Date of Award
Master of Science (MS)
Environmental Engineering and Science
Powell, Brian A
DeVol , Timothy A
Molz , Fred
Understanding how radionuclides interact in the subsurface is important for the remediation of contaminated sites, assessment of risk due to radioactive waste disposal, and designing new radioactive waste management strategies. The current understanding of the geochemical behavior of radionuclides in the subsurface and more specifically the vadose zone has been developed through reactive transport modeling supplemented by laboratory experiments. Interactions between radionuclides with the mineral particles and organic matter in the vadose zone can be very complex and while laboratory experiments produce valuable data, few controlled, intermediate scale transport studies have been performed. In order to accurately predict vadose zone behavior of radionuclides, intermediate to field scale experiments must be performed. This work is intended to provide a summation of the data collected from the first two years of the Radionuclide Field Lysimeter Experiment (RadFLEx) at the Savannah River Site (SRS) in Aiken, South Carolina. The radionuclides studied in this work were 99Tc, 60Co, 137Cs, 133Ba, 152Eu, 237Np, and 239,240Pu. Non-radioactive iodine was also studied as an analogue for its radioactive isotopes 129I and 131I. Two types of sources were chosen. The first, which was chosen in order to simulate soil contamination, was prepared by pipetting the radionuclides onto a small mass of the soil used to pack the lysimeters. The second, which was chosen to represent the release from the cementitious waste form at the SRS, was prepared by amending the radionuclides into the pre-mixture batch components of the cementitious material (Roberts et al., 2012). The sources were placed in the lysimeters at the midpoint of the soil column and packed with a sandy loam soil that is representative of the majority of the vadose zone at the site (Roberts et al., 2012). The lysimeter were then exposed to natural rainfall and environmental conditions and effluent samples were collected on a quarterly basis. The work was divided into two phases. The first included measurement of the effluent from the lysimeters using a variety of analytical and radio-analytical detection methods to monitor for any breakthrough that may have occurred. Sampling occurred on a quarterly basis, and this work includes the compilation of data from six sampling events ranging from October 2012 to February 2014. Only 60Co, 237Np, and 99Tc, and have been measured in appreciable quantities in the lysimeter effluent. Six of the lysimeters containing 60Co in the source have had appreciable breakthrough (or measurable activity in the effluent) with cumulative activities ranging from 372 - 8038 Bq (0.01 - 0.26% of the initial activity added) measured in the effluents. Breakthrough of approximately 1597 Bq (2.5% of the initial activity added) has been measured from one of the lysimeters containing 237Np (added as Np(V)). The most significant breakthrough occurred in the lysimeters containing 99Tc with a total of 1 - 12 MBq (10-58% of the initial activity added) measured in the effluent of these lysimeters. The study is designed to run for 10 years and sampling will continue for the duration. The second phase of this work included the analyses of the soil columns and sources from two of the lysimeters containing cementitious sources amended with 99Tc. The lysimeters were opened and segmented into 2.5 cm thick sections as a function of depth. Analysis of the soil columns showed both downward and upward migration of 99Tc in both of the lysimeters. There was little difference between the distribution profiles of both lysimeters as well. The Tc concentrations in each core were within one order of magnitude. Analysis of sources from the two lysimeters included the extraction of a representative number of subsamples that could be used in computational estimations of both the total activity remaining and the Tc distribution. The sources were also split in half in order to verify the conceptual model for release from cementitious materials. The combination of the visual and computational analyses provided valuable knowledge regarding this conceptual model but further development of the computational analysis component will be necessary in order to increase the accuracy of its estimations.
Witmer, Michael, "An Examination of Radionuclide Transport in the Vadose Zone Using Field Lysimeters" (2014). All Theses. 1855.