Validation of the operational emergency response model at the Swedish Meteorological and Hydrological Institute using data from etex and the chernobyl accident

The Eulerian atmospheric tracer transport model MATCH (Multiscale Atmospheric Transport and Chemistry model) has been extended with a Lagrangian particle model treating the initial dispersion of pollutants from point sources. The model has been implemented at the Swedish Meteorological and Hydrological Institute in an emergency response system for nuclear accidents and can be activated on short notice to provide forecast concentration and deposition fields.The model has been used to simulate the transport of the inert tracer released during the ETEX experiment and the transport and deposition of ¹³⁷Cs from the Chernobyl accident. Visual inspection of the results as well as statistical analysis shows that the extent, time of arrival and duration of the tracer cloud, is in good agreement with the observations for both cases, with a tendency towards over-prediction for the first ETEX release. For the Chernobyl case the simulated deposition pattern over Scandinavia and over Europe as a whole agrees with observations when observed precipitation is used in the simulation. When model calculated precipitation is used, the quality of the simulation is reduced significantly and the model fails to predict major features of the observed deposition field.     

Influence of trees on the dispersion of pollutants in an urban street canyon—Experimental investigation of the flow and concentration field

Flow field and concentration measurements have been performed in an idealized model of an urban street canyon with one row of trees arranged along the center axis. The model was set up in an atmospheric boundary layer wind tunnel and the approach flow was directed perpendicular to the street axis. A line source embedded in the bottom of the street was used to release tracer gas for the simulation of traffic exhaust emissions. Trees with spherical crowns were modeled and positioned inside the street canyon, varying crown diameter, crown permeability, trunk height and tree spacing. Traffic-induced turbulence was simulated by rotating belts with thin plates. Concentrations were measured at the facades of the street canyon. For small tree crowns, only little changes in concentration were measured, however, increasing crown diameters led to increasing concentrations at the leeward street canyon wall associated with a reduction of local concentrations at the windward wall. For some cases, a variation of trunk height led to a modification of the concentration pattern on the walls. Increasing the tree spacing resulted in a noticeable concentration decrease. When compared to the situation with standing (but emitting) traffic, the traffic-induced turbulence by two-way car movements always contributed to a more homogenous concentration field inside the street canyon yielding to reduced mean concentration levels.     

Simulations of flow around a cubical building: comparison with towing-tank data and assessment of radiatively induced thermal effects

A three-dimensional (3-D) computational fluid dynamics (CFD) model, coupled with a meteorological radiation and surface physics package, is used to model the mean flow field and tracer dispersion in the vicinity of an idealized cubical building. We first compare the simulations with earlier numerical studies as well as towing-tank laboratory experiments, where radiation effects were not included. Our simulations capture most of the features revealed by the towing-tank data, including the variation of the flow reattachment point as a function of Froude number and the induction of a prominent lee wave in the low Froude number regime. The simulated tracer concentration also compares very favorably with the data.We then assess the thermal effects due to radiative heating on the ground and building including shading by the building, on the mean flow and tracer dispersion. Our simulations show that convergence within and beyond the cavity zone causes a substantial lofting of the air mass downstream from the building. This lofting results from the combination of thermal heating of the ground and building roof, and vortex circulation associated with the horseshoe eddy along the lateral sides of the building. The specific effect of shading on the flow field is isolated by comparing simulations for which the radiative heating and shading patterns are kept constant, but the environmental wind direction is altered. It is found that the shading exerts local cooling, which can be combined into the overall thermodynamic interaction, described above, to effectively alter the circulation downstream from the building.     

Validation studies of turbulence closure schemes for high resolutions in mesoscale meteorological models – A case of gas dispersion at the local scale

This work is a contribution to a large project, aimed at the development of an advanced environmental assessment modelling system to be used in Japan. The modelling system here considered consisted of the RAMS and HYPACT coupled models. The RAMS code was modified to properly simulate local scale phenomena using a fine mesh size of 250 m. In this direction, the main aim here was to investigate the effect of the choice of the turbulence closure scheme on the dispersion of pollutants. Our modified version of the RAMS/HYPACT model chain was validated using field experiments which were carried out by the Japan Atomic Energy Research Institute (JAERI) in the area of Mt. Tsukuba (Japan). The mean flow, turbulence and concentration fields obtained using two alternative turbulence closure schemes are compared. A discussion on the different performances of the turbulence closures is presented and the influence of the closure schemes over the plume dispersion is investigated.     

Modelling of nonreactive tracer dipole tests in a shear zone at the Grimsel test site

The investigation of the migration of a high pH plume in a fractured shear zone is foreseen by a long-term experiment at the Grimsel rock laboratory. In order to characterise the initial conditions for the long-term experiment and to evaluate an optimal hydraulic in situ set-up, several dipole experiments with nonreacting tracers have been performed. The dipole experiments differ in geometry, pumping rates and orientation to the background water flow. Several single and double-porosity models have been applied to fit the results of these dipole tracer tests in order to extract values for some transport parameters and discriminate for certain transport processes. A two-dimensional porous medium approach was successfully used to fit tracer breakthrough curves measured for a dipole experiment. A model based on a one-dimensional dual porous medium approach was also successful, although the applied hydraulic dipole, with similar injection and extraction rates, suggests the existence of an extended two-dimensional flow field. For the two-dimensional porous medium approach, tracer breakthrough could only be fitted with a complex flow field geometry within the heterogeneous fractured shear zone. The heterogeneity was generated by heterogeneous porosity and hydraulic permeability distributions. Predictions for further dipole geometries and a sorbing tracer have been calculated by means of both models using the flow and transport parameters deduced from fits for a single dipole experiment. This allows for comparison with the measured breakthrough of sorbing tracers. The foreseen experiment with sorbing (radionuclide) tracers will help decide on the appropriate approach that should be used to describe such dipole experiments in this shear zone. Additionally, the migration and spreading of a solution with high pH has been calculated taking into account mineral dissolution and precipitation in a two-dimensional porous medium approach in order to estimate the amount and character of the mineral reactions induced by the interaction between the high pH solution and the rock.     

On the influence of the urban roughness sublayer on turbulence and dispersion

The concept of the urban roughness sublayer is discussed and this lowest atmospheric layer over a rough surface is shown to have a non-negligible vertical extension over typical urban surfaces. The existing knowledge on the turbulence and flow structure within an urban roughness sublayer is reviewed, focusing on the height dependence of turbulent fluxes and a scaling approach for turbulence statistics, such as velocity variances, in the above-roof part of the roughness sublayer. Finally, the implication of this turbulence and flow structure upon dispersion characteristics is investigated. The most prominent difference of explicitly taking into account the roughness sublayer in a dispersion simulation (as compared to assuming a `constant flux layer') is a clearly enhanced ground level concentration far downwind from the source. For the example of a tracer release experiment over a (sub) urban surface (Copenhagen) it is shown that introducing the roughness sublayer clearly improves the model performance.     

Numerical calculation of flow and stack-gas concentration fluctuation around a cubical building

A numerical simulation model was developed to predict the instantaneous concentration fluctuation of a plume and applied to stack-gas diffusion around a cubical building. The flow field, including an instantaneous velocity component, was predicted using the large eddy simulation (LES) method in the developed numerical model. Then, the instantaneous concentration fluctuation was predicted using the obtained unsteady flow field. Concentration was calculated using the finite difference method, in which the LES is expanded for concentration, and the puff method, in which small volumes of the tracer gas are divided and combined according to the calculation mesh sizes. In order to avoid numerical viscous effects, a puff method and finite difference method were applied separately in the regions near and far from the stack-gas release point, respectively. Then, the flow field around a cubical building and the diffusion of stack-gas, emitted from an elevated point source at an upstream position of the building, were calculated using the model mentioned above. Numerical calculation results were compared with those obtained in wind tunnel experiments in which concentration fluctuation was measured using high-response flame ionization detectors. Although there were some discrepancies in the flow field between the calculated results and those of wind tunnel experiments, e.g., the calculated windward length of a cavity region behind the building, the calculated mean velocity and turbulent intensity showed good agreement with those of the wind tunnel experiments. Furthermore, the calculated concentration fluctuation showed good agreement with that in the wind tunnel, not only regarding the features of fluctuating concentration signals, but also statistic quantities, viz., mean concentration, fluctuation intensity and high-concentration values.     

Flow and transport in unsaturated fractured rock: effects of multiscale heterogeneity of hydrogeologic properties

The heterogeneity of hydrogeologic properties at different scales may have different effects on flow and transport processes in a subsurface system. A model for the unsaturated zone of Yucca Mountain, Nevada, is developed to represent complex heterogeneity at two different scales: (1) layer scale corresponding to geologic layering and (2) local scale. The layer-scale hydrogeologic properties are obtained using inverse modeling, based on the available measurements collected from the Yucca Mountain site. Calibration results show a significant lateral and vertical variability in matrix and fracture properties. Hydrogeologic property distributions in a two-dimensional, vertical cross-section of the site are generated by combining the average layer-scale matrix and fracture properties with local-scale perturbations generated using a stochastic simulation method. The unsaturated water flow and conservative (nonsorbing) tracer transport through the cross-section are simulated for different sets of matrix and fracture property fields. Comparison of simulation results indicates that the local-scale heterogeneity of matrix and fracture properties has a considerable effect on unsaturated flow processes, leading to fast flow paths in fractures and the matrix. These paths shorten the travel time of a conservative tracer from the source (repository) horizon in the unsaturated zone to the water table for small fractions of total released tracer mass. As a result, the local-scale heterogeneity also has a noticeable effect on global tracer transport processes, characterized by an average breakthrough curve at the water table, especially at the early arrival time of tracer mass. However, the effect is not significant at the later time after 20% tracer mass reaches the water table. The simulation results also verify that matrix diffusion plays an important role in overall solute transport processes in the unsaturated zone at Yucca Mountain.     

潜流式湿地系统停留时间分布实验结果分析

鉴于水力停留时间对人工湿地系统处理效果和构建设计的重要性,通过示踪实验测定了潜流式湿地系统的停留时间分布,结合反应器理论,对实验测定的不同进水流量范围的停留时间响应结果,分别以多釜串联模型以及移位正态分布函数进行拟合,并进一步对系统的水力平均停留时间、系统的流动形态以及非理想性进行了分析和讨论,为湿地系统的水力学优化设计和实际运行提供理论基础和技术参数.     

A numerical model for the simulation of SO2 concentrations in the Kyongin region,Korea

A method for solving the non-reactive tracer continuity equation using a time splitting technique and a Galerkin technique with chapeau functions as finite elements for the horizontal advection has been developed and employed to simulate SO₂ concentrations in the Kyongin region in Korea for a synoptic case of high pollution potential days in autumn with the relatively strong southwesterly geostrophic wind at the 850 hPa pressure level. The paired comparisons between hourly observed and the simulated SO₂ concentrations are made to test the model performance. The result indicates that the present model simulates quite well horizontal distribution patterns of SO₂ concentration. However, the simulated concentrations depend largely on the emission rate, suggesting the importance of accurate source identification for the accurate simulation of the concentration field.     

Characterization of atmospheric contaminant sources using adaptive evolutionary algorithms

The characteristics of an unknown source of emissions in the atmosphere are identified using an Adaptive Evolutionary Strategy (AES) methodology based on ground concentration measurements and a Gaussian plume model. The AES methodology selects an initial set of source characteristics including position, size, mass emission rate, and wind direction, from which a forward dispersion simulation is performed. The error between the simulated concentrations from the tentative source and the observed ground measurements is calculated. Then the AES algorithm prescribes the next tentative set of source characteristics. The iteration proceeds towards minimum error, corresponding to convergence towards the real source.The proposed methodology was used to identify the source characteristics of 12 releases from the Prairie Grass field experiment of dispersion, two for each atmospheric stability class, ranging from very unstable to stable atmosphere. The AES algorithm was found to have advantages over a simple canonical ES and a Monte Carlo (MC) method which were used as benchmarks.     

The field campaigns of the European Tracer Experiment (ETEX): overview and results

As part of the European Tracer Experiment (ETEX) two successful atmospheric experiments were carried out in October and November, 1994. Perfluorocarbon (PFC) tracers were released into the atmosphere in Monterfil, Brittany, and air samples were taken at 168 stations in 17 European countries for 72 h after the release. Upper air tracer measurements were made from three aircraft. During the first experiment a westerly air flow transported the tracer plume north-eastwards across Europe. During the second release the flow was eastwards. The results from the ground sampling network allowed the determination of the cloud evolution as far as Sweden, Poland and Bulgaria. This demonstrated that the PFT technique can be successfully applied in long-range tracer experiments up to 2000 km. Typical background concentrations of the tracer used are around 5–7 fl ?^-1 in ambient air. Concentrations in the plume ranged from 10 to above 200 fl/?^-1. The tracer release characteristics, the tracer concentrations at the ground and in upper air, the routine and additional meteorological observations at the ground level and in upper air, trajectories derived from constant-level balloons and the meteorological input fields for long-range transport models are assembled in the ETEX database. The ETEX database is accessible via the Internet. Here, an overview is given of the design of the experiment, the methods used and the data obtained.     

浅水海湾排污扩散器初始稀释度的计算

流速和水深是影响浅水型海湾扩散器初始稀释能力的主要环境因素,其长时间序列的实测资料在现实中通常很难得到,而平面二维水动力模型的引入可为初始稀释度的计算提供必要和准确的水动力要素。通过建立二维水动力模型模拟受纳海域的流场,利用实测水文资料进行模型的率定和验证,模拟设计水文条件下附近水域的流场,然后提取排污口处的水深、流速和流向的时间序列,再利用近区模型计算任一时刻的初始稀释度,最后进行累积频率分析得到设计保证率下的初始稀释度。将该方法应用于杭州湾某排污扩散器初始稀释能力的评估,研究表明,利用远区模型为扩散器近区模拟提供环境水文要素,弥补了实际工程计算中缺乏实测水文资料的不足,并大大提高了近区模拟的计算精度。     

Characterisation of HTO diffusion properties by an in situ tracer experiment in Opalinus clay at Mont Terri

A long-term single borehole diffusion experiment using tritiated water as tracer was carried out in Opalinus clay, an argillaceous rock formation that is accessible at the Mont Terri Underground Research Laboratory, situated in the Swiss Jura. The tracer was diluted in reconstituted formation water and introduced into a packed-off section of a borehole located in saturated rock. Pressure in this interval was maintained equal to the pore pressure of the surrounding rock in order to prevent any hydraulic gradient around the borehole and to avoid advective transport processes. The evolution of the tracer concentration in the injection system was monitored over time. After 1 year of diffusion, the claystone surrounding the interval was retrieved by overcoring the whole borehole and packer system, and by an adjacent oblique borehole. Compressed air was used as drilling fluid to reduce rock disturbances. The recovered overcore was sampled along profiles perpendicular to the borehole wall with a view to determining the tracer-concentration profiles in the rock. To avoid further evaporation of tritiated water, subsamples were immediately transferred into polyethylene bottles and disaggregated by adding a known amount of tracer-free water. Fifteen profiles were determined and showed a decreasing tracer concentration with distance into the rock. The pore-water contents were constant along those profiles, confirming that only very little water was lost during overcoring operations. The evolution of tritium-tracer concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation of the experiment. That allowed for the identification of the transport parameters (orthotropic diffusion tensor and porosity) by minimising the relative quadratic error between the experimental and simulated data. The fitting is good and the results are consistent with data obtained on drill-core samples. The result of tritiated water is discussed regarding (1) the potential effect of mechanical and/or chemical disturbances around the injection borehole and (2) the specific behaviour of tritiated water.     

Quantitative imaging of contaminant distributions in heterogeneous porous media laboratory experiments

Intermediate-scale laboratory experiments on heterogeneous porous media have been increasingly used for the study of saturated and unsaturated ground water systems. While the ability to reproduce field-scale heterogeneity in these experiments has advanced, the use of visualization or image analysis methods to characterize the spatial distribution of solute concentrations has largely remained at the homogeneous media level. To advance these imaging techniques we developed a generic image analysis package that, for the first time, automatically segments regions in photographic images that require unique concentration calibration curves due to varying porous media properties or lighting nonuniformities. As a robust test, our image analysis package was applied to an intermediate-scale flow tank experiment characterized by a correlated random permeability field with unprecedented resolution. Twenty-five distinct classes of porous media were developed and binned to the synthetic permeability field, creating an experimental field of 3456 rectangular cells and thereby ensuring the emplaced field closely matched the statistics of the original continuous distribution. Concentration distributions were determined for an experimental tracer run and the corresponding dispersion parameters were calculated. The closeness of the experimental, image-processed longitudinal dispersivity (4.6 x 10(-2) m) to that obtained from the field statistics (9.1 x 10(-2) m) verifies our image analysis technique.     

3-D numerical evaluation of density effects on tracer tests

In this paper we present numerical simulations carried out to assess the importance of density-dependent flow on tracer plume development. The scenario considered in the study is characterized by a short-term tracer injection phase into a fully penetrating well and a natural hydraulic gradient. The scenario is thought to be typical for tracer tests conducted in the field. Using a reference case as a starting point, different model parameters were changed in order to determine their importance to density effects. The study is based on a three-dimensional model domain. Results were interpreted using concentration contours and a first moment analysis. Tracer injections of 0.036 kg per meter of saturated aquifer thickness do not cause significant density effects assuming hydraulic gradients of at least 0.1%. Higher tracer input masses, as used for geoelectrical investigations, may lead to buoyancy-induced flow in the early phase of a tracer test which in turn impacts further plume development. This also holds true for shallow aquifers. Results of simulations with different tracer injection rates and durations imply that the tracer input scenario has a negligible effect on density flow. Employing model cases with different realizations of a log conductivity random field, it could be shown that small variations of hydraulic conductivity in the vicinity of the tracer injection well have a major control on the local tracer distribution but do not mask effects of buoyancy-induced flow.     

3-D numerical evaluation of density effects on tracer tests

In this paper we present numerical simulations carried out to assess the importance of density-dependent flow on tracer plume development. The scenario considered in the study is characterized by a short-term tracer injection phase into a fully penetrating well and a natural hydraulic gradient. The scenario is thought to be typical for tracer tests conducted in the field. Using a reference case as a starting point, different model parameters were changed in order to determine their importance to density effects. The study is based on a three-dimensional model domain. Results were interpreted using concentration contours and a first moment analysis. Tracer injections of 0.036 kg per meter of saturated aquifer thickness do not cause significant density effects assuming hydraulic gradients of at least 0.1%. Higher tracer input masses, as used for geoelectrical investigations, may lead to buoyancy-induced flow in the early phase of a tracer test which in turn impacts further plume development. This also holds true for shallow aquifers. Results of simulations with different tracer injection rates and durations imply that the tracer input scenario has a negligible effect on density flow. Employing model cases with different realizations of a log conductivity random field, it could be shown that small variations of hydraulic conductivity in the vicinity of the tracer injection well have a major control on the local tracer distribution but do not mask effects of buoyancy-induced flow.     

Design of aquifer remediation systems: (1) describing hydraulic structure and NAPL architecture using tracers

Aquifer heterogeneity (structure) and NAPL distribution (architecture) are described based on tracer data. An inverse modelling approach that estimates the hydraulic structure and NAPL architecture based on a Lagrangian stochastic model where the hydraulic structure is described by one or more populations of lognormally distributed travel times and the NAPL architecture is selected from eight possible assumed distributions. Optimization of the model parameters for each tested realization is based on the minimization of the sum of the square residuals between the log of measured tracer data and model predictions for the same temporal observation. For a given NAPL architecture the error is reduced with each added population. Model selection was based on a fitness which penalized models for increasing complexity. The technique is demonstrated under a range of hydrologic and contaminant settings using data from three small field-scale tracer tests: the first implementation at an LNAPL site using a line-drive flow pattern, the second at a DNAPL site with an inverted five-spot flow pattern, and the third at the same DNAPL site using a vertical circulation flow pattern. The Lagrangian model was capable of accurately duplicating experimentally derived tracer breakthrough curves, with a correlation coefficient of 0.97 or better. Furthermore, the model estimate of the NAPL volume is similar to the estimates based on moment analysis of field data.     

Design of aquifer remediation systems: (1) Describing hydraulic structure and NAPL architecture using tracers

Aquifer heterogeneity (structure) and NAPL distribution (architecture) are described based on tracer data. An inverse modelling approach that estimates the hydraulic structure and NAPL architecture based on a Lagrangian stochastic model where the hydraulic structure is described by one or more populations of lognormally distributed travel times and the NAPL architecture is selected from eight possible assumed distributions. Optimization of the model parameters for each tested realization is based on the minimization of the sum of the square residuals between the log of measured tracer data and model predictions for the same temporal observation. For a given NAPL architecture the error is reduced with each added population. Model selection was based on a fitness which penalized models for increasing complexity. The technique is demonstrated under a range of hydrologic and contaminant settings using data from three small field-scale tracer tests: the first implementation at an LNAPL site using a line-drive flow pattern, the second at a DNAPL site with an inverted five-spot flow pattern, and the third at the same DNAPL site using a vertical circulation flow pattern. The Lagrangian model was capable of accurately duplicating experimentally derived tracer breakthrough curves, with a correlation coefficient of 0.97 or better. Furthermore, the model estimate of the NAPL volume is similar to the estimates based on moment analysis of field data.