Environ. Sci. Technol. 1982, 16, 214-217
Environmental Behavior of Technetium in Soil and Vegetation. Implications for Radiological Assessments F. Owen Hoffman*
Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830,and Graduate Program in Ecology, University of Tennessee, Knoxville, Tennessee 379 16 Charles T. Garten, Jr., and Deborah M. Lucast
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 John W. Huckabee
Electric Power Research Institute, Palo Alto, California 94303 Field studies on technetium in soil and vegetation were performed to derive parameter values used in environmental assessment models. Measured values of field soil-to-plant transfer factors were 1-2 orders of magnitude less than values obtained from previously reported laboratory potted-plant experiments. The observed dynamics of technetium in contaminated field vegetation indicates the absence of a significant long-term bioaccumulation in terrestrial food chains. The dynamic behavior of technetium appears to be the consequence of the combined effects of growth dilution, removal of technetium from vegetation, and decreasing availability to vegetation over time of the technetium in soil. These results indicate that the radiological impact of technetium releases to the environment may be substantially less than predicted by assessment models using data derived solely from laboratory experiments.
Introduction Technetium is a radioactive element of potential longterm importance in the environment because the half-life of the long-lived isotope ggTcis 210000 yr. The heptavalent oxidation state is common for technetium. In this state, technetium can be present as the pertechnetate (Tc04-) anion, which is highly soluble in water, and is thus potentially mobile in biogeochemical cycles. Although technetium is produced naturally through the spontaneous fission of uranium, the predominant sources of geTc in the environment are nuclear weapon tests and recycling of spent nuclear reactor fuels; additional quantities are also attributable to the application of 9 9 m T in~medicine (1). Major sources of current 99’c releases to the environment are gaseous diffusion facilities processing recycled reactor fuels. Releases to the atmosphere from these facilities have been reported at 18-220 GBq/yr ( 2 , 3 ) . (37 GBq is equal to 1 Ci.) The only data on the behavior of 99’c in food chains are a few recent laboratory studies that quantify the relationship between wTc in potted soil and vegetation (4-7). In these studies, the measured concentration in vegetation is divided by the measured concentration in soil to produce a quantity typically referred to as a concentration ratio (CR). The vegetation/soil CR is used in radiological assessments to predict the concentration of a radionuclide in vegetation by multiplying the CR value times an estimated concentration of the radionuclide in soil. The CR value has a direct influence on the calculated radiological dose when direct or indirect consumption of contaminated ‘Present address: Department of Zoology, The Ohio State University, Columbus, OH 43202. 214
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vegetation is the primary mode of radiation exposure and when the soil is the primary contributor of radionuclides in vegetation (8). An example of indirect consumption of vegetation is the consumption of food products derived from herbivorous animals. In the absence of experimental data, a technetium CR value of 0.25 was generically assumed in past radiological assessments (9-11). This CR value was derived by assuming a similarity between the behavior of technetium and iodine. The dry-weight equivalent of the generic CR value is 1.0, assuming that 25% of fresh-weight vegetation is dry matter. The 99Tc vegetation/soil CR values obtained from potted-plant experiments are 2-3 orders of magnitude higher than this generic CR value. In potted-plant experiments, CR values ([99Tc/(kg of dry vegetation)]/ [99Tc/(kgof dry soil)]) range from 95 to 1490 for wheat seedlings (4,12),from 67 to 380 for soybeans and wheat (5), and from 82 to 881 for cheatgrass and tumbleweed grown in pots of sand and associated subsoil (6). Till et al. (8)demonstrated that, if a CR value typical of laboratory observations were used in environmental assessments, an atmospheric release of 99Tcon the order of 67 GBq/yr could approach, if not exceed, the Uranium Fuel Cycle Standards promulgated by the US Environmental Protection Agency (13). The analysis by Till et al. assumed that an adult human was located 1.6 km downwind from the source of a 37 GBq/yr release and that the release and subsequent buildup of 99Tcin soil would be continuous over a time span of 15 yr. The calculated q c doses were almost entirely due to the ingestion of contaminated foods assumed to have been produced at the same location as the human receptor. The assumed CR value was equivalent to 200 on a dry-weight basis. Although Till et al. questioned the relevancy of laboratory data to actual field conditions, no additional source of data was available for their analysis.
Field Studies Our studies constitute the first known field investigations on the behavior of technetium in soil and vegetation. Technetium-99 was measured in mixed herbaceous vegetation (dominated by fescue grass) and soils collected near the three operating gaseous diffusion facilities in the United States for the purpose of estimating the vegetationto-soil concentration ratio (14). These facilities are located near Portsmouth, OH, Paducah, KY, and Oak Ridge, TN. Radiochemical techniques were developed by the Analytical Chemistry Division, Oak Ridge National Laboratory (ORNL), to detect 99Tcconcentrations below those expected to occur in soil and vegetation in the vicinity of these facilities. Results from radiochemical techniques were tested against the more sensitive isotope-dilution
0013-936X/82/0916-O214$01.25/0
0 1982 American Chemical Society
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mass-spectrometry (IDMS) method (15) to ensure the validity of the detected concentrations. The detection limit using radiochemical analysis was 0.03 Bq. The detection limit by isotope-dilutionmass spectrometry was 0.0006 Bq, or 1pg of 9gTc. The limited sensitivity of available analytical techniques for detecting the low-energy,@-emitting 99Tcisotope (maximum 0.292 MeV) precluded previous attempts at investigating the field behavior of %Tcin food chains. In addition to the measurement of concentrations of 99Tc in soil and vegetation in the vicinity of the operating gaseous diffusion facilities, a solution of 95mT~04was applied as simulated rainfall to field plots containing mixed herbaceous vegetation also dominated by fescue grass (Festuca sp.) and Lespedeza. Two sets of experiments were performed. Field plots, each 1 m2 in size, were located on the Department of Energy (DOE) Oak Ridge Reservation. In the first experiment, the aboveground vegetation was removed from the plots and the solution was sprayed onto the bare soil. The soil is a Captina silt loam, with pH 5.0 (in 0.01 M CaClJ, an organic-matter content of about 6-8%, and a cation-exchange capacity of 14 mequiv/(100 g of soil). Fifteen plots were each contaminated with -7 MBq of 95mT~Odon September 26,1978. Radioactivity was measured in soil and in the vegetation, which subsequently grew out of the plots of bare soil. This experiment was designed to ensure that 95mT~ in vegetation would come predominantly from the soil. In the second experiment, stands of mixed herbaceous vegetation were sprayed at various times during the year. One plot received 4 MBq on September 26, 1978, one plot received 0.4 MBq on April 25,1979, and three plots each received 0.4 MBq on July 17, 1979. This experiment resulted in a simultaneous exposure of both vegetation and soil. The radioisotope 95mT~, unlike q c , is a y-ray emitter (61-day half-life) and is thus easily detected at low concentrations. Time histories of 99Tc activity concentrations in soil and vegetation were simulated by correcting the observed 9 5 m Tconcentrations ~ for radioactive decay. Additional details of the procedures for both experiments
are published elsewhere (16).
Results and Discussion In the field experiments using 95mT~, concentrations in vegetation and soil decreased with time after an initial application of the isotope (Figure 1). This tendency was still evident after correcting for radioactive decay and plotting the technetium inventory in vegetation vs. time to eliminate the effects of dilution by plant growth (Figure 1, a and b). The decrease of radioactivity in vegetation with time exceeds the rate of decrease of activity in soil; decreased with therefore, the observed CR value for g5mT~ time in both experiments (Figure IC). Losses of technetium were observed within the 0-15-cm soil profile over the time period (213 days) of the first experiment ( P < 0.01), although most of the technetium resided in the top 0-2 cm throughout the course of this experiment. In the second experiment only the top 1 cm of soil showed a decrease in 96mT~ concentration with time, although a downward movement of the applied Tc could be detected by slight increases in Tc concentrations with time at the 1-2-, 5-8-, and 14-15-cm soil depths. The initial activity applied was insufficient to allow the second experiment to be carried out long enough to observe a significant loss of Tc from the entire 0-15-cm soil profile. Further details about the interception and retention of technetium by vegetation and soil pertaining to these specific experiments is discussed in another publication (16).
Decreased plant uptake of 95mT~ from soil was probably due to chemical reduction of soluble Tc(VI1) to less soluble oxidation states, although other studies (4, 12, 17, 18) suggest that the organic-matter fraction of soil may also affect technetium soil/plant dynamics. Reduction of Tc(VI1) to Tc(V1, V, and/or IV) is suspected because, in our laboratory, only 33% of the activity in the soil could be leached by using distilled water. Of that remaining, only 2% could be extracted with 0.1 N NaC1; however, after leaching with 0.1 N NaCl, 46% more of the activity was removed from the soil by a cold, 5% solution of Hz02, Environ. Sci. Technol., Vol. 16, No. 4, 1982
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Table I. Technetium Soil-to-Plant CRa Values from Gaseous Diffusion Facilities (GDP), Field Studies at ORNL, and Published Potted-Plant Experiments source geometric mean for all gaseous diffusion facilities geometric mean for Portsmouth GDPC geometric mean for Paducah GDPC geometric mean for Oak Ridge GDPC 95mTcfield expt Id 95mTcfield expt 2d 99Tclaboratory experiment 99Tclaboratory experiment 99Tclaboratory experiment
CR valueb 9.5 (6.5-14)
vegetation type
mixed herbs (Festuca sp. dominant ) 7.4 (3.1-18) mixed herbs (Fesfucasp. dominant) mixed herbs (Festuca sp. 1 6 (8.9-29) dominant) 7.0 (4.5-11) mixed herbs (Festuca sp. dominant ) 6.7 (1.4-8.4) mixed herbs (Festuca sp., Lespedeza sp. dominant) 22 (13-36) mixed herbs (Festuca sp., Lespedeza sp. Dominant) 680 (570-810) wheat seedlings 180 (110-290) soybeans and wheat 240 (160-360) cheatgrass and tumbleweed
soil type
ref
fine sandy-to-silt loams
14
Monongahela-Philo-Tyler silt loam Henry silt loam
14 14
Dewey silt loam, Sequatchie 14 fine sandy loam Captina silt loam this study Captina silt loam this study 4, 12 numerous Minnesota soils Ritzville silt loam 5 Rupert loamy sand 6
a CR = [Tc/(kg o f dry-weight vegetation)]/[Tc/(kg of dry-weight soil)]. Values in parentheses represent a 95% confidence interval about the geometric mean or median value. Eight CR values obtained from each gaseous diffusion plant (GDP). Steady-state CR values calculated by using environmental half-times given in Figure 1, a and b; in experiment 1 only the soil was initially contaminated with 95mTc;in experiment 2 both the soil and standing vegetation received the initial application of 95mTc.
which would oxidize Tc(1V) to Tc(VI1). The CR value as defined in radiological assessement models is relevant to steady-state conditions resulting from a continuous rather than a single application of technetium to the soil-plant system. Therefore, the rate constants for the decrease of technetium in vegetation and soil (from Figure 1, a and b) were used in a model to estimate a steady-state CR value. The steady-state CR value was calculated by estimating the simulated time-integrated concentration of g g Tin~ vegetation and dividing by the simulated time-integrated concentration of 9 9 Tin~ soil. The simulated time-integrated concentration of 99Tcin vegatation was calculated as the initial activity of 9 5 m T ~ deposited onto (or taken up by) the vegetation (Bq/kg) divided by the decay-corrected rate of activity concentration decrease observed in vegetation (day-l). The simulated time-integrated concentration of 99Tcin soil was deposited ~ onto calculated as the initial activity of 9 5 m T the soil (Bq/kg) divided by the decay-corrected rate of activity concentration decrease observed in soil (day-'). For the first experiment in which 95mT~04was applied directly to the soil, the initial activity in vegetation was 1MBq/kg with a decay-corrected rate constant of 0.016 day-'. The initial activity in soil was 34 kBq/kg with a decay-corrected rate constant of 0.003 65 day-l. Thus, the calculated steady-state CR value is 6.7. For the second experiment in which 95mT~04was applied directly to both soil and vegetation, the initial concentration in vegetation was 0.41 MBq/kg and the decay-corrected rate constant was 0.0424 day-l. The initital concentration in the 0-15-cm root zone of soil was 1.6 kBq/kg with a decay-corrected rate constant taken from the first experiment of 0.003 65 dag1. This rate constant was assumed because downward movement of Tc was observed in soil during the second experiment, although the length of the experiment was insufficient to reliably estimate a value for the entire 0-15-cm depth of soil. Thus, the calculated steady-state CR value for the second experiment is 22. The differences between these calculated steady-state CR values can be attributed to several factors. In the first experiment vegetation obtained technetium only from the soil, while in the second experiment both the effects of direct deposition onto the surface of vegetation and uptake from soil were included. In the second experiment, growth 216
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dilution was more important than in the first experiment because the second experiment was conducted during periods of rapid growth in late spring and early summer. Most of the first experiment was during the relatively dormant periods of late fall, winter, and early spring. Sampling of 99Tcin vegetation and soil near the three gaseous diffusion facilities produced 24 vegetation/soil CR values with a geometric mean of 9.5 and a geometric standard deviation of 2.4 (14). No differentiation could be made between the g g Tin~ vegetation obtained from direct aerial deposition and that obtained from the soil. It is assumed that these CR values are in steady state as multiple sampling over a period of 1yr did not detect large differences in the concentrations reported for a given sampling location. Concentrations of 99Tc in selected samples, which were checked by using isotope-dilution mass spectrometry, confirmed the accuracy of the radiochemical analysis of 99Tc in soil and vegetation using low-level /3 counting techniques (14). Vegetation-to-soil CR values for 99Tc near operating gaseous diffusion facilities as well as CR values obtained from published laboratory potted-plant experiments and steady-state CR values derived from our field experiments are presented in Table I. There is convincing evidence of a major difference between technetium concentration ratios obtained from growth-chamber or greenhouse studies and those obtained from field studies. In general, field-derived CR values for technetium are 1-2 orders of magnitude less than CR values derived from potted-plant experiments. We recognize that a direct comparison of the CR values in Table I is precluded by differences in experimental methods, soil types, and plant species. Nevertheless, the difference between laboratory and field CR values is substantial. Aside from differences in soil types and plant species, the results might be explained through the influence of several factors. For example, in laboratory experiments a greater ratio of root-to-soil volume may increase the availability of technetium for root uptake from potted soil, and the additions of fertilizers may affect sorption reactions. In addition, the soils used in pot experiments may not adequately simulate the redox variations of an undisturbed soil. Furthermore, vegetation grown in the laboratory is protected from wind, fog, and rain, all of which could remove technetium from field
vegetation (19-21). Last, field concentrations of technetium in soil pg/g) are far below typical concentrations used in potted-plant studies and are also substantially less than concentrations capable of inducing toxicity in plants. Plant toxicity has been reported at soil concentrations above 0.1 pg/g (1). Radiological Implications
The time history of CR values in the field experiments results from the combined effect of several using 95mT~04 processes, including uptake from soil, dilution of radioactivity as the result of increasing biomass with vegetation growth, conversion of technetium in soil to a biologically less available chemical form, leaching from soil, and leaching from vegetation. Allowances for this dynamic behavior of Tc in radiological assessment models would result in estimated 9eTcconcentrations in vegetation and the subsequent calculated dose to people being much less than if adjustments were made only to account for differences in laboratory and field-derived CR values. For example, a vegetation/soil CR value of 10, instead of the value of 200 assumed by Till et al. (8),would reduce dose estimates by a factor of 20. These reduced dose estimates would be an order of magnitude less than the US EPA Uranium Fuel Cycle Standards for releases on the order of 37 GBq/yr. However, if the observed environmental half-times for technetium in vegetation (Figure 1, a and b) are assumed to represent a net result of the processes of growth dilution, leaching, uptake from the soil, and conversion of the technetium in soil to a less mobile form, the dose estimates calculated by Till et al. (8)would be decreased by approximately 2 orders of magnitude (14). This is a substantial reduction in the estimated radiological impact that was previously based only on data obtained from potted-plant experiments (8). The observed environmental half-times in vegetation tend to preclude a substantial long-term buildup of technetium in vegetation exposed to a continuous low-level release, although the relevancy of these observations, to other locations and vegetation types and to time periods longer than those encountered in this study, still remains to be verified. These results emphasize the significance of performing field experiments on parameters used in environmental assessment models. The potential for misprediction may be large when laboratory studies constitute the only source of available data. It is our contention that derivation of parameter values through field experiments must be considered when a given parameter is crucial to an environmental assessment. Acknowledgments
We thank Tom Scott and Ray Walker, Analytical Chemistry Division, ORNL, for analyzing our samples for 99Tc and E. A. Bondietti, C. W. Francis, D. DeAngelis, and J. Beauchamp, ORNL, for help with various phases of this work.
Literature Cited (1) Wildung, R. E.; McFadden, K. M.; Garland, T. R., J. Environ. Qual. 1979, 8, 156-61.
(2) US Energy Research and Development Administration, ERDA-1542, 1976. (3) US Energy Research and Development Administation, ERDA-1549, 1977. (4) Landa, E. R.; Hart Thorvig, L. J.; Gast, R. G. In “Biological Implications of Metak in the Environment”, CONF-750929, ERDA Symposium Series 42; National Technical Information Service: Springfield, VA, 1977; pp 390-401. ( 5 ) Wildung, R. E.; Garland, T. R.; Cataldo, D. A. Health Phys. 1977, 32, 315-17. (6) Cataldo, D. A. Richland, WA, 1979, Pacific Northwest Laboratory Report PNL-2740. (7) Cataldo, D. A.; Wildung, R. E.; Garland, T. R. In “Environmental Chemistry and Cycling Processes”, CONF-760429; Adriano, D. C., Brisbin, I. L., Eds.; National Technical Information Service: Springfield, VA, 1978; pp 538-49. (8) Till, J. E.; Hoffman, F. 0.;Dunning, D. E., Jr. Health Phys. 1979, 36, 21-30. (9) US Nuclear Regulatory Commission. “Regulatory Guide 1.109, Calculation of Annual Doses to Man from Routine Releases of Reactor Effluents for the Purpose of Evaluating Compliance with 10 CFR Part 50, Appendix I”; US NRC: Washington, DC, 1977. (10) Baker, D. A,; Hoenes, G. R.; Soldat, J. K. Richland, WA, 1976, Battelle Northwest Laboratory Report BNWL-SA5523. (11) Killough, G. G.; McKay, L. R. Oak Ridge, TN, 1976, Oak Ridge National Laboratory Report ORNL-4992. (12) Gast, R. G.; Landa, E. R.; Thorvig, L. J. Springfield, VA, 1976, University of Minnesota Report USERDA-COO2447-5. (13) US Environmental Protection Agency. Fed. Regist. 1977, 42, 2858-61. (14) Hoffman, F. 0.;Huckabee, J. W.; Lucas, D. M.; Garten, C. T., Jr.; Scott, T. G.; Walker, R. L.; Gouge, P. S.; Holmes, C. V. Oak Ridge, TN, 1980, Oak Ridge National Laboratory Report ORNL/TM-7386. (15) Anderson, T. J.; Walker, R. L. Anal. Chem. 1980,52,709-13. (16) Hoffman, F. 0.;Garten, C. T., Jr.; Huckabee, J. W.; Lucas, D. M. J. Environ. Qual., 1982, 11, 134-41. (17) Landa, E. R.; Thorvig, L. H.; Gast, R. G. J. Environ. Qual. 1977, 6 , 181-7. (18) Sheppard, M. I.; Keith Reid, J. A.; Thibault, D. H.; Chen, J. D.; Vandergraaf, T. T. “Technetium Uptake by and Yield of Swiss Chard Grown on Peat and Sand”, presented at the International Symposium on Migration in the Terrestrial Environment of Long-Lived Radionuclides from the Nuclear Fuel Cycle, Knoxville, TN, July 27-31, 1981, IAEASM-257. (19) Myttenaere, C.; Daoust, C.; Roucoux, P. Enuiron. Exp. Bot. 1980,20, 415-19. (20) Tukey, H. B., Jr. Annu. Rev. Plant Physiol. 1970, 21, 305-24. (21) Beauford, W.; Barber, J.; Barringer, A. R. Science 1977,195, 571-3. Received for review April 6,1981. Revised manuscript received, October 23, 1981. Accepted December 31, 1981. Research sponsored by the US Environmental Protection Agency under Interagency Agreement EPA-79-D-XO626 and the Office of Health and Environmental Research, US Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corp. Publication No. 1942, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN.
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