Joseph S. Y. Wang

Field tests and model analyses of seepage into drift

Abstract This paper focuses on field test results and model analyses of the first set of five niche seepage tests conducted in the Exploratory Studies Facility at Yucca Mountain. The test results suggest that (1) a niche opening (short drift excavated for this study) acts as a capillary barrier; (2) a seepage threshold exists; and (3) the seepage is a fraction of the liquid released above the ceiling. Before seepage quantification, air injection and liquid release tests at two niche locations were conducted to characterize the fracture flow paths. Nearly two-order-of-magnitude changes in air permeability values were measured before and after niche excavation. The dyed liquid flow paths, together with a localized wet feature potentially associated with an ambient flow path, were mapped during dry excavation operations. After niche excavation, the seepage is quantified by the ratio of the water mass dripped into a niche to the mass released above the opening at selected borehole intervals. For the first set of five tests conducted at Niche 3650 site, the ratios range from 0% (no dripping for two tests) to 27.2%. Changes in flow path distributions and water accumulation near seepage threshold were observed on the niche ceiling. The seepage test results compare reasonably well with model results without parameter adjustments, using capillary barrier boundary condition in the niche and two-dimensional and three-dimensional conceptualizations to represent discrete fracture and fracture network for the flow paths. Model analyses of the niche tests indicate that the seepage is very sensitive to the niche boundary condition and is moderately sensitive to the heterogeneity of the fracture flow paths and to the strengths of matrix imbibition. Strong capillary strength and large storage capacity of the fracture flow paths limit the seepage. High permeability value also enhances diversion and reduces seepage for low liquid release rate.

Liquid-release tests in unsaturated fractured welded tuffs: I. Field investigations

Wetting-front movement, flow-field evolution, and drainage of fracture flow paths were evaluated within the Topopah Spring welded tuff at Yucca Mountain, Nevada. Equipment and techniques were developed for in situ quantification of formation intake rates, flow velocities, seepage rates, and volumes of fracture flow paths. Localized injections of liquid into a low-permeability zone (LPZ) and a high-permeability zone (HPZ) along a borehole were detected in two boreholes below the point of injection. For the LPZ tests, water did not seep into an excavated slot that defined the lower boundary of the test bed, and the liquid-intake rate under constant-head conditions was observed to steadily decrease by two orders of magnitude. In the HPZ, liquid-intake rates under constant-head conditions were significantly higher and did not exhibit a strong systematic decline. HPZ tests were also conducted under a range of constant-flow conditions. Slot seepage rates showed intermittent responses and the percentage of injected water recovered in the slot increased as each test progressed. A maximum of 80% of the injected water was recovered during high-rate injection tests. The flow path volumes were found to increase during the course of each HPZ test. The data collected during these field tests has provided useful information for developing an understanding of liquid flow in unsaturated fractured welded tuff. It has also provided a basis for quantitative comparisons with numerical simulations of liquid-release experiments, as demonstrated in a companion paper [J. Hydrol. 256 (2002)].

Fault-matrix interactions in nonwelded tuff of the Paintbrush Group at Yucca Mountain

To investigate the potential for fast flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility (ESF). During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. Once wetted, the matrix is able to retain the moisture over a period of months. As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.

Characterizing unsaturated diffusion in porous tuff gravel

latest developments in instrumentation and techniques. Improved understanding of unsaturated diffusion in Evaluation of solute diffusion in unsaturated porous gravel is very gravel will help in the characterization and remediation important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high-level effort in gravel deposits at the Hanford Reservation nuclear waste repository at Yucca Mountain, Nevada). For a porous (Washington). It will also help in the invert diffusion aggregate medium such as granular tuff, the total water content is barrier concept for the potential underground high-level comprised of surface water and interior water. The surface water radioactive waste repository at Yucca Mountain, Necomponent (water film around grains and pendular water between vada, where tuff gravel has been considered as an invert the grain contacts) could serve as a predominant diffusion pathway. material (material filling the bottom of a tunnel having a To investigate the extent to which surface water films and contact circular cross-section) to contain radionuclide transport. points affect solute diffusion in unsaturated gravel, we examined the The invert placed between the waste package or drip configuration of water using X-ray computed tomography (CT) in shield and the tuff host rock at Yucca Mountain is an partially saturated gravel and made quantitative measurements of difintegral component of the repository’s performance. If fusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2to 4-mm granular tuff at effective, an invert diffusion barrier (caused by slow multiple water contents were calculated from electrical conductivity radionuclide diffusion through the invert) can greatly (EC) measurements using the Nernst–Einstein equation. In the secenhance waste-isolation capacity. Conca and Wright ond, we used laser ablation with inductively coupled plasma–mass (1992) measured effective diffusion coefficients (De) in spectrometry (LA/ICP-MS) to perform microscale mapping, allowing unsaturated soil, gravel, bentonite, and whole rock for the measurement of diffusion coefficients for a mixture of chemical a wide range of volumetric water contents (this free tracers for tuff cubes and tetrahedrons having two contact geometries water content does not include interlayer water in clays (cube–cube and cube–tetrahedron). The X-ray computed tomography or other structural water; Conca and Wright, 2000). images show limited contact between grains, and this could hinder They found that De values in all media were primarily the pathways for diffusive transport. Experimental results show the a function of volumetric water content and not material critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water characteristics. CRWMS M&O (2000b) reported that content of 1.5%, the measured solute diffusion coefficient is as low this diffusion data set was well correlated in terms of a as 1.5 10 14 m2 s 1 for tuff gravel. Currently used diffusion models power-dependence (Archie’s Law type) on the volumetrelating diffusion coefficients to total volumetric water content inaderic water content, and in this study a resultant “univerquately describe unsaturated diffusion behavior in porous gravel at sal” power function was used to represent diffusive very low water contents. transport of radionuclides through the invert. However, in waste emplacement drifts, characterized by a humid environment with or without the presence of liquid S of flow and transport in gravels have rewater, crushed porous rock may provide unique characcently received attention because of the importance teristics that vary greatly from this generic power funcof gravel aquifers, the need to understand contamination (Wang et al., 2001; Hu and Wang, 2003). For examtion characterization and remediation of gravel deposits ple, Conca (1990) placed four different size fractions of in the vadose zone, and the use of gravel as capillary bartuff gravel samples (2–4, 4–6.3, 6.3–9.5, and 15–25.4 mm) riers for waste isolation. As stated in Tokunaga et al. for equilibrium inside a chamber with a nearly 100% (2003), relatively little information is available on the humidity atmosphere. After equilibrating about 70 d, unsaturated hydraulic properties of gravels; this is also all samples of individual grains were observed to be dry, true for transport processes in unsaturated gravel sysdespite the 2.7% intragranular water content. No EC tems. Conca and coworkers published pioneering work could be measured on these samples, resulting in an examining chemical diffusion behavior in porous gravinferred diffusion coefficient below 10 15 m2 s 1, which els, but this work was conducted more than 10 yr ago is the detection limit reported by Conca (1990) using (Conca, 1990; Conca and Wright, 1990, 1992). With the EC for estimating the diffusion coefficient. The inferred improved understanding of water distribution in gravel, low diffusion value of 10 15 m2 s 1 at this water content there is a strong need to investigate the diffusion prodeviates significantly from the “universal” power funccesses in unsaturated porous gravel by employing the tion with a diffusion coefficient (≈2.8 10 12 m2 s 1), which is obtained with continuous fluid introduction. In other words, at the same water content, diffusion in samQ. Hu and J.J. Roberts, 7000 East Ave., MS L-231, Lawrence Livermore National Laboratory, Livermore, CA 94550; T.J. Kneafsey, L. ples prepared using high humidity (without fluid source Tomutsa, and J.S.Y. Wang, 1 Cyclotron Road, MS 90-1116, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Received 12 Nov. Abbreviations: CT, computed tomography; EC, electrical conductiv2003. Original Research Paper. *Corresponding author (hu7@llnl.gov). ity; ICP-MS, inductively coupled plasma-mass spectrometry; LA/ICPMS, laser ablation with inductively coupled plasma-mass spectromePublished in Vadose Zone Journal 3:1425–1438 (2004).

Migration of a water pulse through fractured porous media

Contaminant transport from waste-disposal sites is strongly affected by the presence of fractures and the degree of fracture matrix interaction. Characterization of potential contaminant plumes at such sites is difficult, both experimentally and numerically. Simulations of water flow through fractured rock were performed to examine the penetration depth of a large pulse of water entering such a system. Construction water traced with lithium bromide was released during the excavation of a tunnel at Yucca Mountain, Nevada, which is located in an unsaturated fractured tuff formation. Modeling of construction-water migration is qualitatively compared with bromide-to-chloride ratio (Br/Cl) data for pore-water salts extracted from drillcores. The influences of local heterogeneities in the fracture network and variations in hydrogeologic parameters were examined by sensitivity analyses and Monte Carlo simulations. The simulation results are qualitatively consistent with the observed Br/Cl signals, although these data may only indicate a minimum penetration depth, and water may have migrated farther through the fracture network.

Field tracer transport tests in unsaturated fractured tuff

This paper presents the results of a field investigation in the unsaturated, fractured welded tuff within the Exploratory Studies Facility (ESF) at Yucca Mountain, NV. This investigation included a series of tests during which tracer-laced water was released into a high-permeability zone within a horizontal injection borehole. The tracer concentration was monitored in the seepage collected in an excavated slot about 1.6 m below the borehole. Results showed significant variability in the hydrologic response of fractures and the matrix. Analyses of the breakthrough curves suggest that flow and transport pathways are dynamic, rather than fixed, and related to liquid-release rates. Under high release rates, fractures acted as the predominant flow pathways, with limited fracture-matrix interaction. Under low release rates, fracture flow was comparatively less dominant, with a noticeable contribution from matrix flow. Observations of tracer concentrations rebounding in seepage water, following an interruption of flow, provided evidence of mass exchange between the fast-flowing fractures and slow- or non-flowing regions. The tests also showed the applicability of fluorinated benzoate tracers in situations where multiple tracers of similar physical properties are warranted.

Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye-stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer-free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer-imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel-distance and mass-distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore-size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.

Evolution of the unsaturated zone testing at Yucca Mountain

The evaluation of the Yucca Mountain site has evolved from intensive surface-based investigations in the early 1980s to current focus on testing in underground drifts. Different periods of site characterization activities and prominent issues concerning the unsaturated zone (UZ) are summarized. Data collection activities have evolved from mapping of faults and fractures to estimation of percolation through tuff layers, and to quantification of seepage into drifts. Evaluation of discrete flow paths in drifts has led to fracture-matrix interaction and matrix diffusion tests over different scales. The effects of tuff interfaces and local faults are evaluated in fractured-welded and porous-nonwelded units. Mobilization of matrix water and redistribution of moisture are measured in thermal tests. Lessons learned from underground tests are used to focus on processes needed for additional quantification. Migration through the drift shadow zone and liquid flow through faults are two important issues that have evolved from current knowledge.

A probe for measuring wetting front migration in rocks.

To facilitate investigations of flow dynamics in unsaturated fractured rocks, we have designed a probe to qualitatively monitor saturation changes in borehole environments. This flexible probe is able to fit the cylindrical shape of boreholes and requires no backfill. Field results suggest that the design of this electrical resistivity probe (ERP) is suitable for determining changes in relative wetness along boreholes in unsaturated fractured rock environments. The performance of these probes was compared with that of psychrometers under a variety of tests; these probes were found to be significantly cheaper in addition to providing data that had a greater spatial and temporal resolution.

Development of a Wet Plume Following Liquid Release along a Fault

To investigate unsaturated flow through a fault located within fractured welded tuff, we performed in situ field experiments in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of approximately 82000 L water for a period of 17 mo directly into a near-vertical fault under both constant positive head (at about 0.04 m) and decreasing fluxes. As water was released into the fault, changes in moisture content were monitored in the formation while a large cavity excavated below the test bed was visually inspected for seepage. We observed that water (introduced along the fault) maintained the fault as the primary vertical flowpath, while the adjacent fractured rock served to move water laterally and vertically. Further, unlike primary flowpaths along the fault, flow was not persistent along the secondary flowpaths. While this field experiment provided preliminary insights about the flow field associated with a fault, it also demonstrated the need to investigate the role of infill material and secondary fractures in diverting flow from gravity-driven fast flow toward flowpaths in which lateral flow may occur.