Vadose Zone Hydrogeology Data Package for Hanford Assessments

Freeman, Eugene; Cantrell, Kirk J.; Fayer, M.J.; Gee, Glendon W.; Nichols, William; Bjornstad, Bruce N.; Horton, Duane G.

doi:10.2172/896355

Cited by 21 publications

(18 citation statements)

References 109 publications

(152 reference statements)

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“…The current tabulation of K d values for the different waste categories and impact zones were presented in Last et al (2006), and further modified by Cantrell et al (2007) to include Integrated Disposal Facility (IDF) waste streams and impact zones beneath the facility (Table 5.2). These values are considered most probable for each waste type and impact level.…”

Section: Generic K D Values Based On Waste Type and Impact Zonesmentioning

confidence: 99%

A Site Wide Perspective on Uranium Geochemistry at the Hanford Site

Zachara¹,

Brown²,

Christensen³

et al. 2007

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Executive SummaryUranium (U) is an important risk-driving contaminant at the Hanford Site. Over 200,000 kg have been released to the vadose zone over the course of site operations, and a number of vadose zone and groundwater plumes containing the uranyl cation [UO 2 2+ , U(VI)] have been identified. U is recognized to be of moderate-to-high mobility, conditions dependent. The site is currently making decisions on several of these plumes with long-lasting implications, and others are soon to come.Uranium is one of nature's most intriguing and chemically complex elements. The fate and transport of U(VI) has been studied over the long lifetime of the Hanford Site by various contractors, along with the Pacific Northwest National Laboratory (PNNL) and its collaborators. Significant research has more recently been contributed by the national scientific community with support from the U.S. Department of Energy's (DOE) Office of Science through its Environmental Remediation Sciences Division (ERSD). This report represents a first attempt to integrate these findings into a cohesive view of the subsurface geochemistry of U at the Hanford Site. The objective is to inform all interested Hanford parties about the in-ground inventory of U and its geochemical behavior. This report also comments on the prospects for the development of a robust generic model to more accurately forecast future U(VI) migration at different Hanford waste sites, along with further research necessary to reach this goal.To accomplish the report objectives, the environmental geochemistry of U at the Hanford Site is discussed in terms of both the vadose and saturated zone, to the extent that it is known. Hexavalent uranium [U(VI)] is the dominant valence form of U under the predominantly oxidizing subsurface conditions at the Hanford Site, and the researchers' analyses consequently emphasize this species. The nature and concentration of background U in Hanford subsurface sediments is identified to place contaminant U(VI) concentrations and behavior in perspective to the natural system. In-ground U-waste inventories are quantified and characterized with regard to source term, to the extent possible, and the most important sites from an inventory perspective are identified. The U-isotopic content of various waste streams are discussed from the perspective of waste-source tracking. The geochemical attenuation processes responsible for slowing the rate of subsurface U migration, relative to the transporting water front, are illustrated through careful consideration of both field characterization studies of existing U vadose-zone and groundwater plumes, and laboratory studies of derived contaminated and uncontaminated sediments. Both empirical and more mechanistic models of these attenuation processes are considered as well as the parameters that define attenuation magnitude. Attention is given to the behavior of contaminant U(VI) that has been in contact with Hanford sediments for extended periods (circa 10-50 years), as long contact imparts unique characte...

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Section: Generic K D Values Based On Waste Type and Impact Zonesmentioning

confidence: 99%

A Site Wide Perspective on Uranium Geochemistry at the Hanford Site

Zachara¹,

Brown²,

Christensen³

et al. 2007

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“…The conceptual model and material properties were based on previous 221-U Building assessments (Rockhold et al 2004, Zhang andXie 2005) and the Hanford vadose zone hydrogeology data package 2.2 (Last et al 2006). This section describes the modeling scenarios, model domain, and assigned model parameters used in this work.…”

Section: Methodsmentioning

confidence: 99%

Development of Waste Acceptance Criteria at 221-U Building: Initial Flow and Transport Scoping Calculations

Freedman¹,

Zhang²,

Keller³

et al. 2007

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SummaryThis report documents numerical flow and transport simulations that were performed under a contract from Fluor Hanford, Inc. to establish initial waste acceptance criteria for the potential waste streams that may be safely sequestered in the 221-U Building and similar canyon structures. Specifically, simulations were executed to identify the maximum loading of contaminant activity (without considering volume limitations) that can be emplaced within the 221-U Building with no more than 1 pCi/m 2 of contaminant exiting the bottom of the structure within a 1,000 year time period. Contaminants considered in the analysis were 3 H, 90 Sr, 99 Tc, 113 Cd, 129 I, 137 Cs, and 238 U. The initial scoping simulations were executed in one dimension to assess important processes, and then two dimensions to establish waste acceptance criteria. Two monolithic conditions were assessed: (1) a grouted canyon monolith and (2) a canyon monolith filled with sand. A three-stage approach was taken to account for different processes that may affect the amount of contaminant that can be sequestered safely in the canyon structure. In the first stage, flow and transport simulations established waste acceptance criteria based on a linear (K d ) isotherm approach. In the second stage, effects of thermal loading were examined, and the differences in waste acceptance criteria were quantified. In the third stage of modeling, precipitation/dissolution reactions were considered using reactive transport simulations, and the subsequent effect on the maximum contaminant loading was evaluated. The reactive transport modeling demonstrates a reactive transport capability that can be used for future performance predictions once site-specific data have been obtained.Results of the simulations demonstrated that contaminants that were transported with water flow and did not undergo retardation (e.g., 99 Tc and 3 H), cannot be safely sequestered in the 221-U Building using the waste stabilization approach assumed in this study. By contrast, the amount of moderately to strongly retarded (e.g., K d ≥ 1.0) contaminants such as 238 U and 113 Cd that can be safely sequestered was immense, far exceeding known inventories at the Hanford Site. For contaminants that were only slightly retarded (e.g., 0 < K d < 1.0), specific limits on waste acceptance were established, but uncertainties in the K d and half-life may introduce significant uncertainties in the estimated maximum loading capacities.The simulations demonstrated that the use of grout or sand as backfill material had little effect on establishment of waste acceptance criteria. A sensitivity analysis of the preclosure barrier recharge rate demonstrated only a small effect on the flux through the contaminant source and hence a small effect on the waste acceptance criteria. This was due to a surface gravel layer that permitted runoff to flow around the concrete structure of the 221-U Building.Heat loading had a more significant effect on the maximum loading due to increases in diffusive transport and...

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“…In principal, this approach could also be used to deal with temporal variation as well. This approach was used in the composite analysis (Last et al 2006) and is discussed in more detail in the following sections.…”

Section: Empirical K D Model and Its Applicability At The Hanford Sitementioning

confidence: 99%

“…The compilation of Hanford K d data was used to develop a generic set of compartmentalized K d values for input to the 2004 Composite Analysis, a Hanford Site wide performance assessment (Last et al 2006). These values are provided in Appendix B.…”

Section: Sources Of Available K D Data For the Hanford Sitementioning

confidence: 99%

“…For the 2004 composite analysis, six waste stream designations were used (Last et al 2006). The first four waste stream designations (Very Acidic, High Salt/Very Basic, Chelate/High Salt, and Low Salt/Near Neutral) were assigned a specific composition (Table 6.1) to better justify the selection of the K d values assigned to these waste streams.…”

Section: Sources Of Available K D Data For the Hanford Sitementioning

confidence: 99%

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Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

Cantrell¹,

Zachara²,

Dresel³

et al. 2007

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Companion reports that review other subject matter areas relevant to contaminant transport within the vadose zone beneath the SST WMAs, the IDF, and groundwater have been recently published. The specific subject areas include the geology of the SST WMAs iv contaminants released to the vadose zone exhibit a wide range of mobility behaviors. Many tank waste contaminants have been strongly retarded by adsorption and precipitation reactions (e.g., cesium, plutonium, americium, and europium), while others have remained mobile (e.g., technetium, nitrate, and selenium). Still others show variable, waste-specific behaviors (e.g., cesium [where sodium concentrations are very high], strontium, chromium, and uranium) that are closely tied to evolving composition of pore water in the sediments, and for chromium, a change in oxidation state. The temperature of in-ground tank waste has moderated by heat exchange with the subsurface sediment, and high concentrations of base (OH -) have been neutralized through reactions with sediment minerals and secondary mineral precipitation. Major geochemical features that may potentially affect the mobility of key contaminants of interest in the vadose zone include oxidation state, aqueous speciation, solubility, and adsorption reactions.Processes suspected of facilitating the far-field migration of immobile radionuclides, such as formation of stable aqueous complex formation and mobile colloids, were found to be potentially operative, but unlikely to occur in the field. An exception is the enhanced migration of cobalt-60 facilitated by the formation of highly stable aqueous-cyanide complexes. Certain fission-product oxyanions (e. • Adsorption is suppressed by large concentrations of tank waste anions (e.g., NO 3 -, OH -, and CO 3 2-)• Surface charge of clay-size minerals is negative and will therefore tend to repel anions• Unlike chromium, their less-soluble, less-mobile reduced forms are unstable in oxidizing environments.Laboratory studies conducted through PNNL's Vadose Zone Characterization Project and basic-science geochemical studies are expected to continue in partnership to provide information to further refine the conceptual model used for risk assessment modeling. Results of these studies can be used to help ascertain uncertainties associated with the refinement of the conceptual models for contaminant migration in the vadose zone beneath the single-shell tanks. Results can also provide improved parameterization, such as distribution coefficients (K d ), and mathematical constructs used by modelers to predict contaminant mobility under these conditions, and to decrease the degree of conservatism incorporated in these models.For various reasons, risk assessment models typically use various approximations to represent the conceptual model developed for the site. Adsorption is frequently approximated using empirical distribution coefficients (K d s). Although the use of empirical distribution coefficients can potentially lead to erroneous results under certain conditions where the...

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Vadose Zone Hydrogeology Data Package for Hanford Assessments

Cited by 21 publications

References 109 publications

A Site Wide Perspective on Uranium Geochemistry at the Hanford Site

A Site Wide Perspective on Uranium Geochemistry at the Hanford Site

Development of Waste Acceptance Criteria at 221-U Building: Initial Flow and Transport Scoping Calculations

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

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