailing (current) climatic conditions at these sites is thought to be representative of the possible range of paleoclimatic conditions in the general area of Yucca Mountain during the Quaternary. Two independent modeling approaches were conducted at each of the analog sites using observed hydrologic data on precipitation, temperature, solar radiation, stream discharge, and chloride-ion water chemistry for a 6-year study period (October 1986 through September 1992). Both models quantify the hydrologic water-balance equation and yield estimates of ground-water recharge, given appropriate input data. Results of the modeling approaches support the conclusion that reasonable estimates of average-annual recharge to ground water range from about 1 to 3 centimeters per year for 3-Springs basin (the drier site), and from about 30 to 32 centimeters per year for East Stewart basin (the wetter site). The most reliable results are those derived from a reduced form of the chloride-ion model because they reflect integrated, basinwide processes in terms of only three measured variables: precipitation amount, precipitation chemistry, and streamflow chemistry.