Transient groundwater chemistry near a river: Effects on U(VI) transport in laboratory column experiments

Abstract

[1] In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA, inorganic carbon and major cations, which have large impacts on U(VI) transport, change on an hourly and seasonal basis near the Columbia River. Batch and column experiments were conducted to investigate the factors controlling U(VI) adsorption/desorption by changing chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity and high Ca concentrations (Hanford groundwater) reduced adsorption and increased aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated using laboratory batch experiments accounted for the decrease in U(VI) adsorption observed with increasing (bi)carbonate concentrations and other aqueous chemical conditions. In the column experiment, alternating pulses of river and groundwater caused swings in aqueous U(VI) concentration. A multispecies multirate surface complexation reactive transport model simulated most of the major U(VI) changes in two column experiments. The modeling results also indicated that U(VI) transport in the studied sediment could be simulated by using a single kinetic rate without loss of accuracy in the simulations. Moreover, the capability of the model to predict U(VI) transport in Hanford groundwater under transient chemical conditions depends significantly on the knowledge of real-time change of local groundwater chemistry.

Updated Conceptual Model for the 300 Area Uranium Groundwater Plume

“…The leaching profile (e.g., dissolved U(VI) versus cumulative water volume) is generally asymptotic, beginning with concentrations that significantly exceed the DWS, and decreasing with volume of groundwater passage to concentrations below the DWS (Figure 5.1). This asymptotic profile has been observed for all 300 Area sediments studied to date Liu et al 2009;Yin et al 2011), indicating that resupply of the plume is expected to follow similar behavior. As is evident for this IFRC site-wide composite (<8 mm), the passage of approximately 75 pore volumes of fluid is required for released U(VI) concentrations to drop below the DWS.…”

“…IFRC investigators have developed a surface complexation model to predict the K d of 300 Area sediments (<2-mm fraction) based on measured surface area and solution composition and pH . This model reveals that adsorption from river water is three to four times stronger than that from ambient groundwater because of reduced bicarbonate concentration (see Yin et al 2011). Accordingly, K d will be used in discussions below to semiquantitatively characterize the adsorptivity of 300 Area sediments Various publications have explored the merits of the equilibrium K d , equilibrium surface complexation, and kinetic variants of these models to describe the reactive transport dynamics of the 300 Area uranium plume (Yabusaki et al 2008;Ma et al 2010).…”

“…Another important 5.2 variable in the data set is time to reach equilibration, which is observed to vary significantly and is inconsistent. The adsorption and desorption process of uranium in contaminated 300 Area sediments has been shown to be slow and kinetically controlled, requiring kinetic K d or kinetic surface complexation models (Yin et al 2011;Stoliker et al In Press 1 ) for appropriate description. This kinetic behavior is believed to result from the integrated effects of several factors: 1) a long in-ground residence time of contaminant uranium has allowed its diffusion into internal pores and fractures of lithic fragments and aggregates of variable size, requiring lengthy periods; and 2) historic waste-sediment reactions in the vadose zone beneath the former waste disposal facilities have created mineral precipitates and coatings that trap uranium in transport-limited domains.…”

Reactive transport modeling of 90Sr sorption in reactive sandpacks

“…Priebe [10] conducted column experiments also using low calcium concentrations of 20 mg L −1 , and obtained K d values comparable with our results (5000-32,000 mL g −1 ). The effect of competitive sorption involving Ca 2+ in radionuclide transport has been well studied [e.g., 1,23,24]. With increasing Ca 2+ concentration, sorption capacity for Sr 2+ tends to decrease due to competitive sorption.…”

Experimental and modeling study of pure terephthalic acid (PTA) wastewater transport in the vadose zone

“…[20][21][22] The main objective of this paper is to study the sorption and degradation behaviour of PTA wastewater with representative soil in the vadose zone, and then simulate the transport of wastewater through the vadose zone with Hydrus-1D. [20][21][22] The main objective of this paper is to study the sorption and degradation behaviour of PTA wastewater with representative soil in the vadose zone, and then simulate the transport of wastewater through the vadose zone with Hydrus-1D.…”