Radium-226 in cattails, Typha latifolia, and bone of muskrat, Ondatra zibethica (L.), from a watershed with uranium tailings near the city of Elliot Lake, Canada

Radionuclide contamination in Canada: A scoping review

Radionuclides were first discovered in the late 1800s, and artificial (anthropogenic) radionuclides in the 1930s. Since then, this group of substances has been increasingly incorporated into various peaceful and non-peaceful applications across Canada and the world, bringing with it both advanced technological and medical benefits, and public concern about the dangers from radiation exposure. As such, a breadth of research on, and monitoring of, radionuclides in the Canadian environment has been generated, the results of which span decades. However, a recent comprehensive review of these is not readily available. This study aims to fill this gap by synthesizing available literature from the last 30 years on the Canadian state and provenance of radionuclide contamination to better understand the context of overall sources and status of contamination. The findings indicate that while regional and temporal variations exist, on average, routine radionuclide exposure in Canada is generally attributed mainly to natural sources and fallout from historical nuclear weapons testing and nuclear accidents (including the Chernobyl and Fukushima power plant accidents) and to a smaller degree to emissions from nuclear facilities, including active and historical uranium mines and mills, nuclear research facilities, and nuclear power plants. Levels of anthropogenic radionuclides in the Canadian environment have declined since the initial cessation of nuclear weapons testing in the 1960s and are generally below guidelines protective of human health. On the national scale, present-day nuclear sector facilities do not appear to be a significant source of routine anthropogenic, nor technically-enhanced naturally occurring radionuclide exposure, though local scenarios may vary. These findings contribute context for evaluating the sustainable management of nuclear technologies, radioactive materials and waste in Canada and globally, in line with UN Sustainable Development Goal 12 and target 12.4: responsible management of chemicals and waste.

Oil & gas produced water retention ponds as potential passive treatment for radium removal and beneficial reuse

Oil and gas (O&G) extraction generates large volumes of produced water (PW) in regions that are often water-stressed. In Wyoming, generators are permitted under the National Pollutant Discharge Elimination System (NPDES) program to discharge O&G PW for beneficial use. In one Wyoming study region, downstream of the NPDES facilities exist naturally occurring wetlands referred to herein as produced water retention ponds (PWRPs). Previously, it was found that dissolved radium (Ra) and organic contaminants are removed within 30 km of the discharges and higher-resolution sampling was required to understand contaminant attenuation mechanisms. In this study, we sampled three NPDES discharge facilities, five PWRPs, and a reference background wetland not impacted by O&G PW disposal. Water samples, grab sediments, sediment cores and vegetation were collected. No inorganic PW constituents were abated through the PWRP series but Ra was shown to accumulate within PWRP grab sediments, upwards of 2721 Bq kg^-1, compared to downstream sites. Ra mineral association with depth in the sediment profile is likely controlled by the S cycle under varying microbial communities and redox conditions. Under anoxic conditions, common in wetlands, Ra was available as an exchangeable ion, similar to Ca, Ba and Sr, and S was mostly water-soluble. ²²⁶Ra concentration ratios in vegetation samples, normalizing vegetation Ra to sediment Ra, indicated that ratios were highest in sediments containing less exchangeable ²²⁶Ra. Sequential leaching data paired with redox potentials suggest that oxic conditions are necessary to contain Ra in recalcitrant sediment minerals and prevent mobility and bioavailability.

Effective radium-226 concentration in rocks, soils, plants and bones

Effective radium-226 concentration, ECRa, is the product of radium activity concentration, CRa, multiplied by the emanation coefficient, E, which is probability of producing a radon-222 atom in the pore spaces. It is measured by accumulation experiments in the laboratory, achieved routinely for a sample mass >50 g using scintillation flasks to measure the radon concentration. We report on 3370 ECRa values obtained from more than 11 800 such experiments. Rocks (n=1351) have a mean ECRa value of 1.9±0.1 Bq kg−1 (90% of data in the range 0.11–35 Bq kg−1), while soils (n=1524) have a mean ECRa value of 7.5±0.2 Bq kg−1 (90% of data between 1.4 and 28 Bq kg−1). Using this large dataset, we establish that the spatial structure of ECRa is meaningful in geology or sedimentology. For plants (n=85), ECRa is generally <1 Bq kg−1, but values of larger than 10 Bq kg−1 are also observed. Dedicated experiments were performed to measure emanation, E, in plants, and we obtained values of 0.86±0.04 compared with 0.24±0.04 for sands, which leads to estimates of the radium-226 soil-to-plant transfer ratio. For most measured animal bones (n=26), ECRa is >1 Bq kg−1. Therefore, ECRa appears essential for radon modelling, health hazard assessment and also in evaluating the transfer of radium-226 to the biosphere.

A review of the potential for radium from luminising activities to migrate in the environment

During the first half of the twentieth century, radium, mixed with other components, was used to luminise many items, including watches, clocks, dials and meters. On many sites, and in particular MoD sites, luminised instruments and paint were disposed of by burning and burial. This paper presents a review of the potential for radium from such sites to migrate in the environment. The most likely mechanisms of migration of radium from former luminising sites in the UK are surface-water erosion and transport, and the action of animals and people. Plant uptake or rainsplash followed by cropping of the plants is another possible mechanism, but the extent of impact is uncertain. The migration of significant quantities of radium through soils or rocks, or due to landsliding or wind transport, is considered to be of minimal importance to most UK sites. A low pH, high salinity (in particular of group II metals) or reducing groundwater would need to be present for migration through soils/rocks to occur and such conditions are unlikely to be present in most shallow aquifer systems in the UK. To reduce the potential for migration to occur it is recommended that luminising wastes at ground surface are removed or covered, that controls are put in place to limit animal activity and that human entry to former luminising sites is restricted.

Tissue metal levels in Muskrat (Ondatra zibethica) collected near the Sudbury (Ontario) ore-smelters; prospects for biomonitoring marsh pollution

An examination of tissue metal levels in Sudbury-area muskrat (Ondatra zibethica) revealed that animals collected in the vicinity of the local ore-smelters contained elevated burdens of Cd and Ni in their liver and kidneys. Respective tissue concentrations averaged 2-fold and 3- to 6-fold higher than background values and are believed to reflect accumulations resulting from food chain contamination in regional marshes, including that reportedly characterizing Typha latifolia stands-their primary food source-and adherent sediments which may be consumed inadvertently while feeding. No evidence of site-influence or enhanced tissue metal levels was seen for Cu, Pb or Zn. While Cd : Ni accumulations were positively correlated in both the liver (r=0.78) and the kidneys (r=0.65), between-tissue comparisons indicated that hepatic : renal burdens were significantly correlated (r=0.75) only in the case of Ni. With the exception of 30-35% lower hepatic Zn levels in females relative to males within the Sudbury population, tissue metal levels did not vary according to sex or age class at either site. Our findings substantiate the potential of muskrat to serve as useful bioindicators/monitors of metal pollution in semi-aquatic environments.

Uranium mill tailings: nuclear waste and natural laboratory for geochemical and radioecological investigations

Uranium mill tailings (UMT) are a high volume, low specific activity radioactive waste typically disposed in surface impoundments. This review focuses on research on UMT and related earth materials during the past decade relevant to the assessment of: (1) mineral hosts of radionuclides; (2) the use of soil analogs in predicting long-term fate of radionuclides; (3) microbial and diagenetic processes that may alter radionuclide mobility in the surficial environment; (4) waste-management technologies to limit radionuclide migration; and (5) the impact of UMT on biota.

Generic performance assessment for a deep repository for low and intermediate level waste in the UK--a case study in assessing radiological impacts on the natural environment

Concentrations of radionuclides in soil and surface water, taken from a generic performance assessment of a repository for low and intermediate level radioactive waste, assumed to be located in the UK, have been used as the basis for a case study in assessing radiological impacts on the natural environment. Simplified descriptions of the terrestrial and aquatic ecosystem types likely to be impacted have been developed. A scoping assessment has identified (226)Ra, (210)Po, (234)U, (230)Th and (238)U as having the highest potential for impact, with doses from internally incorporated alpha emitters as being potentially of particular importance. These nuclides, together with (36)Cl and (129)I (which have proved to be of importance in radiological risk assessments for humans) were included in a more detailed dose assessment. A basic methodology for dose assessment of ecosystems is described, and has been applied for the defined impacted ecosystems. Paucity of published data on concentration factors prevented a more detailed assessment for terrestrial ecosystems. For the aquatic ecosystem, a more detailed assessment was possible and highest calculated absorbed dose rates (weighted for the likely higher biological effectiveness of alpha radiation) were about 6.5 microGy h(-1). We conclude that harm to the impacted ecosystems is unlikely and make the observation that the lack of concentration factor or transfer factor data for a sufficiently wide range of species, ecosystems and nuclides appears to be the principal obstacle to establishing a comprehensive framework for the application of radiological protection to ecosystems.

U- and Th-series radionuclides in snowshoe hare (Lepus americanus) taken near U mill tailings close to Elliot Lake, Ontario, Canada

Snowshoe hare (Lepus americanus) trapped near U tailings had higher concentrations of (226)Ra in their bones (250 +/- 94 mBq g(-1) dry wt) than those from local control sites 3-15 km from the tailings (20-30 mBq g(-1) dry wt) and those from a distant control site 880 km away from the U mining area, which were below the detection limit (DL) (3.7 mBq g(-1) dry wt). Most chyme (stomach content) samples contained 226Ra below DL. Concentration ratios of 226Ra from tissues of local plants, considered important in the hare's diet, to bone ranged from 0.22 to 8.60. Concentrations of 210Pb and 210Po (95-245 mBq g(-1) dry wt) were not significantly different among tailings and control site populations. Disequilibrium between these isotopes and their precursors was noted. No significant accumulation of U and Th was noted at any site. Higher concentrations of 228Th compared to 232Th are attributed to accumulation of 228Ra in a manner similar to that of 226Ra. Based on bone 226Ra and 210Po contents, the maximum internal dose rates to the skeleton and the maximum life-time dose of hare living near tailings were 3.9 x 10(-5) Gy d(-1) and 4.2 x 10(-2) Gy, respectively. These rates were below the threshold required to produce osteosarcoma in other mammals and were considered unlikely to adversely affect hare during their lifetime. Radionuclide uptake by the animals was concluded to have no environmental significance in the transport of radionuclides from tailings to other locations.