Field-scale effective matrix diffusion coefficient for fractured rock: results from literature survey

Matrix diffusion is an important mechanism for solute transport in fractured rock. We recently conducted a literature survey on the effective matrix diffusion coefficient, D_m^e, a key parameter for describing matrix diffusion processes at the field scale. Forty field tracer tests at 15 fractured geologic sites were surveyed and selected for the study, based on data availability and quality. Field-scale D_m^e values were calculated, either directly using data reported in the literature, or by reanalyzing the corresponding field tracer tests. The reanalysis was conducted for the selected tracer tests using analytic or semi-analytic solutions for tracer transport in linear, radial, or interwell flow fields. Surveyed data show that the scale factor of the effective matrix diffusion coefficient (defined as the ratio of D_m^e to the lab-scale matrix diffusion coefficient, D_m, of the same tracer) is generally larger than one, indicating that the effective matrix diffusion coefficient in the field is comparatively larger than the matrix diffusion coefficient at the rock-core scale. This larger value can be attributed to the many mass-transfer processes at different scales in naturally heterogeneous, fractured rock systems.Furthermore, we observed a moderate, on average trend toward systematic increase in the scale factor with observation scale. This trend suggests that the effective matrix diffusion coefficient is likely to be statistically scale-dependent. The scale-factor value ranges from 0.5 to 884 for observation scales from 5 to 2000 m. At a given scale, the scale factor varies by two orders of magnitude, reflecting the influence of differing degrees of fractured rock heterogeneity at different geologic sites. In addition, the surveyed data indicate that field-scale longitudinal dispersivity generally increases with observation scale, which is consistent with previous studies. The scale-dependent field-scale matrix diffusion coefficient (and dispersivity) may have significant implications for assessing long-term, large-scale radionuclide and contaminant transport events in fractured rock, both for nuclear waste disposal and contaminant remediation.     

Study on anisotropy of effective diffusion coefficient and activation energy for deuterated water in compacted sodium bentonite

To quantify the effects of temperature on the diffusivity of deuterated water (HDO) in compacted sodium bentonite, through-diffusion experiments were conducted at elevated temperatures ranging from 298 to 333 K. Kunipia F (Na-montmorillonite content>98 wt.%; Kunimine Industries) was compacted to a dry density of 0.9 or 1.35 Mg/m(3). As montmorillonite particles were oriented perpendicular to the direction of compaction, the anisotropy of diffusivity was investigated both parallel and normal to the preferred orientation of the montmorillonite. The effective diffusion coefficient D(e) of HDO was larger when the diffusional direction was parallel as opposed to normal to the preferred orientation for both dry densities. The magnitude of D(e) and the anisotropy for HDO were in good accordance with previously reported results for tritiated water at room temperature. Activation energies of D(e) were isotropic and increased with increasing dry density over the range of 19-25 kJ/mol. This relationship was considered to be due to both pore structure development and the high activation energy of water near the montmorillonite surface.     

Influence of the mode of matrix porosity determination on matrix diffusion calculations

The theoretical basis for matrix diffusion in fractured rocks and the methodology for the determination of diffusion coefficients in the laboratory are well established. One significant problem, however, remains in that it is difficult to quantify the degree of sample disturbance affecting the geometrical, geophysical and hydraulic properties of the rock matrix. A new technique, with in situ rock impregnation with resin, for examining the diffusion-accessible rock matrix has been developed and successfully adopted to the rock matrix behind a water-conducting fracture in host crystalline rocks at Nagra's Grimsel Test Site in Switzerland and JNC's Kamaishi In Situ Test Site in Japan. In line with the results of a large number of natural analogue and laboratory studies, the existence of an in situ interconnected pore network was substantiated. Matrix porosities determined on the laboratory samples from both the sites are 1.5-3 times higher than in situ values, irrespective of the technique applied. On the Grimsel granodiorite matrix, matrix porosity existing in situ and artefacts of stress release and physical disturbance, induced by sampling and sample preparation, were clearly distinguished, allowing in situ porosity to be quantified. Laboratory work with conventional techniques tends to overestimate the porosity of the rock matrix, hence leading to an overestimation of in situ matrix diffusion. The implications of these differences to a repository performance assessment are assessed with a couple of examples from existing assessments, and recommendations for future approaches to the examination of in situ matrix porosity are made.     

有机蒸气-空气分子扩散系数实测数据及分析

利用自行研发的气体扩散系数测试装置,测定了烷烃、环烷烃、芳香烃、含氧化合物（酯类、醇类、酮类等）等21种挥发性有机化合物在空气中的分子扩散系数。测定结果表明：互为同系物的有机化合物在相同温度下,分子扩散系数随着分子链的增长而减小,与其沸点呈负相关;互为同分异构体的有机化合物在相同温度下,有支链的分子扩散系数比没有支链的要大。另外,将实验实测值与FSG方程计算值以及文献值进行了对比分析。通过比较实测值与计算值,认为大部分分子结构全部为单键并且不含环状结构的有机化合物的实测值与计算值吻合度比较高,但分子结构中含苯环、碳环和烷烃支链的有机化合物,实测值与计算值吻合度不是很高。建议FSG方程进一步考虑分子结构差异性的影响。     

有机蒸气-空气分子扩散系数测定的影响因素分析

利用自行研发的气体扩散系数测试装置及测定方法,分别测定和分析了常温常压下,不同扩散管径、扩散距离、体系温度、气体进气方式及其流量、扩散时间等对气体扩散系数测定的影响情况。基于拟稳态扩散模型,得出气体扩散系数测定的合理条件：扩散距离为9~14 cm;吹气流量为0.22~0.33 m3·h-1;体系温度应低于液体沸点且不要太接近沸点;扩散时间根据液体挥发性情况,一般不小于12 h,具体以能够明显观测到测定前后液面高度差为基准;扩散管管径应在保证扩散管截面积较小、液面以上气体以单向自然扩散为主、液体蒸发速度相对较慢的范围内选取。研究结果确定了气体扩散系数测定的合理条件,对加深油气空间传质机理的认识以及确保扩散系数测定的准确度具有重要指导意义。     

The dependence of the beta coefficient of REA system with dynamic deadband on atmospheric conditions

We simulated the REA system with dynamic deadband to study numerical value and the effect of atmospheric conditions on the empirical constant beta which relates vertical flux to concentration difference between updrafts and downdrafts. We found that the value of beta depends only weakly on the friction velocity and atmospheric stability. In agreement with previous studies, the median value obtained for a system with dynamic deadband proportional to 0.5 times the running mean of the standard deviation of vertical wind speed was beta=0.42+/-0.03. For a single half-hour measurement one has to consider the large uncertainty of +/-0.2. According to our study, the dynamic deadband enables the use of a constant value of beta in flux calculation.     

Molecular insight into the diffusion/flow potential properties initiated by methane adsorption in coal matrix: taking the factor of moisture contents into account

Environmental Science and Pollution Research - Coal seam permeability is one of the key parameters affecting coalbed methane (CBM), and plays an important role in resource evaluation and regional...     

Simultaneous estimation of effective and apparent diffusion coefficients in compacted bentonite

Effective diffusion coefficients (D(e)) are usually measured by means of "through-diffusion" experiments in which steady state is reached, and the "time-lag" methods are used to estimate the apparent diffusion coefficient (D(a)). For sorbing radionuclides (as caesium), the time needed to reach steady-state conditions is very large, and the precision in D(a) determinations is not satisfactory. In this paper, a method that allows determining simultaneously effective and apparent diffusion coefficients in compacted bentonite without reaching steady-state conditions is described. Basically, this method consists of an "in-diffusion" experiment in which the concentration profile in the bentonite sample is used to estimate D(a), and the temporal evolution of the solute concentration in the reservoir is used to estimate D(e). This method has several advantages over the typical "through-diffusion" experiments, in particular: (a) experiment duration is significantly shorter, (b) D(a) values are measured with greater precision and (c) it is not necessary to maintain a constant solute concentration in the reservoir. This new method has been used to estimate the effective and apparent diffusion coefficients for caesium in FEBEX bentonite and in order to validate it, the results have been compared with results previously obtained with standard methods. Experimental results have been satisfactorily modelled using a simple model of diffusion in porewater and the measured value of D(e)(Cs) is very similar to D(e)(HTO) in the same bentonite. There is no evidence of "surface diffusion" in FEBEX bentonite for caesium.     

An axisymmetric diffusion experiment for the determination of diffusion and sorption coefficients of rock samples

Diffusion anisotropy is a critical property in predicting migration of substances in sedimentary formations with very low permeability. The diffusion anisotropy of sedimentary rocks has been evaluated mainly from laboratory diffusion experiments, in which the directional diffusivities are separately estimated by through-diffusion experiments using different rock samples, or concurrently by in-diffusion experiments in which only the tracer profile in a rock block is measured. To estimate the diffusion anisotropy from a single rock sample, this study proposes an axisymmetric diffusion test, in which tracer diffuses between a cylindrical rock sample and a surrounding solution reservoir. The tracer diffusion between the sample and reservoir can be monitored from the reservoir tracer concentrations, and the tracer profile could also be obtained after dismantling the sample. Semi-analytical solutions are derived for tracer concentrations in both the reservoir and sample, accounting for an anisotropic diffusion tensor of rank two as well as the dilution effects from sampling and replacement of reservoir solution. The transient and steady-state analyses were examined experimentally and numerically for different experimental configurations, but without the need for tracer profiling. These experimental configurations are tested for in- and out-diffusion experiments using Koetoi and Wakkanai mudstones and Shirahama sandstone, and are scrutinized by a numerical approach to identify favorable conditions for parameter estimation. The analysis reveals the difficulty in estimating diffusion anisotropy; test configurations are proposed for enhanced identifiability of diffusion anisotropy. Moreover, it is demonstrated that the axisymmetric diffusion test is efficient in obtaining the sorption parameter from both steady-state and transient data, and in determining the effective diffusion coefficient if isotropic diffusion is assumed. Moreover, measuring reservoir concentrations in an axisymmetric diffusion experiment coupled with tracer profiling may be a promising approach to estimate of diffusion anisotropy of sedimentary rocks.     

Measurement of effective air diffusion coefficients for trichloroethene in undisturbed soil cores

In this study, we measure effective diffusion coefficients for trichloroethene in undisturbed soil samples taken from Picatinny Arsenal, New Jersey. The measured effective diffusion coefficients ranged from 0.0053 to 0.0609 cm2/s over a range of air-filled porosity of 0.23-0.49. The experimental data were compared to several previously published relations that predict diffusion coefficients as a function of air-filled porosity and porosity. A multiple linear regression analysis was developed to determine if a modification of the exponents in Millington's [Science 130 (1959) 100] relation would better fit the experimental data. The literature relations appeared to generally underpredict the effective diffusion coefficient for the soil cores studied in this work. Inclusion of a particle-size distribution parameter, d10, did not significantly improve the fit of the linear regression equation. The effective diffusion coefficient and porosity data were used to recalculate estimates of diffusive flux through the subsurface made in a previous study performed at the field site. It was determined that the method of calculation used in the previous study resulted in an underprediction of diffusive flux from the subsurface. We conclude that although Millington's [Science 130 (1959) 100] relation works well to predict effective diffusion coefficients in homogeneous soils with relatively uniform particle-size distributions, it may be inaccurate for many natural soils with heterogeneous structure and/or non-uniform particle-size distributions.     

Temperature dependence of the infinite dilution activity coefficient and Henry's law constant of polycyclic aromatic hydrocarbons in water

The water solubility of 9,10-dihydroanthracene was experimentally determined between 278.12 and 313.17 K. Determinations were carried out by an experimental procedure developed in our laboratory, which is a modification of the dynamic coupled column liquid chromatographic technique. The uncertainty of the experimental determinations ranged from +/- 0.50% to +/- 3.10%. These data, as well as the water solubility data of other five polycyclic aromatic hydrocarbons (PAHs) previously studied, were used to calculate the temperature dependence of the infinite dilution activity coefficient of 9,10-dihydroanthracene, anthracene, pyrene, 9,10-dihydrophenanthrene, m-terphenyl, and guaiazulene in water. Molar excess enthalpies and entropies at infinite dilution, at 298.15 K, were also derived. The temperature dependence of the infinite dilution activity coefficients was used, together with literature values of the vapor pressures of supercooled liquid PAHs (p(B)(sc)), to estimate their Henry's law constants (HLC). Only HLC for anthracene, pyrene, and 9,10-dihydrophenanthrene were calculated, since no p(B)(sc) data were available in the literature for 9,10-dihydroanthracene, m-terphenyl, and guaiazulene. From the observed temperature dependence of the Henry's law constants the enthalpy and entropy of the phase change from the dissolved phase to the gas phase were also derived for anthracene, pyrene, and 9,10-dihydrophenanthrene.     

新型高效气体扩散电极的制备与性能

评述了原位产生H₂O₂的电化学法污水处理技术中使用的主要电极,研究了用石墨制备高效气体扩散电极的方法。对石墨用HNO₃与H₃PO₄的混合液改性,石墨与聚四氟乙烯的质量比为2∶1,电极经350℃煅烧1 h,对溶解O₂还原产生H₂O₂的催化活性最高。仅在pH值很低时,电极的活性较差,在pH=3～9时,电极的活性受pH值变化影响较小。在pH=3时电解150 min,电极的电流效率始终高于72%,H₂O₂浓度达到了274.5 mg/L。     

An interpretation of Taylor's statistical analysis of particle dispersion

We interpret Taylor's equation for plume growth in homogeneous turbulence in terms that have ready intuitive and physical appeal. We show how an understanding of the physical meaning of the Lagrangian time-scale allows a simple derivation of the linear and square root growth laws, which form the basis of most plume spread parametenzations. We then go on to show how this understanding can be used to derive the commonly used expressions for the effects of wind shear on plume spread.     

Estimating temperature dependence of solubility and octanol-water partition coefficient for organic compounds using RP-HPLC.

Temperature dependence data for physical-chemical properties is increasingly required for modelling the fate of chemicals in the environment. Solubility and octanol-water partition coefficient (Kow) are among the most important parameters. A simple and fast method is presented to determine solubility and Kow of organic chemicals at different temperatures (5 degrees C, 15 degrees C, 25 degrees C, 35 degrees C) utilising a variable temperature RP-HPLC column. Correlations between capacity factors (k') and solubility and Kow were determined for some halogenated and methylated benzenes and showed that this approach could be used to predict acceptable results. New values for solubility and Kow as function of temperature for several compounds are presented.     

The effects of matrix diffusion on radionuclide migration in rock column experiments

Rock column experiments were performed to examine the effects of matrix diffusion and hydrodynamic dispersion on the migration of radionuclides at the laboratory scale. Tritiated water and chloride transportation was studied in intact mica gneiss and in altered more porous tonalite columns with narrow flow channels. The column diffusion properties were estimated prior to water flow experiments using the gas diffusion method with helium as the tracer gas. The numerical compartment model for advection and dispersion, with and without matrix diffusion, was used to interpret the tracer transport in the columns. Matrix diffusion behavior was also distinguished from dominating hydrodynamic dispersion in rock column experiments at the slowest water flow rates.     

Modeling potential vadose-zone transport of nitrogen from onsite wastewater systems at the development scale

Water in the urban front-range corridor of Colorado has become an increasingly critical resource as the state faces both supply issues as well as anthropogenic degradation of water quality in several aquifers used for drinking water. A proposed development (up to 1100 homes over two quarter-quarter sections) at Todd Creek, Colorado, a suburb of Westminster located about 20 miles northeast of Denver, is considering use of onsite wastewater systems (OWS) to treat and remove domestic wastewater. Local health and environmental agencies have concerns for potential impacts to local water quality. Nitrogen treatment in the vadose zone and subsequent transport to ground water at a development scale is the focus of this investigation. The numerical model HYDRUS 1D was used, with input based on site-specific data and several transport parameters estimated from statistical distribution, to simulate nitrate concentrations reaching ground water. The model predictions were highly sensitive to mass-loading of nitrogen from OWS and the denitrification rate coefficient. The mass loading is relatively certain for the large number of proposed OWS. However, reasonable values for the denitrification rate coefficients vary over three orders of magnitude. Using the median value from a cumulative frequency distribution function, based on rates obtained from the literature, resulted in simulated output nitrate concentrations that were less than 1% of regulatory maximum concentrations. Reasonable rates at the lower end of the reported range, corresponding to lower 95% confidence interval estimates, result in simulated nitrate concentrations reaching groundwater above regulatory limits.     

Tracer diffusion from a horizontal fracture into the surrounding matrix: measurement by computed tomography

The vertical diffusion of NaI solution from a horizontal fracture into and within the surrounding matrix was tracked and quantified over time using an artificially fractured chalk core (30x5 cm) and a second-generation X-ray computed tomography (CT) scanner. The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix. The enhanced transport in the matrix below the fracture was related to the Rayleigh-Darcy instability induced by the density differences between the heavier tracer solution in the fracture (1.038) and the distilled water that had initially resided in the matrix. Our observations suggest that below the fracture, the tracer is propagated by an advection-diffusion process that is characterized by both higher rates and higher concentrations relative to its propagation by diffusion above the fracture. The experimental results suggest that the prediction of contaminant migration in a rock intersected by both vertical and horizontal (e.g. along bedding planes) fractures is difficult because of density effects that result in different solute-penetration rates.