Published in J. Environ. Qual. 32:1759-1763 (2003).
© 2003 ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Plant and Environment Interactions
Uptake Rates of Thorium Progeny in a Semiarid Environment
Yvonne McClellan*,a,
Robert Augustb,
James Goszc,
Steve Gannd,
Robert Parmenterc,
Martin Nelsone and
Mark Harpere
a Sandia National Laboratories, Mail Stop 0748, Albuquerque, NM 87185
b Naval Research Laboratory (NRL), Washington, DC 20375
c University of New Mexico, Department of Biology, Albuquerque, NM 87131
d NRL S&T Reserve Unit, Chattanooga, TN 37406
e U.S. Naval Academy, Annapolis, MD 21402
* Corresponding author (ymcclel{at}sandia.gov).
Received for publication November 19, 2002.
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ABSTRACT
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The release rates and transformation processes that influence the mobility, biological uptake, and transfer of radionuclides are essential to the assessment of the health effects in the food chain and ecosystem. This study examined concentrations of 232Th in both soil and vegetation at a closed military training site, Kirtland Air Force Base (KAFB), New Mexico. Brazilian sludge was intentionally introduced into the topsoil in the early 1960s to simulate nuclear weapon accidents. Soil (60) and vegetation (120) samples were collected from 1996 to 2000 and analyzed for radionuclides and progeny. High-resolution 
-ray spectroscopy was used to determine radionuclide activities. The results indicate that the thorium progeny were the predominant contaminant in soil and vegetation. Concentration ratios (CRs) were calculated based on actinium levels.
Abbreviations: CR, concentration ratio • KAFB, Kirtland Air Force Base
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INTRODUCTION
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THE MOVEMENT and subsequent off-site dispersal of radionuclides are essential factors for risk assessments and determining remedial actions at hazardous, radioactive waste sites. One potential transport path from the soil column is through the uptake and deposition of plant material to surface soils. The behavior and mobility of radionuclides in soil is a major consideration for plant uptake and is influenced by many variables (Romney et al., 1974; Poston et al., 1995). The uptake characteristics of radionuclides have been observed to vary depending on soil properties such as texture, organic matter content, bacterial action, pH, redox potential, and physicochemical speciation (Poston et al., 1995; White and Dunaway, 1975; Romney et al., 1978; Han and Lee, 1997; Salbu and Oughton, 1995).
Biological availability of uranium and plutonium has been studied extensively in reference to the human food chain and the soil-to-plant CR (Ng and Hoffman, 1980; Shaeffer and Hoffman, 1979; Ng et al., 1982; McClellan et al., 1991). Past radionuclide CRs typically have been extremely low values, ranging from 10-8 for Pu to 10-4 for 234Th (Travis and Arms, 1988; USEPA, 1985; National Information Services Corporation, 2001). In recent years, other factors shown to influence the radionuclide uptake were plant species, Kd values, chemical speciation, and the specific contaminant (Poston et al., 1995). It has been reported that the mobility of thorium is restricted in plants because of adsorption on cell wall material, and that thorium plant concentrations were typically several orders of magnitude lower than soil concentrations (CR values in annual grass species range from 0.001–0.05) (Zararsiz et al., 1997). Risk assessment, remediation, and dose reconstruction models are based on concentration ratios that in the case of thorium may be significantly underestimated. The objective of this study was to measure thorium progeny in soil and vegetation and the corresponding CRs derived from these measurements. The relative thorium uptake from soil by plants was calculated as a CR, defined as the ratio (unitless) of the plant concentration to the soil concentration.
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MATERIALS AND METHODS
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Study Site
The experimental study site is located on KAFB, Albuquerque, NM. Kirtland Air Force Base is located at the juncture of four major North American physiogeographic and biotic provinces: the Great Basin, the Rocky Mountains, the Great Plains, and the Chihuahuan Desert. The study site is located in a semiarid desert with an average annual precipitation near 20 cm. Soils in this area are typically sandy loams to gravelly sandy loams with less than a 5% slope. The dominant grass species found on KAFB include galleta (Pleuraphis jamesii Torr.), dropseed (Sporobolus spp.), grama (Bouteloua spp.), muhly (Muhlenbergia spp.), Indian ricegrass [Achnatherum hymenoides (Roem. & Schult.) Barkworth], and fluffgrass [Dasyochloa pulchella (Kunth) Willd. ex Rydb.] (IT Corporation, 1992).
The five acres (Site 5) were treated in early 1960s with Brazilian sludge, which was spread and raked into the ground to resemble the aftermath of a radioactive release from a nuclear weapons accident. These are relatively short-lived compared with the parent 232Th that achieves secular equilibrium in approximately 60 yr; the Brazilian sludge was in secular equilibrium before being used at the study site. This site was used to train personnel for potential accidents, recovery, and cleanup operations (i.e., a "Broken Arrow" scenario). The site has since served as an emergency operation instruction arena by the Interservice Nuclear Weapons School for training of U.S. Department of Defense, U.S. Department of Energy, Federal Emergency Management Agency, and other federal and state personnel.
The study began with an in-depth field investigation to identify soil contamination using geophysical, radiological, and chemical characterization activities between December 1996 and September 1998 (August et al., 1998, 1999). After site characterization was completed, soil and plant samples were collected from corresponding contaminated areas. Soil sampling was performed in late summer of 1996 through 1999 and vegetation sampling was conducted from 1998 to 1999. Control samples (soil and vegetation) were collected around KAFB (with no known history of contamination).
Soil Sampling
The geophysical survey, using a portable magnetometer sensor array and a sodium iodide (NaI) detector (Bicron Surveyor 50; Thermo Electron, Reading, UK), initially identified highly radioactive areas within the site and guided further soil-sampling activities. Sixty soil samples (60) and confirmative samples were taken via hand corer, dried at 60°C for 24 h, and analyzed using an on-site -ray spectroscopy system (Au et al., 1975). To characterize the depth of the contamination, five additional depth interval samples were taken at 8 cm (width) x 15 cm (length) to a depth of 152 cm. Standard procedures were taken to prevent cross-contamination in all experimental sampling. The site was subsequently divided into six areas (Areas 1–6) based on the extensive sampling performed.
Vegetation Samples
Vegetation samples (120 ranging from 10 to 600 g dry wt.) were collected at the soil sampling locations (0- to 15-cm depth), agitated, washed, and then placed in clean plastic zip-lock bags. Samples were dehydrated at 60°C for 24 h, then ashed by placing in a muffle furnace at 150 to 200°C for approximately two to four hours (depending on the sample size). The vegetation ashing was necessary to obtain a consistent sample matrix needed to normalize plant constituent volume and surface area differences. Vegetation samples were analyzed using a -ray spectroscopy system. The concentration ratio was calculated using the actinium concentration in vegetation and soil (30-cm upper soil layer). The CRs are defined as activity per weight unit of plant material (dry weight) divided by the activity per weight unit of dry substrate or reference material.
Radioisotope Analyses
The isotopes in the soil and vegetation samples were analyzed by a high-resolution -ray spectroscopy system, constructed with a mechanically cooled lead-shielded germanium detector (approximately 40% efficiency), associated electronics, and workstation. Procedures to prepare and count standards for counting followed the requirements of Skoog (1985). A measurement of naturally occurring background radiation was prepared and used for each sample. Calibration checks were performed with a known spiked radioactive standard, with an efficiency curve for each geometry, prepared by Isotope Products Laboratory specifically for this detector system. Once a week the system was recalibrated and the curves checked for drift. Radionuclide concentrations were evaluated by year and used to provide descriptive statistics and an analysis of variance (
= 0.05) to test differences (soil and vegetation) between contaminated and control sites.
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RESULTS
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The best indicators for 232Th using spectroscopy are 228Ac, 212Pb, 212Bi, and 208Tl. Actinium exhibits a number of lines distributed across the energy spectrum and is an excellent indication of thorium content. Therefore, 228Ac was used as a surrogate for 232Th. The mean 228Ac soil activity in Areas 1 through 4 and 6 ranged from 0.2 Bq/g (45- to 60-cm depth) to more than 6.5 Bq/g (0- to 15-cm depth) with trace amounts of other radionuclides (Table 1). The soil contamination was higher in the top 0- to 15-cm depth profile than all lower depths for Areas 1 through 4 and 6. Historical records and photographs were examined to determine a possible cause for increasing soil core contamination with depth in all Area 5 soil samples. Photographs from Sandia National Laboratory's Geographical Information System (circa 1964) showed a large area of disturbed soil and a pit at this location. This correlates with a subsequent exploration to approximately 2.1 m that uncovered military debris and high radiation (228Ac > 42 Bq/g), identified at the Area 5 (see Table 1). The thorium progeny (212Pb, 212Bi, and 208Tl) showed elevated levels of activity in the top three soil depth profiles as well (see Table 2). Site soil samples analyzed contained thorium progeny and other isotopes well above the soil screening level and the soil action levels (USEPA, 1996). The International Atomic Energy Agency (1994) reports transfer factors for pasture grasslands of 1.1-10 (Bq/g) in temperate environments. A soil summary of the mean background is presented in Table 3.
The mean 228Ac concentrations in vegetation for both sampling years (1998 and 1999) ranged between 1.10 and 1.35 Bq/g; background concentrations ranged between 0.09 and 0.06 Bq/g, respectively (see Table 4). Individual vegetation samples (dry weight) ranged from 0.037 to 9.54 Bq/g (mean = 1.43; SD = 60) and background concentrations ranged from 0.018 to 0.185 Bq/g (mean = 0.01; SD = 1.3). Site vegetation for both years contained mean activities significantly higher (P = 0.005 and 0.01) than controls. Although radionuclide concentrations were found in all site samples (soil and vegetation), significantly higher radionuclide soil and vegetation concentrations were found at the Area 5 location.
Concentration Ratios
The CRs were calculated based on the 228Ac activities in soil and vegetation. The mean CRs for site vegetation ranged from 0.015 to more than 9 for both years, while background CRs ranged from 0.001 to 0.49. Concentration ratios in 1998 vegetation samples ranged from 0.08 to 3.2 and from 0.07 to 5.68 for 1999 vegetation samples. The data show significantly higher CRs in all plants collected on the contaminated area relative to background. The spatial distributions of representative concentration ratios and the relative soil contamination (shaded areas represent high contamination) across the site are presented in Fig. 1
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Fig. 1. Mean concentration ratios based on 228Ac measurements and representative soil concentrations for areas within Site 5.
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DISCUSSION
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The highest concentration of contamination was found in the surface soils (0–15 cm). Although some of the radionuclides were found at deeper depths, these data suggest that after 40 years the majority of contaminants were relatively immobile. The CRs derived from this study are substantially higher than those previously reported and presumably used in various risk assessments and exposure calculations. This significant difference in the measured amount of thorium progeny in vegetation suggests that previous dose and health risk assessments for thorium progeny may have been underestimated for similar environments.
Literature review indicates that a variety of mechanisms govern the uptake of radionuclides by roots, involving many interacting soil and plant factors (D'Souza and Mistry, 1970; Ng et al., 1982; Howard and Fuller, 1985; Sheppard and Evenden, 1988; Saric et al., 1995; Zararsiz et al., 1997; Bunzl and Trautmannsheimer, 1999). These results may indicate that soil concentrations may influence corresponding thorium progeny concentrations in plants. The CRs from this study were considerably higher than other CRs for thorium in two plants (1.4 x 10-3 to 2.9 x 10-3) and plant concentrations decreased with corresponding thorium concentrations in soils and also differed among soil and plant types (Zararsiz et al., 1997).
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REFERENCES
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ABSTRACT
INTRODUCTION
