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1.
Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less‐mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore‐fluid sampling and geoelectrical measurements can be used to quantify less‐mobile porosity exchange dynamics using the time‐varying relation between fluid and bulk electrical conductivity. For this study, we use macro‐scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less‐mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less‐mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less‐mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow‐dependent effects directly impact local less‐mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less‐mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low‐to‐moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model‐based results show that less‐mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less‐mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales.  相似文献   

2.
Abstract

Models on flow and transport in surface water sediments currently neglect compaction, although it is well understood that compaction is one of the major processes below the free fluid-sediment interface. Porosity changes in the sediment layers, as a result of compaction, are measured in almost all probes: porosity decreases with the distance from the surface water-sediment interface. This paper provides a rigorous derivation of basic flux terms for a frame of reference that is moving with the fluid-sediment interface. It is shown how burial rate, interface velocity, velocities of fluid and solid phase and porosity are connected—under steady-state conditions. It turns out that porosity and the velocities in a one-dimensional column can be directly computed from each other. These findings are important not only for the understanding of compaction-driven flow itself; they are crucial for all studies on storage and transport of chemical components in sediments. As mass fluxes across the sediment-water interface may be affected, there is an indirect link on surface water quality, making these findings relevant also for research on eutrophication of surface water bodies and/or on biogeochemical cycles.  相似文献   

3.
Porosity and permeability in sediment mixtures   总被引：1，自引：1，他引：0  
Porosity in sediments that contain a mix of coarser- and finer-grained components varies as a function of the porosity and volume fraction of each component. We considered sediment mixtures representing poorly sorted sands and gravely sands. We expanded an existing fractional-packing model for porosity to represent mixtures in which finer grains approach the size of the pores that would exist among the coarser grains alone. The model well represents the porosity measured in laboratory experiments in which grain sizes and volume fractions were systematically changed within sediment mixtures. Permeability values were determined for these sediment mixtures using a model based on grain-size statistics and the expanded fractional-packing porosity model. The permeability model well represents permeability measured in laboratory experiments using air- and water-based permeametry on the model sediment mixtures.  相似文献   

4.
Diffusive mass exchange into immobile water regions within heterogeneous porous aquifers influences the fate of solutes. The percentage of immobile water is often unidentified in natural aquifers though. Hence, the mathematical prediction of solute transport in such heterogeneous aquifers remains challenging. The objective of this study was to find a simple analytical model approach that allows quantifying properties of mobile and immobile water regions and the portion of immobile water in a porous system. Therefore, the Single Fissure Dispersion Model (SFDM), which takes into account diffusive mass exchange between mobile and immobile water zones, was applied to model transport in well‐defined saturated dual‐porosity column experiments. Direct and indirect model validation was performed by running experiments at different flow velocities and using conservative tracer with different molecular diffusion coefficients. In another column setup, immobile water regions were randomly distributed to test the model applicability and to determine the portion of immobile water. In all setups, the tracer concentration curves showed differences in normalized maximum peak concentration, tailing and mass recovery according to their diffusion coefficients. These findings were more pronounced at lower flow rates (larger flow times) indicating the dependency of diffusive mass exchange into immobile water regions on tracers' molecular diffusion coefficients. The SFDM simulated all data with high model efficiency. Successful model validation supported the physical meaning of fitted model parameters. This study showed that the SFDM, developed for fissured aquifers, is applicable in porous media and can be used to determine porosity and volume of regions with immobile water. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

5.
Fine sediment deposition in streambeds can reduce pore water fluxes and the overall rate of hyporheic exchange, producing deleterious effects on benthic and hyporheic ecological communities. To increase understanding of the factors that control the reduction of hyporheic exchange by fine sediment deposition, we conducted experiments in a laboratory flume to observe changes in the rates of solute exchange and kaolinite clay deposition as substantial amounts of kaolinite accumulated in the streambed. Two long‐term experiments were conducted, with durations of 14 days and 29 days. Use of a laboratory flume system allowed steady stream flow conditions to be maintained throughout both experiments, and alternating injections of known quantities of kaolinite and a sodium chloride tracer were used to assess the effect of clay accumulation on hyporheic exchange directly. In the first experiment, there was no bed sediment transport and kaolinite deposition formed a highly clogged near‐surface layer that greatly reduced hyporheic exchange. Application of a fundamental model for advective hyporheic exchange indicated that the effective permeability and porosity of the streambed decreased substantially during the course of the experiment. In the second experiment, the kaolinite was prepared with different surface properties to be more mobile, and the experiment was conducted with a small degree of bed sediment transport. As a result, no distinct clogged layer developed, and the rate of hyporheic exchange was found to remain approximately constant throughout the experiment (29 days). These results indicate that increasing fine sediment loads, e.g. those that occur from changes in land use, can have substantially different impacts on hyporheic exchange and associated ecological processes depending on the stream flow conditions, the rate and frequency of bed sediment transport, and the extent of interaction of the introduced fines with bed sediments. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

6.
Porosity measurement is an important step for petrophysical characterization of reservoir rocks, as porosity is essential for analysing elastic properties and estimating the capacity of rocks to hold hydrocarbon resources. Commonly used porosity measurement methods fail to work on heavy oil sands, because of the loose fragile sand frame and solid-like irreducible heavy oil. We develop three methods to measure the porosity of heavy oil sands, which are feasible and affordable in most laboratories. Method 1 involves calculating the bulk volume of the sample by measuring its physical dimensions directly. Method 2 uses calibrated sample weight and volume (after removing the protecting foil cover and metal clips). Method 3 first applies pressurizing on samples and then measures the weight and volume of the bare samples. The measurement results of the three methods are then compared and method 3 is determined as the most reliable, which is also verified by the porosity log. Finally, an analysis of the potential sources of errors associated with method 3 is performed.  相似文献   

7.
Using a slab of Massillon Sandstone, laboratory-scale solute tracer experiments were carried out to test numerical simulations using the Advection–Dispersion Equation (ADE). While studies of a similar nature exist, our work differs in that we combine: (1) experimentation in naturally complex geologic media, (2) X-ray absorption imaging to visualize and quantify two-dimensional solute transport, (3) high resolution transport property characterization, with (4) numerical simulation. The simulations use permeability, porosity, and solute concentration measured to sub-centimeter resolution. While bulk breakthrough curve characteristics were adequately matched, large discrepancies exist between the experimental and simulated solute concentration fields. Investigation of potential experimental errors suggests that the failure to fit solute concentration fields may lie in loss of intricate connectivity within the cross-bedded sandstone occurring at scales finer than our property characterization measurements (i.e., sub-centimeter).  相似文献   

8.
Transport processes that lead to exchange of mass between surface water and groundwater play a significant role for the ecological functioning of aquatic systems, for hydrological processes and for biogeochemical transformations. In this study, we present a novel integral modeling approach for flow and transport at the sediment–water interface. The model allows us to simultaneously simulate turbulent surface and subsurface flow and transport with the same conceptual approach. For this purpose, a conservative transport equation was implemented to an existing approach that uses an extended version of the Navier–Stokes equations. Based on previous flume studies which investigated the spreading of a dye tracer under neutral, losing and gaining flow conditions the new solver is validated. Tracer distributions of the experiments are in close agreement with the simulations. The simulated flow paths are significantly affected by in- and outflowing groundwater flow. The highest velocities within the sediment are found for losing condition, which leads to shorter residence times compared to neutral and gaining conditions. The largest extent of the hyporheic exchange flow is observed under neutral condition. The new solver can be used for further examinations of cases that are not suitable for the conventional coupled models, for example, if Reynolds numbers are larger than 10. Moreover, results gained with the integral solver provide high-resolution information on pressure and velocity distributions at the rippled streambed, which can be used to improve flow predictions. This includes the extent of hyporheic exchange under varying ambient groundwater flow conditions.  相似文献   

9.
Flach GP 《Ground water》2012,50(2):216-229
Dual-domain solute transport models produce significantly improved agreement to observations compared to single-domain (advection-dispersion) models when used in an a posteriori data fitting mode. However, the use of dual-domain models in a general predictive manner has been a difficult and persistent challenge, particularly at field-scale where characterization of permeability and flow is inherently limited. Numerical experiments were conducted in this study to better understand how single-rate mass transfer parameters vary with aquifer attributes and contaminant exposure. High-resolution reference simulations considered 30 different scenarios involving variations in permeability distribution, flow field, mass transfer timescale, and contaminant exposure time. Optimal dual-domain transport parameters were empirically determined by matching to breakthrough curves from the high-resolution simulations. Numerical results show that mobile porosity increases with lower permeability contrast/variance, smaller spatial correlation length, lower connectivity of high-permeability zones, and flow transverse to strata. A nonzero non-participating porosity improves empirical fitting, and becomes larger for flow aligned with strata, smaller diffusion coefficient, and larger spatial correlation length. The non-dimensional mass transfer coefficient or Damkohler number tends to be close to 1.0 and decrease with contaminant exposure time, in agreement with prior studies. The best empirical fit is generally achieved with a combination of macrodispersion and first-order mass transfer. Quantitative prediction of ensemble-average dual-domain parameters as a function of measurable aquifer attributes proved only marginally successful.  相似文献   

10.
This study demonstrates that positron emission tomography (PET) and neutron radiography (NR) techniques are complementary methods for determining the fluid pathway and porosity in crystalline rock. After preliminary injection of an organic solvent (e.g. isopropanol) front followed by the injection of the polymer solution (e.g. epoxy used for both techniques) and resin hardening, rock cutting may be performed. Flow pathway may be imaged by using a β+ emitter (e.g. 68Ga) in the resin. With a high-resolution PET camera, determination of the original water carrier features is possible in granodiorite pieces 20 cm in size and in simulated features with porosities of the order of 0.2. The use of a β+ tracer and the camera field, however, limit the lateral resolution of the technique (10 mm). Neutron radiography makes it possible to visualize the simulated porous phases by neutron transmission. The transmission process depends on the neutron scattering properties of the hydrogen-rich material (e.g. epoxy resin). Combination of 2D pictures may rebuild the 3D pattern. Lateral resolution may be in the range of 1 mm; however, the thickness of the rock sample must not exceed 10 cm. Complementarity of these techniques is discussed and they are compared with other methods used to determine porosity.  相似文献   

11.
It is argued in this commentary that, in order to understand better the physical mechanisms that generate boundary shear stress over water‐worked gravel beds, flow velocity data should be re‐evaluated by spatial averaging the Reynolds equations to produce time‐ and space‐averaged (double‐averaged) momentum equations. A series of laboratory experiments were conducted in which the flow velocities were measured using a PIV system over two water‐worked gravel deposits. Combined with detailed data on the bed surface topography and vertical porosity, the physical components of shear stress were obtained. This enabled the various momentum transfer mechanisms present above, within and at the interface of a porous, fluvial deposit, to be quantified. This included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer. It is demonstrated that double‐averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment–water interface exchange mechanisms within the roughness layer in water‐worked gravel deposits. By considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel–sand mixtures and the transfer of solutes into porous, gravel deposits. This analysis reveals the importance of obtaining co‐located, high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

12.
Exchange of groundwater and lake water with typically quite different chemical composition is an important driver for biogeochemical processes at the groundwater‐lake interface, which can affect the water quality of lakes. This is of particular relevance in mine lakes where anoxic and slightly acidic groundwater mixes with oxic and acidic lake water (pH < 3). To identify links between groundwater‐lake exchange rates and acid neutralization processes in the sediments, exchange rates were quantified and related to pore‐water pH, sulfate and iron concentrations as well as sulfate reduction rates within the sediment. Seepage rates measured with seepage meters (?2.5 to 5.8 L m‐2 d‐1) were in reasonable agreement with rates inverted from modeled chloride profiles (?1.8 to 8.1 L m‐2 d‐1). Large‐scale exchange patterns were defined by the (hydro)geologic setting but superimposed by smaller scale variations caused by variability in sediment texture. Sites characterized by groundwater upwelling (flow into the lake) and sites where flow alternated between upwelling and downwelling were identified. Observed chloride profiles at the alternating sites reflected the transient flow regime. Seepage direction, as well as seepage rate, were found to influence pH, sulfate and iron profiles and the associated sulfate reduction rates. Under alternating conditions proton‐consuming processes, for example, sulfate reduction, were slowed. In the uppermost layer of the sediment (max. 5 cm), sulfate reduction rates were significantly higher at upwelling (>330 nmol g‐1 d‐1) compared to alternating sites (<220 nmol g‐1 d‐1). Although differences in sulfate reduction rates could not be explained solely by different flux rates, they were clearly related to the prevailing groundwater‐lake exchange patterns and the associated pH conditions. Our findings strongly suggest that groundwater‐lake exchange has significant effects on the biogeochemical processes that are coupled to sulfate reduction such as acidity retention and precipitation of iron sulfides. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
孔隙率是评价多孔材料相关性能的重要因素之一,是检测材料质量的一项重要指标。针对孔隙率值受分辨率影响较大的问题,在能谱与材质信息均未知的条件下,本文基于多能谱CT与（NC-POCS）非凸-凸集投影算法,结合非负矩阵求解方法,实现分解重建,以多孔材料泡沫铝为例,分析其在不同分辨率下的孔隙率变化情况。实验结果表明,本文提出的算法具有抑制射束硬化伪影的效果,并且测得的孔隙率值受分辨率影响较小。   相似文献   

14.
Flow in many bedrock aquifers is through fracture networks. Point to point tracer tests using applied tracers provide a direct measure of time of travel and are most useful for determining effective porosity. Calculated values from these tests are typically between 10−4 and 10−2 (0.01% to 1%), with these low values indicating preferential flow through fracture and channel networks. Tracer tests are not commonly used in site investigations, and specific yield is often used as a proxy for effective porosity. The most popular methods have used centrifuge measurements, water table fluctuations, pumping tests, and packer tests. Specific yield varies substantially with the testing method. No method is as reliable as tracer testing for providing estimates of effective porosity, but all methods provide complementary insights on aquifer structure. Temporal and spatial scaling effects suggest that bedrock aquifers have hierarchical structures, with a network of more permeable fractures and channels, which are connected to less permeable fractures and to the matrix. Consequences of the low effective porosities include groundwater velocities that often exceed 100 m/d and more frequent microbial contamination than in aquifers in unconsolidated sediments. The large uncertainty over the magnitude of effective porosity in bedrock aquifers makes it an important parameter to determine in studies where time of travel is of interest.  相似文献   

15.
Water exchange between surface water and groundwater can modulate or generate ecologically important fluxes of solutes across the sediment‐water interface. Seepage meters can directly measure fluid flux, but mechanical resistance and surface water dynamics may lead to inaccurate measurements. Tank experiments were conducted to determine effects of mechanical resistance on measurement efficiency and occurrence of directional asymmetry that could lead to erroneous net flux measurements. Seepage meter efficiency was high (average of 93%) and consistent for inflow and outflow under steady flow conditions. Wave effects on seepage meter measurements were investigated in a wave flume. Seepage meter net flux measurements averaged 0.08 cm/h—greater than the expected net‐zero flux, but significantly less than theoretical wave‐driven unidirectional discharge or recharge. Calculations of unidirectional flux from pressure measurements (Darcy flux) and theory matched well for a ratio of wave length to water depth less than 5, but not when this ratio was greater. Both were higher than seepage meter measurements of unidirectional flux made with one‐way valves. Discharge averaged 23% greater than recharge in both seepage meter measurements and Darcy calculations of unidirectional flux. Removal of the collection bag reduced this net discharge. The presence of a seepage meter reduced the amplitude of pressure signals at the bed and resulted in a nearly uniform pressure distribution beneath the seepage meter. These results show that seepage meters may provide accurate measurements of both discharge and recharge under steady flow conditions and illustrate the potential measurement errors associated with dynamic wave environments.  相似文献   

16.
Porosity, or void space, of large wood jams in stream systems has implications for estimating wood volumes and carbon storage, the impacts of jams on geomorphic and ecological processes, and instream habitat. Estimating porosity and jam dimensions (i.e. jam volume) in the field is a common method of measuring wood volume in jams. However, very few studies explicitly address the porosity values in jams, how porosity is calculated and assessed for accuracy, and the effect such estimates have on carbon and wood budgets in river corridors. We compare methods to estimate jam porosity and wood volume using field data from four different depositional environments in North America (jam types include small in-channel jams, large channel-margin jams, a large island apex jam, and a large coastal jam), and compare the results with previous studies. We find that visual estimates remain the most time-efficient method for porosity estimation in the field, although they appear to underpredict back-calculated porosity values; the accuracy of jam porosity, and thus wood volume, estimates are difficult to definitively measure. We also find that porosity appears to be scale invariant, dictated mostly by jam type, (which is influenced by depositional processes), rather than the size of the jam. Wood piece sorting and structural organization are likely the most influential properties on jam porosity, and these factors vary according to depositional environment. We provide a framework and conceptual model that uses these factors to demonstrate how modeled jam porosity values differ and give recommendations as a catalyst for future work on porosity of wood jams. We conclude that jam type and size and/or the study goals may dictate which porosity method is the most appropriate, and we call for greater transparency and reporting of porosity methods in future studies. © 2020 John Wiley & Sons, Ltd.  相似文献   

17.
A general one-dimensional equation for interstitial transport in accumulating and compacting sediments under non-steady state conditions is derived. As a consequence of compaction the metric along the path of a given horizon, i.e. the spatial distance between neighbouring particles, changes continually. Special emphasis is put on the treatment of advection caused by compaction. The resulting partial differential equation for the interstitial concentration of a given solute contains terms which can be evaluated based on data from a single sediment core. In addition, an integral over the time-derivative of porosity appears which would make it necessary to compare cores from the same site but at different times. Under quite general assumptions this last term may, however, be transformed into a form for which evaluation from a single core becomes possible. Several special solutions are treated such as total steady state, steady state at the surface, non-constant sedimentation rate with steady state compaction, and non-steady state with steady state compaction. The last case applies, e.g., to diffusion under the influence of changing boundary conditions at the water/sediment interface while the accumulation process remains in steady state.  相似文献   

18.
储层岩石流动电位频散特性的数学模拟   总被引：1，自引：1，他引：0       下载免费PDF全文
利用储层岩石流动电位的频散特性评价复杂储层已经成为勘探地球物理领域关注的热点,但是目前还没有形成基于储层岩石储渗特性及电化学性质的具有普遍指导意义的理论方法和数学模型.本文利用微观毛管理论,通过随时间谐变条件下渗流场和电流场的耦合模型,建立了描述储层岩石流动电位频散特性的数学方法,定量分析了频率域储层岩石动态渗透率、动电耦合系数和流动电位耦合系数随储层岩石孔隙度、溶液浓度和阳离子交换量的变化规律.研究结果表明:储层岩石流动电位频散特性是储层流体惯性力与流体黏滞力相互作用的结果.储层岩石孔隙度越大,储层维持流体原有运动状态的能力越大,临界频率越小;储层岩石的溶液浓度和阳离子交换量对临界频率没有影响.储层岩石的孔隙度越大,流体流动能力越强,流动电位各耦合系数的数值越大;溶液浓度越小或阳离子交换量越大,孔隙固液界面的双电层作用越强,各耦合系数的数值越大.  相似文献   

19.
Origin of dolostone remained a controversial subject, although numerous dolomitization models had been proposed to date. Because of the dolomitization’s potential to be hydrocarbon reservoirs, one debatable issue was the role of dolomitization in porosity construction or destruction. Based upon case studies of dolostone reservoirs in various geological settings such as evaporative tidal flat (Ordos Basin, NW China), evaporative platform (Sichuan Basin, SW China), and burial and hydrothermal diagenesis (Tarim Basin, NW China), here we systematically discuss the origin of porosity in dolostone reservoirs. Contrary to traditional concepts, which regarded dolomitization as a significant mechanism for porosity creation, we found two dominant factors controlling reservoir development in dolostones, i.e., porosity inherited from precursor carbonates and porosity resulted from post-dolomitization dissolution. Actually, dolomitization rarely had a notable effect on porosity creation but rather in many cases destroyed pre-existing porosity such as saddle dolostone precipitation in vugs and fractures. Porosity in dolostones associated with evaporative tidal flat or evaporative platform was generally created by subaerial dissolution of evaporites and/or undolomitized components. Porosity in burial dolostones was inherited mostly from precursor carbonates, which could be enlarged due to subsequent dissolution. Intercrystalline porosity in hydrothermal dolostones was either formed during dolo- mitization or inherited from precursor carbonates, whereas dissolution-enlarged intercrystalline pores and/or vugs were usually interpreted to be the result of hydrothermal alteration. These understandings on dolostone porosity shed light on reservoir prediction. Dolostone reservoirs associated with evaporative tidal flat were laterally distributed as banded or quasi-stratified shapes in evaporite-bearing dolostones, and vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Dolostone reservoirs associated with evaporative platform commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones and/or evaporite-bearing dolostones of reef/shoal facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/shoal facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was obviously controlled by conduits (e.g., faults, unconformities), along which lenticular reservoirs could develop.  相似文献   

20.
Changes in the water table level result in variable water saturation and variable hydrological fluxes at the interface between the unsaturated and saturated zone. This may influence the transport and fate of contaminants in the subsurface. The objective of this study was to examine the impact of a decreasing and an increasing water table on solute transport. We conducted tracer experiments at downward flow conditions in laboratory columns filled with two different uniform porous media under static and transient flow conditions either increasing or decreasing the water table. Tracer breakthrough curves were simulated using a mobile–immobile transport model. The resulting transport parameters were compared to identify dominant transport processes. Changes in the water table level affected dispersivities and mobile water fractions depending on the direction of water table movement and the grain size of the porous media. In fine glass beads, the water flow velocity was similar to the decline rate of the water table, and the mobile water fraction was decreased compared with steady‐state saturated conditions. However, immobile water was negligible. In coarse glass beads, water flow was faster because of fingered flow in the unsaturated part, and the mobile water fraction was smaller than in the fine material. Here, a rising water table led to an even smaller mobile water fraction and increased solute spreading because of diffusive interaction with immobile water. We conclude that changes of the water table need to be considered to correctly simulate transport in the subsurface at the transition of the unsaturated–saturated zone. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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