共查询到20条相似文献，搜索用时 15 毫秒
1.
Can we use tracer tests to obtain data for performance assessment of repositories for nuclear waste?
With regard to the long-term safety of repositories for spent nuclear fuel, tracer tests have commonly been used in site characterisation (SC) for finding information that can be later used for performance assessment (PA). The question arises as to whether data obtained in tracer tests performed over a time scale of weeks or months are relevant for PA calculations. As part of a study overseen by the Äspö Task Force, the mechanisms that determine the radionuclide residence time under SC and PA conditions are addressed, given that they influence the validity of data transference from SC to PA. The results show that radionuclide transport in SC and PA, although governed by the same retardation mechanisms, are dominated by different sub-processes. In a practical sense this means that the parameter values (typically fracture apertures and “in situ” apparent retention data) determined in tracer tests, performed as a part of the SC program, should not be used directly for making PA predictions. The emphasis in SC should focus more on the determination of other parameters of relevance at PA timescales. The PA-specific flow rate, flow connectivity, and flow-wetted surface to flow ratio are given here as examples. 相似文献
2.
The water retention curve (WRC) is an important hydraulic property of unsaturated soil needed for seepage analysis. Experimental evidence shows that the WRC is affected by various factors such as net stress and soil type. Many attempts have been made to describe the effects of net stress by including the void ratio in a water retention model. But the void ratio (i.e., soil density) is not the only parameter altered by the application of net stress. The pore structure, including the pore size distribution, pore shape and pore orientation, is also changed. Thus the influence of net stress on the WRC should not be treated as equivalent to density effects. In this study, it is verified that the inclusion of the void ratio cannot adequately capture the effects of net stress on the water retention behaviour. A new and simple water retention model is thus developed by considering the stress effects on the void ratio as well as the pore structure. The model is then applied to simulate the WRCs of three different soils tested over a wide range of stress conditions, including isotropic and anisotropic stress conditions. The results show that better predictions of experimental data can be made by incorporating the effects of net stress on both the void ratio and the pore structure. 相似文献
3.
4.
Gabriel C. Rau Martin S. Andersen Andrew M. McCallum Richard Ian Acworth 《Hydrogeology Journal》2010,18(5):1093-1110
Two methods applying natural heat as a tracer to quantify surface water–groundwater exchange were evaluated using field data. Arrays capable of monitoring and recording the streambed response to diurnal temperature variations in the surface water were deployed for a 2-month period in three locations in perennial pools at Maules Creek, New South Wales, Australia. Multi-level array design, field deployment and parameter estimation are discussed. The applicability of analytical solutions derived from the heat transport equation to the streambed environments was analysed using the recorded temperature time series. The stream was found to lose water to the aquifer, which was supported by simultaneously recorded hydraulic gradients. However, the one-dimensional (1D) analytical solutions did not adequately describe the observed streambed thermal response at two locations. The resulting artefacts in the estimated flow velocities are discussed. It was hypothesised that the artefacts originate from model limitation due to streambed heterogeneity and application of 1D solutions to multi-dimensional and dynamic streambed flow. This consequently imposes limitations on the field applicability of the methods. Nevertheless, in combination with time series of surface water and streambed water levels, the use of heat as a tracer provided a powerful tool for better understanding the shallow hydrogeological system. 相似文献
5.
A comparison of estimated and calculated effective porosity 总被引：1，自引：1，他引：0
Daniel B. Stephens Kuo-Chin Hsu Mark A. Prieksat Mark D. Ankeny Neil Blandford Tracy L. Roth James A. Kelsey Julia R. Whitworth 《Hydrogeology Journal》1998,6(1):156-165
Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field
or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared
to estimates derived from particle-size distributions and soil–water characteristic curves. The agreement was poor and it
seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity
estimated from particle-size distributions and soil–water characteristic curves. A field tracer test in a sand-and-gravel
aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from
textural data, moisture retention, and published values were approximately 50–90% greater than the field calibrated value.
Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best
obtained by laboratory or field tracer tests.
Received, March 1997 · Revised, August 1997 · Accepted, August 1997 相似文献
6.
深层地热能是一种清洁环保的可再生能源,尾水回灌是深层地热可持续开发利用的重要保证,但目前国内地热尾水回灌还处在起步阶段,地热尾水回灌方案优化设计的理论模型尚不成熟。文章结合示踪试验,提出了基于等效渗流通道模型的热储参数反演与开采井热突破预测的完整理论框架。应用多孔介质溶质运移理论,修正了等效渗流通道中的溶质浓度解析解,并联合移动渐近线法提出了热储参数反演理论模型;推导了等效渗流通道中的对流传热解析模型,可对开采井中的水温变化进行预测。依托山东省德州市平原县魏庄社区地热对井示踪试验,应用该理论框架进行了示踪试验反演及开采井热突破预测,并对影响开采井热突破的主要因素进行了敏感性分析。 相似文献
7.
M. J. CRICKMORE 《Sedimentology》1967,8(3):175-228
The paper describes tests made on a small river to examine the practicability of measuring quantitatively, by tracer methods, sand transport occurring over a rippled bed in a natural channel. Three principal tracer methods, all previously verified under steady flow conditions in a laboratory channel, and identified as the spatial integration, time integration and steady dilution methods, were examined by introducing radioactive and fluorescent tracers to represent a narrow size fraction of the bed sand. The serious limitations of the time integration and steady dilution methods under conditions of unsteady solids discharge were exposed by a sudden flood discharge occurring during the course of the study. An approximate method, based on the time integration approach, but also requiring some spatial tracer data, is suggested, whereby an estimate of the transport is possible in the case of highly variable sand discharge. However, it is seen that in the sand case, the spatial integration method offers the most workable and informative tracer method for use in natural rivers. The results, by this method, were generally consistent with those given by the summation of the measured suspended-solids flux and the transport occurring either by surface creep or in close vicinity to the bed. The latter quantity is determined from the downstream advance of the ripple bed-forms, and methods are described for its derivation from ultrasonic echo sounder records of bed profiles. Finally sand transport values are calculated for a number of river discharges and for two laboratory channel tests using various well-known computational procedures—Einstein, Bagnold, modified Einstein, Meyer-Peter and Muller, and Laursen procedures. Significant differences in the computed values for individual flow conditions are observed. 相似文献
8.
An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
9.
最小孔隙比是确定岩土体的密实程度与孔隙特征的有效物理指标,如何快速有效地确定岩土体的最小孔隙比,可为岩土体的固结与稳定提供可靠参数。多数估算细粗混合材料最小孔隙比的模型参数与细粗粒径比一一对应,导致估算困难。在分析尾矿粒度组成、沉积规律和固结稳定的基础上,以8种不同粗细粒径,7～9种不同细粒含量尾矿为试验对象,拟合得到不同粒组尾矿最小孔隙比分布模型参数的函数关系;基于混合尾矿颗粒的粒组特征,给出了确定参数幂函数关系的指数值。分别采用模型参数要求粒组范围内的其他6组岩土材料和非粒组范围内的3组岩土材料进行验证。结果表明,考虑粒组分类影响下的模型,参数简单,对不同材料的最小孔隙比估算准确率较高,给出的最小孔隙比的分布规律合理,可为岩土工程领域最小孔隙比估算提供可靠的计算方法。 相似文献
10.
G. Muraleedharan Mourani Sinha A. D. Rao N. Unnikrishnan Nair P. G. Kurup 《Natural Hazards》2009,49(2):165-186
We used field and model wave data to investigate that zero crossing and average wave period distribution follow Gamma distribution.
Since Gamma CDF is an infinite power series, further mathematical treatment is difficult. Hence its shape parameter is approximated
to the nearest integer to arrive at Erlang distribution. An expression is derived from Erlang distribution to estimate various
mean wave periods and significant wave period and validated by relative root mean square (RRMS) error criteria. It is shown
by mathematical logic that the significant wave period distribution follows Erlang (or Gamma) distribution and is validated.
The average of one-third and one-tenth highest wave periods (T
s
) estimated from Erlang distribution are in accordance with the values computed from recorded buoy and numerical coastal wave
model wave period data. The predicted T
s
values from coastal wave model wave period data underestimates the values from buoy wave period data. 相似文献
11.
土壤的孔隙是具有连续分形性质的物理结构,根据土壤孔隙分形结构建立了非饱和水力传导度模型。模型包括综合系数、分形维数和临界体积比3个参数,综合系数为不同土壤基质势对应的非饱和水力传导度与饱和传导度之间的水力联系,与土壤质地有关;分形维数反映土壤孔隙结构对于非饱和水力传导度的作用,土壤不同尺寸孔隙之间的连通性则通过临界体积加以描述。模型具有较为明确的物理解释。将模型应用于5种不同土壤的结果表明,所提出的非饱和水力传导度模型具有较好的模拟效果。 相似文献
12.
The retention (or release) of a liquid compound on a solid controls the mobility of many substances in the environment and has been quantified in terms of the “sorption isotherm”. This paper does not review the different sorption mechanisms. It presents the physical bases underlying the definition of a sorption isotherm, different empirical or mechanistic models, and details several experimental methods to acquire a sorption isotherm. For appropriate measurements and interpretations of isotherm data, this review emphasizes 4 main points: (i) the adsorption (or desorption) isotherm does not provide automatically any information about the reactions involved in the sorption phenomenon. So, mechanistic interpretations must be carefully verified. (ii) Among studies, the range of reaction times is extremely wide and this can lead to misinterpretations regarding the irreversibility of the reaction: a pseudo-hysteresis of the release compared with the retention is often observed. The comparison between the mean characteristic time of the reaction and the mean residence time of the mobile phase in the natural system allows knowing if the studied retention/release phenomenon should be considered as an instantaneous reversible, almost irreversible phenomenon, or if reaction kinetics must be taken into account. (iii) When the concentration of the retained substance is low enough, the composition of the bulk solution remains constant and a single-species isotherm is often sufficient, although it remains strongly dependent on the background medium. At higher concentrations, sorption may be driven by the competition between several species that affect the composition of the bulk solution. (iv) The measurement method has a great influence. Particularly, the background ionic medium, the solid/solution ratio and the use of flow-through or closed reactor are of major importance. The chosen method should balance easy-to-use features and representativity of the studied natural conditions. 相似文献
13.
修正剑桥模型参数对计算结果的影响 总被引：9，自引：1，他引：8
修正剑桥模型是土工计算中广泛应用的弹塑性模型,模型参数的取值是计算分析的重要内容之一,运用实测位移资料采用反分析方法可以有效地确定模型参数。通过有限元计算分析了模型中各参数的变化引起的钻孔灌注桩码头前沿的水平位移的改变,计算结果表明,影响水平位移的主要参数是与弹性模量有关的无量纲参数,与弹性模量有关的指数,泊松比和原始各向等压加荷试验常数,其中尤以与弹性模量有关的无量纲参数对计算结果的影响最为显著,这为反分析参数的选择提供了依据。 相似文献
14.
Effect of water residence time on annual export and denitrification of nitrogen in estuaries: A model analysis 总被引：1，自引：0，他引：1
Edward H. Dettmann 《Estuaries and Coasts》2001,24(4):481-490
A simple model of annual average response of an estuary to mean nitrogen loading rate and freshwater residence time was developed
and tested. It uses nitrogen inputs from land, deposition from the atmosphere, and first-order calculations of internal loss
rate and net export to perform a steady-state analysis over a yearly cycle. The model calculates the fraction of total nitrogen
input from land and the atmosphere that is exported and the fraction that is denitrified or lost to other processes within
the estuary. The model was tested against data from the literature for 11 North American and European estuaries having a wide
range of physical characteristics, nitrogen loading rates, and geographical and climatic settings. The model shows that the
fraction of nitrogen entering an estuary that is exported or denitrified can be predicted from the freshwater residence time.
The first-order rate constant for nitrogen loss within an estuary, as a fraction of total nitrogen in the water column, is
0.30 mo−1. Denitrification typically accounts for 69–75% of the total annual net nitrogen removal from the water column by processes
within the estuary. The model makes explicit the dependence of nitrogen concentration in the water column on the loading rate
of nitrogen, water residence time, estuary volume, and the rate constant for loss within the estuary. 相似文献
15.
Singular spectrum isolates significant principal components in a time series from the embedded noise. This tool-kit is used
to reconstruct trend-free individual time series, formed by restricting the mean monthly hourly values of geomagnetic field
to one hour at a time at a low latitude station Alibag (dipole latitude 9.5°N). Each reconstructed component is extrapolated
over the next 12 values using an autoregressive model based on Burg’s maximum entropy algorithm. Details of a numerical approach
to increase the reliability of extrapolation are highlighted. The extrapolated reconstructed components are then combined
to generate predicted monthly values for each hour. The mean diurnal variation for any month obtained from the extrapolated
individual hourly time series compares favorably with the observations. This approach to Sq(H) modelling incorporating both
long and short term variations will be beneficial in the derivation of Dst index. 相似文献
16.
Daniel C. Reed Katherine Huang Filip J.R. Meysman 《Geochimica et cosmochimica acta》2006,70(23):5855-5867
The intensity of biogenic sediment mixing is often expressed as a “biodiffusion coefficient” (Db), quantified by fitting a diffusive model of bioturbation to vertical profiles of particle-bound radioisotopes. The biodiffusion coefficient often exhibits a dependence on tracer half-life: short-lived radioisotopes (e.g. 234Th) tend to yield notably larger Db values than longer-lived radioisotopes (e.g. 210Pb). It has been hypothesized that this is a result of differential mixing of tracers by particle-selective benthos. This study employs a lattice-automaton model of bioturbation to explore how steady-state tracers with different half-lives are mixed in typical marine settings. Every particle in the model is tagged with the same array of radioisotopes, so that all tracers experienced exactly the same degree of mixing. Two different estimates of the mixing intensity are calculated: a tracer-derived Db, obtained in the standard way by fitting the biodiffusion model to resulting tracer profiles, and a particle-tracking Db, derived from the statistics of particle movements. The latter provides a tracer-independent measure of mixing for use as a reference. Our simulations demonstrate that an apparent Db tracer-dependence results from violating the underlying assumptions of the biodiffusion model. Breakdown of the model is rarely apparent from tracer profiles, emphasizing the need to evaluate the model’s criteria from biological and ecological parameters, rather than relying on obvious indications of model breakdown, e.g., subsurface maxima. Simulations of various marine environments (coastal, slope, abyssal) suggest that the time scales of short-lived radioisotopes, such as 234Th and 7Be, are insufficient for the tracers to be used with the biodiffusion model. 210Pb appears an appropriate tracer for abyssal sediments, while 210Pb and 228Th are suitable for slope and coastal sediments. 相似文献
17.
Djuro Novakovic Christopher D. White Rucsandra M. Corbeanu William S. Hammon III Janok P. Bhattacharya George A. McMechan 《Mathematical Geology》2002,34(7):857-893
Ground-penetrating radar (GPR) surveys, outcrop measurements, and cores provide a high-resolution 3D geologic model to investigate the hydraulic effects of shales in marine-influenced lower delta-plain distributary channel deposits within the Cretaceous-age Ferron Sandstone at Corbula Gulch in central Utah, USA. Shale statistics are computed from outcrop observations. Although slight anisotropy was observed in mean length and variogram ranges parallel and perpendicular to pale of low
, the anisotropy is not statistically significant and the estimated mean length is 5.4 m. Truncated Gaussian simulation was used to create maps of shales that are placed on variably dipping stratigraphic surfaces interpreted from high-resolution 3D GPR surveys, outcrop interpretations, and boreholes. Sandstone permeability is estimated from radar responses calibrated to permeability measurements from core samples. Experimentally designed flow simulations examine the effects of variogram range, shale coverage fraction, and trends in shale coverage on predicted upscaled permeability, breakthrough time, and sweep efficiency. Approximately 1500 flow simulations examine three different geologic models, flow in the 3 coordinate directions, 16 geostatistical parameter combinations, and 10 realizations for each model. ANOVA and response models computed from the flow simulations demonstrate that shales decrease sweep, recovery, and permeability, especially in the vertical direction. The effect on horizontal flow is smaller. Flow predictions for ideal tracer displacements at Corbula Gulch are sensitive to shale-coverage fraction, but are relatively insensitive to twofold variations in variogram range or to vertical trends in shale coverage. Although the hydraulic effects of shale are statistically significant, the changes in flow responses rarely exceed 20%. As a result, it may be reasonable to use simple models when incorporating analogous shales into models of reservoirs or aquifers. 相似文献
18.
深锥浓密机是膏体技术中尾矿浓缩工艺的关键设备,深锥浓密机底流质量分数与泥层压力密切相关,孔隙比是表征底流质量分数的重要物理参数,但是孔隙比随泥层压力的变化规律并不清晰。针对这一问题,首先,提出了尾矿可浓缩性能表征的物理概念--有效孔隙比,即孔隙比减去饱和孔隙比;其次,采用某矿全尾砂进行了动态压密试验,结果表明,当泥层高度为31～200 mm,对应泥层压强为2 477～4 410 Pa,获得的底流质量分数范围为73.26%～78.30%,该质量分数范围对应的有效孔隙比为0.433～0.191。回归有效孔隙比与泥层压强数学关系得知,二者遵循幂函数Allometric模型;最后,提出了膏体动态压密数学模型,并根据模型曲线,将膏体动态压密分为3个区域：（1）线性压缩区,孔隙比随泥层压强增大而基本呈线性关系,孔隙比变化幅度较大;（2）衰减压缩区,随着压强继续增大,孔隙比下降幅度变缓,膏体趋于饱和;（3）恒定压缩区,该区域膏体达到饱和状态,孔隙比随着压强增大而基本恒定。该研究揭示了尾矿浓缩过程中孔隙比随泥层压强的变化规律,为浓密机设计及运行提供理论依据。 相似文献
19.
Martin Larsson Christine Doughty Chin-Fu Tsang Auli Niemi 《Hydrogeology Journal》2013,21(8):1691-1700
The single-well injection-withdrawal (SWIW) tracer test is a method used to estimate the tracer retardation properties of a fracture or fracture zone. The effects of single-fracture aperture heterogeneity on SWIW-test tracer breakthrough curves are examined by numerical modelling. The effects of the matrix diffusion and sorption are accounted for by using a particle tracking method through the addition of a time delay added to the advective transport time. For a given diffusion and sorption property (P m) value and for a heterogeneous fracture, the peak concentration is larger compared to a homogeneous fracture. The cumulative breakthrough curve for a heterogeneous fracture is similar to that for a homogeneous fracture and a less sorptive/diffusive tracer. It is demonstrated that the fracture area that meets the flowing water, the specific flow-wetted surface (sFWS) of the fracture, can be determined by matching the observed breakthrough curve for a heterogeneous fracture to that for a homogeneous fracture with an equivalent property parameter. SWIW tests are also simulated with a regional pressure gradient present. The results point to the possibility of distinguishing the effect of the regional pressure gradient from that of diffusion through the use of multiple tracers with different P m values. 相似文献
20.
为探讨含水层非均质性不同刻画方法对地下水流和溶质运移预测的影响,基于非均质含水层砂箱实验,分别用传统等效均质模型、克立金插值和水力层析刻画含水层渗透系数场,并探讨了先验信息对水力层析结果的影响.将不同方法估算的渗透系数场用以预测地下水流和溶质运移过程,以此判断不同方法估算结果的优劣,分析含水层非均质性对地下水流和溶质运移的影响.结果表明:与克立金插值法相比,水力层析法可以更好地刻画含水层非均质性,较准确地预测地下水流和溶质运移过程;钻孔岩心渗透系数样本值作为先验信息可以提高水力层析法估算结果的精度;传统等效均质模型无法准确预测地下水流和溶质运移过程.含水层非均质性的增强将导致溶质污染羽分布形态和运移路径的空间变异性增强,并且优势通道直接决定溶质的分布及运移路径. 相似文献