The Yucca Mountain Project: Ambient Testing: Drift-to-Drift (Alcove 8 – Niche 3) and Surface-to-Drift (Alcove 1) Tests
drift-to-drift tests at Alcove 8 - Niche 3 and the completed surface-to-drift tests at Alcove 1 provide data over large scales in the range of 20 to 30 m. This is the relevant scale to relate site-scale processes of infiltration and percolation with drift-scale processes of diversion and seepage. Along long flow paths, fracture-matrix interaction is shown to be an important component of transport, with the matrix contributing to delays in water and tracer movements through the unsaturated units. The data from Alcove 8 - Niche 3 fault tests were used to partly validate the UZ flow model and UZ radionuclide transport model. Data from the Alcove 1 tests were used to corroborate estimates of hydrogeologic properties.
Alcove 8 - Niche 3 Test
This test was aimed at determining matrix diffusion, seepage response and flow within the unsaturated zone at Yucca Mountain. Niche 3 (located in the main drift of the ESF) serves as the test site for collection of water and monitoring of wetting front and tracer migration from Alcove 8 (in the ECRB). Alcove 8 was excavated for liquid releases through a fault and a network of fractures.
The Niche 3 cavity was been instrumented with water collection trays and a series of boreholes surrounding the Niche have been instrumented with sensors to detect the arrival of a wetting front. Additional boreholes were drilled in Alcove 8 and Niche 3 for geophysical measurements and/or other tests to characterize tracer plume migration. Numerical modeling studies were used for designing the test, interpretation of data, calibration of rock properties and validation of numerical approaches used in the UZ models.
The near-vertical fault that intercepts the formation between Alcove 8 and Niche 3 provided a unique opportunity to evaluate two important issues:
matrix diffusion and
extent of area subject to fracture-matrix interactions.
By using cocktails of tracers with different molecular diffusion coefficients scientists at LBNL and the USGS will attempt to evaluate the importance of matrix diffusion from observations of breakthrough times, and to determine the F/M interfacial area . This information will be useful for understanding fracture fingering flow, F/M interactions and film water flow within fractures.
Infiltration rates along the fault reached quasi–steady-state conditions approximately 45 days after water was introduced to the infiltration zones, and the infiltration rates varied at different locations along the fault.
Observations of saturation changes within the fault indicated the velocity of the wetting front (vertically along the fault) was approximately 0.65 m/s.
Seepage observations indicated that quasi–steady-state conditions may have been reached two months after the initial releases into the fault.
Radar data collected in support of the Alcove-8/Niche-3 (Niche 3107) infiltration experiment suggested that this method was appropriate for investigating subsurface anomalies that may have been related to moisture migration.
Experimental results indicated that matrix diffusion had an important effect on solute transport.
An observed low seepage recovery rate from the fault implied good communication between the fault and the surrounding fracture networks.
Similar tracer arrival times (corresponding to the peak concentration values) for most flow paths suggests that macrodispersion may not be important for solute transport in unsaturated fractured rock.
The observation of the first seepage spot in Niche 3 (Niche 3107), 21 days after water was introduced along the nonfaulted section of Alcove 8, suggests that wetting-front velocity was approximately 1.0 m/day below the large plot test bed.
The relatively long flow distance of approximately 20 m between the injection plot in Alcove 8 and the ceiling of Niche 3 provides possibilities for water to be diverted through discrete geological features (such as fractures or the contact between the upper lithophysal and middle nonlithophysal zones), thus bypassing the collection system in the niche. The fact that only a portion of the released water can be accounted for leads to confidence that the physical processes that were expected to impact seepage are indeed effective. However, a lack of a mass balance between the water introduced in Alcove 8 and water arrival at Niche 3 leads to uncertainties arise in detailed interpretations.
Salve R.; Liu H.H.; Cook P.; Czarnomski A.; Hu Q.; Hudson D. 2004. Unsaturated Flow and Transport through a Fault Embedded in Fractured Welded Tuff. Water Resources Research, 40. Washington, D.C.: American Geophysical Union.
Salve R.; Ghezzehei, T.A.; Jones, R. 2008. Infiltration into Fractured Bedrock. Water Resources Research, 44. Washington, D.C.: American Geophysical Union.
Salve R.; 2005. Observations of Preferential Flow During a Liquid Release Experiment in Fractured Welded Tuffs. Water Resources Research, 41. Washington, D.C.: American Geophysical Union.
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Alcove 1 Test
Alcove 1 is located near the North Portal of the ESF in the upper lithophysal zone of the Tiva Canyon Tuff, near the Bow Ridge fault and other faults and fault intersections. The alcove is approximately 5.5 m high and 5.8 m wide. In the Alcove 1 tests, water was applied in a plot on the ground surface approximately 30 m directly above the alcove. The size of the infiltration plot was 7.9 m by 10.6 m. Irrigation drip tubing, with 490 drippers uniformly distributed within the infiltration plot, was used to apply the water.
Alcove 1 surface-to-drift testing included two phases. Phase I was a water infiltration test. LiBr tracer was added to the infiltration water during Phase II.
Water was applied for Phase 1 over a 5-month period, and was collected by a series of 1-foot-square (approximately 0.3 m by 0.3 m) drip trays inside Alcove 1. The irrigated area was 83.7 m2 and the collection area was 40.2 m2. The amount of seepage as a percentage of the water applied on the surface depends on the areas chosen for comparison and on the time of observations:
2.9 percent of the total applied water seeped into Alcove 1;
6.1 percent of the water applied vertically above the collection area seeped in Alcove 1;
5.4 percent of the total applied water seeped into Alcove 1 after the rock above Alcove 1 became wet; and
11.1 percent of the water applied vertically above the collection area seeped into Alcove 1 after the rock above Alcove 1 was wetted.
Both Phase I and Phase II data were used to interpret the test, with seepage rate data from Phase I used for calibration, and seepage and tracer data from Phase II used to test the ability of the model to make predictions. The study of LiBr tracer data shows that matrix diffusion may have a significant effect on overall transport behavior in unsaturated fractured rocks.
Liu, H-H.; Haukwa, C.B.; Ahlers, C.F.; Bodvarsson, G.S.; Flint, A.L.; and Guertal, W.B. 2003. "Modeling Flow and Transport in Unsaturated Fractured Rock: An Evaluation of the Continuum Approach." Journal of Contaminant Hydrology, 62-63, 173-188. New York, New York: Elsevier. TIC: 254205.
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Earth Sciences Division