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John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California

Unsaturated-Zone Case Study at the Idaho National Engineering and Environmental Laboratory: Can Darcian Hydraulic Properties Predict Contaminant Migration?. John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California. Geological Society of America Denver, Colorado

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John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California

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  1. Unsaturated-Zone Case Study at the Idaho National Engineering and Environmental Laboratory: Can Darcian Hydraulic Properties Predict Contaminant Migration? John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California Geological Society of America Denver, Colorado November 9, 2004

  2. Idaho Eastern Snake River Plain INEEL Subsurface Disposal Area (SDA)

  3. Subsurface Disposal Area Fractured Basalt Interbedded with Thin Layers of Coarse To Fine Sediments 200 m to Water Table

  4. 2 km June 21-23, 1999: Apply tracer to spreading areas. 1999-2000: Sample available wells in unsaturated zone and aquifer (symbols). Big Lost River Diversion SDA

  5. Chemical Tracer • Previously applied in geothermal applications • Conservative in subsurface materials • Detectable to 0.2 ppb

  6. June 21-23, 1999: Applied 725 kg of tracer

  7. Aquifer Basalt Sediment Perched Water Subsurface Disposal Area Spreading area A-B Interbed Depth to aquifer approximately 200 meters B-C Interbed Basalt C-D Interbed Ground water mound Snake River Plain Aquifer Prevailing ground water flow direction

  8. Sampling Results SDA 1 km B-C (34 m) Detection Non-detect C-D (73 m) Detection Non-detect Aquifer (200 m) Detection Non-detect

  9. C-D and Aquifer Well Detections Aquifer (200 m depth; 0.2 km away) CD Interbed (73 m depth; 1.3 km away)

  10. Speed of Travel = 30 (± 10) m/day (7 ± 2) days • Vertical (at edge of SAB): 200 m qvertical*= 3  10-2 cm/s • Horizontal (SAA to SDA): 2.1 km qhorizontal* = 4  10-2 cm/s * Flux density for effective porosity of 0.3 = 35 (± 17) m/day (60 ± 30) days

  11. X (m) Water Content Sediment Ksat= 5.8 x 10-3 cm/s Z (m) Basalt Ksat= 1.7 cm/s Porosity= 0.33 104 days Numerical modeling by Richards’ Equation (VS2DT code)

  12. Model Sensitivity

  13. Driving Force in Fractured BasaltExample: Spreading Area A to SDA on CD Interbed 2.1 km SAA Land Surface Perched Water Well USGS-92 9.4 m Sloping Interbed Gradient: 9.4 m / 2100 m = 0.0045

  14. Horizontal Flow Along Sloping Interbeds Distance From Spreading Area (km) 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 0.00E+00 B-C Interbed, No Detection B-C Interbed, Tracer Detected -1.00E-03 C-D Interbed, No Detection C-D Interbed, Tracer Detected -2.00E-03 Average Gradient of Interbed from Spreading Area to Detection Point -3.00E-03 -4.00E-03 -5.00E-03 -6.00E-03

  15. Darcy’s law calculationExample: Spreading Area A to SDA on CD Interbed q = 4  10-2 cm/s, inferred from observation Gradient = 0.0045, based on interbed elevation data • K  9 cm/s

  16. Estimated Maximum Effective Hydraulic Conductivity

  17. Conclusions for Prediction of Long-Range Horizontal UZ Transport There is a feature of the INEEL UZ, probably associated with basalt-sediment interfaces, that conducts fast and continuous flow over km-scale distances. The INEEL UZ must have extreme anisotropy, in excess of previous estimates. A simple Darcy’s law calculation predicts tracer arrival as well as, or better than, detailed numerical modeling based on Richards’ equation.

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