3-D URANIUM FATE AND TRANSPORT MODELING IN THE NORTHEASTERN NEGEV DESERT SUBSURFACE, ISRAEL

This work is the product of a collaborative project between the US National Nuclear Security Administration (NNSA) and the Israel Atomic Energy Commission (IAEC) to assess the suitability of a potential site for subsurface radioactive waste disposal in the Negev Desert, Israel. To this end, we created a detailed 3-D probabilistic flow and transport model of the Negev Desert vadose zone informed by rock properties and uranium sorption characteristics. The model includes data from uranium sorption and desorption experiments that were performed on representative Negev Desert rock specimens. Due to the strong buffering capacity of Negev Desert carbonates studied, a linear partitioning coefficient (Kd) for uranium sorption is an adequate approximation and greatly reduces computational complexity. All samples exhibited intermediate to high uranium sorption capacities (i.e., Kd = 10 – 420 L/kg) suggesting that the 150 m thick inter-layered shallow marine sequence at a depth of 150 m to 300 m represents a natural barrier to radionuclide migration. This was confirmed in our results, using a geostatistical representation of a proposed intermediate borehole disposal within this depth range. In the model, thin, laterally discontinuous lenses capture the variability in porosity and permeability of interlayered sequences. Measured Kd values for the rock layers are input into the model for each hydrogeologic unit. Constrained by these data, we simulate a cannister breach scenario where 1000 moles of uranium migrate from the disposal borehole under different infiltration conditions that represent present day and future climate scenarios. Results show that: (1) under a moderate infiltration flux (30 mm/y), the uranium source will remain in the nearfield domain (X m) for over 1000 years; (2) for extreme precipitation conditions (210 mm/y infiltration rate), uranium migrates away from the source (Y m) in @ 200 years; and (3) for a surface ponding scenario, a uranium plume exits the nearfield after 700 years but at undetectable concentrations (< Z mg/L). Eventually, the uranium migrates to the lower phosphorite unit where it is strongly adsorbed (Kd ³ 400). Since multiple statistical realizations result in significant differences in lateral dispersion, further characterization can improve these results.