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

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

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

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

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

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

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

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

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

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

Numerical modeling by Richards’ Equation (VS2DT code) Water Content X (m) Z (m) Basalt K sat = 1.7 cm/s Porosity= 0.33 Sediment K sat = 5.8 x cm/s 104 days

Model Sensitivity ParameterInitial ValueModified ValueSensitivity Surficial Sediment K sat (cm/s)5.79 x x High Combination of Surficial Sediment and Basalt K sat (cm/s) 5.79 x and x and 1.7 High Basalt Porosity.23.33Low Basalt Residual Moisture Content00.1None Surficial Sediment Van Genuchten  Low Combination of Surficial Sediment and Basalt Van Genuchten  and and Low Surficial Sediment Van Genuchten n Low Combination of Surficial Sediment and Basalt Van Genuchten n 1.36 and and Low Surficial Sediment Thickness (m): 2 Cases and 0 High Ponding Depth (m)2.04.0Low

Driving Force in Fractured Basalt Example: Spreading Area A to SDA on CD Interbed Gradient: 9.4 m / 2100 m = Perched Water Sloping Interbed SAA 9.4 m Well USGS km Land Surface

Horizontal Flow Along Sloping Interbeds -6.00E E E E E E E Distance From Spreading Area (km) Average Gradient of Interbed from Spreading Area to Detection Point B-C Interbed, No Detection B-C Interbed, Tracer Detected C-D Interbed, No Detection C-D Interbed, Tracer Detected

Darcy’s law calculation Example: Spreading Area A to SDA on CD Interbed q = 4  cm/s, inferred from observation Gradient = , based on interbed elevation data  K  9 cm/s

Estimated Maximum Effective Hydraulic Conductivity Medium & SourceMethod K horiz (cm/s) 1-cm Gravel (Fayer and others, 1992) Lab measurement 0.35 INEEL UZ (this study) Darcy calculation 9 INEEL UZ (this study) RE numerical model > 1.7 INEEL UZ (Wood & Norrell, 1996) Large-Scale Infiltration Test of INEEL UZ (Magnuson & Sondrup, 1998) TETRAD calibration INEEL Sat. Zone (Anderson and others, 1999) Single-well aquifer tests 11

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.