Quantitative assessment of the long term behaviour of 90Sr in Lake Uruskul,Southern Urals,Russia

The present paper is aimed at assessing the long term behaviour of 90Sr migration from water to bottom sediments of Lake Uruskul, Southern Urals, Russia. The lake was contaminated following the nuclear accident at the Mayak nuclear complex in 1957 (the Kyshtym accident). Some transfer parameters relevant to the behaviour of 90Sr in the water-sediment system were evaluated: a) the radionuclide migration velocity from the water column to the bottom sediment, b) the radionuclide migration rate from bottom sediment to water, and c) the radionuclide migration rate from bottom sediment to deep sediment. The estimated values of the above parameters were 6.4 x 10(-1) m s(-1), 5.7 x 10(-10) s(-1) and 5.2 x 10(-10) s(-1), respectively. These values were compared with data obtained for some Italian lakes contaminated by 90Sr after the nuclear weapons tests fallout. The relatively low radionuclide migration from water to sediment of these lakes is reflected by the values of the ratio migration velocity/migration rate from sediment to water (4 m and 12 m) that are significantly lower than the corresponding value in the Russian lake (112 m). The peculiar hydrochemical conditions of Lake Uruskul (high pH, high mineralisation, etc.) are considered to be responsible for the high radionuclide migration from water to sediment.     

Levels of 137Cs in muddy sediments on the seabed in the Bay of Cadiz (Spain). Part II. Model of vertical migration of (137)Cs

This second part of the study reports the development of a model to describe the vertical migration of the artificial radioisotope (137)Cs in the sediment column on the seabed of the Bay of Cadiz. The application of the model provides an overall picture of the process of sedimentation in the Inner Bay of Cadiz. The spatial distribution of the rate of sedimentation enables us to study the sources of sediments and the means by which the sediments have been transported. A method has been derived from the rate of sedimentation to perform the dating of the layers of sediment. The model describes the behaviour of (137)Cs in the area under study, taking into account the time of residence in the zones that are the source of accumulation, the origin of the sedimentary material, together with the diffusion of the radionuclide in the sediment of the seabed.     

Vertical distribution, migration rates, and model comparison of actinium in a semi-arid environment

Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites on Kirtland Air Force Base (KAFB), New Mexico to determine the vertical downward migration of radionuclides in a semi-arid environment. The surface soils (0-15 cm) were intentionally contaminated with Brazilian sludge (containing (232)Thorium and other radionuclides) approximately 40 years ago, in order to simulate the conditions resulting from a nuclear weapons accident. Site grading consisted of manually raking or machine disking the sludge. The majority of the radioactivity was found in the top 15 cm of soil, with retention ranging from 69 to 88%. Two models, a compartment diffusion model and leach rate model, were evaluated to determine their capabilities and limitations in predicting radionuclide behavior. The migration rates of actinium were calculated with the diffusion compartment and the leach rate models for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the leach rate models were similar to those using the diffusion compartment model and did not increase with depth (0.045-0.076, 0.0 cm/yr). The research found that the physical and chemical properties governing transport processes of water and solutes in soil provide a valid radionuclide transport model. The evaluation also showed that the physical model has fewer limitations and may be more applicable to this environment.     

On the radiocesium carbonate barrier in organics-rich sediments of Lake Juodis, Lithuania

Radiocesium vertical profiles in organics-rich sediments of running shallow eutrophic Lake Juodis (Lithuania) were studied in relation to seasonal variations of vertical profiles (in water column and sediments) of standard variables (pH, redox potential, temperature, oxygen concentrations, conductivity). It is shown that the sedimentation rate, radiocesium mobility and its vertical profiles in sediments are controlled by the vital cycle (processes of the growth, accumulation and decomposition) of green algae covering the main bottom areas of the lake. It is also shown that calcite deposits are formed in the shallow bottom areas that are oxygenated throughout the year because of the photosynthetic activity of the green algae covering the sediment. Formation of the calcite coatings on freshly accumulated organics is remarkable for causing elevated densities of sediment solids in the upper part of the respective vertical profiles. These calcite deposits behave as a barrier for radiocesium backward flux to the bottom water making the respective bottom areas a radionuclide sink. Together with the jelly-structured sediments lying below these deposits, the calcite preserves the shape of the primary radiocesium vertical profiles formed due to free-ion diffusion after the deposition event. It was determined that bottom areas anaerobic in winter are the main radiocesium source in the water column and cause characteristic radiocesium redistribution in surface sediments.     

A new general dynamic model predicting radionuclide concentrations and fluxes in coastal areas from readily accessible driving variables

This paper presents a general, process-based dynamic model for coastal areas for radionuclides (metals, organics and nutrients) from both single pulse fallout and continuous deposition. The model gives radionuclide concentrations in water (total, dissolved and particulate phases and concentrations in sediments and fish) for entire defined coastal areas. The model gives monthly variations. It accounts for inflow from tributaries, direct fallout to the coastal area, internal fluxes (sedimentation, resuspension, diffusion, burial, mixing and biouptake and retention in fish) and fluxes to and from the sea outside the defined coastal area and/or adjacent coastal areas. The fluxes of water and substances between the sea and the coastal area are differentiated into three categories of coast types: (i) areas where the water exchange is regulated by tidal effects; (ii) open coastal areas where the water exchange is regulated by coastal currents; and (iii) semi-enclosed archipelago coasts. The coastal model gives the fluxes to and from the following four abiotic compartments: surface water, deep water, ET areas (i.e., areas where fine sediment erosion and transport processes dominate the bottom dynamic conditions and resuspension appears) and A-areas (i.e., areas of continuous fine sediment accumulation). Criteria to define the boundaries for the given coastal area towards the sea, and to define whether a coastal area is open or closed are given in operational terms. The model is simple to apply since all driving variables may be readily accessed from maps and standard monitoring programs. The driving variables are: latitude, catchment area, mean annual precipitation, fallout and month of fallout and parameters expressing coastal size and form as determined from, e.g., digitized bathymetric maps using a GIS program. Selected results: the predictions of radionuclide concentrations in water and fish largely depend on two factors, the concentration in the sea outside the given coastal area and/or adjacent coastal areas and the ecological half-life of the radionuclide in the sea. Uncertainties in these factors generally dominate all other uncertainties, e.g., concerning the surface water retention time, the settling velocity of the particulate fraction, the distribution coefficient regulating the fluxes in dissolved and particulate phases, the catchment area influences and the factors regulating biouptake and excretion of the radionuclide in fish. This means that the conditions in the sea are of paramount importance for the conditions in the coastal area, even for relatively enclosed coastal areas. This coastal model may be regarded as a tool for testing working hypotheses on the relative roles of different processes in different coastal areas. Such information is essential for getting realistic expectations of various remedial measures, such as coastal dredging discussed in this work.     

Modeling 137Cs migration processes in lake sediments

Lake sediments can be considered as the "diary" of the lake: new layers are continuously deposited on top of the sediments. The migration behavior of radionuclides, particularly of (137)Cs, can be analyzed by modeling the input into and the vertical distribution within the sediment. For this purpose, a model consisting of sedimentation-diffusion equations was developed. Solved with a finite element method, this model can cover the time period from the nuclear weapons testing to the present. It takes into account fixation and redissolution, compaction of sediments, and the influence of competing ions on the retarded diffusion within the sediments. The outcome of the model is compared to water and sediment measurements from Lago Maggiore. The results of an optimization process can produce the sedimentation rate and (137)Cs distribution coefficients which determine the uptake of activity into the sediment and also the retarded diffusion within the sediment.     

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.     

The role of physical processes controlling the behaviour of radionuclide contaminants in the aquatic environment: a review of state-of-the-art modelling approaches

This paper is aimed at presenting and discussing the methodologies implemented in state-of-the-art models for predicting the physical processes of radionuclide migration through the aquatic environment, including transport due to water currents, diffusion, settling and re-suspension. Models are briefly described, model parameter values reviewed and values recommended. The different modelling approaches are briefly classified and the advantages and disadvantages of the various model approaches and methodologies are assessed.     

河流底泥冲刷悬浮对水质影响途径的实验研究

进入天然水体的大部分污染物会在河流，湖泊底部形成富含各种污染物的沉积物层，在一定的水流流速及紊动作用下发生底泥冲刷及再悬浮。大量的污染物被重新释放出来造成河湖水体的二次污染，这种情况是外源污染得到治理后影响水质的主要因素。建立这种影响条件下的水质模型需要对沉积物的污染释放机理有充分的认识。采用富含污染物的天然河湖底泥，通过衡温静态培养等手段，在不同的水力条件。瞬彰服，连续悬浮和静止状态下，研究了污染物的释放，并对实验结果进行了分析。确定了底泥冲刷悬浮影响水质的主要途径，即在底泥间隙水污染物浓度接近平衡时，其与上层大水体的混合作用是影响水质的主要作用，其次是下部底泥的静态释放，而悬浮颗粒的污染物扩散释放作用则较小，这为底泥冲刷悬浮影响下的水质模型的建立提供了理论依据。     

On the sensitivity of a marine dispersion model to parameters describing the transfers of radionuclides between the liquid and solid phases

The sensitivity of a marine dispersion model for non-conservative radionuclides, previously developed and validated for the English Channel, to parameters describing the exchanges between the liquid and solid phases (suspended matter and bottom sediments) has been studied using a Monte Carlo method. A probability distribution is assigned to each parameter. They are sampled to obtain a set of model parameters and a model run is carried out. This process is repeated to obtain a distribution of model outputs. Partial correlation coefficients are calculated to assess the relative influence of each parameter on model output. Errors are also assigned to model results. Three situations are studied: an instantaneous release of radionuclides, a continuous release and the case of a contaminated sediment behaving as a long-term source of radionuclides. Calculations have also been carried out for two radionuclides with different geochemical behaviour: (137)Cs and (239,240)Pu. The results indicate that all parameters are relevant, depending on the phase we are interested in obtaining the result and on the source term (instantaneous, continuous or due to sediments). However, parameters that are, in general, more influential are kinetic rates, mixing depth in the sediment and mean radius of suspended and sediment particles. This suggests that including several particle sizes in future radionuclide dispersion models could lead to an improvement in model results. Differences have also been found with respect to the relevance of some parameters depending on the geochemical behaviour of the radionuclide.     

A general methodology for structuring models to predict the long-term migration of radionuclides from catchments

In the present paper a generic model for predicting the long-term migration of radionuclides and heavy metals from catchments is described. The model subdivides the catchment into a number of homogeneous, infinitesimal sub-catchments and integrates the radionuclide contributions from such sub-catchments to calculate the total flux of contaminant. It relates the radionuclide behaviour to the statistical distribution of the pollutant partition coefficient on the "ensemble" of sub-catchments. The methodology was validated for 90Sr and 137Cs by using data for water contamination in some European rivers. Values of migration parameters for Pu, Tc, I and Cd isotopes were obtained.     

Estimating sediment and caesium-137 fluxes in the Ribble Estuary through time-series airborne remote sensing

High spatial and temporal resolution airborne imagery were acquired for the Ribble Estuary, North West England in 1997 and 2003, to assess the application of time-series airborne remote sensing to quantify total suspended sediment and radionuclide fluxes during a flood and ebb tide sequence. Concomitant measurements of suspended particulate matter (SPM) and water column turbidity were obtained during the time-series image acquisition for the flood and ebb tide sequence on the 17th July 2003 to verify the assumption of a vertically well mixed estuary and thus justifying the vertical extrapolation of spatially integrated estimate of surface SPM. The ¹³⁷Cs activity concentrations were calculated from a relatively stable relationship between SPM and ¹³⁷Cs for the Ribble Estuary. Total estuary wide budgets of sediment and ¹³⁷Cs were obtained by combining the image-derived estimates of surface SPM and ¹³⁷Cs with estimates of water volume from a two-dimensional hydrodynamic model (VERSE) developed for the Ribble Estuary. These indicate that around 10,000 tonnes of sediment and 2.72 GBq of ¹³⁷Cs were deposited over the tidal sequence monitored in July 2003. This compared favourably with bed height elevation change estimated from field work. An uncertainty analysis on the total sediment and ¹³⁷Cs flux yielded a total budget of the order of 40% on the final estimate. The results represent a novel approach to providing a spatially integrated estimate of the total net sediment and radionuclide flux in an intertidal environment over a flood and ebb tide sequence.     

A particle-tracking method for simulating the dispersion of non-conservative radionuclides in coastal waters.

A particle-tracking method has been used to simulate the dispersion of non-conservative radionuclides in the sea. Three dimensional turbulent diffusion and the interactions between water, suspended matter and bottom sediments are simulated using a stochastic method. Kinetic transfer coefficients, as in finite difference models, are used to describe the transfers between the liquid and solid phases. Deposition of suspended matter and erosion of sediment are also included in the model. The method has been applied to simulate the dispersion of 137Cs and (239,240)Pu in the English Channel and the results have been compared with those of a finite difference model. The results from both techniques are, in general, in good agreement.     

A modelling study on 137Cs and 239,240Pu behaviour in the Alborán Sea, western Mediterranean

A model for simulating the dispersion processes of 137Cs and 239,240Pu in the Alborán Sea is described. The model consists of two hydrodynamic models: a 2D depth-averaged model and a two-layer model which provide tidal and geostrophic currents, respectively; a sediment transport model which provides suspended particle concentrations and sedimentation rates over the domain; and the radionuclide dispersion model including interactions of dissolved radionuclides with suspended particles and bed sediments. These processes are formulated using kinetic transfer coefficients. The hydrodynamic and sediment models are run and validated in advance, and their results are then used to simulate the dispersion of 137Cs and 239,240Pu, which are introduced from atmospheric fallout. Radionuclide concentrations in the water column and distributions in bed sediments have been compared with measurements in the sea. Both set of data are, in general, in agreement. The model has also been applied to calculate radionuclide fluxes through the Strait of Gibraltar. These computed fluxes have been compared with previous estimations as well.     

Modelling the physico-chemical speciation of plutonium in the eastern Irish Sea: a further development

A numerical full three-dimensional model previously developed to simulate the physicochemical speciation of plutonium in the eastern Irish Sea has been improved. The model solves simultaneously the hydrodynamic equations, the suspended matter equation and the equations that give the time evolution of Pu concentrations in water, suspended matter and bottom sediments. It is considered that Pu may exist in each phase in two different oxidation states. Redox reactions are also considered. In the earlier version of the model, a one-step kinetic model was used to describe the transfers of radionuclides between the dissolved and solid phases. Although with this kind of model the contamination of the sediments can be properly simulated, it is clearly not able to describe the re-dissolution of radionuclides from a contaminated sediment once the external source to the sea is reduced. Thus, the model has been improved by substituting the one-step model with a two-step kinetic model consisting of two consecutive reversible reactions. Now it is possible to simulate both the sediment contamination and the re-dissolution processes.     

岷江老木孔航电枢纽库区泥沙淤积计算与分析

老木孔航电枢纽工程是岷江乐山-宜宾段航电梯级开发方案中的第一级,是以航运为主,航电结合的水利工程。首先建立平面二维水流泥沙数学模型,并由实测资料验证其相似性;再根据入库水沙资料,计算分析水库建成后不同时期的泥沙淤积量及其淤积分布,并初步分析分布不均形成的原因;讨论在各级频率洪水作用下,库区泥沙淤积对乐山城区及乐山大佛的影响;并分析泥沙淤积对库区建港环境的影响。通过物模实测值和数模计算值的比较,再次验证了该数学模型的适用性。结果表明:该数学模型可较好的模拟河段水流的运动及库区泥沙的淤积过程;泥沙淤积主要发生在老江坝尾至坝址河段,且估算了该段淤积量占淤积总量的85%;库区淤积后,频率洪水不会对乐山城区及乐山大佛产生影响。     

鄱阳湖丰水期不同底质类型下氮、磷含量分析

以2011年7月份(丰水期)鄱阳湖实测数据为基础,通过不同底质在遥感影像上的光谱特征,将鄱阳湖底质分为沙滩、泥滩和草洲3种类型,并就不同底质类型鄱阳湖水体和底泥总氮(TN)、总磷(TP)含量进行分析。结果表明:(1)底质类型不同时,鄱阳湖表层水体TN、TP含量以及底泥磷含量平均值:沙滩泥滩草洲。底泥TN含量平均值:草洲泥滩沙滩。(2)底质为沙滩的站点主要分布在饶河入湖口至都昌段、赣江北支入湖口至星子段和入江水道前段,饶河、赣江高含量氮、磷的输入,以及沿途陆源污染物的排放,可能是沙滩底质表层水体TN、TP含量较高的一个因素;泥滩底质的站点,有一部分分布在星子至湖口段,该区间陆源污染的影响是泥滩底质表层水体TN、TP含量居次原因之一;而底质为草洲时,草洲对水体中氮、磷的吸收使得草洲底质的站点表层水体TN、TP含量最低。     

Performance assessment studies of models for water flow and radionuclide transport in vegetated soils using lysimeter data

Increasingly the burial of nuclear waste in deep underground repositories is being regarded as a safe long-term solution for disposal. However, to support this safety assessment models of the associated risks are required. An important component of these models is the upward migration of radionuclides from a contaminated water table into arable and pasture crops. A five-year experiment to investigate the processes which control these transfers has been undertaken at Imperial College. Selected data from this experiment were made available to participants of the BIOMOVS II programme in order to allow them to perform blind hydrological and radionuclide transport simulations. The results show the importance of correctly characterising the soil hydrology and indicate that model conceptualisations derived from surface contamination studies may not adequately capture the various processes which influence the upward movement of radionuclides in the vadose zone. These include not only the water movement, but also chemical and biological processes. Finally, the difficulty and importance of a priori parameter selection is highlighted.