ASSESSMENT OF URANIUM ATTENUATION REMEDY PERFORMANCE AT THE 300 AREA OF THE DEPARTMENT OF ENERGY’S HANFORD SITE

Uranium contamination exists in the 300 Area of the Hanford Site. Part of the selected remedy is enhanced attenuation of uranium over a 12,140 m² (3-acre) area by injecting phosphate solutions in the variably saturated portion of the vadose zone where uranium is present in the sediments to precipitate phosphate-bearing mineral phases and sequester uranium via multiple mechanisms.

Scale-dependent 3-D numerical models were developed to evaluate the fate and transport of uranium in a variably saturated media from in-situ phosphate treatment (conducted in 2015 and 2018) and to predict the groundwater uranium concentrations over the long term. The models incorporated residual uranium soil distribution along with distribution of lithofacies represented as equivalent homogeneous medium (for both vadose and saturated zone) and daily averaged Columbia River stage fluctuations. Uranium transport modeling was conducted using a single-site kinetic sorption-desorption parameter model based on pre- and post-injection leaching characteristics. The models were calibrated based on water-level measurements, river-groundwater mixing ratios, and uranium concentrations at monitoring wells.

The simulated average uranium concentration in selected monitoring wells compared well with the observed trends and magnitude prior to and following the phosphate solution injection indicating reasonableness of the modeling parameters. The spatial distribution of uranium concentrations outside the zone of influence of injection indicates existence of local residual uranium contamination sources where uranium leaches out from periodic rewetting. Longer-term simulations were conducted to predict future concentrations over the next 20 years (to Year 2040). The spatial plume distribution near the end of the simulation indicates overall reduction in uranium concentrations in the aquifer below the clean-up levels with localized areas of higher concentrations.