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Ra-226 concentrations in blueberries Vaccinium angustifolium Ait. Near an inactive uranium tailings site in Elliot Lake, Ontario, Canada
Environmental Pollution (Series B) 10 (1985) 301-314
Ra-226 Concentrations in Blueberries Vaccinium angustifolium Ait. Near an Inactive Uranium Tailings Site in Elliot Lake, Ontario, Canada N. K. Dav6, T. P. Lim & N. R. Cloutier Elliot Lake Laboratory, CANMET, Energy, Mines and Resources Canada, Elliot Lake, Ontario, Canada, P5A2J6
ABSTRACT Ra-226 concentrations were measured in blueberries (Vaccinium angustifolium Ait., late low blueberry) growing around the Stanrock uranium tailings area near Elliot Lake, Ontario, Canada. Elevated levels of total Ra-226 ranging between 20-290mBqg-1 (dry weight) were observed in samples collected within 500 m from the tailings. Highest levels, ~ 285 mBq g - 1, were observed in a sample collected on a tailings spill. For sites located more than 500 m away in the upwind direction, and those situated at distances greater than 1kin downwind from the waste pile, the total Ra-226 concentrations approached background levels which were measured as 2-6 mBq g- 1. Approximately 17 % of the total Ra-226 measured was removable by washing the samples with distilled water. Wind dispersal of the tailings material and its deposition in the form of dust on blueberries was believed to be responsible for the external contamination. Based on the ICRP recommended dose limits for oral intake of Ra-226, it was calculated that approximately 160kga -1, 3350 kg a- 1and 47 kg a- 1 of washed blueberries (wet weight)from inside (less than 500m) and outside (greater than l km downwind) the influenced zone, and from the tailings spill site (where the highest Ra-226 levels in blueberries were measured), respectively, wouM need to be consumed before the individual annual limit jor the general public was exceeded.
INTRODUCTION The Elliot Lake area (Fig. 1), situated within the Great Lakes-St Lawrence mixed hardwood forest (Rowe, 1972), is the major uranium 301 ~" Crown Copyright, 1985
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mining region of Ontario, Canada. There are approximately 120 million tonnes of uranium tailings, placed over an area of 600 ha in the vicinity of Elliot Lake. About 25 million tonnes of these tailings, covering an area of approximately 250 ha, are located at sites which have been inactive for the past 17 years or so. Most of these inactive sites have been quite successfully stabilised through revegetation, which has greatly reduced wind and water erosion and has resulted in aesthetic improvement. For other sites, the tailings surface was stabilised chemically by the application of lime, which neutralised acidity produced as a result of the oxidation of pyrite in the tailings material, and formed a hard surface
R a - 2 2 6 in b l u e b e r r i e s n e a r an u r a n i u m site
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crust, thereby resisting erosional forces to a certain extent. Vegetation growing on and around uranium tailings is known to have elevated levels of radionuclides, such as Th-230, Ra-226, Pb-210, and chemical Pb resulting from higher contaminant levels in the substrate (Moffett & Tellier, 1977; Marple, 1980; Skinner, 1982; Cloutier et al., 1983; Dav6 et al., 1984). The effect is more pronounced for non-stabilised uranium tailings where wind erosion has been quite considerable. Breslin & Glauberman (1970) demonstrated clear relationships between the distance from a uranium tailings pile and the concentrations of uranium and Pb-210 in the dust downwind from the waste pile. Snelling& Shearer (1969) measured concentrations of Ra-226 and Th-230 exceeding the recommended concentration guides downwind from a uranium tailings pile, and stabilisation of the waste area was recommended based on those results. Skinner (1982) determined that Ra-226 concentrations in soil and total vegetation approached background levels at a distance of 1.1 and 6.6 km, respectively, from a uranium tailings pile. It is therefore possible to use, as indicators, contaminant levels in vegetation growing around uranium tailings piles to evaluate the zone of influence of the waste heap on the surrounding environment and the impact, if any, of a successful reclamation programme. In the present study, Ra-226 concentrations were measured in blueberries Vaccinium angustifolium Ait. growing around an inactive uranium tailings pile near Elliot Lake. The berries are a native shrub and grow in abundance in shallow soil of the region. Samples were collected at various locations around the tailings site and analysed for Ra-226 in both washed and unwashed samples. The results are discussed in terms of wind-related dispersion factors and their effects on both the external and internal contamination, and on the zone of influence. Following the International Commission on Radiological Protection (ICRP) recommendations for exposure to radioactivity by ingestion for the general public, annual intake limits of blueberries growing around such a waste site were calculated. METHODS
Study site and field procedure The Stanrock tailings area (Fig. 1), located about 20 km northeast of the town of Elliot Lake, was chosen as a suitable site for the present
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N. K. DavO, T. P. Lim, N. R. Cloutier
investigation. Approximately 8 million tonnes of neutralised tailings were discharged to the basin from Stanrock and Can-Met mines from 1957 to 1964 and cover an area of 71 ha. The tailings were impounded by dams, constructed of classified tailings sands on the southeast and west sides. One of the dams on the west side failed, causing tailings to spill into an adjacent swamp and creek. In 1978, the tailings surface was stabilised by spraying with lime in order to control wind erosion. This has provided neutral surface conditions which have encouraged invasion by natural vegetation into the tailings area. Most of the immediate area around the tailings is open and has shallow soil which supports white pine, white spruce, white birch and local shrubs, including blueberries. Abundant blueberry patches were observed all around the tailings area, on windblown tailings present on the dams, and on outcrops in the main tailings area. Sampling sites, each of approximate dimensions 300 x 300 m, were located all around the tailings area and at various distances from tailings along the prevalent wind direction (Fig. 2). The control site, located 30km northwest of the Stanrock tailings (Fig. 1), was approximately 20km northwest of the closest uranium tailings present in the Elliot Lake area. At the control site, samples were collected at two different locations (Fig. 1) and then pooled together to make a composite sample. All of the samples were hand-picked in late July-early August, 1983. Only ripe berries were picked and, where possible, approximately I kg of sample was collected at each location. The summer of 1983 was very hot and dry, and at many locations blueberry plants were all parched before the berries were fully ripe. The crop yield was therefore low and at some locations, away from tailings on high grounds, only 300400 g of sample could be collected. Plants growing on or close to the tailings near the impoundment dams and on outcrops had more moisture available and the best crop yield. All samples were stored in pre-washed containers, sealed, labelled and transported to the laboratory within 8 h.
Sample preparation and digestion Immediately after arrival at the laboratory, leaves, twigs, etc., were sorted out from the samples and discarded. Each sample was then divided into two parts according to its wet weight. The first half of the sample was thoroughly washed with distilled water and the second half was left unwashed. Both the washed and unwashed portions were placed, respectively, in pre-weighed beakers (acid-washed), weighed and then
Ra-226 in blueberries near an uranium site
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Fig. 2. Samplinglocations at the Stanrock railings site. oven dried at 100 °C until a constant weight was reached. The dry weights were recorded. To concentrate Ra-226, the oven-dried samples (in their respective beakers) were ashed in a muffle furnace using the following heating programme: 3 h at 200 °C and 5 h at 300 °C. The samples were then cooled slowly and the weight of ash was recorded. For each sample, 5 g of ash was placed in a beaker (acid-washed) and 40ml of Ba-133 tracer solution (approximately 1850 Bq) was added to determine the recovery factor for Ra-226 analysis. Subsequently, the samples were wet ashed by digesting with 5 ml of HC10 4, 20 ml of H N O 3 and 2 ml of HzSO 4. The mixture was heated until all the organic matter was oxidised and a clear solution was obtained. The digested samples were then dissolved in 20 ~o HCI and vacuum-filtered using 0-45 #m membrane filters. The filtrate (-~ 200 ml) for each sample was stored in a plastic bottle. A small amount of white residue, believed to be silica and KC10 4
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precipitate, was present on the filter paper for all the samples. It was observed that about 60-80 % of Ba-133 and Ra-226 had absorbed on the residue, which could not be acid solubilised or leached even after breaking down silica with HF treatment. The filter papers were therefore also ashed and digested as follows: dry-ashed in a platinum crucible at 500 °C for 12 h in a muffle furnace, cooled slowly and digested with 10 ml HF at low heat, followed by dissolution in 20 % HC1. This treatment provided a clear solution with no residue left after a second filtration. All filter papers were checked for Ba-133 activity before discarding to ensure that no material was lost. The overall recovery at this stage was observed to be close to 100 %. For each sample, all the filtrates were added together and the final solution was brought to 1 litre. Data concerning the recoveries of Ra-226 and Ba-133 at various stages of the above-mentioned sample treatment programme will be published separately elsewhere.
Analytical methods Ra-226 concentrations in solution samples were measured, following a chemical separation of radium as Ra-Ba sulphate, by alphaspectrometry, where the 4.78 MeV alpha-decay peak of Ra-226 was counted. Details of the analytical procedure and measurement technique are reported in a previous paper (Lim & Dav6, 1981). As mentioned above, a known amount of Ba-133 tracer solution is added to the starting solid sample to measure the overall recovery for Ra-226 analysis. All analyses were done in duplicate, where a 250ml aliquot of the l-litre sample was used for each analysis. For calibration purposes, certified Ra-226 Standard Solutions from the National Bureau of Standards were used. In order to quantify Ra-226 levels in organic samples and to test the overall precision and accuracy of the method, several vegetative samples were spiked with 1 ml of a standard Ra-226 solution containing 388.5 _+ 48.1 mBq activity and carried through the above sample preparation and analysis technique. Ra-226 levels in the spiked samples were measured as 386.3 + 10.0 mBq, giving an overall recovery of 99.4 %. RESULTS A N D DISCUSSION The results obtained for Ra-226 concentrations for both washed and unwashed blueberry samples around the tailings site are summarised in
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Fig. 3. Ra-226 levels (mean, + 1SD, mBq g- 1 dry weight) measured in unwashed (U) and washed (W) blueberry samples collected at various sites around the Stanrock tailings.
Fig. 3. All the values are reported in terms of activity as mBq g - 1 (dry weight). The measured concentrations varied between 20-290 mBq g-1 for samples located on outcrops of the tailings area and its immediate vicinity (within 500m). The highest values, 284 and 2 8 6 m B q g -1, respectively, for washed and unwashed samples, were obtained at site B15, where blueberries were growing on a tailings spill. Intermediate values of 70 m B q g - t were obtained for sites B-2 and B-7, located on small outcrops totally within the main tailings impoundment. Low values of 25 mBq g - 1 were obtained for site B-11, which was located within 250 m north of the tailings area and at a higher elevation (Fig. 2). For other sites: B-3, located at a higher elevation and more than 500 m northwest of the waste site; B-16 in the southeast direction; and B-17 in the south, which
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were both at a distance greater than I km, the measured concentrations were comparable to those obtained for the control sites ranging between 2-6 mBq g-1. The average Ra-226 concentration in the tailings coarse material (sands), observed near some of the sites close to the tailings, was measured as 3670 + 8 9 0 m B q g - 1 (Bigu et al., 1984). The windrose pattern for Elliot Lake area is shown in Fig. 4. The data were collected between January 1980 and December 1982 at the Elliot Lake airport, which is located 13 km south of the Stanrock tailings site.
Fig. 4. Windrose pattern observed at the Elliot Lake Airport (located 13 km south of Stanrock tailings) based on 18513 regular recordings between 1 January 1980 and 31 December 1982 (courtesy of Elliot Lake Airport Management and Wallace and Associates). Winds having a velocity over 18.5, 37.0 and 55.6 km h - ~accounted for 31.0, 6.7 and 0.6 °o of the record, respectively.
The above results (Fig. 3) show that Ra-226 levels were higher in unwashed samples than in the distilled water washed samples. Figure 5 shows a plot of Ra-226 concentrations in unwashed versus washed blueberry samples. A statistical analysis of the data showed a linear relationship of the form y = 1 . 1 7 x - 1 . 0 5 (coefficient of regression r = 0"91) between unwashed (y) and washed (x) Ra-226 concentrations.
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Ra-226 CONCENTRATIONSImBq/g dry weight] IN WASHEO BLUEBERRIES
Fig. 5. Plot of Ra-226 concentrations (mBq g - 1 dry weight) in washed and unwashed samples. A is the theoretical line y = x , where unwashed and washed Ra-226 concentrations are the same (i.e. no external contamination). B is the linear regression line y = 1.17x - 1.05 fitted to the observed data (R = 0-91), where y and x are, respectively, Ra226 concentrations in unwashed and washed samples. C are the 95 % confidence limits to the regression line.
Also plotted in Fig. 5 are 95 % confidence limits for the linear regression line and the theoretical line y = x for no external contamination. A t-test on the slope of the regression line showed that for 95 ~o confidence limits, Ra-226 concentrations in unwashed samples were 17 % higher (P < 0.05) than those in the washed samples, indicating a substantial external contamination. In the above regression analysis the results for site B-15,
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where the blueberries were growing in an area covered by a previous tailings spill and contained mostly fine railings material (slimes), were not included. Approximately 60-80 9/0 of the total Ra-226 contained in the original tailings is associated with fines, particles smaller than 44/~m (Skeaff, 1977), which makes the spill site unique in relation to the main tailings impoundment. The observed external contamination in blueberries could have two origins: wind-deposited tailings and soil splash. Wind dispersal of tailings is a very common phenomenon and around the Stanrock site there were visual indications of wind erosion during dry periods, although the tailings surface was stabilised with lime. At some sites close to the impoundment, B- l, B-2, B-4 to B- 10, B- 12 and B- 13, tailings material was readily visible on the ground during summer months, and on the snow during winter. At most of these sites, the data showed evidence of external Ra-226 contamination, making wind-deposited tailings dust a probable cause of the observed contamination. Furthermore, the presence of tailings in the soil substrate could also contribute as external contamination in the form of soil splash during periods of heavy rainfall. However, the summer of 1983 was abnormally dry and the external contamination was believed to have resulted primarily from the windblown dust. It should be mentioned that external contamination refers here to the fraction which was removable by a simple water wash. Tailings particles have very sharp edges in comparison to natural sand due to their crushed nature. Wind-blown particles are therefore believed to embed, to some extent, in the berries. Such contamination is probably not removed in the washing process and was considered as a part of internal contamination. The extent of contamination by wind dispersal of tailings depends upon various factors, such as: average wind velocity and frequency; local terrain and surface conditions; physical characteristics of tailings (average moisture content, particle size distribution and surface conditions); radium concentration of tailings and the presence, or absence, of a snow cover during winter. If it is assumed that the wind frequency distribution pattern at the Stanrock site was similar to one obtained at the Elliot lake airport (Fig. 4), then maximum dispersal of the tailings material is expected along the south, southeast and east sides of the impoundment area. The present data show a similar dispersal trend where, in the downwind direction, Ra-226 contamination in blueberries was limited to a distance of approximately 1 km from the impoundment
Ra-226 in blueberries near an uranium site
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Fig. 6. Plot of Ra-226 concentration (mBq g - ~ dry weight) in unwashed berries versus wind dispersal factor (Wd), obtained from Fig. 4, The solid line was obtained by linear regression of the plotted data with correlation coefficient R = 0.65.
boundaries. Figure 6 shows a plot of the observed radium concentration (in a given direction) versus wind dispersal factor (WI) defined as: 0=s
d o
where fo and V o are, respectively, wind frequency and velocity along the angular direction '0' with respect to the north, s is the angular sector over which the data are integrated (for the present case s was taken as 45 °) and d is the average distance of the sampling site from the impoundment boundary. It should also be mentioned in context with the above plot that for many locations Ra-226 concentration data size was limited to those of available sampling sites in the field. Skinner (1982) reported that Ra-226 levels in total vegetation (Artemisia tridentata) approached background levels at a distance of 6.6 km downwind from a US tailings site which had higher radium levels and was drier than the Stanrock tailings. Because of local site and climatic conditions, the extent of wind dispersal from Stanrock tailings was limited to a distance l km downwind. Interestingly, wine made from blueberries collected around B-17 site, which was more than 1 km away from tailings downwind, had an average Ra-226 concentration of 392mBq litre-1. This, after conversion, corresponded to a Ra-226
312
N. K. Dark, T. P. Lira, N. R. Cloutier
concentration of 4.8 mBq g- 1 dry weight, which is comparable to values reported for site B-17 above. For the oral intake of Ra-226 radioisotope, the ICRP has recommended an annual intake limit (ALL) of 2 × 105Bqa -1 for radiation workers for stochastic effects (ICRP, 1979b). The recommended dose equivalent limits for exposure to radionuclides for individual members of the general public are ~ to 3!6othat of radiation workers based on exposure to critical groups where the lower limit is for exposure over prolonged periods of time (ICRP, 1977, 1979a, 1984). Assuming that the above dose equivalent limits are true for ingestion of radionuclides and using the lower exposure limits, the annual intake limit for individual members of the general public for Ra-226 can be taken as of 2 x 105Bqa -1 = 4 0 0 0 B q a -1. Based on the average Ra-226 concentration of 85-2 mBq g-1 dry weight in washed blueberries collected within 500 m from the tailings (including those growing on the tailings and the spill site) it was calculated that approximately 160 kg a-1 of wet blueberries (70 ~ moisture by weight) from Stanrock tailings site need to be consumed by a member of the general public before exceeding the above oral intake limit for Ra-226. Similarly, it was calculated that annual consumptions in excess of 47 kg a - ~ and 3350 kg a - ~ of wet-washed blueberries collected at the spill site, where the highest level of Ra-226 was detected (285 mBqg-1 dry weight), and from outside the zone of influence, 1 km away from the waste site (4-0 mBq g-x dry weight), are required, respectively, to exceed the recommended intake dose limits. It is not believed that casual consumption of the blueberries by the public will cause the above limits to be exceeded.
CONCLUSIONS At the Stanrock tailings site in Elliot Lake, Ontario, elevated Ra-226 levels were observed in late low blueberries Vaccinium angustifolium Ait. Total Ra-226 levels in samples collected within 500 m from the tailings were elevated compared to background levels. At distances greater than 1 km downwind from the waste pile, wind dispersal of the tailings material was considered insufficient to increase radium levels in blueberries above background levels. Wind dispersal of the tailings material and its deposition in the form of dust on blueberries is believed to be the cause of the observed external contamination. Approximately 17 ~o of the total
Ra-226 in blueberries near an uranium site
313
Ra-226 present in blueberries was easily removable by a simple distilled water wash. Following the ICRP recommended dose limits for the oral intake of Ra-226, an excess of 160 kg a - 1, 3350 kg a - 1 and 47 kg a - 1, respectively, of washed blueberries (wet weight) from inside and outside the zone of influence (extent of I km downwind from the waste pile), and from the tailings spill site, where the highest Ra-226 levels in blueberries were measured, need to be consumed by a member of the general public before exceeding the annual intake dose limit.
ACKNOWLEDGEMENTS The authors wish to thank Mr S. Lundberg, Manager, Elliot Lake Airport, for making the wind data available, and Mr J. L. Chakravatti, Chief Environmental Engineer, Denison Mines Ltd, for his assistance and many helpful suggestions.
REFERENCES Bigu, J., Grenier, M., Dav6, N. K., Lim, T. P. & Chakravatti, J. L. (1984). Study of radon gas concentration, surface radon flux and other radiation variables from uranium mine tailings areas. Uranium, 1,257-77. Breslin, A. J. & Glauberman, H. (1970). Investigation of radioactive dust dispersal from uranium tailings piles. In Environmental surveillance in the vicinity of nuclear facilities, ed. by W.C. Reinig, 249-53. Springfield, Illinois, Thomas. Cloutier, N. R., Clulow, F. V., Vivyurka, A., Dav6, N. K. & Lim, T. P. (1983). Metals ( Cu, Ni, Fe, Co, Zn, Pb ) and radionuclide (Ra-226) levels in meadow voles (Microtus pennsylvanicus) established on revegetated copper and nickel tailings in Copper Cliff and uranium tailings in Elliot Lake. Division
Report MRP/MRL 83-53(TR), CANMET, Energy, Mines and Resources Canada. Dav6, N. K., Lim, T. P. & Cloutier, N. R. (1984). Radionuclide uptake by various plants growing on uranium tailings, Elliot Lake, Ontario. Division Report MRP/MRL 84-19(TR), CANMET, Energy, Mines and Resources Canada. ICRP (1977). Recommendations of the International Commission on Radiological Protection, ICRP Publication, 26, Annals of the ICRP, 1, No. 3. ICRP (1979a). Limits for intakes of radionuclides by workers. ICRP Publication, 30, Part L Annals of the ICRP, 2, No. 3/4. ICRP (1979b). Limits for intakes of radionuclides by workers. ICRP Publication, 30, Supplement to Part L Annals of the ICRP, 3, No. 1--4.
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ICRP (1984). Principles for limiting exposure of the public to natural sources of radiation. ICRP Publication, 39, Annals of the ICRP, 14, No. 1. Lim, T. P. & Dav6, N. K. (1981). A rapid method of Ra-226 analysis in water samples using an alpha-spectroscopic technique. C I M Bulletin, 74, 97-105. Marple, M. L. (1980). Radium-226 in vegetation and inactive uranium mill sites. PhD thesis, University of New Mexico. Moffett, D. & Tellier, M. (1977). Uptake of radioisotopes by vegetation growing on uranium tailings. Can. J. Soil Sci., 57, 417-24. Rowe, J. S. (1972). Forest regions of Canada. Can. Forest Service No. 1300. Skeaff, J. M. (1977). Distribution of Ra-226 in uranium tailings. Division Report MRP/MSL 79-3400). CANMET, Energy, Mines and Resources Canada. Skinner, D. J. (1982). Ra-226 contamination of soil and foliage as a function of distance downwind from uranium tailings. US Department of Environment Report DOE/EV. 10305-9. Snelling, R. N. & Shearer, S. D. (1969). Environmental survey of uranium mill tailings pile, Tuba City, Arizona. Radiological health data and reports. US Health Service, 475-87.
Ra-226 Concentrations in Blueberries Vaccinium angustifolium Ait. Near an Inactive Uranium Tailings Site in Elliot Lake, Ontario, Canada N. K. Dav6, T. P. Lim & N. R. Cloutier Elliot Lake Laboratory, CANMET, Energy, Mines and Resources Canada, Elliot Lake, Ontario, Canada, P5A2J6
ABSTRACT Ra-226 concentrations were measured in blueberries (Vaccinium angustifolium Ait., late low blueberry) growing around the Stanrock uranium tailings area near Elliot Lake, Ontario, Canada. Elevated levels of total Ra-226 ranging between 20-290mBqg-1 (dry weight) were observed in samples collected within 500 m from the tailings. Highest levels, ~ 285 mBq g - 1, were observed in a sample collected on a tailings spill. For sites located more than 500 m away in the upwind direction, and those situated at distances greater than 1kin downwind from the waste pile, the total Ra-226 concentrations approached background levels which were measured as 2-6 mBq g- 1. Approximately 17 % of the total Ra-226 measured was removable by washing the samples with distilled water. Wind dispersal of the tailings material and its deposition in the form of dust on blueberries was believed to be responsible for the external contamination. Based on the ICRP recommended dose limits for oral intake of Ra-226, it was calculated that approximately 160kga -1, 3350 kg a- 1and 47 kg a- 1 of washed blueberries (wet weight)from inside (less than 500m) and outside (greater than l km downwind) the influenced zone, and from the tailings spill site (where the highest Ra-226 levels in blueberries were measured), respectively, wouM need to be consumed before the individual annual limit jor the general public was exceeded.
INTRODUCTION The Elliot Lake area (Fig. 1), situated within the Great Lakes-St Lawrence mixed hardwood forest (Rowe, 1972), is the major uranium 301 ~" Crown Copyright, 1985
302
N. K. DavO, T. P. Lim, N. R. Cloutier
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mining region of Ontario, Canada. There are approximately 120 million tonnes of uranium tailings, placed over an area of 600 ha in the vicinity of Elliot Lake. About 25 million tonnes of these tailings, covering an area of approximately 250 ha, are located at sites which have been inactive for the past 17 years or so. Most of these inactive sites have been quite successfully stabilised through revegetation, which has greatly reduced wind and water erosion and has resulted in aesthetic improvement. For other sites, the tailings surface was stabilised chemically by the application of lime, which neutralised acidity produced as a result of the oxidation of pyrite in the tailings material, and formed a hard surface
R a - 2 2 6 in b l u e b e r r i e s n e a r an u r a n i u m site
303
crust, thereby resisting erosional forces to a certain extent. Vegetation growing on and around uranium tailings is known to have elevated levels of radionuclides, such as Th-230, Ra-226, Pb-210, and chemical Pb resulting from higher contaminant levels in the substrate (Moffett & Tellier, 1977; Marple, 1980; Skinner, 1982; Cloutier et al., 1983; Dav6 et al., 1984). The effect is more pronounced for non-stabilised uranium tailings where wind erosion has been quite considerable. Breslin & Glauberman (1970) demonstrated clear relationships between the distance from a uranium tailings pile and the concentrations of uranium and Pb-210 in the dust downwind from the waste pile. Snelling& Shearer (1969) measured concentrations of Ra-226 and Th-230 exceeding the recommended concentration guides downwind from a uranium tailings pile, and stabilisation of the waste area was recommended based on those results. Skinner (1982) determined that Ra-226 concentrations in soil and total vegetation approached background levels at a distance of 1.1 and 6.6 km, respectively, from a uranium tailings pile. It is therefore possible to use, as indicators, contaminant levels in vegetation growing around uranium tailings piles to evaluate the zone of influence of the waste heap on the surrounding environment and the impact, if any, of a successful reclamation programme. In the present study, Ra-226 concentrations were measured in blueberries Vaccinium angustifolium Ait. growing around an inactive uranium tailings pile near Elliot Lake. The berries are a native shrub and grow in abundance in shallow soil of the region. Samples were collected at various locations around the tailings site and analysed for Ra-226 in both washed and unwashed samples. The results are discussed in terms of wind-related dispersion factors and their effects on both the external and internal contamination, and on the zone of influence. Following the International Commission on Radiological Protection (ICRP) recommendations for exposure to radioactivity by ingestion for the general public, annual intake limits of blueberries growing around such a waste site were calculated. METHODS
Study site and field procedure The Stanrock tailings area (Fig. 1), located about 20 km northeast of the town of Elliot Lake, was chosen as a suitable site for the present
304
N. K. DavO, T. P. Lim, N. R. Cloutier
investigation. Approximately 8 million tonnes of neutralised tailings were discharged to the basin from Stanrock and Can-Met mines from 1957 to 1964 and cover an area of 71 ha. The tailings were impounded by dams, constructed of classified tailings sands on the southeast and west sides. One of the dams on the west side failed, causing tailings to spill into an adjacent swamp and creek. In 1978, the tailings surface was stabilised by spraying with lime in order to control wind erosion. This has provided neutral surface conditions which have encouraged invasion by natural vegetation into the tailings area. Most of the immediate area around the tailings is open and has shallow soil which supports white pine, white spruce, white birch and local shrubs, including blueberries. Abundant blueberry patches were observed all around the tailings area, on windblown tailings present on the dams, and on outcrops in the main tailings area. Sampling sites, each of approximate dimensions 300 x 300 m, were located all around the tailings area and at various distances from tailings along the prevalent wind direction (Fig. 2). The control site, located 30km northwest of the Stanrock tailings (Fig. 1), was approximately 20km northwest of the closest uranium tailings present in the Elliot Lake area. At the control site, samples were collected at two different locations (Fig. 1) and then pooled together to make a composite sample. All of the samples were hand-picked in late July-early August, 1983. Only ripe berries were picked and, where possible, approximately I kg of sample was collected at each location. The summer of 1983 was very hot and dry, and at many locations blueberry plants were all parched before the berries were fully ripe. The crop yield was therefore low and at some locations, away from tailings on high grounds, only 300400 g of sample could be collected. Plants growing on or close to the tailings near the impoundment dams and on outcrops had more moisture available and the best crop yield. All samples were stored in pre-washed containers, sealed, labelled and transported to the laboratory within 8 h.
Sample preparation and digestion Immediately after arrival at the laboratory, leaves, twigs, etc., were sorted out from the samples and discarded. Each sample was then divided into two parts according to its wet weight. The first half of the sample was thoroughly washed with distilled water and the second half was left unwashed. Both the washed and unwashed portions were placed, respectively, in pre-weighed beakers (acid-washed), weighed and then
Ra-226 in blueberries near an uranium site
305
Fig. 2. Samplinglocations at the Stanrock railings site. oven dried at 100 °C until a constant weight was reached. The dry weights were recorded. To concentrate Ra-226, the oven-dried samples (in their respective beakers) were ashed in a muffle furnace using the following heating programme: 3 h at 200 °C and 5 h at 300 °C. The samples were then cooled slowly and the weight of ash was recorded. For each sample, 5 g of ash was placed in a beaker (acid-washed) and 40ml of Ba-133 tracer solution (approximately 1850 Bq) was added to determine the recovery factor for Ra-226 analysis. Subsequently, the samples were wet ashed by digesting with 5 ml of HC10 4, 20 ml of H N O 3 and 2 ml of HzSO 4. The mixture was heated until all the organic matter was oxidised and a clear solution was obtained. The digested samples were then dissolved in 20 ~o HCI and vacuum-filtered using 0-45 #m membrane filters. The filtrate (-~ 200 ml) for each sample was stored in a plastic bottle. A small amount of white residue, believed to be silica and KC10 4
306
N. K. Dark, T. P. Lira, N. R. Cloutier
precipitate, was present on the filter paper for all the samples. It was observed that about 60-80 % of Ba-133 and Ra-226 had absorbed on the residue, which could not be acid solubilised or leached even after breaking down silica with HF treatment. The filter papers were therefore also ashed and digested as follows: dry-ashed in a platinum crucible at 500 °C for 12 h in a muffle furnace, cooled slowly and digested with 10 ml HF at low heat, followed by dissolution in 20 % HC1. This treatment provided a clear solution with no residue left after a second filtration. All filter papers were checked for Ba-133 activity before discarding to ensure that no material was lost. The overall recovery at this stage was observed to be close to 100 %. For each sample, all the filtrates were added together and the final solution was brought to 1 litre. Data concerning the recoveries of Ra-226 and Ba-133 at various stages of the above-mentioned sample treatment programme will be published separately elsewhere.
Analytical methods Ra-226 concentrations in solution samples were measured, following a chemical separation of radium as Ra-Ba sulphate, by alphaspectrometry, where the 4.78 MeV alpha-decay peak of Ra-226 was counted. Details of the analytical procedure and measurement technique are reported in a previous paper (Lim & Dav6, 1981). As mentioned above, a known amount of Ba-133 tracer solution is added to the starting solid sample to measure the overall recovery for Ra-226 analysis. All analyses were done in duplicate, where a 250ml aliquot of the l-litre sample was used for each analysis. For calibration purposes, certified Ra-226 Standard Solutions from the National Bureau of Standards were used. In order to quantify Ra-226 levels in organic samples and to test the overall precision and accuracy of the method, several vegetative samples were spiked with 1 ml of a standard Ra-226 solution containing 388.5 _+ 48.1 mBq activity and carried through the above sample preparation and analysis technique. Ra-226 levels in the spiked samples were measured as 386.3 + 10.0 mBq, giving an overall recovery of 99.4 %. RESULTS A N D DISCUSSION The results obtained for Ra-226 concentrations for both washed and unwashed blueberry samples around the tailings site are summarised in
307
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Fig. 3. All the values are reported in terms of activity as mBq g - 1 (dry weight). The measured concentrations varied between 20-290 mBq g-1 for samples located on outcrops of the tailings area and its immediate vicinity (within 500m). The highest values, 284 and 2 8 6 m B q g -1, respectively, for washed and unwashed samples, were obtained at site B15, where blueberries were growing on a tailings spill. Intermediate values of 70 m B q g - t were obtained for sites B-2 and B-7, located on small outcrops totally within the main tailings impoundment. Low values of 25 mBq g - 1 were obtained for site B-11, which was located within 250 m north of the tailings area and at a higher elevation (Fig. 2). For other sites: B-3, located at a higher elevation and more than 500 m northwest of the waste site; B-16 in the southeast direction; and B-17 in the south, which
308
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were both at a distance greater than I km, the measured concentrations were comparable to those obtained for the control sites ranging between 2-6 mBq g-1. The average Ra-226 concentration in the tailings coarse material (sands), observed near some of the sites close to the tailings, was measured as 3670 + 8 9 0 m B q g - 1 (Bigu et al., 1984). The windrose pattern for Elliot Lake area is shown in Fig. 4. The data were collected between January 1980 and December 1982 at the Elliot Lake airport, which is located 13 km south of the Stanrock tailings site.
Fig. 4. Windrose pattern observed at the Elliot Lake Airport (located 13 km south of Stanrock tailings) based on 18513 regular recordings between 1 January 1980 and 31 December 1982 (courtesy of Elliot Lake Airport Management and Wallace and Associates). Winds having a velocity over 18.5, 37.0 and 55.6 km h - ~accounted for 31.0, 6.7 and 0.6 °o of the record, respectively.
The above results (Fig. 3) show that Ra-226 levels were higher in unwashed samples than in the distilled water washed samples. Figure 5 shows a plot of Ra-226 concentrations in unwashed versus washed blueberry samples. A statistical analysis of the data showed a linear relationship of the form y = 1 . 1 7 x - 1 . 0 5 (coefficient of regression r = 0"91) between unwashed (y) and washed (x) Ra-226 concentrations.
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Fig. 5. Plot of Ra-226 concentrations (mBq g - 1 dry weight) in washed and unwashed samples. A is the theoretical line y = x , where unwashed and washed Ra-226 concentrations are the same (i.e. no external contamination). B is the linear regression line y = 1.17x - 1.05 fitted to the observed data (R = 0-91), where y and x are, respectively, Ra226 concentrations in unwashed and washed samples. C are the 95 % confidence limits to the regression line.
Also plotted in Fig. 5 are 95 % confidence limits for the linear regression line and the theoretical line y = x for no external contamination. A t-test on the slope of the regression line showed that for 95 ~o confidence limits, Ra-226 concentrations in unwashed samples were 17 % higher (P < 0.05) than those in the washed samples, indicating a substantial external contamination. In the above regression analysis the results for site B-15,
310
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where the blueberries were growing in an area covered by a previous tailings spill and contained mostly fine railings material (slimes), were not included. Approximately 60-80 9/0 of the total Ra-226 contained in the original tailings is associated with fines, particles smaller than 44/~m (Skeaff, 1977), which makes the spill site unique in relation to the main tailings impoundment. The observed external contamination in blueberries could have two origins: wind-deposited tailings and soil splash. Wind dispersal of tailings is a very common phenomenon and around the Stanrock site there were visual indications of wind erosion during dry periods, although the tailings surface was stabilised with lime. At some sites close to the impoundment, B- l, B-2, B-4 to B- 10, B- 12 and B- 13, tailings material was readily visible on the ground during summer months, and on the snow during winter. At most of these sites, the data showed evidence of external Ra-226 contamination, making wind-deposited tailings dust a probable cause of the observed contamination. Furthermore, the presence of tailings in the soil substrate could also contribute as external contamination in the form of soil splash during periods of heavy rainfall. However, the summer of 1983 was abnormally dry and the external contamination was believed to have resulted primarily from the windblown dust. It should be mentioned that external contamination refers here to the fraction which was removable by a simple water wash. Tailings particles have very sharp edges in comparison to natural sand due to their crushed nature. Wind-blown particles are therefore believed to embed, to some extent, in the berries. Such contamination is probably not removed in the washing process and was considered as a part of internal contamination. The extent of contamination by wind dispersal of tailings depends upon various factors, such as: average wind velocity and frequency; local terrain and surface conditions; physical characteristics of tailings (average moisture content, particle size distribution and surface conditions); radium concentration of tailings and the presence, or absence, of a snow cover during winter. If it is assumed that the wind frequency distribution pattern at the Stanrock site was similar to one obtained at the Elliot lake airport (Fig. 4), then maximum dispersal of the tailings material is expected along the south, southeast and east sides of the impoundment area. The present data show a similar dispersal trend where, in the downwind direction, Ra-226 contamination in blueberries was limited to a distance of approximately 1 km from the impoundment
Ra-226 in blueberries near an uranium site
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boundaries. Figure 6 shows a plot of the observed radium concentration (in a given direction) versus wind dispersal factor (WI) defined as: 0=s
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where fo and V o are, respectively, wind frequency and velocity along the angular direction '0' with respect to the north, s is the angular sector over which the data are integrated (for the present case s was taken as 45 °) and d is the average distance of the sampling site from the impoundment boundary. It should also be mentioned in context with the above plot that for many locations Ra-226 concentration data size was limited to those of available sampling sites in the field. Skinner (1982) reported that Ra-226 levels in total vegetation (Artemisia tridentata) approached background levels at a distance of 6.6 km downwind from a US tailings site which had higher radium levels and was drier than the Stanrock tailings. Because of local site and climatic conditions, the extent of wind dispersal from Stanrock tailings was limited to a distance l km downwind. Interestingly, wine made from blueberries collected around B-17 site, which was more than 1 km away from tailings downwind, had an average Ra-226 concentration of 392mBq litre-1. This, after conversion, corresponded to a Ra-226
312
N. K. Dark, T. P. Lira, N. R. Cloutier
concentration of 4.8 mBq g- 1 dry weight, which is comparable to values reported for site B-17 above. For the oral intake of Ra-226 radioisotope, the ICRP has recommended an annual intake limit (ALL) of 2 × 105Bqa -1 for radiation workers for stochastic effects (ICRP, 1979b). The recommended dose equivalent limits for exposure to radionuclides for individual members of the general public are ~ to 3!6othat of radiation workers based on exposure to critical groups where the lower limit is for exposure over prolonged periods of time (ICRP, 1977, 1979a, 1984). Assuming that the above dose equivalent limits are true for ingestion of radionuclides and using the lower exposure limits, the annual intake limit for individual members of the general public for Ra-226 can be taken as of 2 x 105Bqa -1 = 4 0 0 0 B q a -1. Based on the average Ra-226 concentration of 85-2 mBq g-1 dry weight in washed blueberries collected within 500 m from the tailings (including those growing on the tailings and the spill site) it was calculated that approximately 160 kg a-1 of wet blueberries (70 ~ moisture by weight) from Stanrock tailings site need to be consumed by a member of the general public before exceeding the above oral intake limit for Ra-226. Similarly, it was calculated that annual consumptions in excess of 47 kg a - ~ and 3350 kg a - ~ of wet-washed blueberries collected at the spill site, where the highest level of Ra-226 was detected (285 mBqg-1 dry weight), and from outside the zone of influence, 1 km away from the waste site (4-0 mBq g-x dry weight), are required, respectively, to exceed the recommended intake dose limits. It is not believed that casual consumption of the blueberries by the public will cause the above limits to be exceeded.
CONCLUSIONS At the Stanrock tailings site in Elliot Lake, Ontario, elevated Ra-226 levels were observed in late low blueberries Vaccinium angustifolium Ait. Total Ra-226 levels in samples collected within 500 m from the tailings were elevated compared to background levels. At distances greater than 1 km downwind from the waste pile, wind dispersal of the tailings material was considered insufficient to increase radium levels in blueberries above background levels. Wind dispersal of the tailings material and its deposition in the form of dust on blueberries is believed to be the cause of the observed external contamination. Approximately 17 ~o of the total
Ra-226 in blueberries near an uranium site
313
Ra-226 present in blueberries was easily removable by a simple distilled water wash. Following the ICRP recommended dose limits for the oral intake of Ra-226, an excess of 160 kg a - 1, 3350 kg a - 1 and 47 kg a - 1, respectively, of washed blueberries (wet weight) from inside and outside the zone of influence (extent of I km downwind from the waste pile), and from the tailings spill site, where the highest Ra-226 levels in blueberries were measured, need to be consumed by a member of the general public before exceeding the annual intake dose limit.
ACKNOWLEDGEMENTS The authors wish to thank Mr S. Lundberg, Manager, Elliot Lake Airport, for making the wind data available, and Mr J. L. Chakravatti, Chief Environmental Engineer, Denison Mines Ltd, for his assistance and many helpful suggestions.
REFERENCES Bigu, J., Grenier, M., Dav6, N. K., Lim, T. P. & Chakravatti, J. L. (1984). Study of radon gas concentration, surface radon flux and other radiation variables from uranium mine tailings areas. Uranium, 1,257-77. Breslin, A. J. & Glauberman, H. (1970). Investigation of radioactive dust dispersal from uranium tailings piles. In Environmental surveillance in the vicinity of nuclear facilities, ed. by W.C. Reinig, 249-53. Springfield, Illinois, Thomas. Cloutier, N. R., Clulow, F. V., Vivyurka, A., Dav6, N. K. & Lim, T. P. (1983). Metals ( Cu, Ni, Fe, Co, Zn, Pb ) and radionuclide (Ra-226) levels in meadow voles (Microtus pennsylvanicus) established on revegetated copper and nickel tailings in Copper Cliff and uranium tailings in Elliot Lake. Division
Report MRP/MRL 83-53(TR), CANMET, Energy, Mines and Resources Canada. Dav6, N. K., Lim, T. P. & Cloutier, N. R. (1984). Radionuclide uptake by various plants growing on uranium tailings, Elliot Lake, Ontario. Division Report MRP/MRL 84-19(TR), CANMET, Energy, Mines and Resources Canada. ICRP (1977). Recommendations of the International Commission on Radiological Protection, ICRP Publication, 26, Annals of the ICRP, 1, No. 3. ICRP (1979a). Limits for intakes of radionuclides by workers. ICRP Publication, 30, Part L Annals of the ICRP, 2, No. 3/4. ICRP (1979b). Limits for intakes of radionuclides by workers. ICRP Publication, 30, Supplement to Part L Annals of the ICRP, 3, No. 1--4.
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ICRP (1984). Principles for limiting exposure of the public to natural sources of radiation. ICRP Publication, 39, Annals of the ICRP, 14, No. 1. Lim, T. P. & Dav6, N. K. (1981). A rapid method of Ra-226 analysis in water samples using an alpha-spectroscopic technique. C I M Bulletin, 74, 97-105. Marple, M. L. (1980). Radium-226 in vegetation and inactive uranium mill sites. PhD thesis, University of New Mexico. Moffett, D. & Tellier, M. (1977). Uptake of radioisotopes by vegetation growing on uranium tailings. Can. J. Soil Sci., 57, 417-24. Rowe, J. S. (1972). Forest regions of Canada. Can. Forest Service No. 1300. Skeaff, J. M. (1977). Distribution of Ra-226 in uranium tailings. Division Report MRP/MSL 79-3400). CANMET, Energy, Mines and Resources Canada. Skinner, D. J. (1982). Ra-226 contamination of soil and foliage as a function of distance downwind from uranium tailings. US Department of Environment Report DOE/EV. 10305-9. Snelling, R. N. & Shearer, S. D. (1969). Environmental survey of uranium mill tailings pile, Tuba City, Arizona. Radiological health data and reports. US Health Service, 475-87.