孔隙尺度多孔介质流体流动与溶质运移高性能模拟

深入探究孔隙尺度下的流体流动特性和溶质运移规律对石油开采、农田养分管理、地下水污染修复有着重要意义。以人工构建的多孔介质结构和同步辐射X射线显微CT扫描的土壤团聚体(分辨率3.7μm)为研究对象,在空间节点数多达64 000 000的情况下,基于格子Boltzmann模型和GPU并行技术计算得到多孔介质流体运动和溶质运移过程的关键参数,并据此探究多孔介质空间异质性对水力学特性的影响。通过对3组不同结构的多孔介质比较发现,结构复杂程度最高的土壤样品和不规则堆叠的圆球结构的渗透率在100 mD(即10^-13m^2)量级,远低于规则堆叠的圆球结构(>20 000 mD);土壤的迂曲度为1.40~1.60,明显高于规则堆叠的圆球结构。研究结果表明,渗透率大的样品具有较小的迂曲度,这与结构的空间异质性有较强的关系;土壤的渗透率和迂曲度呈现各向异性;在水力梯度一定的前提下,渗透率较大的样品,纵向弥散系数也较大;同时,结构的异质性也会影响溶质的穿透曲线。本研究提出的模拟方法可在土壤结构中进行高效的水流运动和溶质运移模拟,可用于土壤多孔介质在孔隙尺度下的水力学特性研究。     

孔隙尺度多孔介质流体流动与溶质运移高性能模拟

深入探究孔隙尺度下的流体流动特性和溶质运移规律对石油开采、农田养分管理、地下水污染修复有着重要意义。以人工构建的多孔介质结构和同步辐射X射线显微CT扫描的土壤团聚体（分辨率3.7 μm）为研究对象，在空间节点数多达64 000 000的情况下，基于格子Boltzmann模型和GPU并行技术计算得到多孔介质流体运动和溶质运移过程的关键参数，并据此探究多孔介质空间异质性对水力学特性的影响。通过对3组不同结构的多孔介质比较发现，结构复杂程度最高的土壤样品和不规则堆叠的圆球结构的渗透率在100 mD（即10^-13m²）量级，远低于规则堆叠的圆球结构（>20 000 mD）；土壤的迂曲度为1.40~1.60，明显高于规则堆叠的圆球结构。研究结果表明，渗透率大的样品具有较小的迂曲度，这与结构的空间异质性有较强的关系；土壤的渗透率和迂曲度呈现各向异性；在水力梯度一定的前提下，渗透率较大的样品，纵向弥散系数也较大；同时，结构的异质性也会影响溶质的穿透曲线。本研究提出的模拟方法可在土壤结构中进行高效的水流运动和溶质运移模拟，可用于土壤多孔介质在孔隙尺度下的水力学特性研究。     

死端孔隙对溶质运移影响的实验研究

拖尾分布是溶质运移规律研究的热点和难点。死端孔隙、透镜体等空间结构形成的非均质性以及密度变化产生的指状流会导致溶质运移拖尾分布。论文选择保守性溶质氯化钠作为示踪剂,采用砂柱实验系统地研究了以死端孔隙导致的非均质性以及由不同浓度和温度形成的变密度条件对溶质运移拖尾分布的影响。实验结果表明:死端孔隙的存在会延迟溶质运移穿透曲线出峰时间,降低峰值及出现拖尾分布,随着死端孔隙介质体积或数量的增加这一现象越明显;随着溶液浓度的增加拖尾现象越明显,且温度的升高会加剧这一现象。对比死端孔隙、温度和浓度三者对溶质运移拖尾分布的影响,其中影响最大的是死端孔隙,其次是浓度和温度。     

海水入侵和地下水水溶质运移规律的定性研究

本文通过对沿海地区海岸带淡，盐水按触带，海水入侵范围和地下水含水层中溶质运移之间的内在联系，以及溶质运移的运动规律和途径的定性分析，从理论上为沿海地区予测海水入侵范围及淡水咸化提供根据，并通过一维地下水流的微分方程的研究。为进一步推导不同边界条件下的海水入侵范围提供研究方向。     

同位素示踪模拟地下水溶质运移速度的研究

本文通过青海某地一废物库的选址，进行了单井同位素地下水流速流向的测定，研究放射性同位素作为地下水的运动介质，模拟污染物质的溶质运移速度，从而预报建库后可能对周围环境产生的不利影响，提供切合实际的新的测试手段与数据。     

波浪作用下考虑海床变形影响的溶质运移数值模拟

波浪会促进海水中溶质向海底沉积物运移,但已有研究大多未考虑海床（海底沉积物）变形效应的影响。为揭示波浪作用下海床土变形对溶质运移过程的影响机制,构建了考虑海床土变形影响的溶质运移计算模型,对波浪作用下溶质向砂质海底沉积物中的运移过程进行模拟。结果表明:海床土变形会增大孔隙水流速,进而增大溶质纵向水动力弥散系数,增强溶质运移的机械弥散作用,促进溶质向沉积物中运移;考虑海床变形时的溶质最大纵向水动力弥散系数可达不考虑海床变形时的8.5倍,约为分子扩散系数的545倍;海床土剪切模量越小,土体变形效应越明显,对溶质运移过程的影响越大;海床土饱和度的降低,会进一步加速波浪作用下溶质向海底沉积物的运移过程。     

地表地下耦合的水量及溶质运移模拟研究进展

地表水与地下水之间存在强烈的相互作用,构建地表水和地下水耦合的水量和溶质运移模型可以揭示地表水和地下水之间的相互作用机制,并提高地表地下水量水质预测的精度。介绍了现有的地表地下耦合的水量及溶质运移模型,对比了各模型的求解方法和适用领域,总结了这些模型的应用现状。其中水量模型中全耦合模型精度较高,是未来模型发展的主流方向,溶质运移模型中溶质反应过程模拟较为欠缺。未来应在优化求解算法、提高溶质反应模拟和加强裂隙介质模拟等方面深入开展相关研究。     

地下水溶质运移三维有限元模拟软件的设计与应用

介绍了含水层系统地了下水溶质运移三维有限元要模拟软件（ＧＴＳ３ｄ）的设计思路，该软件充分考虑到从模型的建立，数据与参数的输入，模拟计算到计算结果可视化与表达的整个过程，通过键盘或鼠标控制下拉式菜单结构而有机地组织在一起，实现了数据输入，计算结果输出的全部图形功能，该软件成功地用于广西北海市涠洲岛含有水层系统的海水入侵模拟研究     

大孔隙位置对饱和土壤水及溶质运移影响的实验研究

大孔隙密度是土壤优势流的重要影响因素.通常大孔隙密度相同时,其位置分布各异,这可能对优势流程度存在影响.本研究采用室内人造大孔隙土柱模拟实验,在保持相同大孔隙密度条件下,依据大孔隙集中程度的不同,设计了完全分离型、分离型、聚集型、完全聚集型四种大孔隙位置关系,并与均质土柱对照,研究了大孔隙位置对饱和土壤水及溶质运移的影...     

Modeling solute transport and sulfate reduction in marsh sediments

The seasonal oscillation in sulfate and chloride concentration profiles in some salt marsh sediments is due to exchange of solutes with water on the surface of the marsh, and to the desiccation of the sediment in summer. Desiccation is manifested by disappearance of surface waters, fluctuations in the location of the water table, and by removal of water from the sediment above the water table. The loss of water from the pore space is commonly accompanied by entry of air into the soil, which oxidizes sulfide. The oxidation causes titratable alkalinity to decrease and results in CO₂ degassing.Diffusion models of salinity can account for the observed profiles but only as long as the marsh is maintained inundated. The complexities introduced to the solute transport equations by sediment desiccation invalidate steady-state modeling of solute transport and diagenesis. The concentration profiles of dissolved products of sulfate reduction, such as bicarbonate, require months to reestablish a steady state after being disrupted. If the profiles of dissolved products of sulfate reduction are disrupted seasonally, such as by a seasonal fluctuation in the water table, they may remain transient throughout the year.     

多孔介质中非均匀流动特性的染色示踪试验研究

通过土壤染色剂进行的4组试验,对不同介质结构条件下的水流和溶质非均匀运动规律,非均匀流动变异信息分布特征关系以及全局性非均匀流动示踪方法进行了研究。结果表明,即使在相对比较均匀的介质条件下,流动也表现出明显的非均匀特性;对数正态分布能够较好的反映水流的运动分布模式,相比水流运动,溶质的运动和分布规律明显不同,表现出更多的不确定性和变异性。     

Modeling preferential water flow and solute transport in unsaturated soil using the active region model

Preferential flow and solute transport are common processes in the unsaturated soil, in which distributions of soil water content and solute concentrations are often characterized as fractal patterns. An active region model (ARM) was recently proposed to describe the preferential flow and transport patterns. In this study, ARM governing equations were derived to model the preferential soil water flow and solute transport processes. To evaluate the ARM equations, dye infiltration experiments were conducted, in which distributions of soil water content and Cl⁻ concentration were measured. Predicted results using the ARM and the mobile–immobile region model (MIM) were compared with the measured distributions of soil water content and Cl⁻ concentration. Although both the ARM and the MIM are two-region models, they are fundamentally different in terms of treatments of the flow region. The models were evaluated based on the modeling efficiency (ME). The MIM provided relatively poor prediction results of the preferential flow and transport with negative ME values or positive ME values less than 0.4. On the contrary, predicted distributions of soil water content and Cl⁻ concentration using the ARM agreed reasonably well with the experimental data, with ME values higher than 0.8. The results indicated that the ARM successfully captured the macroscopic behavior of preferential flow and solute transport in the unsaturated soil.     

Ammonium and phosphate dynamics in a Virginia salt marsh

Experimental chambers were used in a Virginia salt marsh to partition the tidal flux of dissolved nutrients occurring at the marsh surface and in the water column. On five dates from June to October 1989, six replicate chambers in the short Spartina alterniflora zone were monitored over complete tidal cycles. When reservoir water, used to simulate tidal flooding in the chambers, was initially low in dissolved nutrients, the marsh surface was a source of both ammonium and phosphate to the water column. Calculations of the physical processes of diffusion and advection could not account for total nutrient release from the marsh surface. We hypothesize the primary source of nutrients was organic matter mineralization in surface sediments, which released nutrients into the flooding water column. Assimilation (uptake) of phosphate measured in water-column incubation experiments was nearly equal to phosphate released from the marsh surface. Surface release of ammonium, however, was somewhat greater than water-column uptake. In this salt marsh, benthic production and release of ammonium and phosphate is comparable in magnitude to pelagic consumption, thereby yielding only a small “net” transfer of these nutrients to the estuary.     

Finite element methods for variable density flow and solute transport

Saltwater intrusion into coastal freshwater aquifers is an ongoing problem that will continue to impact coastal freshwater resources as coastal populations increase. To effectively model saltwater intrusion, the impacts of increased salt content on fluid density must be accounted for to properly model saltwater/freshwater transition zones and sharp interfaces. We present a model for variable density fluid flow and solute transport where a conforming finite element method discretization with a locally conservative velocity post-processing method is used for the flow model and the transport equation is discretized using a variational multiscale stabilized conforming finite element method. This formulation provides a consistent velocity and performs well even in advection-dominated problems that can occur in saltwater intrusion modeling. The physical model is presented as well as the formulation of the numerical model and solution methods. The model is tested against several 2-D and 3-D numerical and experimental benchmark problems, and the results are presented to verify the code.     

Effects of sea level induced disturbances on high salt marsh metabolism

Salt marshes, which provide a transition between the marine and terrestrial environments around much of the temperature world, will be the first ecosystem to feel the effects of an increased rate of sea level rise. This study examined the metabolic responses of a high salt marsh to increased inundation and wrack deposition associated with sea level rise. We measured changes in ecosystem and soil photosynthesis and respiration by analyzing carbon dioxide fluxes in the light and dark. Data from seasonal flux measurements were combined with continuously measured light and temperature data to develop a model that estimated annual production and respiration. Results suggested that increased inundation will reduce respiration rates to a greater extent than production, yielding a moderate net loss of organic carbon from the high marsh. The model also predicted a substantial loss of organic carbon from wrack-affected areas. This decreased organic carbon input may play an important role in the ability of the marsh to maintain elevation relative to sea level rise.     

Halophyte recruitment in a salt marsh restoration site

In restored salt marshes, seedling recruitment can be limited where large areas of soil are exposed and physical conditions are harsh. On a 0.7-ha excavated marsh plain, we studied recruitment as a function of abiotic (elevation) and biotic factors in 2 × 2 m plots planted with 0, 1, 3, or 6 species from the pool of 8 native halophytes. The random draws of 3-species and 6-species assemblages produced approximately equal numbers of plants per species for the experiment as a whole, yet only three species recruited> 10 seedlings per plot.Salicornia virginica andSalicornia bigelovii each produced> 15,000 seedlings in 1998, andSuaeda esteroa produced> 2,500 seedlings in 1999. For these 3 species, seedling recruitment increased with elevation in 1998, but this trend weakened in 1999, when species richness affected recruitment (fewer seedlings in more species-rich plots). Abiotic effects preceded biotic interactions in determining seedling recruitment patterns early in the development of the salt marsh. Effects of species richness appeared to be scale-dependent in that having all species present in the site likely enhanced overall recruitment (all species had 2 or more seedlings), while plantings of 6 species in a 2 × 2 m plot reduced seedling density.S. virginica was the only species that increased its presence and relative cover in the experimental site over the 4-yr study. Protocols for planting southern California salt marsh restoration sites could omit this species, but all others probably need to be introduced to restore diverse vegetation.     

Algal mat productivity: Comparisons in a salt marsh

This paper documents the role of salt marsh algal mats in the productivity of a southern California tidal wetland. The productivity of the mats, which are composed of filamentous bluegreen and green algae and diatoms, varies both temporally and spatially in relation to tidal inundation and overstory vegetation. The estimates of net primary productivity (NPP) were highest under the canopy ofJaumea carnosa (Less.) Gray (341 g C m^?2 yr^?1) at low elevation. Elsewhere, NPP appeared to be limited by low light (276 g C m^?2 yr^?1 underSpartina foliosa Trin.) and desiccation (185 g C m^?2 yr^?1 underBatis martima L. and 253 g C m^?2 yr^?1 underMonanthochloe littoralis Engelm). Algal NPP was from 0.8 to 1.4 times that of the vascular plant overstory NPP. It is hypothesized that the arid environment of southern California and resulting hypersaline soils reduce vascular plant cover, which leads to high algal productivity.     

Preferential water and solute transport through sandy soil containing artificial macropores

Macropores resulting from soil pedogenesis and biological activity play important roles in soil water and chemical transport. Numerous studies have examined individual macropores and the effects of their size on solute transport, but few have assessed the effects of macropore continuity and of neighboring macropores. This paper describes a laboratory investigation of the effects of macropores, with varying degrees and types of continuity, on the transport and distribution of solutes in a sandy soil from the northern Loess Plateau, China. Breakthrough curves were obtained from 60 cm tall, 2-D columns containing standardized artificial macropores using an input solution of 1,190 mg/L KBr and 100 mg/L FD&C Blue #1 under a constant hydraulic head of 8 cm. The types of macropore were: open at both the surface and bottom of the soil column (O–O); open at the surface, closed at the bottom (O–C); and closed at the surface, open at the bottom (C–O). Columns with no macropores served as a control. In the O–O columns the solution reached the bottom 10–50 times faster than in any other treatment, bypassing most of the soil matrix. The presence of an O–C macropore resulted in weak retardation and much deeper penetration of the bromide and FD&C Blue #1 solution than in the control columns. However, the C–O macropore had little effect on either breakthrough curves or solute distributions. In further experiments that considered neighboring macropores effects, an inclined macropore strongly affected solute concentrations in the profile around a nearby vertical macropore. It was concluded that the length, type and position of single macropores, and the presence of neighboring macropores, all affect soil water flow and solute infiltration parameters in a sandy loam soil.     

Freshwater requirements of a semi-arid supratidal and floodplain salt marsh

When rivers are impounded, the reduction in downstream flow can produce important and often adverse effects, especially in the estuarine environment. One or more dams have been proposed for the Olifants River system in the Western Cape, South Africa. This estuary has an extensive area of salt marsh that was examined to see whether it required occasional flooding with freshwater to wash out accumulated salts. The dominant salt marsh species,Sarcocornia pillansii, occurred in supratidal and floodplain areas where the water table was shallowest, the soil moisture highest, and the soil electrical conductivity lowest. Aerial photographs and simulated runoff data showed that no flood had covered the floodplain during the previous 80 years. The data indicate that salt marsh plants use saline groundwater during the dry months of the year in order to survive, but use the short season winter rainfall period with low salinity conditions to grow and reproduce. This study demonstrated that live roots ofS. pillansii reached the water table during the dry season. Tissue and soil water potentials, the relationship between vegetation cover, depth to the water table, and electrical conductivity of the groundwater support the conclusion that saline groundwater is the only source of water during the drier months of the year. Freshwater flooding of the river in winter may be important because it covers the supratidal area with less saline water and reduces the depth to the water table on the floodplain. This makes the groundwater more accessible to the halophytes growing on the floodplain.