14C and 210Pb in NE atlantic sediments: Evidence of biological reworking in the context of radioactive waste disposal

Scavenging by the seabed and by sedimentary particles in the deep ocean may have a significant effect on the removal of artificial radionuclides released to the water column as a result of radioactive waste disposal operations. Biological activity in the upper layers of the sediment column will enhance the rate of removal for those particle-reactive radionuclides with a short half-life relative to the turnover time of the upper mixed layer. For longer-lived radionuclides the rate of sediment accumulation will determine the ultimate rate of removal.The rate of sediment accumulation and extent of biological mixing of deep-sea sediment from three areas of the NE Atlantic Ocean have been investigated using ¹⁴C and ²¹⁰Pb data. Treatment of the radiocarbon with a simple box model provided estimates of sedimentation rate (ω), mixed layer depth (L), mixed layer age (T_ML) and the age of material arriving at the surface (T₀), which were broadly similar to previously published values from other ocean basins. Box cores from the Iberia Abyssal Plain, Madeira Abyssal Plain and from the NEA low-level radioactive waste dumpsite yielded sedimentation rates in the range 0.8 to 2.2 cm ky^?1 over the upper 16–25 cm. Continuous particle mixing appears to be taking place to a depth of 4 to 6 cm below the present sediment-water interface. Closely spaced vertical sampling of core 161-2 for ²¹⁰Pb allowed a biodiffusion coefficient (D_B) to be calculated (4 × 10^?9cm²s^?1), treating bioturbations as a diffusive term and sedimentations as an advective term in a simple mathematical model. In general, values of D_B in deep ocean sediments fall within the range 1–10 × 10^?9cm²s^?1, two orders of magnitude lower than nearshore values. From a review of published data it is concluded that biological mixing takes place extensively in the deep ocean; it appears to be fairly constant on a basin-wide scale and amenable to incorporation in mathematical models of radionuclide behaviour in the water column.     

Sediment problems and lake restoration in Wisconsin

Twelve dredging projects are underway or will soon begin in Wisconsin. These include both natural and man-made lakes, with lake size and sediment removal up to 205 ha and 1,720,250 m³, respectively. Solids content of the sediments ranges from 70%–80% to 1%–5%. The projects were designed using a mixture of on-site data collection, predictive models, and professional judgement. Sediment disposal has limited project implementation, with arsenic being a special problem. Theoretically, sediment concentrations below 4 μg/g could still produce unacceptable contamination of groundwater at the disposal site. One project is being held up, pending completion of laboratory testing. Organic sediments from Lilly Lake were deposited in an active gravel pit and within diked areas on agricultural land. Passage through a spray irrigation system proved impractical. Rapid infiltration of water into the bottom and sides of the settling basins was short-lived due to the self-sealing characteristics of these sediments. Studies are now underway to determine the effect of lake sediment application to upland soils on corn production.     

Radionuclides in the Ecosystem of Lake Tygish in the Zone of the Eastern Ural Radioactive Trace

The radioecological situation in Lake Tygish is described. The lake is situated on the central axis of the Eastern Ural Radioactive Trace (EURT), which was formed after the nuclear accident in the Southern Urals in 1957. The distributions of ⁹⁰Sr and ¹³⁷Cs among the main components of the water body and the results of the measurement of tritium concentration in the water are presented. Mathematical models are described that have made it possible to estimate changes in the concentrations of radionuclides and their amounts accumulated in the water and bottom sediments of the lake during the long period after the accident and to predict the development of the radioecological situation in the lake until 2057. Based on the mathematical models, more accurate data on the initial ecological situation in the lake in the year of the accident have been obtained.     

Time history of trace elements in sediments from Standley Lake,Colorado

Eighteen trace elements were analyzed in sections from a 50 cm long sediment core from Standley Lake, Colorado. The core was dated by the use of bomb-produced ¹³⁷Cs. Ten of the elements measured (Ag, Cd, Cu, Ga, Hg, Pb, Se, Sn, Tl, and Zn) appear to be enriched in this sediment; seven (Al, As, Be, Co, Cr, Ni, and V) are probably of local geological origin; and Te was not detected. Several of the constituents increase with depth in the sediment, and the concentrations of some compare to levels in lake sediments near highly industrialized centers. Based upon comparisons with other work, fluxes, enrichment factors, benzo(a)pyrene concentrations, and other evidence we conclude that the primary source of the “excess” elemental concentrations is local stream pollution.     

Applications of a hydro-biogeochemical model and long-term simulations of the effects of logging in forested watersheds

We simulated hydrological and biogeochemical responses to logging in a forested watershed to determine the vulnerability and/or resiliency of the forest ecosystems in the Lake Shumarinai Basin in northern Hokkaido, Japan. We used a biogeochemical model (PnET-CN) and a rainfall–runoff model (HYCYMODEL) to predict ecosystem responses. The PnET-CN model simulated well the observed NO₃ ⁻ concentrations in streamwater, particularly at high concentrations during snowmelt; however, the model could not simulate small increases in NO₃ ⁻ during the summer. By considering hydrological processes within the watershed and combining the model with the HYCYMODEL (PnET + HYCYMODEL), the seasonality of streamwater NO₃ ⁻ concentrations was better simulated. Using these models, the long-term effects of logging were simulated for coniferous, deciduous, and mixed forests. NO₃ ⁻ concentrations in streamwater increased in response to the logging disturbance in both coniferous and deciduous forests. In the coniferous forest, NO₃ ⁻ concentrations reached a maximum 10 years after logging, and high concentrations persisted for 30 years. In contrast, NO₃ ⁻ concentrations in the deciduous forest reached a maximum within 3–4 years and recovered to pre-disturbance levels after 15 years. We also used the models to determine the effects of different sizes and types (coniferous, deciduous, and mixed forest) of logging areas on Lake Shumarinai. The model results indicated that large areas of cutting require more than 100 years for complete lake recovery. Whereas the annual discharge to the lake minimally increased, the annual NO₃ ⁻ load greatly increased. Our simulation results elucidate the vulnerability and resiliency of forest ecosystems and provide valuable information for ecosystem management.     

Time pattern of off-site plutonium deposition from rocky flats plant by lake sediment analyses

A sediment core from a lake downwind of the Rocky Flats Plant, where nuclear weapons components are produced, was used to reconstruct a time pattern of off-site plutonium deposition. Core sections were dated by analyses of ¹³⁷Cs, ^239,240Pu, ²³⁸Pu, and ²⁴¹Am fallout from nuclear testing and ²³⁸Pu fallout from a satellite failure. A peak in transuranic concentrations occurred in late 1969 which was attributable to the Plant. This was confirmed by mass isotopic analysis of plutonium isotopes in selected core segments where the global fallout and Plant contributions could be differentiated. The 18 nCi ^239,240Pu per m² from the Plant that had accumulated in the sediment is reasonable when compared to soil analyses.     

Association of organic matter, iron and inorganic phosphorus in lake waters

Gel permeation chromatography was used to fractionate ³²PO₄³⁻ labelled components by molecular size in filtered, destabilized lake foam, concentrated lake water, and lake sediment extracts. Evidence is presented for the abiotic formation of organic matter iron inorganic P complexes in lake foam, water and sediments. ³²PO₄^staggered was found to excahnge with low molecular weight, dissolved P fraction, probably an organic matter -Fe- inorganic P complex, but not with a high molecular weight, non-inorganic dissolved reactive P pool, hypothesized to be similar to the dimer. Our experiments suggest that inorganic P binding by high molecular weight organic matter may play a significant role in the P cycle. If the bound P is unavailable to algae and bacteria, the complexes could explain Rigler's (1968) hypothesis that the molybdenum blue technique may overestimate the free orthophosphate concentration by 10–100 times.     

Migration of Cs in tributaries,lake water and sediment of Lago Maggiore (Italy,Switzerland) – analysis and comparison with Lago di Lugano and other lakes

This paper describes the behaviour of ¹³⁷Cs in Lago Maggiore and other pre-alpine lakes as a consequence of atmospheric nuclear weapons testing fallout and the fallout from the nuclear accident in Chernobyl. It presents data on the ¹³⁷Cs distribution in tributaries, lake water, bottom sediments and reveals the role of ¹³⁷Cs as a marker of the sedimentation processes. The run-off of ¹³⁷Cs from the watershed to the lake is described with a simple compartment model. Measurements of the activity concentration of ¹³⁷Cs in sediments are compared with the output of a model (diffusion–convection type) which describes the input of ¹³⁷Cs into and its vertical distribution within the sediment. Varying sedimentation rates (0.05–0.90 g (cm² y)⁻¹) in Lago Maggiore are compared with data of other authors. Sedimentation rates and total distribution coefficients (of about 10⁵ L kg⁻¹) in Lago Maggiore are discussed and compared with those of Lago di Lugano, Lake Constance, and Lake Vorsee.     

Assessing radiation impact at a protected coastal sand dune site: an intercomparison of models for estimating the radiological exposure of non-human biota

This paper presents the application of three publicly available biota dose assessment models (the ERICA Tool, R&D128/SP1a and RESRAD-BIOTA) to an assessment of the Drigg coastal sand dunes. Using measured ⁹⁰Sr, ⁹⁹Tc, ¹³⁷Cs, ²³⁸Pu, ^239+240Pu and ²⁴¹Am activity concentrations in sand dune soil, activity concentration and dose rate predictions are made for a range of organisms including amphibians, birds, invertebrates, mammals, reptiles, plants and fungi. Predicted biota activity concentrations are compared to measured data where available. The main source of variability in the model predictions is the transfer parameters used and it is concluded that developing the available transfer databases should be a focus of future research effort. The value of taking an informed user approach to investigate the way in which models may be expected to be applied in practice is highlighted and a strategy for the future development of intercomparison exercises is presented.     

Fukushima fallout in Northwest German environmental media

Traces of short- and long-lived fallout isotopes (¹³¹I, ¹³⁴Cs and ¹³⁷Cs) were found in environmental samples collected in Northwest Germany (rain water, river sediment, soil, grass and cow milk) from March to May 2011, following the radioactivity releases after the nuclear accident in Fukushima, Japan. The measured concentrations are consistent with reported concentrations in air, amount of rainfall and expected values applying simple radioecological models. The [¹³⁴Cs]/[¹³⁷Cs] ratio reported for air (about 1) allows for discrimination between “recent” and “old” ¹³⁷Cs. Expected ¹³⁶Cs values fell below the detection limits of the instrumentation, despite large sample masses and long counting times.     

Application of the ERICA Assessment Tool to freshwater biota in Finland

In recent years there has been growing international interest in the assessment of doses and risks from ionising contaminants to biota. In this study the ERICA Tool, developed within the EC 6th Framework Programme, was applied to estimate incremental dose rates to biota in freshwater ecosystems in Finland mainly resulting from exposure to the Chernobyl-derived radionuclides ¹³⁷Cs, ¹³⁴Cs and ⁹⁰Sr. Data sets consisting of measured activity concentrations in fish, aquatic plants, lake water and sediment for three selected lakes located in a region with high ¹³⁷Cs deposition were applied in the assessment. The dose rates to most species studied were clearly below the screening level of 10 μGy h⁻¹, indicating no significant impact of the Chernobyl fallout on these species. However, the possibility of higher dose rates to certain species living on or in the bottom sediment cannot be excluded based on this assessment.     

Distribution of radionuclides among the main components of Lake Chervyanoe in the Eastern Ural Radioactive Trace

Radioecological conditions have been estimated in Lake Chervyanoe located on the central axis of the Eastern Ural Radioactive Trace (EURT) about 100–110 km northeast of the Mayak Production Association. Data on the ⁹⁰Sr and ¹³⁷Cs contents in the main components of the lake, the distribution of these radionuclides over the profile of bottom sediments, and their amounts in the lake are presented. The experimental data have been used to construct mathematical models for estimating changes in the radionuclide concentrations and amounts in the water and bottom sediments of the lake over a long period after the nuclear accident, as well as the tritium content of the lake water in different periods of time.     

Development of a dynamic transfer model of (14)C from the atmosphere to rice plants

A dynamic compartment model was investigated to describe ¹⁴C accumulation in rice plants exposed to atmospheric ¹⁴C with temporally changing concentrations. In the model, rice plants were regarded to consist of three compartments: the ear and the mobile and immobile carbon pools of the shoot. Photosynthetically fixed carbon moves into the ear and the mobile carbon pool, and these two compartments release a part of this carbon into the atmosphere by respiration. Carbon accumulated in the mobile carbon pool is redistributed to the ear, while carbon transferred into the immobile carbon pool from the mobile one is accumulated there until harvest. The model was examined by cultivation experiments using the stable isotope, ¹³C, in which the ratios of carbon photosynthetically fixed at nine times during plant growth to the total carbon at the time of harvest were determined. The model estimates of the ratios were in relatively good agreement with the experimental observations, which implies that the newly developed compartment model is applicable to estimate properly the radiation dose to the neighboring population due to an accidental release of ¹⁴C from nuclear facilities.     

Carbon-14 transfer into rice plants from a continuous atmospheric source: observations and model predictions

Carbon-14 ((14)C) is one of the most important radionuclides from the perspective of dose estimation due to the nuclear fuel cycle. Ten years of monitoring data on (14)C in airborne emissions, in atmospheric CO(2) and in rice grain collected around the Tokai reprocessing plant (TRP) showed an insignificant radiological effect of the TRP-derived (14)C on the public, but suggested a minor contribution of the TRP-derived (14)C to atmospheric (14)C concentrations, and an influence on (14)C concentrations in rice grain at harvest. This paper also summarizes a modelling exercise (the so-called rice scenario of the IAEA's EMRAS program) in which (14)C concentrations in air and rice predicted with various models using information on (14)C discharge rates, meteorological conditions and so on were compared with observed concentrations. The modelling results showed that simple Gaussian plume models with different assumptions predict monthly averaged (14)C concentrations in air well, even for near-field receptors, and also that specific activity and dynamic models were equally good for the prediction of inter-annual changes in (14)C concentrations in rice grain. The scenario, however, offered little opportunity for comparing the predictive capabilities of these two types of models because the scenario involved a near-chronic release to the atmosphere. A scenario based on an episodic release and short-term, time-dependent observations is needed to establish the overall confidence in the predictions of environmental (14)C models.     

Modelling the reworking effects of bioturbation on the incorporation of radionuclides into the sediment column: implications for the fate of particle-reactive radionuclides in Irish Sea sediments

A microcosm laboratory experiment was conducted to determine the impact of biological reworking by the ragworm Nereis diversicolor on the redistribution of particle-bound radionuclides deposited at the sediment-water interface. Over the course of the 40-day experiment, as much as 35% of a ¹³⁷Cs-labelled particulate tracer deposited on the sediment surface was redistributed to depths of up to 11 cm by the polychaete. Three different reworking models were employed to model the profiles and quantify the biodiffusion and biotransport coefficients: a gallery-diffuser model, a continuous sub-surface egestion model and a biodiffusion model. Although the biodiffusion coefficients obtained for each model were quite similar, the continuous sub-surface egestion model provided the best fit to the data. The average biodiffusion coefficient, at 1.8 ± 0.9 cm² y⁻¹, is in good agreement with the values quoted by other workers on the bioturbation effects of this polychaete species. The corresponding value for the biotransport coefficient was found to be 0.9 ± 0.4 cm y⁻¹. The effects of non-local mixing were incorporated in a model to describe the temporal evolution of measured ⁹⁹Tc and ⁶⁰Co radionuclide sediment profiles in the eastern Irish Sea, influenced by radioactive waste discharged from the Sellafield reprocessing plant. Reworking conditions in the sediment column were simulated by considering an upper mixed layer, an exponentially decreasing diffusion coefficient, and appropriate biotransport coefficients to account for non-local mixing. The diffusion coefficients calculated from the ⁹⁹Tc and ⁶⁰Co cores were in the range 2-14 cm² y⁻¹, which are consistent with the values found by other workers in the same marine area, while the biotransport coefficients were similar to those obtained for a variety of macrobenthic organisms in controlled laboratories and field studies.     

Effect of liming on the behaviour of (90)Sr and (137)Cs in a lake ecosystem

Liming of lakes is considered one possible remedial action to reduce the accumulation of radionuclides into fish in the case of a radiological accident. These responses were tested in field conditions in a small acidified lake that was divided into two parts: one limed with CaCO₃ and the other half left as an unlimed control. The transfer of ⁹⁰Sr from water into fish decreased on average by 50% during the first year after liming. However, at the same time the ⁹⁰Sr concentration in water increased, reaching a maximum within 6 months after liming. Approximately 50% more ⁹⁰Sr was detected in water in the limed part of the lake than on control side during the first year. ⁹⁰Sr was most probably released from the sediment as the Ca concentration and pH of the water increased. As a result of these two processes, which counterbalanced each other (increased release of ⁹⁰Sr into water from sediment and decreased transfer of ⁹⁰Sr from water into fish), the ⁹⁰Sr concentration in fish did not notably differ between the limed and control sides of the lake. Liming may only be suitable as a remedial action if carried out immediately after a radiological accident, before significant amounts of radionuclides have been deposited in lake sediments. In the case of ¹³⁷Cs, the effect of liming was less pronounced. ¹³⁷Cs activity concentration in water increased in the first year by 20% and uptake by fish decreased by 20%.     

Recent changes in Red Lake (Romania) sedimentation rate determined from depth profiles of Pb and Cs radioisotopes

This work presents a first estimation of the sedimentation rate for the Red Lake (Romania). The sediment accumulation rates were determined by two well-known methods for recent sediment dating: ²¹⁰Pb and ¹³⁷Cs methods. Both techniques implied used the gamma emission of the above-mentioned radionuclides. The ²¹⁰Pb and ¹³⁷Cs concentrations in the sediment were measured using a gamma spectrometer with a HpGe detector, Gamma-X type. Activities ranging from 41 ± 7 to 135 ± 34 Bq/kg were found for ²¹⁰Pb and from 3 ± 0.5 to 1054 ± 150 Bq/kg for ¹³⁷Cs. The sediment profile indicates acceleration in sedimentation rate in the last 18 years. Thus, the sedimentation process for the Red Lake can be divided in two periods, the last 18 years, and respectively, the period before that. Using the Constant Rate of ²¹⁰Pb Supply method values between 0.18 ± 0.04 and 1.85 ± 0.5 g/cm² year (0.32 ± 0.08 and 2.83 ± 0.7 cm/year) were obtained. Considering both periods, an average sedimentation rate of 0.87 ± 0.17 g/cm² year (1.17 cm/year) was calculated. Considering an average depth of 5.41 m for the lake and the sedimentation rate estimated for the last 18 years, it could be estimated that the lake will disappear in 195 years.     

Modelling the long-term dynamics of radiocaesium in closed lakes

During the years after the Chernobyl accident the radioceasium activity concentration in most contaminated aquatic ecosystems decreased markedly. Lakes with no permanent inflows and outflows (closed lakes), however, still present a radioecological problem which is expected to continue for some time. In this paper, a mechanistic model for the long-term prediction of radiocaesium behaviour in closed lakes is developed. The model of Prokhorov (Radiokhimiya (Radiochemistry) 11 (1969) 317) was modified to describe the effects of bottom sediment bioturbation, surface runoff from the catchment and suspended solids formation and sedimentation. The model input parameters are the effective diffusion coefficient in bottom sediments, depth of the completely mixed layer, the distribution coefficient in the sediment-water system, the runoff coefficient, sedimentation rate, and deposition density. Values of all these parameters can be independently estimated or measured in a short-term experiment. Given negligible runoff and sedimentation, the dynamics of radiocaesium in lake water is described by a simple equation with only one unknown parameter. This allows us to make long-term predictions on the basis of a series of measurements carried out during the relatively short period. The model was tested against 137Cs activity concentrations measured between 1993 and 1999 in Svyatoe lake in the Bryansk region of Russia. Calculated and measured activity concentrations are in good agreement.     

Variations of anthropogenic CO2 in urban area deduced by radiocarbon concentration in modern tree rings

Radiocarbon concentration in the atmosphere is significantly lower in areas where man-made emissions of carbon dioxide occur. This phenomenon is known as Suess effect, and is caused by the contamination of clean air with non-radioactive carbon from fossil fuel combustion. The effect is more strongly observed in industrial and densely populated urban areas. Measurements of carbon isotope concentrations in a study area can be compared to those from areas of clear air in order to estimate the amount of carbon dioxide emission from fossil fuel combustion by using a simple mathematical model. This can be calculated using the simple mathematical model. The result of the mathematical model followed in this study suggests that the use of annual rings of trees to obtain the secular variations of ¹⁴C concentration of atmospheric CO₂ can be useful and efficient for environmental monitoring and modeling of the carbon distribution in local scale.     

Climate change impacts on vegetation and water cycle in the Euro-Mediterranean region,studied by a likelihood approach

Scientific community and policy-makers share the common interest in identifying and evaluating potential impacts of climate change on ecosystems, relying mainly on probabilistic methods of exploring the risks. In this perspective, the concept of ensemble forecasting makes possible to handle uncertainties associated with climate risk analysis by focusing on a range of potential or probable impact scenarios rather than actualizing a single case. In this paper, an ensemble of simulations based on the Lund-Potsdam-Jena (LPJ) model was used to investigate the uncertainty upon predictions of the future Euro-Mediterranean vegetation distribution, carbon dynamics, and water budget. Twenty simulations from past to future were based on the combination of different climate inputs, vegetation model parameterizations, and configurations. The evaluation of results combined the separate deterministic future projections from the LPJ model into a single probabilistic projection, associating a likelihood degree in accordance with the most recent Intergovernmental Panel on Climate Change terminology. Results projected a general critical situation in terms of water availability, made more serious if considering that also the occurrence of extreme-related events, e.g., fires, is expected to become more frequent as favored by more recurrent drought episodes. Although more uncomfortable climate conditions were projected for vegetation, net primary production (NPP) was predicted to increase due to the potential enrichment of CO₂ in atmosphere and its fertilization effects on vegetation. The combination of rising NPP and fire frequency may shape the carbon cycle components, as the carbon losses by fire also were projected to increase.