A Fully Transient Model for Long‐Term Plutonium Transport in the Savannah River Site Vadose Zone: Root Water Uptake

Abstract

Improved understanding of flow and radionuclide transport in vadose zone sediments is fundamental to future planning involving radioactive materials. To that end, long‐term experiments were conducted at the Savannah River Site in South Carolina, where a series of lysimeters containing sources of plutonium (Pu) in different oxidation states was placed in the shallow subsurface and exposed to the environment for 2 to 11 yr. Then, sediment Pu activity concentrations were measured along vertical cores taken from the lysimeters. Results showed anomalous activity distributions below the source, with significant migration of Pu above the source. A previously developed reactive transport model with surface‐mediated redox reactions successfully simulated the lysimeter results below the source, assuming a steady, net downward flow. To simulate more realistic field conditions, a transient variably saturated flow model with root water uptake is developed and coupled to the reactive transport model. Overall, the fully transient analysis shows results nearly identical to the much simpler steady flow analysis. Thus, the surface‐mediated redox hypothesis remains consistent with the below‐source experimental data, the most important variable affecting transport being the oxidation state of the source material. However, none of the models studied could produce the upward Pu transport observed in the data. The hydraulic and chemical mechanisms tested as potential causes for upward migration—a modified root distribution, hysteresis, and air‐content dependent oxidation—yielded little or no enhancement of the upward Pu movement. This suggests another transport mechanism such as root Pu uptake and translocation in the transpiration stream.

LLNL SFA FY11 Program Management and Performance Report: Environmental Transport of Plutonium

“…Generally, existing transport models have ignored the coupled factors of redox chemistry and microbial activity and have relied on empirical equilibrium or rate-limited distribution coefficient (K d ) models to address actinide retardation and colloidfacilitated transport (e.g., Glynn, 2003;Pickett, 2005;Tien and Jen, 2007). Recent experimental data from Powell and others Powell et al, 2005;Kaplan et al, 2006;Demirkanli et al, 2008) that these simplified models cannot adequately predict Pu transport because they do not capture the coupled processes controlling Pu sorption and desorption.…”

Variability of Kd Values in Cementitious Materials and Sediments

“…A Portlandite saturated leaching solution was prepared for use in the desorption tests to simulate young/moderately aged cement pore solution (Kaplan et al 2008). DI water was used for the preparation of Ca(OH) 2 -saturated solutions.…”

“…Recently, and not reflected in the data set presented in Table 12, was a series of studies conducted in lysimeters on the SRS (Demirkanli et al, 2008(Demirkanli et al, , 2009Demirkanli et al, 2007;Kaplan et al, 2010;. These experiments reflect a higher-pedigree measurement of how Pu would behave under SRS conditions than in a shortterm laboratory batch setting.…”

Laboratory And Lysimeter Experimentation And Transport Modeling Of Neptunium And Strontium In Savannah River Site Sediments

“…The advective term is the product of the dispersivity (α L ) and the seepage velocity (v). The dispersivity was determined to be 0.3 cm based on column experiments (Demirkanli et al, 2008). The molecular dispersion term is the product of the tortuosity (ω) and the molecular diffusion coefficient (D d ).…”

“…The lysimeter sediment and aqueous chemistry most closely mirror the clayey sediment at the lower ionic strength concentration. Using this K d value of 32 L kg -1 , the average water content (θ) for the lysimeter experiments of 0.25 (Demirkanli et al, 2008), and a bulk density (ρ) of 1.6 kg m 3 and Equation (3), a retardation factor of 200 (unitless) is obtained. We will discuss this value more in Section 3.2.7.…”