NUCLEAR TASK FORCE
PART II
This section addresses individual elements and nuclides with respect to bioaccumulation and biomagnification factors for freshwater biota. The data presentations draw from many studies for the Great Lakes and other locations. A few selected data are presented for terrestrial species of radionuclide importance.
A secondary purpose of this work is to provide an extensive data base of elemental compositions from which to discern appropriate values of bioaccumulation and biomagnification factors radionuclides of various elements and to confirm which factors from tables from Radioactivity in the Marine Environment (National Academy of Sciences 1971); the listing of Chapman et al. are the most appropriate for Great Lakes work.
TRITIUM
Because of its widespread presence in the effluents (both atmospheric and aquatic) of nuclear power plants, tritium was discussed in detail in the Inventory report. Most of the Task Force data were taken from a symposium volume prepared by the IAEA, Behaviour of Tritium in the Environment. The symposium volume paper has three valuable papers on tritium uptake by aquatic biota: a paper by Blaylock and Frank (1979) on some plants and animals from a pond on the Oak Ridge Reserve (Tennessee), including two invertebrates and three warmwater fishes; a paper of Kirchmann et al. (1979) on several freshwater and marine species to the array, including a salmonid species and important information on the distribution of tritium in cellular compartments; and lastly, a paper by Adams et al. (1979) on tritium uptake by the aquatic biota in a freshwater marsh system in Lake Erie and near the Davis-Besse nuclear power plant. This third paper considers organisms found in the Great Lakes.
These papers showed that tritium levels in organisms track the tritium levels of the environment. Bioaccumulation factors of unity for tritium are conservative. Some investigators note that tritium in organismal tissues quickly "equilibrates" with the tritium content of the surrounding water, but that view is a bit simplistic because "equilibration" masks isotope exchange of tritium from bound water to stable hydrogen on biomolecules. From the available information, a bioaccumulation factor of unity (1) is probably conservative.
GROUP Ia AND Ib ELEMENTS: SODIUM, POTASSIUM, LITHIUM, RUBIDIUM, CESIUM, FRANCIUM AND COPPER, SILVER, AND GOLD
The univalent positive nuclides belong to Groups Ia and Ib of the Periodic Table and consist of sodium, potassium, lithium, rubidium, cesium, and francium (Group Ia) and copper, silver, and gold (Group Ib). Francium is an artificial element, which might be important in high-level waste production, but there is no indication that it is released to the Great Lakes and thus requires no inventory.
Copper and gold bioaccumulate and have important biological properties. However, they do not have long-lived radionuclides that require inventories. The main radionuclide of copper, 64Cu, which occurs as an activation product and from fission product, has a half-life of only 12.5 hours. Thus, the Task Force considers only sodium, potassium, lithium, rubidium, cesium, and silver.
Sodium and potassium
Biologically, potassium and sodium are major elements in tissues (macronutrients). Sodium levels typically exceed potassium levels in natural media (freshwaters, estuaries, and marine waters) as well as in the tissues of vertebrates. The situation typically reverses in aquatic plants and invertebrates with potassium exceeding sodium. This reversal of enrichment acquires special significance given that levels of sodium and potassium in biological tissues are typically of the order of parts per hundred or per cent, rather than parts per million for the trace elements.
Two radionuclides of sodium, 22Na and 24Na, result from both cosmogenic processes and fission, although fission yields are relatively low compared with the levels of cosmogenic production. At least for 22Na, the cosmogenically produced nuclide is believed to contribute to the background dose of radiation received by biota (UNSCEAR 1977). Therefore, some portion of the sodium content of biota may be radioactive. Further, environmental studies of Group Ia elements may require some comparisons with sodium.
The naturally occurring radionuclide of potassium, 40K, is important in any assessment of radioactivity for the Great Lakes. It accounts for as much as 40% of the radioactivity present in all biological tissues (human included), and is one of the two major nuclides that contribute to the natural background dose of radiation, the other one being 14C. Biological measurements of 40K in Great Lakes biota are available from various monitoring studies, and pertinent data appear in several tables that discuss nuclides monitored in the vicinity of discharges from nuclear facilities or in the discussions of other nuclides according to how the original investigators obtained and reported their findings.
Tables 6 and 7 presents Cowgill's data on the uptake of sodium, potassium, lithium, cesium, rubidium, silver, chlorine, bromine, and iodine in aquatic biota. These are the univalent elements of concern in this report. The tables include calculations of bioaccumulation and biomagnification factors. The tabular presentation aims to maintain some integrity of her various data sets as well as present the maximum elemental data economically. Only sodium and potassium are discussed here. The remaining elements are discussed later in their own subsections.
Note that the enrichment of sodium and potassium in the tissues of the two species of plants presented in Tables 6 and 7 shows a reversal of what is observed in source materials. Further note that the plants tend to keep the sodium in the stems and stalks rather than the leaves and flowers. If the aphids (Rhopalosiphum nymphaea) consume mainly the leaves and flowers, they are denied a major source of sodium. The theories interpreting the observed sodium enrichment and depletion are not discussed herein because they do not relate to the calculation of inventories for radionuclides of sodium in the Great Lakes.
From Tables 6 and 7, it appears that the bioaccumulation factors for sodium in plant parts are of the order of magnitude of 100-2500 relative to the aquatic environment and the biomagnification factor for the aphid relative to the plant parts is about 1-4. For potassium the bioaccumulation factors for the plant parts relative to the water are 1000-3000 and for the aphid relative to the plant parts is about 1-2.
UPTAKE OF UNIVALENT ELEMENTS: Na, K, Li, Rb, Cs, Ag, Cl, Br, AND I BY AQUATIC BIOTA PART I. LINSLEY POND (adapted from the data of Cowgill) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UPTAKE OF UNIVALENT ELEMENTS: Na, K, Li, Rb, Cs, Ag, Cl, Br, AND I BY AQUATIC BIOTA PART II. CULTURES (adapted from the data of Cowgill) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UPTAKE OF UNIVALENT ELEMENTS: Na, K, Li, Rb, Cs, Ag, Cl, Br, AND I BY AQUATIC BIOTA PART III. OTHER STUDIES (adapted from the data of various authors) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Lithium
Lithium has small enough atomic and ionic radii to replace sodium and potassium in various chemical matrices. The element has a metabolic role with respect to neuron function in animal cells. Because it can produce radionuclides of beryllium under a number of different simple nuclear reactions and is itself sometimes the stable daughter product of the radioactive decay of certain beryllium radionuclides, it would appear logical to treat lithium and beryllium together. However, their geochemical coherence is not obvious in biological materials, and therefore, lithium is discussed with the other univalent positive elements, and beryllium, with divalent positive elements.
Lithium data for plants are limited. Cannon's work (1960) on lithium accumulator plants for geobotanical prospecting is important for terrestrial species. Cowgill's work is probably the most important and complete data set for lithium in aquatic species. Her lithium data are included in Tables 6 and 7.
Rubidium
Rubidium has sufficiently small atomic and ionic radii that it can substitute for potassium and sodium in some chemical matrices. Rubidium's geological rarity somewhat limits its biological availability.
The substitution of rubidium for potassium or sodium suggests a possible environmental risk factor associated with its uptake and the likelihood that it will bioaccumulate to greater levels than cesium. Existing evidence supports both statements. Rubidium accumulates to levels about 100 times greater than cesium. Rubidium has acted as a growth stimulant and electrolyte balancing element in tissues that were temporarily depleted or impoverished with potassium (Luckey and Venugopal 1978).
There are many important radionuclides of rubidium: 83Rb, 85Rb, 86Rb, 87Rb, and 88Rb. The first three nuclides come either directly from fission of uranium or as an immediate daughter isotope of another direct fission product. Thus, they appear in the effluents of nuclear power plants either directly or indirectly. The fourth radionuclide is primordial. Assessments of the health risk from rubidium uptake usually examine the primordial nuclide because it is assumed to form a significant part of the natural background radioactivity to which all life is exposed. Yet according to information provided by UNSCEAR (1977) reports, the nuclide was not itself the subject of any uptake or metabolic studies. Rather, data from studies with stable rubidium isotopes formed a basis to infer the metabolic risks associated with the naturally occurring radionuclide.
Radionuclides of rubidium are not measured in biota and bioconcentration factors for them do not appear in the two main lists previously cited in this chapter. Therefore, one must rely on data from other studies, of which Cowgill's data are the most comprehensive early data.
Cesium
Although its chemistry parallels that of sodium and potassium, cesium does not routinely replace either sodium or potassium in various chemical matrices. Cesium's atomic and ionic radii are considerably larger than those of sodium and potassium, sometimes making such replacement difficult. This large size of the cesium nucleus would thus suggest a rather small biotic uptake for the element. Indeed, the data from Cowgill given Tables 6 and 7 suggest 0.1-5 parts per million (ppm) levels, but the rarity of cesium as an element in the earth's crust also limits its biological availability and could imply that the observed ppm levels are higher than might otherwise be expected.
Cesium's radionuclides are important for two reasons:
Very few radionuclide tracer studies on freshwater species have emphasized long-lived artificial nuclides. Early radionuclide tracer studies used 32P (half-life 14.28 days) because this nuclide was the first to become commercially available to research scientists. Other studies used the long-lived 14C (half-life 5730 years), especially in work on the "geochronology" (dating) of the materials, and some work on the environmental cycling of carbon. 137Cs (half-life 30 years) became the first long-lived isotope related to nuclear fallout intensively studied in aquatic organisms. Given the 30-year half-life of the isotope, it became a prime candidate for biological monitoring in studying radiation effects.
137Cs emits a distinctive radiation signal that permits its analysis by gamma radiation spectroscopy. The nuclide is usually detectable if present, and it is often the only artificial nuclide which is detected in certain types of samples. Thus, the quantity of data for this nuclide probably exceeds that of all nuclides monitored from fallout.
The early studies of cesium uptake emphasized marine biota as part of the tracking of the element to ocean repositories. Very few definitive studies exist for cesium uptake in freshwater biota. Some studies of cesium uptake in algae in special cultures came from researchers at the nuclear weapons laboratories, but studies of radiocesium uptake in macrophytes and aquatic invertebrates are far fewer. The most famous work on radiocesium accumulation is the study of the uptake of cesium from nuclear fallout in the Arctic by lichens, the consumption of the lichens by reindeer or caribou, and the exposure of human populations who consume the cesium-contaminated meat.
Given the previous information on the extent of studies using 137Cs, it should not be surprising that very high-quality bioaccumulation data for this nuclide are available in research studies. In fact, the Task Force considers the research data for bioaccumulation of this nuclide to be the most reliable of all of the bioaccumulation data for the artificial radionuclides.
Geochemical interpretations of cesium data often use its congener element, potassium, as a reference element. This permits the consideration of the possibility that cesium and potassium exhibit "coherence," a geochemical term which suggests that two elements move together environmentally or follow similar kinetic processes and mechanisms even if they do not move together environmentally. Since potassium has a major role in plant and animal metabolism, the comparisons may provide insight into the metabolic behavior of cesium relative to potassium. Cowgill found that relative to potassium, cesium accumulation in aquatic plants exhibited a geochemical ratio (Cs/K) of 0.25 X 10-4. Taylor (1964) suggested that the cesium to potassium geochemical ratio of the "accessible lithosphere" was 1.4 X 10-4. Thus, while cesium clearly accumulates in biological materials, it does not necessarily accumulate to levels in excess of that found naturally in the source mineral substrates. Also, while the two geochemical ratios have the same order of magnitude, they differ sufficiently to suggest that cesium uptake does not occur mechanistically by processes that mimic uptake and behavior of potassium. The differences are in the metabolism of the two elements. Since cesium does not necessarily substitute for potassium, cesium's metabolic behavior is more limited than that of potassium.
Studies on radioactive cesium in plants have included environmental behavior in a variety of species: ranging from cesium uptake by algae in laboratory cultures, to the cesium levels in "periphyton" (reported by some nuclear power plant dischargers in their monitoring work) to extensive research data on terrestrial plants (i.e., forest stands, agricultural plots) and emergent wetland plants (those that root in a mud or aquatic system but emerge and grow in the terrestrial environment or at the water's edge). The most comprehensive of the early studies of the 1970s were reported in various conference proceedings of the United States Department of Energy.
The deposition of radionuclides of cesium from the atmosphere in the northern hemisphere favors the northernmost latitudes. Some of the ecological systems in these latitudes are rather limited because of the climate conditions. An important system in these regions is the lichen - reindeer/caribou - human food chain. Lichens, a plant-fungus symbiont, can readily accumulate metallic elements because some of the plant material behaves like an ion-exchange resin. This food chain pathway to man of the radionuclides of cesium has been studied in various Arctic regions, including northern Canada, Alaska, Denmark, the Faroë Islands, Sweden, Norway, Finland, and Commonwealth of Independent States (formerly Russia). Some of the data on caribou are presented in Table 8. Following the accident at the nuclear power plant at Chernobyl (Ukraine) in 1986, researchers and Government agencies undertook new studies of atmospheric deposition in the Arctic. Many of the results of these studies appeared in 1995 in a special symposium on pollution in the Arctic (Volume 160/161, Science of the Total Environment). The research findings updated important information on 137Cs levels in Arctic species, including some species not previously considered.
ELEMENTAL ANALYSES (INCLUDING RADIONUCLIDES) OF ARCTIC BIOTA | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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As previously noted, there is no inherent reason to assume that either Sr and Cs will bioaccumulate in plants in the same ratios as they appear in nuclear fallout. In fact, in aquatic species of interest to the Great Lakes, the accumulations of these two elements differ by at least an order of magnitude. Thus, it may seem surprising that the two major radionuclides of Sr and Cs from fallout appear to accumulate in mosses and lichens in ratios that track their relative distribution in nuclear fallout. Because mosses and lichens behave like living versions of ion-exchange resins and can exchange strontium and cesium ions for hydrogen ions. A two-step accumulation process occurs: a stoichiometric binding as a rapid and dominant first step, and possible incorporation of a nuclide into tissue as a second step. The Task Force would not expect this ion-exchange behavior in Great Lakes biota.
The important compilations on radiocesium uptake come from the work of Blaylock (1982), Joshi (1984), and Hesslein and Slaviek (1984). The latter two references uniquely address Great Lakes fishes. Some of the materials in those compilations come from monitoring studies, but much of the data collection was research motivated.
Tables 9 and 10 present data from previous IJC compilations on radioactivity in the Great Lakes with respect to bioaccumulation studies for 137Cs and includes some limited citations from Joshi's (1984) paper for the bioaccumulation factors. The original data compilations presented both mass of the fish collected and their activity per unit mass. The activity per unit mass stays relatively constant over a fourfold range of mass (1.5-6.0 kg) of the fish. Further, the mean activity per unit mass for fishes in the two lakes are within a similar range of numbers, although the mean activities for the two species shown for Lake Huron are not identical by a two-tailed t test, while the mean activities for the species in Lake Ontario are likely the same by the two-tailed t test. Text Box 6 shows the dosages in microsieverts that would be ingested by a person eating the affected fish.
The Ganaraska River flows into Lake Ontario and carries the effluents from the CAMECO fuel processing site. Since the cesium activity data from the fish in the river do not differ strongly from the cesium activity of the data from fishes within Lake Ontario stations, the Task Force suggests that the source of radiocesium is probably nuclear fallout and not direct radionuclide discharge from the CAMECO operations. The radium data for Ganaraska River and the other Lake Ontario stations (not presented here) do show specific source inputs of the CAMECO site.
CESIUM ACCUMULATION IN GREAT LAKES BIOTA PART I -- 137Cs IN GREAT LAKES FISHES (adapted from International Joint Commission (1983, 1987), Joshi (1984), and other agency reports) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Notes: Numbers in parentheses indicate number of fishes in sample used in the averaging. If no number appears in parentheses, then either only one measurement was reported, or the source of the data did not qualify the information in some manner. This is especially important for the data on the Ganaraska River for 1976 to 1978, where the sources used reported means but not uncertainties or standard errors in the compilation. Radioactive measurements were made on a wet weight basis. Data are from both Canadian (IJC 1983, 1987; Joshi 1984; Environment Canada, personal communication) and United States (New York State Department of Health 1983-1993) sources. Bioaccumulation factors from Joshi (1984) relate to open water levels or ambient levels at collection site, depending on the available data. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
All of the bioaccumulation factors cited have the same order of magnitude, although fishes in Lake Huron have higher bioaccumulation factors than fishes in Lake Ontario. These factors should not be assumed to hold for the two Lakes over all periods of time. Joshi (1984) calculated bioaccumulation factors from several fish studies going back to 1976 (some of the data from which appear in Table 10). His calculations suggest that bioaccumulation factors were much higher in early years and have exhibited a downward trend with time. His work also indicated that fishes in lakes not receiving direct discharges of radionuclides (Lake Superior) sometimes had higher bioaccumulation factors than fishes from lakes that had direct discharges of radionuclides. There are several possible explanations for such an observation, but because the data bases are not sufficiently extensive to perform a statistically comprehensive interlake comparison of bioaccumulation factors of cesium or test any hypotheses or reasons for trends or differences in apparent factors from lake to lake, the Task Force cautions that no particular emphasis should be placed on any trend suggested by the data; they merely illustrate what has been reported and observed.
CESIUM ACCUMULATION IN GREAT LAKES BIOTA PART II -- 137Cs IN GREAT LAKES FISHES: STUDIES AT HWR NUCLEAR GENERATING STATIONS (adapted from operating reports of Ontario Hydro) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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RADIATION DOSES RESULTING FROM RADIONUCLIDE LEVELS IN FISH (TABLE 10) | ||||||||||||||||||||||||
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A simple comparison of the data from Tables 9 and 10 shows that Arctic species accumulate radionuclides of cesium over two orders of magnitude greater than Great Lakes species. Unfortunately, data for Lake Superior are not present. This is the largest of the Great Lakes with the largest surface area. Since fallout increases with surface area, it is difficult to tell whether Arctic species have a greater radiocesium inventory than Great Lakes species. Arctic species only receive nuclear fallout, while the Great Lakes species receive fallout, gaseous and liquid discharges of nuclear power plants, and the runoff of deposited materials from nearby areas in the watershed. Fallout fluxes depend on latitude band, and the latitude band for the Great Lakes has a higher flux than the latitude band for the Arctic, but the Arctic region has a greater surface for deposition than the Great Lakes. What is clear is that both regions, Arctic and Great Lakes receive a high quantity of radiocesium, and that the biota in both regions reflect this.
Silver
One radionuclide of silver, 110mAg, consistently appears in the nuclear discharges to the Great Lakes. The nuclide has been reported every year in the gaseous and liquid discharges of at least eight nuclear power plants in the Great Lakes over the period of 1980-1993. Its half-life of 253 days assures that silver can cycle through Great Lakes biota, if bioaccumulated, over a measurable period of a year following its discharge. The isotope decays by emission to form a stable nuclide of cadmium, but silver and cadmium do not usually move geochemically together; thus, silver is not discussed with cadmium. The isotope also undergoes internal nuclear rearrangement (note the letter "m" designation in the isotope), to produce the isotope 110Ag (half-life 24.4 seconds), which also decays by emission.
Very little is known about the environmental cycling of silver. It is highly toxic to aquatic microorganisms and fishes. The Task Force decided to include a presentation of data for silver in its report and estimate a biological inventory for its important radionuclide. Again, Cowgill's data are the most comprehensive for freshwaters systems and appeared in Table 7.
GROUP IIa ELEMENTS: BERYLLIUM, MAGNESIUM, CALCIUM, STRONTIUM, AND BARIUM
The Group IIa positive divalent elements are beryllium, magnesium, calcium, strontium, barium, and radium. Radium, a naturally occurring radioactive element is treated separately with the transuranic elements. Data on the uptake of Group IIa elements are given in Table 11.
Beryllium
Beryllium accumulates in plants. Two main radionuclides of beryllium, 7Be and 10Be, were previously discussed because they are cosmogenically produced. When these nuclides originate from cosmogenic processes, they usually do not last long enough to reach freshwater systems and be accumulated by biota, although this can happen under favorable conditions of atmospheric deposition. The nuclides also are produced in nuclear reactors and is a likely source of radioactive beryllium, which can accumulate in plants and animals in the Great Lakes, and possibly from radiation interactions with source materials of the Basin. Indeed, the Task Force has noted the occurrence of 7Be in the effluents (gaseous and liquid) of nuclear power plants. However, animals reject beryllium in the food supply. The calculated bioaccumulation factors for beryllium from the data sets available in this report are of the order of 0.06 for animals relative to plants. Thus, other than a source of internal radiation, the radioactive beryllium levels in tissues would not appear to be a bioaccumulation problem. On the other hand, humans who breathe in particulate matter containing beryllium are at risk from beryllosis and lung cancers. Airborne levels of beryllium are of concern in cancer risk assessments for hazardous air pollutants, but none of the pathways of beryllium exposure have thus far been shown to be significantly impacted by bioaccumulation related processes.
Calcium and Magnesium
Calcium and magnesium are macronutrient elements in biological tissues. They have roles in maintenance of ionic balance, energy metabolism, production of DNA and RNA, and neurological function (animal cells). Calcium is the primary cation in skeletal tissue of vertebrates and invertebrates, and magnesium is the essential inorganic element in chlorophyll.
Neither the radionuclides of calcium nor magnesium require their inventories in biological compartments of the Great Lakes because these nuclides do not have sufficiently long half-lives for consideration. Both elements, however, are needed in discussing the behavior of the radionuclides of other elements cycling within the Great Lakes, especially in developing inventories for the radionuclides of beryllium, strontium, and barium.
Strontium
Strontium can replace calcium in the bone tissue of vertebrates. The radionuclides of strontium are mostly beta and gamma emitters, making them a major source of internal radiation when incorporated into tissues. In humans and other vertebrates, radionuclides of strontium affect the mineral integrity of bone tissue, and the internal radiation affects bone cells and bone marrow (the blood forming organ found in bones).
Of the several radionuclides of strontium formed in fission either directly or as the decay products of other nuclides (mainly noble gas radionuclides of krypton), 90Sr is especially important because of its long half-life of 30 years. A second nuclide of concern, 89Sr, also a fission product, occasionally appears in nuclear fallout.
Nuclear power plants produce many strontium nuclides. Nuclear power plants in the United States within the Great Lakes Basin reported 89Sr in liquid effluents 117 times during the period of 1980-1993, with at least seven nuclear power plants reporting the isotope each year. The maximum number of United States nuclear power plants in the Great Lakes Basin which reported this isotope in liquid effluents was 11, and that occurred in 1991. All United States and Canadian nuclear power plants report discharges of 90Sr. One or two power plants yearly report releases of 92Sr, and very rarely, perhaps once every 5 or 6 years, a power plant reports releases of 91Sr. Among the other strontium radionuclides reported as fission products for Canadian plants with respect to inventories for high-level waste disposal are 86Sr and 87Sr. The stable nuclide, 88Sr, is reported in high-level waste but not reported in radioactive effluents.
Barium
Only 140Ba is important in inventories of radionuclides. This fission product decays to 140La, and these two nuclides are sometimes treated together. Some sources even report the data for the two isotopes as a combined activity, without indicating which fraction of the activity belongs to each isotope. Only barium is discussed in this section. Lanthanum, a rare earth element, is treated with other rare earth elements, notably radionuclides of cerium.
Barium accumulates only slight in organisms, the most definitive freshwater studies being those of Cowgill. Its accumulation strongly depends on levels of calcium both within the environment and within organism tissues. High calcium levels in tissues tends to block accumulation and deposition of barium.
UPTAKE OF GROUP IIA AND GROUP IIIA ELEMENTS: Be, Mg, Ca, Sr, Ba, B, AND Al VARIOUS STUDIES (adapted from work of various authors) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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A recent data set from Yan et al. (1989) provides uptake data for the net zooplankton for a series of lakes in the Canadian Shield. Since the Canadian Shield is a region currently under consideration by the Government of Canada as a possible site for a high-level nuclear waste repository, the biota of this region are of Task Force interest. Furthermore, the region is within one day's automobile travel of the Great Lakes, and depending on a variety of climate and other geophysical factors, the possibility of radionuclides in this region reaching the Great Lakes cannot be discounted. Further, and quite fortunately, most of biota found in the lakes studied also occur in the upper Great Lakes, making the zooplankton studies of these lakes relevant to an understanding of the biological cycling of elements in comparable biota in the Great Lakes. These very important data appear in Table 12.
The data suggest that strontium and barium have very different uptake characteristics relative to the Group II elements. Strontium uptake and levels do not appear to depend on the uptake and tissue levels of calcium, but barium uptake and levels show strong statistical correlations with calcium levels. The correlations between barium and calcium levels in zooplankton are particularly pronounced when the calcium level exceeds 10,000 ppm dry weight of tissue.
The separation of lakes into Sudbury and non-Sudbury area reflects the investigators' concerns about the discharges of nickel from the Sudbury smelter as a point source of pollution. This pollution source strongly influences the overall elemental uptake of metals by zooplankton in nearby lakes. Yan et al. (1989) chose lakes that were not themselves known to be polluted from other point sources or nonpoint sources, paying careful attention to obtaining a range of chemical conditions with respect to alkalinity, pH (a concern about lake acidity, which again is strongly influenced by smelter releases of sulfur oxide gases and aerosols), organic content, and diversity of zooplankton species. Thus non-Sudbury area lakes would not be expected to have zooplankton with high tissue levels of nickel unless atmospheric transport and deposition of nickel to those lakes occurred. The Task Force did not consider whether a calcium correlation with nickel influenced a calcium correlation with strontium or barium because of the inadequacy of the statistical tools available for such comparisons with these special data.
ELEMENTAL COMPOSITION OF THE BIOTA FROM LAKES IN THE CANADIAN SHIELD. PART I -- UPTAKE OF STRONTIUM, BARIUM AND RELATED ELEMENTS BY NET PLANKTON (adapted from Yan et al. 1989) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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ELEMENTAL COMPOSITION OF THE BIOTA FROM A SERIES OF LAKES IN THE CANADIAN SHIELD -- NET ZOOPLANKTON (adapted from Yan et al. 1989) | ||||||||||||||||||||||||||||||||||||||||
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GROUP IIb ELEMENTS: ZINC, CADMIUM, AND MERCURY
Zinc, cadmium, and mercury
The Group IIb elements, zinc, cadmium, and mercury, are important heavy metal contaminants of the Great Lakes. Each element has at least one radionuclide potentially requiring an inventory for the Great Lakes (65Zn, 113Cd, 203Hg), but actually only the radionuclide of zinc needs a separate inventory for biological compartments.
65Zn is an activation product with a long enough half-life (244 days) to cycle within the Great Lakes. Zinc is also a required trace metal in many metabolic processes, and thus, zinc will accumulate quite easily in biological tissues.
A nuclide of cadmium occurs naturally and the radioactive portion of this nuclide found naturally in biota can be obtained by a procedure similar to the one used for 40K. Another nuclide of cadmium, 109Cd (half-life 450 days), occasionally appears in discharges of nuclear facilities and is used in laboratory research (see discussions of the Task Force of Tier II nuclear facilities). Although its half-life assures its cycling through biological compartments, the source data on its discharge into the Great Lakes are inadequate to produce an estimated inventory.
The nuclide of mercury, although it is a fission product, has a very low fission yield and does not routinely occur in nuclear effluents or wastes. It has been used in laboratory and field research, but inadequate data preclude preparation of an estimate of its inventory in Great Lakes biotic compartments.
GROUP IIIa ELEMENTS: BORON, ALUMINUM, GALLIUM, INDIUM, AND THALLIUM
The Group IIIa elements are boron, aluminum, gallium, indium, and thallium. Of these elements, only boron and aluminum are of concern in developing inventories of radionuclides. Gallium, indium, and thallium do not have nuclides that are produced in nuclear systems that are likely to be discharged to the Great Lakes, although Cowgill has noted that gallium is accumulated by a variety of organisms.
Boron
Boron is a component of moderator materials in the reactors of nuclear power plants because of its high neutron absorption abilities. There are no reports of radionuclides of boron released to the Great Lakes. Rather the products of nuclear reactions of boron are nuclides of other elements, the most important of which is tritium. Boron is a plant nutrient, and in excess, a plant toxicant. Since few compounds of boron are water soluble, the range of concentrations of boron in source material exerting effects on organisms is quite small.
Aluminum
Aluminum is a reference element often chosen to describe geochemical coherence of elements originating in soils and minerals. Plants can accumulate aluminum, and an early study by Hutchinson and Wollack (1943) examined aluminum accumulator plants in considerable detail. Also, two radionuclides of aluminum, 26Al and 28Al, are produced cosmogenically. Data for aluminum may assist in the understanding of the behavior of other elements, but no separate bioaccumulation factors for aluminum are given.
GROUP IIIb ELEMENTS: SCANDIUM, YTTRIUM, LANTHANUM, AND RARE EARTHS
The Group IIIb elements of the Periodic Table are rather unusual. All are rare in nature. The low molecular weight scandium often shows up in particulate matter sampled in the upper atmosphere and has two nuclides which form cosmogenically. It has also been studied by Cowgill in plants, but the data on potential discharges to the Great Lakes are scant.
Yttrium and lanthanum and the other rare earth elements, are well represented among the nuclides formed in the nuclear fuel cycle. Many rare earth elements have primordial radionuclides among their isotopes, and almost all of the isotopes of the rare earth elements are mildly radioactive. The behavior of this group of elements in biological materials is not well understood. Cowgill's studies of the rare earth elements in various plant species suggested that organisms may exert considerable selectivity on which elements they accumulate, that they can accumulate considerable quantities relative to the low content of these elements in source materials, and that the elements can remain largely undetectable by present methods, suggesting levels below 1 ppm in either source materials or biological tissues.
Except for scandium, radionuclide inventories in biological compartments are needed for all of the Group IIIb elements and rare earths, although some scandium data are provided for reference purposes. The data are presented in Table 13.
Yttrium
Radionuclides of yttrium form directly as fission products and as the decay products of other fission products, notably radionuclides of Sr. Two nuclides of yttrium have been reported in discharges to the Great Lakes, 90Y and 91Y. The former has a very short half-life, but the latter has a sufficiently long half-life to call for an inventory. Yttrium can accumulate in organisms, but the available data base is sparse. Cowgill's data presents the most comprehensive review of yttrium in aquatic organisms. The environmental cycling of yttrium appears to follow that of the rare earth elements described below.
Lanthanum
Lanthanum is one of the most toxic elements to aquatic organisms. Nevertheless, it is detectable in small amounts in aquatic biota along with other rare earth elements.
Lanthanum phosphate is very insoluble in water. The precipitate is quantitative with a very high gravimetric constant, thus providing an analytical method for the element. This information suggests that lanthanum would not be expected to be a water pollution problem for most waters given their phosphate content. Further, the rarity of the element makes its recovery from aquatic systems financially worthwhile rather than discharging it in a waste effluent.
The accumulation of lanthanum in biota seems to depend on the presence of calcium. Lanthanum can sometimes substitute for calcium in a biouptake process from a calcium-impoverished medium. Since calcium phosphates are also insoluble, this may offer a partial explanation of the behavior of lanthanum.
One radionuclide of lanthanum important for Great Lakes work is 140La. It forms directly as a fission product and as the decay product of 140Ba. This latter fact explains why several dischargers tend to report the combination of 140Ba/140La , without separating the relative proportions of the isotopes.
Cerium
Cerium is only other rare earth element besides lanthanum that is routinely detected in biota. Cowgill's data probably provides the most complete set of information on the stable forms of the element in aquatic biota. Radionuclides of cerium are major fission products and have been detected in Great Lakes waters. The two major radionuclides of interest are 141Ce and 144Ce.
Other Lanthanides
As previously noted, organisms may be capable of accumulating rare earth elements selectively. Except for lanthanum and praeseodymium, Cowgill (1974a) found that the macrophytes of Linsley Pond only accumulated elements of even atomic number. She did not observe the pattern repeated in her studies of the giant Amazon River water lily, Victoria amazonica. For that species, the only detected rare earth elements were lanthanum and cerium (Table 14).
In her studies of the rare earth elements accumulated by Daphnia, Cowgill found europium in the organisms, but not in the source materials, suggesting that it was highly impoverished in the source materials composing the Daphnia medium, but that the organisms could still bioaccumulate and concentrate to a great degree. With respect to bioaccumulation processes in general, the lanthanides act sufficiently coherently that one can treat the total accumulation of all of those elements as a single element for bioaccumulation analyses.
In several studies the behavior of unusual elements that cycle in lakes and the oceans through sorption to the surfaces of bioparticles, a radionuclide of gadolinium was used. However, the authors of that study could not provide background data on gadolinium levels in the aquatic environment to assist in the interpretation of their work. It is not clear whether gadolinium should be considered an element which cycles on an organism as opposed to in an organism.
UPTAKE OF RARE EARTH (RE) ELEMENTS AND THEIR CONGENERS BY AQUATIC BIOTA (from Cowgill 1976) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UPTAKE OF RARE EARTH (RE) ELEMENTS AND THEIR CONGENERS BY AQUATIC BIOTA (from Cowgill 1976) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UPTAKE OF RARE EARTH (RE) ELEMENTS AND THEIR CONGENERS BY AQUATIC BIOTA: Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb (adapted from data of Cowgill) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UPTAKE OF RARE EARTH ELEMENTS AND CHEMICAL CONGENERS BY AQUATIC BIOTA: Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb (adapted from data of Cowgill) | |||||||||||||||||||||||||||||||||
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GROUP IVa ELEMENTS: CARBON, SILICON, GERMANIUM, TIN, AND LEAD
The Group IVa elements are carbon, silicon, germanium, tin, and lead. Although they share a common chemical grouping, their chemistry differs very markedly from element to element. For that reason, each element is discussed separately without any major prefatory statement. Data on uptake of Group IVa elements is presented in Table 15.
Carbon
The major radionuclide of carbon is 14C, and it requires consideration all by itself. However, bioaccumulation factors for carbon would require a consideration of all the compounds which are formed. Thus, bioaccumulation factors are not developed for 14C, per se, but specific inorganic and organic compounds that contain the element.
Silicon
Silicon is an essential element in the skeletal structure of diatoms and Foraminafera, usually in the amorphous mineralogical form of opaline phytoliths. The bioaccumulation of silica suggests that it might be a macronutrient in plants other than diatoms, but as earlier indicated, many silica analyses may be faulty because of analyses of silica on the material rather than silica in the material.
The biological cycling of silicon in the Great Lakes (because of diatoms) has been intensively studied by Stoermer and his coworkers. Wahlgren et al. (1980) used the cycling of silica to explain much of the dynamics of the movement of transuranics, especially plutonium, in Lake Michigan, particularly the movement of plutonium to sediments and seasonal distributions. The silicon-plutonium connection thus has particular importance in establishing inventories of transuranic radionuclides to the Great Lakes.
Two radionuclides of silicon, 31Si and 32Si, are produced cosmogenically. There is some limited production by fission, but no indication that the fission isotopes are released to the Great Lakes. The same procedures used to develop biocompartment inventories for the cosmogenically produced nuclides of aluminum can be used to produce biocompartment inventories for the radionuclides of silicon.
Germanium
Germanium does accumulate in biota, but there are many problems with analytical procedures and other technical concerns that mitigate making the most cursory estimates of germanium bioaccumulation factors. Germanium has not been detected or reported on in Great Lakes water or biota, but its presence is sufficiently difficult to measure that it might be overlooked. Germanium isotopes are found in fuel elements and therefore of possible concern, although as previously noted, the discharge of germanium to the Great Lakes is largely undocumented.
The analytical procedures for germanium are quite difficult, and there are many technical problems in detecting and quantifying this element in aquatic media. Some of these methodological difficulties cast doubt on the confidence of some of the data in the literature. Given the very limited data available for the element, there seems little justification in attempting what would be a highly speculative estimate of an inventory for germanium for biocompartments.
Tin
Tin is also a very difficult element of Group IV to consider. Very few data exist on its occurrence in aquatic systems and even less in biological compartments. Analytical methods to quantify tin in environmental media and substrates call for considerable skill and instrumental sophistication.
Two radionuclides of tin, 113Sn (half-life 115 days) and 117mSn (half-life 14 days), occasionally are reported in the liquid emissions to the Great Lakes. Both nuclides have sufficiently long half-lives to undergo cycling through Great Lakes compartments. They also exhibit unusual decay patterns: 113Sn decays by orbital electron capture to give a stable nuclide of indium, while 117mSn decays by an internal transition that produces a stable nuclide of tin. Thus, at least one tin nuclide continues to cycle environmentally unchanged. The nuclides originate from fission and as activation products of the zircalloy (a zirconium-tin alloy) sheathing for nuclear fuel elements, making them important nuclides from the nuclear fuel cycle.
Tin has a rather unusual chemistry and a very interesting toxicology. It forms several highly toxic metalorganic compounds (i.e., tributyl tin). The methylation property permits biological mobilization of otherwise relatively water-insoluble tin compounds in the environment, permitting them to cross biological membranes and deposit in lipid tissue reserves. Further, selected organotin compounds are volatile.
Cowgill's data probably comprise the most extensive set of tin analyses on aquatic biota for nonradioactive materials, but her data base is small.
Lead
Isotopes of lead are the final products from the decay of transuranics. The long life of 210Pb and the concern about lead as a toxicant and air pollutant have led researchers to study this element and this particular radionuclide intensively. Thus the information base on radioactive and stable lead is rather large, but the biological data on the radionuclide form of lead is not very extensive.
UPTAKE OF GROUP IV, V, AND VI ELEMENTS BY AQUATIC BIOTA: Si, Ge, Sn, Pb, S , Se, P (ppm) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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GROUPS IVb AND Vb ELEMENTS: TITANIUM, ZIRCONIUM, HAFNIUM, VANADIUM, ANTIMONY, AND TANTALUM
The Group IVb and Group Vb elements include titanium, zirconium, hafnium, vanadium, antimony, and tantalum. Other than for antimony and tantalum, for which biological data are either exceedingly rare or nonexistent, the other elements in these groups present an interesting combination with respect to bioaccumulation. Uptake data for some of these elements are presented in Table 16.
Titanium
Titanium is among the least biological mobile of the elements and, thus, is a favorite choice of geochemists as a reference element for studies of elements that move preferentially with soil or mineral materials rather than with biological materials. Nevertheless, titanium accumulates in organisms, and some recent data from Yan et al. (1989) on a lake in the Canadian Shield region show an unusually enriched titanium level in zooplankton. The great insolubility of titanium in water can sometimes suggest that high titanium levels found in organisms may possibly originate as external titanium contamination of materials (titanium on rather than titanium in biological tissues) or the ingestion of particles that become trapped in some storage matrix in the alimentary canal of organisms without mobilization to other tissues. On the other hand, titanium citrate is soluble in aqueous solution at the levels of titanium detected in biota and the water column, and the presence of citrate as a major organic compound in all organismal tissue offers an alternative hypothesis to explain the apparently anomalous titanium data. The data from titanium analyses in freshwater biota do not indicate whether or not titanium in biological tissues is trapped in a storage matrix and thus biologically inert.
There are no indications that radionuclides of titanium are discharged to the Great Lakes, although several are produced as fission products and would be included in the inventory of nuclides for used fuel elements. Bioaccumulation data for titanium are used to assist in estimation of properties of other elements.
Zirconium and niobium
Zirconium and niobium are metals associated with the preparation of shielding material to contain fuel elements in nuclear power plants. Their isotopes form by both neutron activation and fission products. Atmospheric testing of nuclear weapons prior to 1963 produced zirconium nuclides, which were detected in atmospheric fallout. Radionuclides of zirconium decay to radionuclides of niobium, and the specific combination of 95Zr/95Nb is sometimes treated as one nuclide in the emission reports of nuclear facilities (the reader should recall the situation with 140Ba/140La). Both 95Zr (half-life 65 days) and 95Nb (half-life 35 days) have sufficiently long half-lives to be of environmental concern. The radionuclide, 94Nb (half-life 20,000 years), is one of the longest lived, but there are no indications that it is discharged directly to the Great Lakes region. Both elements bioaccumulate in tissues. In the few available studies, niobium accumulates to a greater extent than zirconium, at least in plants. The available data for each element are quite limited, with Cowgill's work probably the most comprehensive source. Cowgill evaluated her niobium data relative to vanadium. This offers a useful statistical approach because the availability of biological data for vanadium far exceeds that of niobium. The methodology studies Nb/V ratios as guides to estimating niobium inventories in unstudied situations.
Hafnium
Hafnium has several long-lived isotopes, including one that is primordial. It also accumulates to a very limited degree in plants. The limited data come mainly from Cowgill.
Vanadium
Vanadium accumulates in organisms (Table 16) and is an essential micronutrient for certain plants and fungi. 50V is a primordial radionuclide. Other radionuclides of vanadium form as both fission and activation products. Recent interest in vanadium by a number of researchers has provided a data base on vanadium uptake in plants and animals that offers possibilities in estimating its behavior in biological compartments. However, there is only limited indication that vanadium radionuclides are released to the Great Lakes. Rather, the importance of vanadium data assist in studying niobium.
UPTAKE OF SELECTED TRANSITION ELEMENTS BY AQUATIC BIOTA: Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Zr, Nb, Mo (adapted from data of various investigators) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| TABLE 16b UPTAKE OF SELECTED TRANSITION ELEMENTS BY MAMMALS: Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Zr, Nb, Mo (adapted from data of various investigators) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Notes: See page 73 and 75 for discussion of iron (Fe) and nickel (Ni) levels, respectively. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
GROUP VIIa ELEMENTS (THE HALOGENS): FLUORINE, CHLORINE, BROMINE, IODINE, AND ASTATINE
The halogens, Group VIIa, of the Periodic Table are elements with univalent negatively charged ions: fluorine, chlorine, bromine, iodine, and astatine. Since astatine is an artificial nuclide, it is not discussed here. The remaining halogen elements exist naturally as diatomic gases or as salts in various minerals. The largest source fluorine is in minerals, especially certain aluminum and phosphate minerals. Chlorine, bromine, and iodine occur both as rock salt minerals as well as dissolved in ocean waters.
Fluorine
Fluorine has a very limited biological uptake. The carbon-fluorine bond is very strong, but it is also very difficult to create. A group of fluorine accumulator plants (terrestrial, and mainly found in Africa) apparently can metabolize and store fluorine through a fluoracetic acid pathway. Fluoracetic acid is very toxic to almost all animal species (terrestrial and aquatic) because it interferes with the Krebs' cycle. Radionuclides of fluorine have virtually no role in the Great Lakes, and there are no inventory calculations for fluorine presented.
Chlorine
Chlorine has several radionuclides, some of which are formed cosmogenically. Also chlorine is a macroelement in biological tissues because of its role in the maintenance of electrolyte balance and osmotic pressure, but studies of radioactive chlorine behavior in environmental systems are almost nonexistent. A review group evaluating the Draft Environmental Impact Assessment for high level waste deposit in a possible Canadian Shield repository noted with some surprise the apparently unexpected presence of detectable 36Cl in reactor material. Although the nuclide can form as a fission product as well as cosmogenically, apparently its fission yield is so small that its appearance in the reactor materials was unexpected. The usual source of this nuclide is the decay of the cosmogenically produced 35S. Consequently, some fraction of biologically retained chlorine would be expected to be radioactive.
Bromine
Bromine accumulates in several organisms and exhibits some unusual biological properties. It is a major inorganic constituent of the neurotoxins produced by several species of marine animals. It has been detected enriched in tumor tissue of a primate (Cowgill 1977), and the sodium or potassium bromide salts have long been used by humans for the treatment of headaches. Radionuclides of bromine form directly as fission products and as a decay product of other fission products. Although elemental bromine is a liquid at room temperature, it vaporizes easily and might accompany chlorine and iodine in gaseous effluents. However, most bromine occurs naturally as anionic material, usually either as bromide ion or bromate ion. Since most bromide and bromate compounds are very water soluble, the element attains a special importance should some accidental breach of nuclear fuel element integrity occur. Bromine would carry other nuclides directly into solution creating a high-level liquid radioactive waste. However, the radionuclides of bromine are very short lived.
Iodine
Iodine is the heaviest natural element of the halogen family, Group VII of the Periodic Table. Its congener elements, fluorine, chlorine, and bromine, have relatively few isotopes of biological significance and accumulation and are thus discussed together in a separate section of this chapter.
Freshwater and marine biota both accumulate iodine, but environmental levels of iodine are greatest in the oceans (Table 17). Thus marine plants, invertebrates, and vertebrates accumulate it to levels greater than freshwater or terrestrial organisms. The low levels of iodine in freshwater make its uptake almost uniquely limited to plants and vertebrates. There are very limited data on the uptake of iodine by freshwater invertebrates.
The biological role of iodine is unclear except in the vertebrates, where it is unique in thyroid gland function. Since the thyroid is a target organ in humans for radioactive iodine, both United States and Canadian regulations call for monitoring at least of the 131I. This isotope is found in gaseous and sometimes liquid emissions from nuclear power plants. A much longer lived isotope, 129I, occurs in the effluents of nuclear fuel reprocessing operations.
GROUP VIa ELEMENTS: SULFUR, SELENIUM, TELLURIUM, AND POLONIUM
The Group VIa elements are not considered. This may surprise people since isotopes of sulfur form by cosmogenic processes; 75Se is a popular radioactive tracer of organismal metabolism for selenium; both sulfur and selenium are essential elements in the nutrition of all species, although selenium in excess is very toxic; tellurium is also quite toxic; and polonium is a radioactive product of uranium decay.
Cosmogenically produced nuclides of sulfur do not have long-term bioaccumulation potential. If they decay to stable sulfur, the formation of sulfate has implications for many environmental processes, but bioaccumulation and biomagnification are not among them. 75Se is neither a fission nor activation product. All radionuclides of selenium produced by nuclear activities in the Great Lakes Region except for 79Se are very short lived, but the residuals of 79Se are important only in considerations of the high-level waste inventories for fuel elements. The Task Force has no documentation that selenium nuclides are released by nuclear facilities to the Great Lakes Basin. Those users of 75Se for research purposes do not release that large a quantity that the cycling of this nuclide need be routinely considered; however, both sulfur and selenium are treated as elements that have importance in establishing the inventories and behaviors for the nuclides of other elements.
Despite the toxicity of tellurium, there is limited accumulation. Cowgill only found the element occurring naturally in plant material from the Middle East, not from her studies in North America (personal communication, April 26, 1996). Nor are radionuclides of tellurium documented in the releases from nuclear power plants, although such nuclides would require consideration in the high level waste inventories for fuel elements. Several isotopes of tellurium form as fission products. Many tellurium compounds are volatile and would appear in the gaseous emissions. Isotopes of tellurium have been documented in the gaseous emissions of fuel reprocessing operations, but not in North America.
| TABLE 17 BIOACCUMULATION FACTORS AND BIOMAGNIFICATION FACTORS FOR VARIOUS ELEMENTS IN AQUATIC BIOTA | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Notes: NA, not available.
(--) The Task Force was unable to find data. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
GROUP VIb ELEMENTS: CHROMIUM, MOLYBDENUM, AND TUNGSTEN
The group VIIb elements are chromium, molybdenum, and tungsten. All three elements have radionuclides formed by activation or fission processes that occur in the discharges to the Great Lakes. All three elements can bioaccumulate in tissues (Table 17).
Chromium
Chromium cycles through biological compartments in several valence states, two of which are very important in aquatic systems. The hexavalent (+6) state is very water soluble and highly toxic to most organisms. The trivalent (+3) state has a much lower water solubility and behaves as a trace micronutrient in certain tissue and organismal systems (Mertz 1967). Most environmental studies do not report the valence state of chromium, rather a "total" chromium level. As a precaution in toxicological and ecological studies, if the environment is aerobic, many investigators and most regulatory agencies assume that the "total chromium" is in the hexavalent state. Other valence states (+4, +10) of chromium are not stable (they can be explosive) and readily attack organic matter, especially under acid conditions.
Neutron activation produces an important radionuclide of chromium, 51Cr. This nuclide behaves as a tracer of the effluent from nuclear power plants and scientists at the Hanford facility and Battelle Northwest have followed the nuclide's migration along the Columbia River to the Pacific Ocean for many years.
Molybdenum
Molybdenum is a trace nutrient requirement for plants, and functions in several enzymes associated with nitrogen fixation and the utilization of iron and sulfur in cellular metabolism. Two nuclides, 95Mo and 99Mo, form as fission and activation products. Although 99Mo has a half-life of only 6 hours, it decays to 99Tc (half-life 212,000 years) and thus plays an important role in estimating inventories for technetium. 99Mo and 99Tc are sometimes treated as a combined pair, similar to the treatment of 140Ba and 140La.
Tungsten
Tungsten has not been routinely detected in freshwater organisms. Most of the studies on biogeochemical cycling of tungsten examines the two radionuclides 181W and 185W, which are fission and activation products in nuclear reactors. These biological studies emphasized obtaining data to predict the exposure by humans to these nuclides through food. The studies emphasized tungsten uptake by terrestrial plants, notably agricultural crops. The data base upon which to make biological judgments about tungsten, in general and in the Great Lakes specifically, is very thin.
GROUP VIIb ELEMENTS: MANGANESE, TECHNETIUM, RHENIUM, IRON, RUTHENIUM, AND OSMIUM
The elements of VIIb come in two groups: (1) manganese, technetium, and rhenium and (2) iron, ruthenium, and osmium. Only manganese in the first subgroup and iron and ruthenium in the second subgroup have important nuclides for which inventories in biological compartments of the Great Lakes are needed. Technetium also requires study, but there is insufficient data and information to estimate its biological inventory in the Great Lakes. Rhenium and osmium, although highly toxic to mammals, are not documented in the discharges to the Great Lakes either directly as radionuclides or stable isotopes. Radionuclides of rhenium, decay products of radionuclides of ruthenium, have very short half-lives.
Iron and manganese
Iron and manganese are elements subject to the caution about "material on a tissue" versus "material in a tissue." Depending on the pH of the freshwater system or the "local pH" on the surface of tissue (plants that may have dew on leaves), and the level of oxygenation of the system (aerobic versus anaerobic environment), iron and manganese hydroxides can form precipitates on the external surfaces of biological materials. Many analyses of plants that report very high iron and manganese levels may actually have reported crystalline ferric and manganese oxides as surface contaminants. Despite these chemical artifacts, both elements have major metabolic roles in organisms and approach a status of macronutrient rather than simple trace nutritional requirement. Iron and manganese atoms occur in many enzymes associated with the metabolism of carbohydrates and lipids, as well as the production of proteins and nucleotides (DNA).
Radionuclides of iron and manganese form as activation products through neutron interactions with the piping and contaminants in the construction materials of fuel elements, reactor housings, etc. In addition, the casings on those nuclear weapons which were atmospherically detonated in the 1950s and early 1960s became activated with the release of radionuclides of these elements. Important nuclides are 55Fe (half-life 2.6 years), 59Fe (half-life 45 days), 54Mn (half-life 303 days), and 56Mn (half-life 2.6 hours). The two nuclides of iron and lower atomic weight nuclide of manganese last long enough to cycle through Great Lakes ecosystems. 55Fe and 54Mn decay by electron capture mechanisms, and 59Fe and 56Mn decay by beta emission. The decay products of 56Mn, a stable nuclide of iron, is itself subject of neutron activation. The remaining possible decay products undergo further decay by beta- and beta+ emissions, further electron capture mechanisms, or some combination thereof. All daughter nuclides from these decay schemes have relatively high thermal neutron capture probabilities (i.e., "cross sections").
The most important biological data on the radionuclides of iron and manganese come from studies of the marine environment and the monitoring of the Hanford facility discharges to the Columbia River and Pacific Ocean.
Ruthenium
Without nuclear technology, there would probably be very little interest in this element. Two radionuclides of ruthenium are produced in significant quantities in nuclear fuel processing and fission processes: 103Ru (half-life 41 days) and 106Ru (half-life 1 year). Both nuclides have sufficient time under a continuing discharge pattern to cycle within the biota of the Great Lakes, but the element has rarely been detected analytically in freshwater biota and reported in the literature.
The Radiation Protection Branch of the New York State Department of Health (1983-1993) has provided something of an exception to the monitoring of ruthenium. This group uses gamma radiation scintillation spectra to scan for selected radionuclides in biological materials collected from several sites in the New York portion of the Great Lakes Region. The problem, however, is that data reported for 106Ru in biological samples show that the nuclide occurred at the lower limit of detection, if at all, and because the lower limit of detection differed from sample to sample, a range of lower limits of detection was observed. This makes inventory calculations very uncertain for ruthenium nuclides. Data for the Springville Dam site on Cattaraugus Creek (the site for monitoring effluents from the West Valley nuclear reprocessing facility that feed into Lake Erie) and Oswego, New York, near the Nine Mile Point nuclear power plant (Lake Ontario) appear in Table 18 and illustrate these issues.
Although ruthenium belongs to the same chemical family as iron and manganese, the Task Force considers it rather unwise to assume that the biological cycling of ruthenium follows the pattern of the biological cycling of iron and manganese. Available studies suggest that ruthenium moves to sediments and that most of the radiation it contributes to ecosystem burdens is gamma radiation from external exposure. The element has no known biological role. The Task Force has chosen not to estimate a biocompartment inventory for this element given its lack of observation in major studies of the elemental composition of freshwater biota.
| TABLE 18 RADIONUCLIDES IN THE GREAT LAKES REGION: 106Ru, 137Cs, and 40K (adapted from the Annual Reports of the Radiation Protection Branch of the New York State Department of Health 1983-1993) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Notes:
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Rhenium and Osmium
What was said of ruthenium generally applies to rhenium and osmium. The radionuclides, except for a primordial nuclide of rhenium (187Re), are very short lived. The elements have no known biological role but are toxic. They are rare in the earth's crust, and despite their toxicity, they are usually not detected in freshwater biota.
Technetium
This artificial element has several isotopes, all radioactive, but the most important isotope is 99Tc (half-life 212,000 years). The remaining isotopes are short lived and not considered. Technetium bioaccumulates in tissues, but its pathways of biological cycling are almost unknown. There have been no comprehensive environmental studies, although its expected chemical form of technetium in aqueous systems is the TcO4-1, most of whose compounds are soluble. This makes the element potentially available for uptake by organisms in the water column.
Radionuclides of technetium result from fission and neutron activation. The isotopes also have use as medical diagnostic agents. Thus, there are many sources of technetium to the Great Lakes, but without a considerable quantity of fundamental data on water levels and biotic levels, no bioaccumulation factors are estimatable.
GROUP VIIIB ELEMENTS: COBALT, RHODIUM, IRIDIUM, NICKEL, PALLADIUM, AND PLATINUM
Similar to the Group VIIb elements, the Group VIIIb elements also come in two subgroupings: (1) cobalt, rhodium, and iridium and (2) nickel, palladium, and platinum. Only cobalt from the first subgrouping and nickel from the second subgrouping have radionuclides that are potentially bioaccumulation problems for Great Lakes biota (Table 17). The radionuclides of both elements are activation products and among those for which environmental monitoring is called for if detected.
Cobalt
All radionuclides of cobalt form as activation products. Four radionuclides, 57Co, 58Co, 59Co, and 60Co are discharged to the Great Lakes. Two of the nuclides, 60Co (half-life 5.26 years) and 57Co (half-life 270 days) are important in the discharges to the Great Lakes. The former has commercial use, and the latter is a major nuclide in research.
Cobalt is an essential micronutrient and is the metallic element that activates vitamin B12. Therefore, cobalt biouptake is expected in all aquatic biota. Since a radioactive version of the vitamin is a source of internal radiation to blood forming organs and the liver in vertebrates, the cycling of cobalt is of both health and ecological interest.
Nickel
Most radionuclides of nickel form as activation products or the decay products of radioactive isotopes of other activation products, notably radionuclides of cobalt. Thus, cobalt and nickel are strongly connected in their radiochemistry. The direct discharge of radionuclides of nickel to the Great Lakes is infrequent, and a continuous discharge of radionuclides of nickel suggests possible reactor shielding operational problems.
Complicating the situation about nickel discharges to the Great Lakes is the INCO smelter at Sudbury, Ontario. The stack on this smelter is the largest discharger of nickel to the environment in North America. A gradient of nickel levels in water and terrestrial vegetation from Sudbury, Ontario, to the Great Lakes tracks these discharges. As an example, compare the nickel level in zooplankton from McFarlane Lake in Table 16 with other lakes in the Sudbury area. (Yan et al. 1989).
Stable nickel isotopes have relatively large thermal neutron absorption cross sections, making the activation of nickel in the environment by other radionuclides are of possible concern. The Task Force raises the question of whether the nonradioactive nickel in the smelter stack effluents that deposit in the Great Lakes following long-range transport are subject en route to nuclear activation. There are several possible mechanisms which the Task Force could postulate, but no information on any hypothesis is available nor is documentation.
The height of the smelter stack provides a high tropospheric discharge port for the gaseous effluents, and various atmospheric circulatory processes may move some of the gas and particulate matter to higher altitudes and exposure to cosmogenically derived neutrons. There may also be an intersection of stack plumes at some location between the smelter gaseous and particulate effluents and the similar gaseous and particulate effluents from a nuclear power plant. Another question relates to a possible activation processes by a within-lake (in situ) source of radionuclides. The Task Force does not have answers to the preceding questions, but raises them mainly because of the uniqueness of the sources of nickel to the Great Lakes.
The main radionuclide of nickel of interest is 63Ni (half-life 92 years). A second and very much longer lived nuclide, 59Ni (half-life 80,000 years), is occasionally of interest.
Nickel accumulates in aquatic organisms. A biological role for nickel as an essential nutrient may exist. Price and Morel (1990) cite nickel as a possible heavy metal cofactor for enzyme urease. DeFilippis and Pallaghy (1994) cite data on the freshwater red alga, Cyanidium caldarium, as showing some toxicity resistance to Cu/Ni combinations in laboratory cultures, suggesting either an exclusion mechanism for Ni uptake or the ability to immobilize the bioaccumulated materials.
TRANSURANIC ELEMENTS
The presence of transuranic elements as natural inorganic constituents of freshwater biota is rare. Cannon (1960) has studied the presence of transuranics, mainly uranium and thorium, in terrestrial plants. Cowgill has provided data on thorium in her study of the macrophytes of Linsely Pond. She also found thorium in the aphid (R. nymphaeae) which feeds on the leaves of the macrophytes and in the sediments, but none in the water column. Thus, the Task Force has not attempted any estimates of thorium uptake in biota from the water column, but has considered uptake estimates relative to sediments. This is actually a preferred approach since previously the Task Force indicated that transuranics move to sediments as their environmental repository. Uptake from the water column would mainly require ingestion to bring the transuranics internally into the organism as opposed to direct osmosis from the water column. It is possible that such ingestion would find the transuranics bound to particulate matter in the water ingested (Table 19).
| TABLE 19 THORIUM IN AQUATIC ORGANISMS (Adapted from Cowgill 1973a, 1973b) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Notes:
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Dr. Jack Cornett of the Chalk River Laboratory of Atomic Energy Canada Limited supplied data for radionuclides in fishes. These data were in the form of the wet weight and therefore require some estimates on what might be the water content of material. To see how this affects calculated biological transfer factors, notably bioaccumulation and enrichment factors, a simple modeling analysis was undertaken. A range of the water contents of the substrates studied that were typical of biological materials were chosen, and the bioaccumulation factors on an assumed dry weight basis were calculated and compared.
Data are presented for 18 elements that have nuclides with half-lives of greater than 40 days. As can be seen, the difference in estimated bioaccumulation factors can vary over as much 1.5 orders of magnitude (Table 20). Several of the elements show unusual bioaccumulation possibilities in freshwater fishes, which have not been observed in other freshwater taxa, notably iodine, antimony, and zinc. Iodine numbers reflect the presence of thyroid hormones, a unique hormonal constituent in the vertebrates. Zinc is found as cofactor in many vertebrate hormonal systems, especially pancreatic materials since it is the activating element of insulin, and in a number of proteins in vertebrate systems which have a "zinc finger" in their structure (a group of four or five nitrogen atoms from amino acid groups bonded covalently to zinc as a means of stabilizing a particular conformation of protein with a finger or appendage design, or even a peninsula effect if the rest of the protein shape is compared to an island. Antimony is unexpected. Freshwater biota, mainly plants (algae) and invertebrates (zooplankton and insects) have shown no evidence of antimony, and the antimony data remain interesting and unexplained.
A number of elements in Table 20 show lower bioaccumulation potential than is exhibited in plants and invertebrates. Notable here are calcium, magnesium, potassium, molybdenum, and vanadium. Calcium, magnesium, and potassium reflect the physiological systems of fishes uniquely. Molybdenum and vanadium appear in vertebrate physiological systems but are more related to plants.
Several elements show greater bioaccumulation potential than would be expected, and this is also related to vertebrate physiology, but these elements have known bioaccumulation possibilities in all taxa, notably cesium, phosphorus, and rubidium. Cesium and phosphorus reflect that vertebrates have a calcium phosphate based skeletal structure, and the crystalline pattern of apatite minerals is adequate to accommodate the atomic and ionic radii of cesium, where it might not otherwise occur. The atomic and ionic radii of rubidium are smaller than comparable radii of cesium and can also fit into the mineral matrix of apatite. In addition, many rubidium compounds are water soluble and allowing the presence of this element in aqueous matrices.
BIOACCUMULATION FACTORS FOR FISHES FROM LAKE HURON FOR VARIOUS ASSUMED WATER CONTENTS OF TISSUES | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Notes:
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