John R. Twining

Generic values for soil-to-plant transfer factors of radiocesium

There is a need for soil-to-plant transfer factors of radionuclides that take into account all possible crops on all soil varieties to support dose assessment studies. Because only limited experimental data exist for worldwide soil systems, such values should necessarily have a generic character. This paper describes a generic system for 137Cs, mainly based on a reference soil-to-plant transfer factor which depends solely on soil properties such as nutrient status, exchangeable K-content, pH and moisture content. Crops are divided into crop groups, cereals serving as reference group. The transfer of other crop groups can be calculated by multiplying data for cereals by a conversion factor. Existing data present in the IUR (International Union of Radioecologists) databank and in large part the work of a FAO (Food and Agriculture Organisation)/IAEA(International Atomic Energy Agency)/IUR project on tropical systems provided the basis for the derivation of the conversion factors and reference values.

Whole-body to tissue concentration ratios for use in biota dose assessments for animals

Environmental monitoring programs often measure contaminant concentrations in animal tissues consumed by humans (e.g., muscle). By comparison, demonstration of the protection of biota from the potential effects of radionuclides involves a comparison of whole-body doses to radiological dose benchmarks. Consequently, methods for deriving whole-body concentration ratios based on tissue-specific data are required to make best use of the available information. This paper provides a series of look-up tables with whole-body:tissue-specific concentration ratios for non-human biota. Focus was placed on relatively broad animal categories (including molluscs, crustaceans, freshwater fishes, marine fishes, amphibians, reptiles, birds and mammals) and commonly measured tissues (specifically, bone, muscle, liver and kidney). Depending upon organism, whole-body to tissue concentration ratios were derived for between 12 and 47 elements. The whole-body to tissue concentration ratios can be used to estimate whole-body concentrations from tissue-specific measurements. However, we recommend that any given whole-body to tissue concentration ratio should not be used if the value falls between 0.75 and 1.5. Instead, a value of one should be assumed.

Assessing doses to terrestrial wildlife at a radioactive waste disposal site: Inter-comparison of modelling approaches

Radiological doses to terrestrial wildlife were examined in this model inter-comparison study that emphasised factors causing variability in dose estimation. The study participants used varying modelling approaches and information sources to estimate dose rates and tissue concentrations for a range of biota types exposed to soil contamination at a shallow radionuclide waste burial site in Australia. Results indicated that the dominant factor causing variation in dose rate estimates (up to three orders of magnitude on mean total dose rates) was the soil-to-organism transfer of radionuclides that included variation in transfer parameter values as well as transfer calculation methods. Additional variation was associated with other modelling factors including: how participants conceptualised and modelled the exposure configurations (two orders of magnitude); which progeny to include with the parent radionuclide (typically less than one order of magnitude); and dose calculation parameters, including radiation weighting factors and dose conversion coefficients (typically less than one order of magnitude). Probabilistic approaches to model parameterisation were used to encompass and describe variable model parameters and outcomes. The study confirms the need for continued evaluation of the underlying mechanisms governing soil-to-organism transfer of radionuclides to improve estimation of dose rates to terrestrial wildlife. The exposure pathways and configurations available in most current codes are limited when considering instances where organisms access subsurface contamination through rooting, burrowing, or using different localised waste areas as part of their habitual routines.

A study of radium uptake by the water-lily Nymphaea violacea (Lehm) from contaminated sediment

Abstract The rate and extent of radium-226 accumulation from sediment by the water-lily Nymphaea violacea (Lehm) were assessed. The plants were collected from Magela Creek, Northern Territory, Australia and grown in 226RaCl2-labelled laboratory sediment over a period of 570 days. The nominal sediment activity of 5 Bq g−1 dry weight was ∼100 times that of the naturally occurring concentration. In the roots and rhizomes, 226Ra accumulated on plant surfaces. This result was confirmed by autoradiographic studies which showed the presence of an iron-containing plaque on the surface of these tissues with which the radium was closely associated. Little radium reached the pith of the rhizomes and acropetal translocation was not detected. The average concentration ratio for growing rhizomes was 0·22. Assuming first order accumulation kinetics and likely incremental environmental concentrations, this Aboriginal dietary tissue could become dose-limiting within 50 years. The foliar accumulation of radium originating from the sediment was predominately due to contamination via the sediment-water-plant pathway. Differences in foliar radium concentrations in plants of different ages were due to differential biomass turnover rates. Plants with faster biomass turnover had higher average radium concentrations.

Biotic, temporal and spatial variability of tritium concentrations in transpirate samples collected in the vicinity of a near-surface low-level nuclear waste disposal site and nearby research reactor.

The results of a 21 month sampling program measuring tritium in tree transpirate with respect to local sources are reported. The aim was to assess the potential of tree transpirate to indicate the presence of sub-surface seepage plumes. Transpirate gathered from trees near low-level nuclear waste disposal trenches contained activity concentrations of (3)H that were significantly higher (up to ∼700 Bq L(-1)) than local background levels (0-10 Bq L(-1)). The effects of the waste source declined rapidly with distance to be at background levels within 10s of metres. A research reactor 1.6 km south of the site contributed significant (p < 0.01) local fallout (3)H but its influence did not reach as far as the disposal trenches. The elevated (3)H levels in transpirate were, however, substantially lower than groundwater concentrations measured across the site (ranging from 0 to 91% with a median of 2%). Temporal patterns of tree transpirate (3)H, together with local meteorological observations, indicate that soil water within the active root zones comprised a mixture of seepage and rainfall infiltration. The degree of mixing was variable given that the soil water activity concentrations were heterogeneous at a scale equivalent to the effective rooting volume of the trees. In addition, water taken up by roots was not well mixed within the trees. Based on correlation modelling, net rainfall less evaporation (a surrogate for infiltration) over a period of from 2 to 3 weeks prior to sampling seems to be the optimum predictor of transpirate (3)H variability for any sampled tree at this site. The results demonstrate successful use of (3)H in transpirate from trees to indicate the presence and general extent of sub-surface contamination at a low-level nuclear waste site.

Bioaccumulation of 137Cs and 85Sr by an Australian sub-tropical freshwater teleost (Bidyanus bidyanus)

Abstract Adult Silver Perch ( Bidyanus bidyanus ) were exposed to 137 Cs and 85 Sr for 8 weeks to determine radionuclide concentration factors (CFs) for use in a dose assessment model database for sub-tropical and tropical ecosystems. At equilibrium, the fresh weight CFs for 137 Cs and 85 Sr in the flesh of B. bidyanus were 12.6 ± 5.2 kg 1 −1 (mean ± S.E.) and 0.73 ± 0.21 kg 1 −1 , respectively. The mean CFs were considerably lower than those expected for temperate freshwater fish, but were consistent with recently reported values for other sub-tropical and tropical freshwater fish. The results indicate that the use of recommended CFs for radiological dose assessment models, based on temperate freshwater fish, may provide overestimates of dose when applied to sub-tropical and tropical fish. The biological half-lives of 137 Cs (19 ± 2 days) and 85 Sr (5 ± 1 days) in the flesh of B. bidyanus , estimated from a one-compartment exponential uptake model, were lower than reported values for four other species of sub-tropical and tropical freshwater fish. Cesium-137 was distributed in B. bidyanus as follows: flesh (63%) > skin/scales (22%) > bone (15%). Conversely, 85 Sr showed similar activities in the bone and skin/scales that were still increasing after eight weeks, while the levels in the flesh remained relatively low. Analysis of the skin/scales component showed that the majority of 85 Sr (∼ 94%) was located in the scales.

Application of synchrotron radiation technique to analysis of environmental samples

Synchrotron radiation experiments were performed at the Australian National Beamline Facility at Photon Factory, Tsukuba, Japan using the X-ray absorption near edge structure technique to investigate changes in redox speciation of iron and manganese in estuarine particles from the Fly River in Papua New Guinea. In low salinity samples manganese was mostly present as Mn(III) and Mn(IV) compounds with some surface-adsorbed Mn(II). Similarly, iron was present as Fe(III) and Fe(II/III) compounds with some adsorbed Fe(II). The proportions of both Mn(II) and Fe(III) significantly increased in higher salinity samples.

Diminished Metal Accumulation in Riverine Fishes Exposed to Acid Mine Drainage over Five Decades

Bony bream (Nematalosa erebi) and black catfish (Neosilurus ater) were sampled from the fresh surface waters of the Finniss River in tropical northern Australia, along a metal pollution gradient draining the Rum Jungle copper/uranium mine, a contaminant source for over five decades. Paradoxically, populations of both fish species exposed to the highest concentrations of mine-related metals (cobalt, copper, lead, manganese, nickel, uranium and zinc) in surface water and sediment had the lowest tissue (bone, liver and muscle) concentrations of these metals. The degree of reduction in tissue concentrations of exposed populations was also specific to each metal and inversely related to its degree of environmental increase above background. Several explanations for diminished metal bioaccumulation in fishes from the contaminated region were evaluated. Geochemical speciation modeling of metal bioavailability in surface water showed no differences between the contaminated region and the control sites. Also, the macro-nutrient (calcium, magnesium and sodium) water concentrations, that may competitively inhibit metal uptake, were not elevated with trace metal contamination. Reduced exposure to contaminants due to avoidance behavior was unlikely due to the absence of refugial water bodies with the requisite metal concentrations lower than the control sites and very reduced connectivity at time of sampling. The most plausible interpretation of these results is that populations of both fish species have modified kinetics within their metal bioaccumulation physiology, via adaptation or tolerance responses, to reduce their body burdens of metals. This hypothesis is consistent with (i) reduced tissue concentrations of calcium, magnesium and sodium (macro-nutrients), in exposed populations of both species, (ii) experimental findings for other fish species from the Finniss River and other contaminated regions, and (iii) the number of generations exposed to likely selection pressure over 50 years.

Principal Coordinate Analysis of the Distribution of Radium-226 Between Water, Sediment and the Waterlily, Nymphaea violacea (Lehm), in the Vicinity of a Uranium Mine in the Northern Territory, Australia

Radium-226 concentrations in water, sediment and Nymphaea violacea (Lehm) root and rhizome samples were strongly correlated over 2 years between three sample sites from Magela Creek, Northern Territory, Australia. The uptake by roots and rhizomes was due primarily to surface accumulation. Radium-226 concentrations in foliage were not correlated with media concentrations. However, foliar tissue senescence was shown to increase radium accumulation across a range of aquatic plant species including N. violacea (P < 0·05). Principal coordinate analysis showed that the distribution of radium and calcium concentrations in the foliar organs of N. violacea were strongly correlated (r = 0·522; P < 0·001). This result supported the hypothesis that radium was accumulated and/or distributed by the mechanisms involved in uptake of the nutrient divalent cation. However, subsequent analyses comparing the ratio of extractable radium and calcium in the supporting media to their ratio in the plant showed no correlation, which suggested that different uptake mechanisms were involved.

Radioecology of Tropical Freshwater Ecosystems: Mechanisms and Kinetics of Bioaccumulation and the Importance of Water Chemistry

Abstract Radionuclides in (sub-) tropical freshwater systems generally behave in a predictable manner, based on what is known from lakes and streams in the better studied temperate climes. Within the water column, the fate and behaviour of radionuclides and their stable element analogs are typically governed by key physicochemical variables such as pH, redox potential, the concentrations of dissolved ions and the presence and type of organic matter. The chemical form (or speciation) of a radionuclide or stable element is generally of greater biological importance (i.e. bioavailability) than the total concentration. This concept is currently being integrated into mechanistic frameworks (e.g. biotic ligand and bioaccumulation models) by national regulators for protecting freshwater ecosystems. Given that the volume of fresh surface waters (i.e. rivers and lakes) is relatively small (0.01%) in comparison to seas and oceans, then the biota living within them can also substantially influence the chemistry of a radionuclide, by effectively acting as “large particles” available for surface complexation. For example, wetlands serve as effective biofilters to remove radionuclides from the water column. Once radionuclides have been accumulated by organisms, their behaviours generally reflect their similarities to essential (macro and micro nutrients) and non-essential elements. Metabolic mechanisms tending toward homeostasis typically make internal organism chemistry less dynamic than that in the external water column. Again, this is to be expected and radionuclide biokinetics generally follow the patterns observed for freshwater organisms in temperate climates. This constancy has enabled models to be developed to describe the rate and extent of radionuclide bioaccumulation. These models can be adapted to include various uptake pathways (from water, food or sediment) via gills, skin or gut and modes of excretion or dilution (e.g. diffusion, egestion, moulting and growth) and different parameters can be estimated for each radionuclide and organism. Key biotic factors known to influence radionuclide bioaccumulation are size, age and gender. There are also differences within and between species that reflect the natural variability within any system. Despite the similarities that exist between tropical and temperate freshwater systems, it should be noted that there is still a paucity of data for (sub-) tropical freshwater organisms and systems, and hence there is the chance for exceptions to the consistency to exist. Data are presented that show that the uptake of Sr and Cs by tropical freshwater fishes is much lower than would be expected based on the studies of freshwater temperate fishes. These observations point to the need to undertake additional site- and species- specific investigations on the radioecology of key radionuclides, whenever future nuclear developments in tropical systems are initiated.