Colloid and Colloid-Facilitated Radionuclide Transport at the Semi-Arid Hanford Site
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Considerable amount of radioactive waste has been released to vadose zone sediments at the Hanford site. Colloids can facilitate the movement of radionuclides through the subsurface. However, most evidence for colloid-facilitated transport of radionuclides is from indirect field observations, models, and laboratory-scale experiments. The objectives of this dissertation were: 1. To quantify transport of Eu colloids in the vadose zone at the semi-arid Hanford site. 2. To investigate whether in situ colloids can be mobilized in undisturbed, deep vadose zone sediments at the Hanford site under typical, semi-arid recharge rates. 3. To simulate long-term water flow and colloid transport with Hanford's meteorological and climatic information. For the first objective, we used four 7.6-m field lysimeters at the Hanford 300 Area. Eu- hydroxy-carbonate colloids (Eu(OH)(CO3)) were applied to the lysimeters, and the transport of the colloids through the sediments was monitored. Small amounts of Eu colloids (<1%) were detected in 2.14 m depth within a few months. While a fraction of Eu moved consistent with long-term recharge estimates, the main mass of Eu remained in the top 30 cm of the sediments. For the second objective, an undisturbed sediment core was sampled from a depth of 17-m below ground at the Hanford 200 Area, and was used as a laboratory lysimeter. The core was exposed to an infiltration rate 18 mm/year. During five years of monitoring, natural colloids like silicates, aluminosilicates, and Fe-oxides were mobilized. The amount of particles mobilized during the experiment was 1.1% of the total dispersible colloids inside the core. Comparison with weathering rates suggests that mineral weathering can be a major source of the mobilized colloids. For the third objective, we used the HYDRUS-1D model to simulate the water flow and colloid transport. The simulations showed that the long-term average water infiltration and recharge was controlled by the upper boundary's climate conditions. Infiltration peaks cor- responded to the periodic transient flow events such as thunderstorms and snowfall melts. Colloid release and mobilization was a slow, yet continuous process. The cumulative mobilized colloids during the 15 years modeling time constituted a small portion of the total available colloids (0.1%).