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

Abstract

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...

Numerical Modeling of 90Sr and 137Cs Transport from a Spill in the B-Cell of the 324 Building, Hanford Site 300 Area

“…Both 90 Sr and 137 Cs are cations known to adsorb strongly to Hanford sediments under most conditions (Cantrell et al 2007;Last et al 2006). According to Stumm and Morgan (1996), for most clays and oxide surfaces, the following order of sorption affinity is usually observed:…”

“…However, in this case, as with many others, limited data, data and source term uncertainties, the larger computational effort of reactive transport simulations, and fiscal constraints dictated the use of the simpler K d approach for 3-D flow and transport modeling. Last et al (2006) and Cantrell et al (2007) have compiled data packages for Hanford waste site assessments. Both reports contain tables listing recommended K d values and ranges for different waste chemistry/source categories.…”

“…Both reports contain tables listing recommended K d values and ranges for different waste chemistry/source categories. The reader is referred to Cantrell et al (2007) for details on how the recommended K d values and ranges were estimated. Table 3.7 shows a partial compilation of results from Cantrell et al (2007) for strontium and cesium.…”

“…The reader is referred to Cantrell et al (2007) for details on how the recommended K d values and ranges were estimated. Table 3.7 shows a partial compilation of results from Cantrell et al (2007) for strontium and cesium. Clearly, a wide range of K d values and sorption behavior is possible for these two species, depending on chemical conditions and sediment characteristics.…”

Conceptual Models for Migration of Key Groundwater Contaminants Through the Vadose Zone and Into the Upper Unconfined Aquifer Below the B-Complex

“…The proper use of the K d construct requires adherence to several key geochemical restrictions (see discussions in EPA 1999a; Cantrell et al 2002Cantrell et al , 2007. It is beyond the scope of this conceptual model report to instruct potential end users in the proper use of the K d construct; however, the following quote from a recent report, Last et al (2009), that assembles Hanford specific data to use in the RESRAD code to replace the generic default values is presented as a useful caveat.…”