COLUMN AND BATCH TRACER TESTING TO SUPPORT RADIONUCLIDE TRANSPORT MODELING IN SATURATED ALLUVIUM NEAR YUCCA MOUNTAIN, NEVADA

Saturated column and batch tests were conducted using alluvial soil material from wells NC-EWDP-19D1 and NC-EWDP-19P, located south of Yucca Mountain within Forty Mile Wash. Tracers used in batch tests include the reactive tracer ion lithium, and the non-reactive tracer ions bromide, molybdate, and radioactive pertechnetate (Tc⁹⁹0₄^-). Two intervals from 19P were tested, 410-415 and 420-425 feet below ground surface (bgs), for comparison with well 19D1 intervals. Five intervals from 19D, distributed throughout the unconsolidated alluvium portion of the well, were batch tested (405-410, 420-425, 500-505, 580-585, and 660-665 ft bgs). Lithium K_D values for finer fractions (200 mesh and finer) of 19P and 19D were greater than for coarser fractions; overall K_D values for 19P were similar to values for 19D.

Column studies tested the material from the upper portion of 19D, from 405-410 and 420-425 ft bgs. Lithium, bromide, and pentafluorobenzoic acid (PFBA) were used as reactive and non-reactive tracers, respectively. Two or more flow rates were tested for each of two or more tracer concentrations. Typically, lithium breakthrough was observed between 4 and 8 pore volumes. Major cations (Na⁺, Ca²⁺, Mg²⁺, and K⁺) exiting the columns were analyzed for input into a reactive tracer transport model. Column and batch isotherm test results were modeled to obtain best parameter estimations from breakthrough curves, and to calculate equilibrium coefficients and incorporate cation exchange capacity data into the analysis. Inclusion of the major cation concentrations along with lithium in the reactive exchange model was found to improve model fit significantly. Replication of results between duplicate columns was excellent. Column and batch interpretations ultimately will be compared with field tracer test results to determine the effectiveness of laboratory tests in predicting operational results in the field.