Effect of NAPL film stability on the dissolution of residual wetting NAPL in porous media: A pore-scale modeling study

Wettability profoundly affects not only the initial distribution of residual NAPL contaminants in natural soils, but also their subsequent dissolution in a flowing aqueous phase. Under conditions of preferential NAPL wettability, the residual NAPL phase is found within the smaller pores and in the form of continuous corner filaments and thick films on pore walls. Such films expose a much greater interfacial area for mass transfer than would be exposed by the same amount of non-wetting NAPL. Importantly, capillary and hydraulic continuity of NAPL filaments and thick films is essential for sustaining NAPL–water counterflow during the course of NAPL dissolution in flowing groundwater—a mechanism which maintains and even increases the interfacial area for mass transfer. Continued dissolution results in gradual thinning of the NAPL films, which may become unstable and rupture causing disconnection of the residual NAPL in the form of clusters. Using a pore network simulator, we demonstrate that NAPL film instability drastically modifies the microscopic configuration of residual NAPL, and hence the local hydrodynamic conditions and interfacial area for mass transfer, with concomitant effects on macroscopically observable quantities, such as the aqueous effluent concentration and the fractional NAPL recovery with time. These results strongly suggest that the disjoining pressure of NAPL films may exert an important, and hitherto unaccounted, control on the dissolution behaviour of a residual NAPL phase in oil wet systems.     

Evaluation of an upscaled model for DNAPL dissolution kinetics in heterogeneous aquifers

Estimates of contaminant fluxes from DNAPL sources as a function of time and DNAPL mass reduction are important to assess the long-term sustainability and costs of monitored natural attenuation and to determine the benefits of partial source removal. We investigate the accuracy of the upscaled mass transfer function (MTF) proposed by Parker and Park [Parker JC, Park E. Modeling field-scale dense nonaqueous phase liquid dissolution kinetics in heterogeneous aquifers. WRR 2004;40:W05109] to describe field-scale dissolved phase fluxes from DNAPL sources for a range of scenarios generated using high-resolution 3-D numerical simulations of DNAPL infiltration and long-term dissolved phase transport. The results indicate the upscaled MTF is capable of accurately describing field-scale DNAPL dissolution rates as a function of time. For finger-dominated source regions, an empirical mass depletion exponent in the MTF takes on values greater than one which results in predicted mass flux rates that decrease continuously with diminishing DNAPL mass over time. Lens-dominated regions exhibit depletion exponents less than one, which results in more step-function like mass flux versus time behavior. Mass fluxes from DNAPL sources exhibiting both lens- and finger-dominated subregions were less accurately described by the simple MTF, but were well described by a dual-continuum model of the same form for each subregion. The practicality of calibrating a dual-continuum model will likely depend on the feasibility of obtaining spatially resolved field measurements of contaminant fluxes or concentrations associated with the subregions using multilevel sampling or some other means.     

文安地震前后的转换函数参数变化分析

为了提取地震前后的磁场异常变化,本文用转换函数方法分析了文安地震前后,距离震中分别为52km和298km的静海地磁台和泰安地磁台不同周期的转换函数参数随时间的变化过程,发现静海地磁台转换函数参数的数据异常点随着时间的向后推移从短周期向比较长的周期迁移,而泰安地磁台的转换函数参数没有出现类似现象。     

岩石压应力传递系数计算及影响机制

钻孔应变观测是地壳岩石应变相对测量的主要方法,对其附加应力场的研究,可以有效分析岩石压应力向探头的传递过程,可对应变仪有关力学参数的设计和观测数据的物理解释提供帮助。考虑到外加应力在岩石、水泥和仪器钢筒等3层介质中传递的实际情况,根据弹性力学中的厚壁圆筒方程,推导轴对称应力作用下岩石压应力的传递系数公式,分析影响机制,并给出有关变化规律曲线和定量指标。     

Simulation of flood hydrographs based on lumped and semi-distributed models for two tropical catchments in Kenya

Abstract

The design and operation of flood management systems require computation of flood hydrographs for both design floods and flood forecasting purposes, since observed data are usually inadequate for these tasks. This is particularly relevant for most developing countries, i.e. mainly for tropical catchments. One possible way of obtaining information about flood hydrographs is through the use of rainfall—runoff models. Two such models, namely the Bochum model and the Nash Cascade—Diskin Infiltration model, which are semi-distributed and lumped models, respectively, were used in the present study. These models were applied to two catchments in Kenya with drainage areas of 6.71 km² and 26.03 km². A set of 13 selected rainfall—runoff events was used to calibrate and validate the models. The physical parameters required by the models were derived from catchment characteristics using GIS and remote sensing data while the conceptual parameters were obtained by optimization. The flood hydrographs simulated using the parameters so derived indicated that it is possible to use the two models in this tropical environment.     

Effects of rate-limited mass transfer on water sampling with partially penetrating wells

Nonequilibrium concentration type curves are numerically developed and sensitivity analyses are performed to examine the relationships between effluent concentrations in partially penetrating monitoring/extraction wells, the vertical plume shape, and the mass transfer characteristics of the aquifer. The governing two-dimensional, axisymmetric nonequilibrium solute transport equation is solved in three stages using an operator-splitting approach. In the first two stages, the advection and dispersion terms are solved with the Eulerian-Lagrangian method, based on the backward method of characteristics for advection and the standard implicit Galerkin finite element method for dispersion. In the third step, the first-order, immobile-mobile domain mass transfer term is computed analytically for both two-site and lognormally distributed, multirate models. Effluent concentration variations with time and contour plots of the pore water concentration distribution in the aquifer are compared for a wide range of field- and laboratory-measured mass transfer rates, various plume shapes, and relevant physical/chemical parameter values, including pumping rate, vertical anisotropy ratio, retardation factor, and porosity. The simulation results show that rate-limited mass transfer can have a significant impact on sample and aquifer pore water concentrations during three-dimensional transport to a partially penetrating well. An alternative dimensionless form of the nonequilibrium solute transport equation is derived to illustrate the key parameter groupings that quantify rate-limited sorption effects and show the relative importance of individual parameters. A hypothetical field application example demonstrates the fitting of dimensional type curves to discrete-interval sampling data in order to evaluate the mass transfer characteristics of an aquifer and shows how type curve superposition can be used to model complex plume shapes.     

Influence of an endothermic phase transition on mass transfer between the upper and the lower mantle

Geochemical data indicate that two major reservoirs 1–2 Ga in age are present in the mantle. The upper mantle, feeding mid-ocean ridges, is depleted in chemical elements carried away into the continental crust. The lower mantle, feeding hotspot plumes, is close in composition to primordial matter. The 660-km depth of an endothermic phase transition in olivine has been considered over the last two decades as a possible boundary between the reservoirs. In this period, many models of mantle convection were constructed that used values of the phase transition parameters, which led to temporal (up to 1 Gyr long) convection layerings and periodic avalanche-induced mantle intermixing events. However, laboratory measurements with new improved instrumentation give other values of the phase transition parameters that require a revision of the majority of the existence of large-scale avalanches in the Earth’s history becomes disputable. The paper is devoted to comprehensive study of the phase transition effect on the structure of mantle flows with different values of phase transition parameters and Rayleigh numbers; in particular, the mass transfer through the phase boundary is calculated for different regimes of steady-state convection.     

Numerical analysis of simultaneous heat and mass transfer during infiltration into frozen ground

This paper concerns heat transfer and water infiltration into a homogeneous, unsaturated, frozen soil. A numerical analysis of the processes, which is based on the local volume averaging formulation of transport phenomena in porous media, is described. Simultaneous heat and mass transfers with phase changes are considered. The results of the simulations provide an insight into the mechanics of the infiltration process. They suggest that infiltration involves primarily two flow regimes, a transient regime and a quasi-steady-state regime. Once the quasi-steady-state regime is reached, the energy needed to increase soil temperature at depth is supplied by latent heat released from freezing of water in the upper layers of soil. The effects of surface saturation, initial soil saturation and initial soil temperature on infiltration are examined.     

Modeling of heat and mass transfer in the hydrothermal system of the Akademii Nauk caldera

Supposing a leading role of free heat convection in the hydrothermal system above the magma chamber, a model of the hydrothermal system of the Akademii Nauk caldera in Kamchatka is suggested. The model is based on available geological, hydrogeological and geothermal data for the area. Three possible variants of the model are discussed. Based on temperature distribution data, the variant best corresponding to the natural conditions has been chosen. The period of hydrothermal system stabilization for all the variants is close to 7000–8000 years.     

统计地震学的两个基本定律对应力的依赖关系

统计地震学长期使用的两个幂律关系式为:描述地震频度-震级关系的古登堡-里克特(Gutenberg-Richter)关系式[1]和描绘主震后余震随时间衰减速率特征的大森-宇津(Omori-Utsu)定律[2]。最近,地震频度-震级关系斜率(b值)与断裂模式的相关性研究确定了应力对b值的影响[3]。在此,我们以类似的方式根据主震的断裂模式对余震序列进行研究。我们发现逆冲型主震的幂律余震衰减速率起始前的延时(c值)一般比正断层型地震的短,走滑型地震的c值则处于二者之间。对断裂模式的这些类似依赖关系表明两个基本幂律都受应力状态控制。只有2%的余震有已知震源机制解。因此,c值和b值是两个独立的估算值,它们可作为推断应力场的新方法来使用,目前应力场依然难以直接测定。     

Analysis of Water Saturation, NAPL Content, Degradation Half-Life, and Lower Boundary Conditions on VOC Transport Modeling: Implications for Closure of Soil Venting Systems

Simulations using a one-dimensional, analytical, vadose zone, solute-transport screening code (VFLUX) were conducted to assess the effect of water saturation, NAPL saturation, degradation half-life, and boundary conditions at the vadose zone/ground water interface on model output. At high initial soil concentrations, model output was significantly affected by input parameters and lower boundary conditions yet still resulted in consistent decision-making to initiate or continue venting application. At lower soil concentrations, however, typical of what is observed after prolonged venting application, differences in model input and selection of lower boundary conditions resulted in inconsistent decision-making. Specifically, under conditions of low water saturation, use of a first-type, time-dependent lower boundary condition indicated that the primary direction of mass flux was from ground water to the vadose zone, suggesting little benefit from continued venting application. Use of a finite, zero-gradient lower boundary condition, though, indicated continued mass flux from the vadose zone to ground water, suggesting a continued need for venting application. In this situation, sensitivity analysis of input parameters, selection of boundary conditions, and consideration of overall objectives in vadose zone modeling become critical in regulatory decision-making.     

On total turbulent energy and the passive and active role of buoyancy in turbulent momentum and mass transfer

Measurements of turbulent fluctuations of horizontal and vertical components of velocity, salinity and suspended particulate matter are presented. Turbulent Prandtl numbers are found to increase with stratification and to become larger than 1. Consequently, the vertical turbulent mass transport is suppressed by buoyancy forces, before the turbulent kinetic energy (TKE) and vertical turbulent momentum exchange are inhibited. With increasing stratification, the buoyancy fluxes do not cease, instead they become countergradient. We find that buoyantly driven motions play an active role in the transfer of mass. This is in agreement with trends derived from Monin–Obukhov scaling. For positive Richardson flux numbers (Ri _f ), the log velocity profile in the near-bed layer requires correction with a drag reduction. For negative Ri _f , the log velocity profile should be corrected with a drag increase, with increasing |Ri _f |. This highlights the active role played by buoyancy in momentum transfer and the production of TKE. However, the data do not appear to entirely follow Monin–Obukhov scaling. This is consistent with the notion that the turbulence field is not in equilibrium. The large stratification results in the decay of turbulence and countergradient buoyancy fluxes act to restore equilibrium in the energy budget. This implies that there is a finite adjustment timescale of the turbulence field to changes in velocity shear and density stratification. The energy transfers associated with the source and sink function of the buoyancy flux can be modeled with the concept of total turbulent energy.     

围压对低渗砂岩渗透率有效应力系数的影响

多孔岩石渗透率有效应力系数的取值至今仍没有一致的认识,影响取值的因素较为复杂.为了探索围压对低渗砂岩渗透率有效应力系数的影响,实验上采用较大的围压范围(10MPa～50MPa),选取两块低渗致密砂岩岩样,进行渗透率测试,并利用二元线性回归方法,计算了不同围压组合下的渗透率有效应力系数值,获取了该系数随围压的变化趋势.实验中,两块岩样的渗透率有效应力系数随围压的增加而减小,但各自的减小趋势略有不同.利用Bernabé和Berryman提出的模型,理论上解释了围压对渗透率有效应力系数的影响,并得到渗透率有效应力系数的下限值为岩样孔隙度.尽管渗透率有效应力系数不是一个常数,但可以表示为压差的函数,进而起到简化分析的作用.     

Nonlinear Dependence of the Solar Flux on Wolf Numbers and Its Variation for Different 11-Year Cycles

Geomagnetism and Aeronomy - The statistical, nonlinear, nonmonotonic dependence of the daily mean values of the solar “constant” on the daily Wolf numbers has been established. The...     

延性系数对SDOF体系地震能量输入历程的影响研究

文中以弹塑性SDOF体系为对象,提出将总输入能、最大瞬时输入能以及能量增长持时作为描述地震能量输入历程的三个特征参数。根据场地条件及地震动峰值速度与峰值加速度比值(V/A)双参数控制选择了不同场地条件下56条强震记录为输入,讨论延性系数的变化对这三个参数的影响。研究表明,随着延性系数的增加,总输入能量谱及最大瞬时输入能量谱在减小谱峰值的同时,谱型整体向短周期移动,形成压缩效应;弹塑性能量增长持时谱较弹性能量增长持时谱更为平滑,其谱值在统计意义上不发生变化;短周期结构延性的增大使得总输入能与最大瞬时输入能增加,但对于中长周期结构,总输入能与最大瞬时输入能随延性系数增加而减少;输入能量在时间轴上的分布模式受延性系数变化的影响不明显,能量时程增长台阶的总长度在各种延性系数条件下趋于一致。     

Effects of organic acids on dissolution of Fe and Mn from weathering coal gangue

Understanding the effects of organic acids(OA) on the transformation of Fe and Mn to surface water from the weathering coal gangue is of great bene?t to risk assessment and remediation strategies for contaminated water and soil. Based on the investigation on surface water in the central coal districts of the Guizhou Province, 18 water samples were collected for heavy metal analysis. The results indicated that the p H value of surface water is low(3.11–4.92), and Fe concentration(1.31–5.55 mg L~(-1)) and Mn concentration(1.90–5.71 mg L~(-1)) were, on average,10.86 and 34.33 times the limit of Surface Water Quality Standards, respectively. In order to evaluate the effects of the OA on the dissolution of Fe and Mn from the weathering coal gangue, column elution and batch leaching experiments were conducted. The results show that the low molecular weight of organic acids(LMWOAs, i.e., oxalic,tartaric, malic and citric acids) and fulvic acids signi?-cantly accelerated the dissolution of Fe and Mn; in addition, when the concentration of OA reached 25 mmol L~(-1),the concentrations of Fe, and Mn were 1.14–67.08 and1.11–2.32 times as high as those in 0.5 mmol L~(-1)OA,respectively. Furthermore, the migration of Fe and Mn was signi?cantly in?uenced by the p H and Eh, especially for Fe; the ion Mn was dissolved from the gangue more easily than the ion Fe in the column leaching, which was contrary to the results of batch leaching.     

Evaporite dissolution and development of karst features on the rolling plains of the texas panhandle

More than 400 dolines, consisting of collapse sinkholes and closed depressions, occur in Hall and Briscoe Counties, Texas. Of these, at least 36 sinkholes and two depressions formed between 1940 and 1972. Collapse sinks are typically circular and range up to 100 m in diameter and 15 m in depth. Closed depressions are irregularly-shaped, internally-drained depressions up to 2.4 km in length. Surface fractures up to 1.5 km in length are associated with certain closed depressions. Karst features are forming as a result of intrastratal dissolution of Upper Permian evaporites, primarily halite and secondarily gypsum. Salt dissolution and overburden collapse are evident from geophysical log cross-sections and occur at depths ranging from 195 to 275 m. Dissolution is progressing down dip and mean dissolution rates for salt beds in the Hall County area range from 0.29 to 7.73 cm/yr. The valley of the Prairie Dog Town Fork of the Red River is an area of ground-water discharge for flow systems moving eastward from topographically high areas of the Rolling Plains and from the eastern rim of the High Plains. Halite beds in contact with these waters undergo dissolution. Salt springs and brine seeps, some associated with sinkholes and depressions, occur in the discharge area and indicate that dissolution is an ongoing process.     

Dependence of reaction rate of pyrite oxidation on temperature, pH and oxidant concentration

Pyrite is a common sulfide mineral, particularly abundant in mining waste. It is a focus of mine envi-ronmental concern because the oxidation of exposed pyrite may lead to acid drainage and poisonous mate-rials (like As and heavy metals) release. In the United States, approximately $1000000 per day is spent in alleviating acid mine drainage[1], while the Canadian mining community has environmental liabilities that exceed $2 billion for disposal, management, and rec-lamation of mine waste[2]. …