Impacts of Reducing Conditions in the Saturated Zone at Yucca Mountain

Abstract

Variations in groundwater redox chemistry in the saturated zone near Yucca Mountain could have significant repository to the accessible environment, This study examines geochemical data relevant to the distribution of redox impacts ON processes associated with the potential transport of redox-sensitive radionuclidesfrom the proposed conditions in the saturated zone, the relationships between redox state and solubility and sorption coefficients for technetium and neptunium, and sensitivity in transport model simulations. Results indicate evidence for a zone of reducing conditions in the volcanic rocks of the saturated zone located to the east and south of the repository and along the iiferredflow paths from the repository. A working hypothesis is that these reducing conditions are related to the presence of minor pyrite in the matrix of some volcanic units. Chemical equilibrium modeling of technetium solubility using EQ3/6 software codes is used to estimate the value of solubility limits as a function of Eh. Surface complexation tnodeling with the EQ3 code is used to estimate neptunium sorption coefficient values as a function of Eh. A general analytical approach, one-dimensional reactive transport modeling, and the three-dimensional saturated zone site-scale transport model using the FEHMsoftware code are used to evaluate the impacts of solubility limits and enhanced , sorption in reducing zones on the simulated transport of technetium and neptunium in the saturated zone. Results show that ifprecipitation occurs in response to flow through a reducing zone, then the peak concentration released to the accessible environment will be restricted to the solubility limit. Simulations also show that enhanced sorption within a reducing zone of modest width leads to significantly greater retardation of radionuclides in the saturated zone.

Assessing field-scale migration of radionuclides at the Nevada Test Site: “mobile” species

“…Tc(IV) species become associated with solid phases through either hydrolysis or formation of strong surface complexes with Al and Fe oxides and clays (Burke et al, 2005) The solubility of TcO 2 $nH 2 O(s) in carbonate-containing groundwater was reported to be about 10 À8 M (Eriksen et al, 1992). Similarly, chemical equilibrium modeling of groundwater at Yucca Mountain indicated a maximum Tc solubility of 4 Â 10 À9 M under reducing condition (Arnold et al, 2006). Experiments conducted on sediment-groundwater samples in Germany showed dramatically different sorption coefficients for Tc among aerobic and anaerobic conditions (Lieser and Bauscher, 1987).…”

“…Meijer and coworkers examined variations in the redox potential in groundwaters pumped from 20 boreholes in the saturated zone along potential transport pathways from Yucca Mountain (BSC, 2004;Arnold et al, 2006). At least three borehole volumes of groundwater were purged prior to sampling.…”

“…Numerical simulations showed the enhanced sorption within a reducing zone of modest width, and associated larger K d values, leaded to considerable retardation of redox-sensitive radionuclides ( 99 Tc) in the saturated zone (Arnold et al, 2006). The understanding of field-scale behavior of radionuclides, such as 99 Tc, obtained from the NTS data can be used to examine their transport behavior, and reduce the conservatism in input parameters adopted in the TSPA analyses for Yucca Mountain repository.…”

“…These values are based on the assumption that Tc and I exist as mobile anionic species [pertechnetate (TcO 4 À ) and I À ] in Yucca Mountain groundwaters and during experimental measurements of K d values. However, both Tc and I are redox-sensitive radionuclides, and their speciation and retardation is sensitive to modest changes in redox conditions (e.g., Lieser and Bauscher, 1987;Arnold et al, 2006). By varying the redox potential, Lieser and Bauscher (1987) observed a change in the K d value of about three orders of magnitude over a small range of Eh at 170 AE 60 mV and a pH of 7 AE 0.5.…”

The effect of high ionic strength on neptunium (V) adsorption to a halophilic bacterium

“…Further, bacteria have been shown to reduce neptunium (V) to neptunium (IV) in aqueous systems in some instances (Lloyd et al, 2000;Rittmann et al, 2002;Gorman-Lewis et al 2005a;Icopini et al, 2007;Law et al, 2010). The solubility of neptunium (V) is generally considerably higher than that of neptunium (IV) resulting in the potential for enhanced aqueous mobility (Bondietti and Francis, 1977;Lemire et al, 2001;Arnold et al, 2006). Aqueous neptunium (V) commonly exists as the highly stable neptunyl species at pH less than approximately 8; at higher pH, when carbonate is present, carbonate complexed species become increasingly important (e.g., Maya, 1983;Neck et al, 1994;Kaszuba and Runde, 1999), hydrolysis may become important at pH greater than 10 in low carbonate systems (e.g., Lierse et al, 1985;Neck et al, 1992), mixed hydroxocarbonato species may also exist at pH greater than 11 (Neck et al, 1997).…”