A laboratory study of tracer tomography

A tracer tomographic laboratory study was performed with consolidated fractured rock in three-dimensional space. The investigated fractured sandstone sample was characterized by significant matrix permeability. The laboratory transport experiments were conducted using gas-flow and gas-tracer transport techniques that enable the generation of various flow-field patterns via adjustable boundary conditions within a short experimental time period. In total, 72 gas-tracer (helium) tests were performed by systematically changing the injection and monitoring configuration after each test. For the inversion of the tracer breakthrough curves an inversion scheme was applied, based on the transformation of the governing transport equation into a form of the eikonal equation. The reliability of the inversion results was assessed with singular value decomposition of the trajectory density matrix. The applied inversion technique allowed for the three-dimensional reconstruction of the interstitial velocity with a high resolution. The three-dimensional interstitial velocity distribution shows clearly that the transport is dominated by the matrix while the fractures show no apparent influence on the transport responses.     

Analytical solutions of tracer transport in fractured rock associated with precipitation-dissolution reactions

Precipitation-dissolution reactions are important for a number of applications such as isotopic tracer transport in the subsurface. Analytical solutions have been developed for tracer transport in both single-fracture and multiple-fracture systems associated with these reactions under transient and steady-state transport conditions. These solutions also take into account advective transport in fractures and molecular diffusion in the rock matrix. For studying distributions of disturbed tracer concentration (the difference between actual concentration and its equilibrium value), effects of precipitation-dissolution reactions are mathematically equivalent to a “decay” process with a decay constant proportional to the corresponding bulk reaction rate. This important feature significantly simplifies the derivation procedure by taking advantage of the existence of analytical solutions for tracer transport associated with radioactive decay in fractured rock. It is also useful for interpreting tracer breakthrough curves, because the impact of a decay process is relatively easy to analyze. Several illustrative examples are presented, which show that the results are sensitive to fracture spacing, matrix diffusion coefficient (fracture surface area), and bulk reaction rate (or “decay” constant), indicating that the relevant flow and transport parameters may be estimated by analyzing tracer signals.     

Understanding the effect of single-fracture heterogeneity from single-well injection-withdrawal (SWIW) tests

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.     

Regional-scale analysis of karst underground flow deduced from tracing experiments: examples from carbonate aquifers in Malaga province,southern Spain

Tracer concentration data from field experiments conducted in several carbonate aquifers (Malaga province, southern Spain) were analyzed following a dual approach based on the graphical evaluation method (GEM) and solute transport modeling to decipher flow mechanisms in karst systems at regional scale. The results show that conduit system geometry and flow conditions are the principal factors influencing tracer migration through the examined karst flow routes. Solute transport is mainly controlled by longitudinal advection and dispersion throughout the conduit length, but also by flow partitioning between mobile and immobile fluid phases, while the matrix diffusion process appears to be less relevant. The simulation of tracer breakthrough curves (BTCs) suggests that diffuse and concentrated flow through the unsaturated zone can have equivalent transport properties under extreme recharge, with high flow velocities and efficient mixing due to the high hydraulic gradients generated. Tracer mobilization within the saturated zone under low flow conditions mainly depends on the hydrodynamics (rather than on the karst conduit development), which promote a lower longitudinal advection and retardation in the tracer migration, resulting in a marked tailing effect of BTCs. The analytical advection-dispersion equation better approximates the effective flow velocity and longitudinal dispersion estimations provided by the GEM, while the non-equilibrium transport model achieves a better adjustment of most asymmetric and long-tailed BTCs. The assessment of karst underground flow properties from tracing tests at regional scale can aid design of groundwater management and protection strategies, particularly in large hydrogeological systems (i.e. transboundary carbonate aquifers) and/or in poorly investigated ones.     

裂隙网络管道模型弥散试验

为了探求不同裂隙几何参数对裂隙网络溶质运移的影响,基于离散裂隙网络思想和优势流、沟槽流理论,建立裂隙管道网络概念模型,搭建不同管径、不同连通方式的管道网络试验装置,进行渗流和溶质运移实验。运用应用广泛的模拟软件CHEMFLO-2000建立等效多孔介质模型,拟合不同几何参数下等效弥散度,定量刻画不同管道网络几何参数对溶质运移的影响,讨论了不同管径、连通方式等与等效弥散度之间的关系。通过进一步分析得出:在连通方式相同的情况下,不同管径的管道网络等效弥散度存在差异,但是差异不大;溶质在小管径中的穿透时间短于大管径,穿透曲线缓和程度相差不大;管道网络连通方式越复杂,其等效弥散度越大、对溶质运移的影响越大、穿透曲线越缓和、路径越长,等效弥散度越大;用这种等效弥散度的方法表征管道网络对溶质运移的影响,与多孔介质弥散度具有相似性;管道数目、管道面数目与等效弥散度成正相关关系,且等效弥散度随尺度的增大而增加。     

基于COMSOL Multiphysics的非饱和裂隙土降雨入渗特性研究

基于COMSOL Multiphysics软件对非饱和裂隙土降雨入渗特性进行数值模拟研究。通过将裂隙和基质分别离散成有限单元,建立了能充分模拟土中裂隙流、基质流以及裂隙-基质流量交换的离散裂隙-孔隙介质模型。结合"空气单元"的概念,对裂隙土的上边界进行模拟。该方法不仅能描述降雨初期雨水沿裂隙优先入渗的现象,还能描述当降雨量大于裂隙土入渗量时雨水沿地表流走的现象。通过对地表以下2 m深度内低渗含裂隙土体进行模拟,分析了裂隙的几何特征、基质的水力特性、前期水分条件以及降雨强度对非饱和裂隙土降雨入渗过程的影响。结果表明,在非饱和裂隙土中,存在两个主要的渗流过程:一是水沿裂隙优先流动;二是水不断从裂隙吸入基质中,基质吸收水的作用抑制了裂隙中优势流的发展。与裂隙的几何特征相比,基质的水力特性对非饱和裂隙土渗流的影响较大。增大基质的饱和渗透系数可能使由裂隙流主导的渗流过程转变为由基质流主导的渗流过程,而基质的非饱和特性与裂隙土的初始含水率改变了土体的储水能力,从而加速或延缓了降雨入渗至某一深度的时间。降雨强度对土体入渗速率和入渗量均有影响,当超过裂隙土的入渗能力时,多余积水沿地表流走,断面入渗率随...     

非均质介质中地下水流动与溶质运移模拟——问题与挑战

对大空间尺度和长时间跨度的地下水流动及污染物质运移进行预测的需求, 使水文地质研究面临异乎寻常的挑战.这些需求来自于对核废料地质储放方法的安全性评价、地下水污染状况评价及其治理方案的选择.流动系统的非均质性是地下水流动及物质运移模拟中最主要的困难之一, 这种困难来自对非均质系统进行特征描述(通过原位观测实现)、概念化及模拟.评述了非均质介质中流动运移模拟的一些重要问题与挑战, 讨论了解决的途径.讨论的主题包括: 动力流动的沟道化, 示踪剂穿透曲线, 裂隙岩石中流体流动的多尺度, 观测的不同尺度, 模拟、预测与非均质性以及系统特征描述和预测性模拟的分析.      

An empirical probabilistic approach for constraining the uncertainty of long-term solute transport predictions in fractured rock using in situ tracer experiments

Although radionuclide tracer tests have been carried out for over 30  years, the role of tracer tests in radioactive waste repository performance assessment (PA) has been questioned due to the differences between the time scales for tracer tests and PA. The possibility of using in situ tracer tests to constrain PA time scale (over 10,000  years) solute transport has been demonstrated using a systematic “microstructural model” approach to define advective and retentive materials. A series of simulations were conducted of the “TRUE-1” sorbing solute transport experiments in a well-characterized block of fractured granite. These experiments were then used to constrain the uncertainty of long-term transport on the same pathways, using a gradient several orders of magnitude smaller. The comparison of uncertainty of this long-term transport, with and without this conditioning step, provided a measure of the ability of tracer tests to reduce PA time-scale uncertainty. Although this approach for quantifying uncertainty reduction is somewhat empirical, it does indicate the potential usefulness of tracer experiments in reducing the uncertainty of the key PA time-scale transport parameters such as the flow wetted surface, provided that immobile zone properties such as sorption (Kd), porosity and diffusivity are available.     

岩溶管道与裂隙介质间溶质暂态存储机制

溶质暂态存储是岩溶地下水溶质运移过程中的普遍现象。为揭示岩溶管道与裂隙介质间溶质暂态存储机制,本文构建室内管道-裂隙物理模型,开展集中补给条件下的定量示踪试验,运用双区对流弥散模型实现溶质运移过程模拟。研究表明：随着集中补给水动力条件的增强,裂隙暂态存储水量呈线性增加趋势,溶质穿透曲线由单峰型向双峰型转变;管道和裂隙中的平均流速呈负相关关系,溶质在管道和裂隙中的滞留时间差决定了穿透曲线的形态;溶质暂态存储引发了穿透曲线的拖尾效应和双峰现象,对岩溶地下水溶质运移过程具有重要的控制作用。     

A semi-analytic model for contaminant migration in a regular two- or three-dimensional fractured network: Conservative contaminants

A new semi-analytical solution for the transport of a conservative contaminant species in a fractured medium having a regular two- or three-dimensional fracture network is presented. The application of the technique and some of the practical implications arising from an examination of contaminant migration in fractured systems is discussed. Particular consideration is given to the effects of Darcy velocity, fracture spacing, matrix porosity, dispersivity and the mass of contaminant available for transport. The implications of uncertainty with respect to fracture opening size and ground-water velocity is also discussed and it is shown that provided one can obtain a reasonable estimate of the hydraulic gradient and hydraulic conductivity for the rock mass, uncertainty regarding the magnitude of the opening size and the groundwater velocity does not have a significant effect on predicted contaminant migration for the class of problems being considered.     

Field characterization of vertical bromide transport in a fractured glacial till

A study of the fracture distribution, hydraulic properties, groundwater levels and the transport of bromide was conducted to characterize vertical transport in the oxidized and reduced zones of a fractured glacial till. Detailed vertical profiles of groundwater levels and solute concentrations were obtained over a 4.5-year period. Vertical migration occurred at several time scales, as a low concentration front was rapidly transported at rates of 100–500 m/year ahead of a slower moving main plume, which advanced at rates of 0.2–0.8 m/year. Concentrations in the leading edge of the plume displayed a high degree of spatial variability over short vertical distances through day 1,000. Late in the test, the influence of matrix diffusion became apparent as concentration patterns developed from being irregular to more uniform distributions. Calculations show that the mass within the low concentration plume front accounts for less than 1% of the total solute mass. Simulation of the breakthrough curves using a simple one-dimensional advection-dispersion model of transport in porous media indicates that vertical transport is dominated by advection. Furthermore, the results indicate that vertical transport of solutes in oxidized and reduced zones of the till can be adequately simulated using an equivalent porous media.     

Using saline tracers to evaluate preferential recharge in fractured rocks,Floyd County,Virginia, USA

Potassium chloride (KCl) and potassium bromide (KBr) tracers were used to explore the role of geologic structure on groundwater recharge and flow at the Fractured Rock Research Site in Floyd County, Virginia, USA. Tracer migration was monitored through soil, saprolite, and fractured crystalline bedrock for a period of 3 months with chemical, physical, and geophysical techniques. The tracers were applied at specific locations on the ground surface to directly test flow pathways in a shallow saprolite and deep fractured-rock aquifer. Tracer monitoring was accomplished with differential electrical resistivity, chemical sampling, and physical monitoring of water levels and spring discharge. KCl, applied at a concentration of 10,000 mg/L, traveled 160 m downgradient through the thrust fault aquifer to a spring outlet in 24 days. KBr, applied at a concentration of 5,000 mg/L, traveled 90 m downgradient through the saprolite aquifer in 19 days. Tracer breakthrough curves indicate diffuse flow through the saprolite aquifer and fracture flow through the crystalline thrust fault aquifer. Monitoring saline tracer migration through soil, saprolite, and fractured rock provided data on groundwater recharge that would not have been available using other traditional hydrologic methods. Travel times and flowpaths observed during this study support preferential groundwater recharge controlled by geologic structure.     

Using Multiple-Point Geostatistics for Tracer Test Modeling in a Clay-Drape Environment with Spatially Variable Conductivity and Sorption Coefficient

This study investigates the effect of fine-scale clay drapes on tracer transport. A tracer test was performed in a sandbar deposit consisting of cross-bedded sandy units intercalated with many fine-scale clay drapes. The heterogeneous spatial distribution of the clay drapes causes a spatially variable hydraulic conductivity and sorption coefficient. A fluorescent tracer (sodium naphthionate) was injected in two injection wells and ground water was sampled and analyzed from five pumping wells. To determine (1) whether the fine-scale clay drapes have a significant effect on the measured concentrations and (2) whether application of multiple-point geostatistics can improve interpretation of tracer tests in media with complex geological heterogeneity, this tracer test is analyzed with a local three-dimensional ground-water flow and transport model in which fine-scale sedimentary heterogeneity is modeled using multiple-point geostatistics. To reduce memory needs and calculation time for the multiple-point geostatistical simulation step, this study uses the technique of direct multiple-point geostatistical simulation of edge properties. Instead of simulating pixel values, model cell edge properties indicating the presence of irregularly shaped surfaces are simulated using multiple-point geostatistical simulations. Results of a sensitivity analysis show under which conditions clay drapes have a significant effect on the concentration distribution. Calibration of the model against measured concentrations from the tracer tests reduces the uncertainty on the clay-drape parameters. The calibrated model shows which features of the breakthrough curves can be attributed to the geological heterogeneity of the aquifer and which features are caused by other processes.     

Assessment of key transport parameters in a karst system under different dynamic conditions based on tracer experiments: the Jeita karst system,Lebanon

Artificial tracer experiments were conducted in the mature karst system of Jeita (Lebanon) under various flow conditions using surface and subsurface tracer injection points, to determine the variation of transport parameters (attenuation of peak concentration, velocity, transit times, dispersivity, and proportion of immobile and mobile regions) along fast and slow flow pathways. Tracer breakthrough curves (TBCs) observed at the karst spring were interpreted using a two-region nonequilibrium approach (2RNEM) to account for the skewness in the TBCs’ long tailings. The conduit test results revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The polynomial relationship between transport velocity and discharge can be related to the variation of the conduit’s cross-sectional area. Longitudinal dispersivity in the conduit system is not a constant value (α?=?7–10 m) and decreases linearly with increasing flow rate because of dilution effects. Additionally, the proportion of immobile regions (arising from conduit irregularities) increases with decreasing water level in the conduit system. From tracer tests with injection at the surface, longitudinal dispersivity values are found to be large (8–27 m). The tailing observed in some TBCs is generated in the unsaturated zone before the tracer actually arrives at the major subsurface conduit draining the system. This work allows the estimation and prediction of the key transport parameters in karst aquifers. It shows that these parameters vary with time and flow dynamics, and they reflect the geometry of the flow pathway and the origin of infiltrating (potentially contaminated) recharge.     

Fracture step structure: geometrical characterization and effects on fluid flow and breakthrough curve

Groundwater flow and solute transport through fractured rock is highly responsive to the hydraulic anisotropy and heterogeneity that are specific to every major fracture. A major fracture is modeled as the combination of some primal master fractures and several splay fractures that branch out from primal master fractures: step structures (or jog parts). Step structures are commonly observed along a major fracture on various scales. Master fractures were formed and developed by shear movement while some splay fractures were formed by extension normal to their wall. This difference in fracturing process may lead to a permeability difference between master fractures and splay fractures which seems to be one of the major factors controlling flow and solute transport through the fracture networks due to its hydraulic anisotropic and heterogeneous features. This study is composed of two major components: (1) identification and characterization of a step structure from borehole data; (2) evaluation of effect of some idealized step structures on breakthrough curve by numerical simulations. The fracture data of four 1000-m boreholes were used to make clear fracture patterns in the Tono area of Japan. Some major fractures were identified using stereographic projection technique. On the basis of these results, several idealized models of a major fracture having a step was constructed for the numerical study. The obtained results from numerical simulations clearly imply that geometry of step structure plays an important role in flow and transport through the fracture networks.     

Review: Mathematical expressions for estimating equivalent permeability of rock fracture networks

Fracture networks play a more significant role in conducting fluid flow and solute transport in fractured rock masses, comparing with that of the rock matrix. Accurate estimation of the permeability of fracture networks would help researchers and engineers better assess the performance of projects associated with fluid flow in fractured rock masses. This study provides a review of previous works that have focused on the estimation of equivalent permeability of two-dimensional (2-D) discrete fracture networks (DFNs) considering the influences of geometric properties of fractured rock masses. Mathematical expressions for the effects of nine important parameters that significantly impact on the equivalent permeability of DFNs are summarized, including (1) fracture-length distribution, (2) aperture distribution, (3) fracture surface roughness, (4) fracture dead-end, (5) number of intersections, (6) hydraulic gradient, (7) boundary stress, (8) anisotropy, and (9) scale. Recent developments of 3-D fracture networks are briefly reviewed to underline the importance of utilizing 3-D models in future research.     

Influence of flow velocity on conservative solute transport in sets of parallel fractures with fracture skin

Analysis of contaminant transport through fractured crystalline rocks has received considerable attention, particularly with regard to subsurface nuclear waste repositories. Most of the studies have employed the dual continuum approach, with the fractures and the rock matrix as the two continuums, assuming that fractures control the overall conductivity of the rock and the porous matrix just provides storage. However, field observations of rock fractures have shown that the real situation can be very complex. Based on some recent investigations, it has been reported that the portion of the rock matrix adjacent to many open fractures is physically and chemically altered. These alterations, referred to as the fracture skin, can have different sorption and diffusion properties compared to those of the undisturbed rock matrix and this may influence the transport of solutes through such formations. In the present study, a numerical model is developed to simulate conservative solute transport in a fractured crystalline rock formation using the triple continuum approach ?? with the fracture, fracture skin and the rock matrix as the three continuums. The model is solved using a fully implicit finite difference scheme. Contaminant migration in the fractured formation with and without skin has been simulated. It is observed that contaminant penetration along the fracture is enhanced at large flow velocities. The effect of flow velocity on conservative solute transport is investigated for different fracture apertures and fracture skin thicknesses. The influence of flow velocity on contaminant transport is demonstrated to be more with change in fracture aperture than with change in skin thickness.     

Effect of chemical reactions on the hydrologic properties of fractured and rubbelized glass media

Understanding the effect of chemical reactions on the hydrologic properties of geological media, such as porosity, permeability and dispersivity, is critical to many natural and engineered sub-surface systems. Influence of glass corrosion (precipitation and dissolution) reactions on fractured and rubbelized (crushed) forms HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted using fractured and rubbelized forms, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig pressure, causing the precipitation of alteration products. Data were analyzed using analytical expressions and CXTFIT, a transport parameter optimization code, for the estimation of the hydrologic characteristics before and after VHT. It was found that glass reactions significantly influence the hydrologic properties of ILAW glass media. Hydrologic properties of rubbelized glass decreased due to precipitation reactions, whereas those of fractured glass media increased due to reaction which led to unconfined expansion of fracture aperture. The results are unique and useful to better understand the effect of chemical reactions on the hydrologic properties of fractured and rubbelized stony media in general and glass media in particular.