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Saturated Zone Anisotropy Near the C-wells Complex

Saturated Zone Anisotropy Near the C-wells Complex. November, 2003 Scott C. James Sandia National Laboratories Geohydrology Department M.J. Umari United States Geological Survey.

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Saturated Zone Anisotropy Near the C-wells Complex

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  1. Saturated Zone Anisotropy Near the C-wells Complex November, 2003 Scott C. James Sandia National Laboratories Geohydrology Department M.J. Umari United States Geological Survey This work was supported by the Yucca Mountain Site Characterization Office as part of the Civilian Radioactive Waste Program, which is managed by the U.S. Department of Energy, Yucca Mountain Site Characterization Project. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

  2. Back to Basics Tx Isotropic Anisotropic Ty Homogeneous Yucca Mountain Heterogeneous

  3. The C-wells Complex Fault Geologic Contact

  4. The Long Term Pumping Test • May 8, 1996 through November 12, 1997 (549 days). • Well UE-25 c#3 was packed around the Bullfrog Tram. • Pumped at an average rate of 9.53 liters/second. • Some intermittency due to power outages.

  5. Wells Used in the Analysis

  6. Data Collected • Although drawdown was measured in the nearest wells (UE-25 c#1 and UE-25 c#2 not shown on the previous slide), those data were not used in the anisotropy analysis because short range features (fractures or high-T connectivity) would obscure the results. • Note that wells range from 843 to 3,526 meters distant from the pumping well. • Also note that wells UE-25 ONC1 and UE-25 wt#3 are 166.3o apart. Because they are nearly 180o apart, anisotropy calculations are extremely sensitive to their locations.

  7. Raw Water Level Data – UE-25 wt#14

  8. Important Observations • Note the significant noise due to atmospheric and tidal influences. • After approximately 50 days of pumping, the well stopped responding to pumping - filtered data even suggests a gradual increase in water level. One interpretation is that the well is affected by a recharge or high-T boundary. However, there is no way to determine when this effect is superimposed on the drawdown. • There are large unexplained head oscillations toward the end of the pumping test (instrumentation?).

  9. Data Filtering • The USGS code, filter.vi, was used to reduce the noise in the water level data by removing semi-diurnal and earth tide effects. • Low-pass filter with frequency 0.8 cycles/day removed earth tides and semi-diurnal barometric pressure changes from water level data. • Semi-diurnal effects also removed from barometric record, leaving long term weather effects. • Using barometric efficiencies (0.5–0.91), long term weather effects were removed from the water level data.

  10. Filtered Data — UE-25 wt#14

  11. Filtered Data — UE-25 wt#14 • Data remain ‘noisy’ even after filtering. • After 50 days, data show a trend of increasing head that must be disregarded in the analytical solution. • The first 50 days of data yields transmissivity, T=318 m2/day, and storativity, S=0.0008. • Literature reports T between 1,300 and2,600 m2/day and S between 0.001 and 0.002 for UE-25 wt#14.

  12. SZ Anisotropy Ratio • Hydraulic tests in the SZ (pumping at C-well) demonstrate preferential flowpaths. • Anisotropy identified as a KTI. • Binomial anisotropy specified in TSPA-SR and SSPA:– 50% 1.0 (isotropic),– 50% 5.0 (5:1 in north-south direction). • Current QAed version of FEHM includes anisotropy, but principle direction is fixed along N-S (minor E-W). • Latin Hypercube Sampling will select an anisotropy ratio from the specified distribution for each unique stochastic realization (input file) supplied to FEHM.

  13. Analytical Solution • Anisotropies of 17:1 → 5:1, and 3.6:1 have been reported (wt#14: T=1,370 m2/day →17:1,T=1,330 m2/day →5:1). • The solution is extremely sensitive to the location and calculated values of T and S for each well (because two wells are nearly 180o apart). • Although preferential flow pathways/directions clearly exist, the strict definition of anisotropy is difficult to apply to fractured heterogeneous media.

  14. Hantush Technique • Assumptions:– homogeneous medium,– radial flow into the pumping well,– confined aquifer. • Transmissivities and storativities for a minimum of three wells are required (they should all be similar in value). • This information, combined with the locations of the wells, can be used to mathematically ‘contour’ the transmissivity ellipse.

  15. Theis Solution Applied to Filtered Data

  16. Theis Results

  17. Hantush Anisotropy Ellipse USW-H4 could not be used in the analysis because its transmissivity was less than half the average of the other wells. Including it yielded an undefined (negative) anisotropy ratio.

  18. Papadopulos Technique • Same assumptions as Hantush technique. • Transmissivities and storativities for a minimum of three wells are calculated. In addition, the analysis uses the average transmissivity value. • USW-H4 was included in the analysis (because of the averaging process). • Geometric interpretation of anisotropy, an ellipse is fit to the cone of depression.

  19. Two Papadopulos Solutions • First technique (constrained T):– assume all transmissivities 1000 m2/day,– calculate corresponding storativities,– perform best fit of anisotropy ellipse. • Second technique (unconstrained T):– allow transmissivities to vary, – calculate corresponding storativities, – find best fit for anisotropy ellipse.

  20. Polar Plot Using Constrained T Anisotropy ratio = 3.5 Principle angle = –79º T=1,000 m2/day S=0.001–0.005

  21. Polar Plot Using Unconstrained T Anisotropy ratio = 11 Principle angle = –35º T=700 – 1,230 m2/day S=0.001–0.005

  22. FEHM analysis • Different values of the anisotropy ratio were supplied to FEHM. • The RMSE between modeled and observed heads is calculated for each anisotropy ratio. • Although the problem is underconstrained, the minimum RMSE corresponded to anisotropy ratios between 10 and 20.

  23. FEHM results

  24. Problems • Applicability of data is questionable. • No numerical tools are QA available. • Preliminary numerical results corroborate that the data do not facilitate an anisotropy analysis. • None of the assumption used in the Hantush/Papadopulos techniques are applicable at the C-Wells Complex.

  25. Unanswered questions Assumption/interpretations/observations: • Is it reasonable to truncate data sets from wells that stop responding to pumping? • Well H4 is more than 2.5 times more distant from C#3 than is ONC1 (and nearly along a radius), why does it show a faster response to pumping? (H4 data not used in Hantush analysis) • Why does ONC1 never show the recharge/high-T effects seen in wt#14 (and later in H4) even after more than 500 days of pumping?

  26. Bottom Line • Engineering judgment suggests that assuming a triangular distribution of anisotropies will be just as reasonable/realistic as a numerically specified anisotropy distribution. • 3 part distribution:0.05–1 (10%)1–5 (50%)5–20 (40%) • FEHM analysis used to investigate the effects of large anisotropies suggests values <20.

  27. Proposed Distribution • Triangular distribution of anisotropy (10% total probability) between 1 and 0.05 decreasing to zero at 0.05 • Uniform distribution of anisotropy (50% total probability) between 1 and 5 • Triangular distribution of anisotropy (40% total probability) between 5 and 20 decreasing to zero at 20 • Principle direction: North-south

  28. Justification • Max anisotropy based on physical/geological arguments and greatest value reported (17). • Max anisotropy, 20:1, is used to specify the minimum anisotropy ratio, 1:20. • 50% probability of N-S anisotropy ratio >1 and <5 corresponds to binomial (1 and 5) distribution used in SSPA. • 40% probability of N-S anisotropy ratio >5 decreasing to zero probability at 20 based on FEHM calibration study

  29. Overview • Anisotropy will be applied to an area of the site scale model comprising more than 88 km2. • Geologic arguments suggest that the principle direction is likely 30° east of north. • Fixed principle direction in FEHM effectively increases the range of allowable anisotropy ratios (30° east of north cannot be specified until the next version of FEHM is available). • Next round of modeling will investigate the sensitivity of particle tracks to anisotropy.

  30. Thank you!

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