Date of Award

12-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Engineering and Earth Science

Committee Chair/Advisor

Dr. Brian Powell

Committee Member

Dr. Daniel Kaplan

Committee Member

Dr. Nicole Martinez

Committee Member

Dr. Fanny Coutelot

Abstract

Considerable concentrations of cesium-137 (137Cs) and other anthropogenic radionuclides have been released into the environment at the Savannah River Site (SRS). Additionally, cesium-containing radioactive waste will be stored onsite for the foreseeable future. Therefore, the goal of this work was to identify mechanisms controlling cesium transport in surface waters and near surface sediments at SRS to further the understanding of 137Cs migration in the vadose zone and reservoirs. It was hypothesized that a multitude of factors impact the mobility of 137Cs at the SRS, but that the clay sediment fraction provides an important contribution in limiting 137Cs mobility due to its elevated cation exchange capacity (CEC) relative to other sediment types and its selectivity for 137Cs. First, we conducted field-scale transport experiments to observe the impact of competing cations on 137Cs mobility in vadose zone sediment. Through this effort, it was observed that under background conditions (pH = 5.5 and I = 0.1 mM), SRS sediment more effectively reduces 137Cs migration than when competing cations are present. A field sampling campaign and ArcGIS mapping were combined to construct Spatial maps of 137Cs concentrations, trace element concentrations, total organic matter, and sediment size distributions at Pond B. This effort aided in identifying correlations between potential factors controlling the distribution of 137Cs and assessing the present sediment 137Cs inventory, determined in this work to be 2.2±1.1×1011 Bq. Similarities between the sediment inventory developed in this work and that developed by Whicker et al., (1990) suggests that Pond B effectively immobilizes 137Cs. Furthermore, we demonstrate that 137Cs and 239Pu are controlled by different mechanisms in Pond B. Subsequent the sediment study, we conducted hydrologic monitoring at Pond B to determine the fraction of the 137Cs inventory exported annually and elucidate factors impacting 137Cs transport (e.g., storm events, discharge rate, seasons). Based on our observations, minor fractions of the 137Cs inventory are exported annually (0.027% of the total inventory). Additionally, windspeed was the parameter observed to have the strongest positive correlation with the 137Cs activity in the outflowing waters, although the water level in Pond B is thought to be a contributing factor., confirming sediment inventory measurements and elucidating factors impacting 137Cs transport. The mapping and hydrologic monitoring studies indicated that the clay-sized sediment fraction was primarily responsible for the spatial distribution in Pond B and that over the last 40 years, the sediment inventory has experienced little loss other than that from radioactive decay. This dissertation focused on the fundamental science regarding the factors that impact the mobility and distribution of 137Cs at the SRS. The understanding of the fundamental science furthered by this work has application in future risk assessments, Additionally, the determination of the Pond B sediment inventory allows this site to be used in the development of future nuclear forensics technologies (e.g., determination of detector sensitivity).

Author ORCID Identifier

0009-0008-8024-1491

Included in

Geochemistry Commons

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