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

Abstract

• Identify the type, quantity, lateral location, and vertical extent of specific contaminants (e.g., uranium, 99 Tc, 137 Cs, and 60 Co) in the vadose zone sediments• Understand the physical processes that affect the transport of contaminants in the sediments• When practical, identify the source(s) of contamination found in the sediment samples• When practical, determine if a link can be made between the observed vadose zone contamination and any known nearby groundwater contamination.Vadose zone characterization studies have been completed for sediment samples from SST waste management areas (WMAs) A-AX, B-BX-BY, C, S-SX, T, TX-TY, and U. Results of these studies are contained in numerous reports summarized in this data package, and have generated much of the geochemistry data reported in the field investigation reports for Hanford Site SST WMAs. The focus of this data package is to summarize the most current geochemical characterization data (as of December 31, 2007) conducted on vadose zone sediments beneath the SST farms and the Integrated Disposal Facility at the Hanford Site. A review of the empirical K d model, its applicability and limitations for use at the Hanford Site, is also presented as it provides a framework for discussing K d values listed in the summaries of the characterization data for the various SST WMAs.Much has been learned during these initial investigations. Results show that mobile contaminants, such as 99 Tc and nitrate, migrate much differently in the subsurface than previously believed by researchers. Conventional thinking was that after mobile contaminants entered the subsurface, they migrated in a nearly vertical path through the soil column. However, the vadose zone sediment characterization studies indicate that the geologic layering of the vadose zone sediments has an important impact on the direction and rate of migration of the waste liquids and dissolved contaminants. Fine-grained sediment lenses have been shown to cause significant horizontal spreading of leaked fluids within the vadose zone.In the sediment column, the zone of caustic attack, due to the interactions of the sediments with the high pH tank waste, can be determined by measuring the pH of the sediment. Soil pH has therefore become one of the key parameters measured when looking for waste discharge locations in the vadose 1 Resource Conservation and Recovery Act of 1976. 1976. Public Law 94-580, as amended, 90 Stat. 2795, 42 USC 6901 et seq. iv zone. Because tank waste is generally considered caustic (in excess of 1M free hydroxide), it is quite common to find elevated soil pHs (between 8.5 and 10) in the vadose zone adjacent to the point of waste release. However, because natural minerals present in the sediment act to neutralize the elevated pH tank waste, the area exhibiting elevated soil pH is considerably smaller than the footprint that has been impacted by more mobile constituents, such as nitrate or 99 Tc.When waste solutions containing high concentrations of dissolved sodium contact the sediment, sodium...

Technetium, Iodine, and Chromium Adsorption/Desorption Kd Values for Vadose Zone Pore Water, ILAW Glass, and Cast Stone Leachates Contacting an IDF Sand Sequence

“…The vadose zone K d values for the effluents and contaminants of concern to this study ( 99 Tc, 129 I, and Cr(VI)) are summarized in Table 2.11. Cantrell et al [2007] a) 0.04 0.1 0.04 to 0.16 Near Field/Cementitious Secondary Wastes -Young Concrete (pH ~ 12.5) Tc 0 0 0 to 2 Cr(VI) 0 0 0 to 2 I (a) 10 20 10 to 20 Near Field/Cementitious Secondary Wastes -Moderately Aged Concrete (pH ~ 10.5) Tc 0 0 0 to 2 Cr(VI) 0 0 0 to 2 I (a) 5 8 5 to 15 Near Field/Cementitious Secondary Wastes -Aged Concrete (pH ~ 8.5) Tc 0 0 0 to 1 Cr(VI) 0 0 0 to 1 I (a) 1 2 1 to 5 Chemically Impacted Far Field in Sand Sequence Tc 0 0 0 to 0.1 Cr(VI) 0 0 0 to 0.1 I (a) 0 0.1 0 to 0.2 Far Field in Sand Sequence with Natural Recharge (No Impact from Wastes) Tc 0 0 0 to 0.6 Cr(VI) 0 0 0 to 0.6 I (a) 0 0.25 0 to 15 Chemically Impacted Far Field in Gravel Sequence (Gravel-Corrected) Tc 0 0 0 to 0.01 Cr(VI) 0 0 0 to 0.01 I (a) 0 0.25 0 to 0.02 Far Field in Gravel Sequence (no impact from waste, Gravel-Corrected) Tc 0 0 0 to 0.06 Cr(VI) 0 0 0 to 0.06 I (a) 0 0.02 0 to 15 (a) Assumed to be applicable to both Iand IO 3 - Cantrell et al (2008) summarized laboratory characterization data on contaminant mobility specific to the IDF and SST WMAs. Excerpts from this report (pages 3.42 through 3.46), with minor edits to improve clarity, succinctly describe the current conceptual understanding of adsorption properties and contaminant migration behavior specific to the IDF.…”

Selection and Traceability of Parameters To Support Hanford-Specific RESRAD Analyses -- Fiscal Year 2008 Status Report

“…Thus, these values provide the most logical basis for Hanford-specific K d values for use with RESRAD, where data specific to the waste site are lacking. Cantrell et al (2008) and Kaplan et al (2000) using gravel correction factors calculated for high K d contaminants (best estimate K d > 10 for sand-dominated sediment) and low K d contaminants (K d < 10) using Equations (6.1) and (6.2), respectively. The estimated gravel fraction for the various soil classes was based on the mean weight-percent gravel provided by Last et al (2006, Table 4.5).…”

“…Best value and ranges of K d values were provided and are intended to be used to estimate the fate and transport of contaminants and their availability for plant and animal uptake in selected non-groundwater scenarios included in Hanford Site environmental impact statements, risk assessments, and specific facility performance assessments. Cantrell et al (2008) summarized the best-estimate K d values (as well as their range) for key contaminants at each of the single-shell tank waste management areas. They estimated the K d values for three different zones depending on the impacts of the waste chemistry (high impact, intermediate impact, no impact) and on the dominant sediment type (sand, silt, or carbonate dominated).…”

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

“…The list reported herein has been shortened to those contaminants that have been measured frequently in the groundwater below the B-Complex and are present over a significant enough area to merit being described as a "plume." Using known reviews of contaminant interactions with sediments (see, for example, EPA 1999a(see, for example, EPA , 1999b(see, for example, EPA , and 2004Cantrell et al 2003Cantrell et al , 2008, many potential contaminants of concern listed in Table 1-7 of WMP-28945 Rev. 1 (Thomas 2008) are found solely in the shallow sediments below the inactive disposal sites and are not expected to reach the groundwater in significant concentrations.…”