RANGE AND DISTRIBUTION OF TECHNETIUM KD VALUES IN THE SRS SUBSURFACE ENVIRONMENT

Performance assessments (PAs) are risk calculations used to estimate the amount of low-level radioactive waste that can be disposed at DOE sites. Distribution coefficients (K{sub d} values) are input parameters used in PA calculations to provide a measure of radionuclide sorption to sediment; the greater the K{sub d} value, the greater the sorption and the slower the estimated movement of the radionuclide through sediment. Understanding and quantifying K{sub d} value variability is important for estimating the uncertainty of PA calculations. Without this information, it is necessary to make overly conservative estimates about the possible limits of K{sub d} values, which in turn may increase disposal costs. Finally, technetium is commonly found to be amongst the radionuclides posing potential risk at waste disposal locations because it is believed to be highly mobile in its anionic form (pertechnetate, TcO{sub 4}{sup -}), it exists in relatively high concentrations in SRS waste, and it has a long half-life (213,000 years). The objectives of this laboratory study were to determine under SRS environmental conditions: (1) whether and to what extent TcO{sub 4}{sup -} sorbs to sediments, (2) the range of Tc K{sub d} values, (3) the distribution (normal or log-normal) of Tc K{sub d} values, more » and (4) how strongly Tc sorbs to SRS sediments through desorption experiments. Objective 3, to identify the Tc K{sub d} distribution is important because it provides a statistical description that influences stochastic modeling of estimated risk. The approach taken was to collect 26 sediments from a non-radioactive containing sediment core collected from E-Area, measure Tc K{sub d} values and then perform statistical analysis to describe the measured Tc K{sub d} values. The mean K{sub d} value was 3.4 {+-} 0.5 mL/g and ranged from -2.9 to 11.2 mL/g. The data did not have a Normal distribution (as defined by the Shapiro-Wilk's Statistic) and had a 95-percentile range of 2.4 to 4.4 mL/g. The E-Area subsurface is subdivided into three hydrostratigraphic layers: Upper Vadose Zone (11 to 30 ft depth), Lower Vadose Zone (30 to 51 ft depth), and aquifer (51 to 95 ft depth). The Upper Vadose Zone generally contains more clay than the Lower Vadose Zone, and the Aquifer tends to be made up of mostly sand layers with clay strata. The mean K{sub d} values of each of these zones did not differ significantly and the K{sub d} values from each zone were not from the Normal distribution. The ranges of values were greatest in the Upper Vadose Zone and least in the Lower Vadose Zone. Previous Best Estimate Tc K{sub d} values for Sandy Sediment and Clayey Sediment were 0.1 and 0.2 mL/g, respectively (Kaplan 2007a). A more thorough review indicates that the Best Estimates for Sandy Sediment is 0.1 mL/g and for Clayey Sediment is 0.8 mL/g (Kaplan 2007b). This current dataset greatly increases the number of Tc K{sub d} values measured with SRS sediments, but perhaps more importantly, provides a better estimate for E-Area sediments, and provides a measure of Tc K{sub d} distributions. Based on this dataset, the best overall Tc K{sub d} value for E-Area is the mean, 3.4 mL/g, with a log-normal distribution between the 95 percentile values of 2.4 to 4.4 mL/g. This document version differs from the earlier version, SRNS-STI-2008-00286, in that it includes some editorial corrections. This version does not contain any technical changes or changes to the conclusions presented in the earlier version. « less