Rate controls on the chemical weathering of natural polymineralic material. I. Dissolution behaviour of polymineralic assemblages determined using batch and unsaturated column experiments

Chemical weathering rates for a mudstone obtained from a mining environment were investigated using a combination of batch reactors and hydrologically unsaturated column experiments. Results of tracer tests were combined with relationships between solute concentrations, mass fluxes, flow rates and residence times, and used to calculate element release rates and infer rate-controlling mechanisms for the two different experimental environments.Elements that eluted from column experiments exhibited either:

• (1)concentrations independent of flow rate and column length coupled with mass flux increasing with flow rate, or
• (2)an inverse relationship between concentration and flow rate coupled with mass flux increasing with both column length and flow rate.

The former is attributed to equilibrium-controlled release of particular elements (Si, Al), while the latter is ascribed to transport-controlled release of others (Mg, Mn, Ca, Na, K, S). Tracer tests using NaBr solutions revealed that some elements were also affected by ion exchange (Mg, Mn, Ca, Na), but these effects were temporary and did not mask underlying dissolution rate-controlling mechanisms. Analysis of characteristic diffusive lengths was used to distinguish between transport rates limited by the transfer of solutes between immobile and mobile water within the columns, and rates limited by slow diffusion across partially reacted mineral layers. The analysis suggests that transfer between immobile and mobile water limited the element release rates, with diffusion hindered by low diffusion coefficients within the unsaturated medium, and by low interfacial areas between mobile and immobile fluids. Results of the batch experiments showed different characteristics. Element concentrations either rose to a plateau or increased linearly with time. Rate-controlling mechanisms associated with these characteristics were equilibrium (Fe) and surface kinetic reactions (Si, Mg, Mn, Ca, K, S), respectively. Surface area-normalized element release rates for Mg, Mn, Ca and S are consistently a factor of 4 higher than those from the column experiments. This is a significant difference and cannot be attributed to differences in mineral preparation or to factors influencing individual minerals. It must, therefore, reflect the difference in the rate-controlling mechanism, that is, transport vs. surface-kinetic control.These results suggest that some proportion of the commonly recorded discrepancy between laboratory and field weathering rates is due to hydrological differences between the two environments, and that hydrological characterization of weathering environments is, therefore, as important as physical–geochemical characterization of reacting solids. An important practical implication of the work is that substantial reservoirs of solutes are held in immobile water in the unsaturated environment, and that these could be released by soil disturbance or flooding.     

A particle tracking transport method for the simulation of resident and flux-averaged concentration of solute plumes in groundwater models

A new numerical technique called the convolution-based particle tracking (CBPT) method is developed to simulate resident or flux-averaged solute concentrations in groundwater models. The method is valid for steady-state flow and linear transport processes such as sorption with a linear sorption isotherm and first-order decay. The CBPT method uses particle tracking to take advantage of the ability of particle-based approaches to maintain sharp fronts for advection-dominated transport problems common in groundwater modeling and because of the flexibility of the random walk method to simulate a wide range of possible forms of the dispersion tensor. Furthermore, the algorithm for carrying out the convolution and superposition calculation from particle tracking results is very efficient. We show that from a single particle tracking run, source term variability, sorption, and decay can all be simulated rapidly without rerunning the underlying transport model unless the flow field or dispersion parameters are changed. A series of verification simulations are presented to demonstrate the accuracy and efficiency of the CBPT method compared to more conventional particle tracking approaches.     

Investigations on Diffusion Characteristics of Granite and Chalk Rock Mass

Contaminant transport through fractured rock mass is predominated by diffusion. This is due to the continuous interaction of the mobile water present in the fracture network and relatively immobile pore water, which is adsorbed on the surface and in the rock matrix itself. Even though the advective flow through the fracture network is high, besides sorption of rock mass, the diffusive exchange into the rock mass leads to significant retardation of contaminant transport. Hence, for describing contaminant transport in fractured rock mass, more precisely, the effect of retardation attributed to the matrix diffusion must be taken in account. With this in view, a methodology, which can be employed for determination of the diffusion characteristics of the rock mass, has been developed and its details are presented in this paper. Validation of the methodology has been demonstrated with the help of Archie’s law.     

Retention properties of flow paths in fractured rock

There is no straightforward way to extrapolate solute retention properties from typical site characterisation scales to typical scales in the performance assessment of the geological disposal of nuclear wastes. Solutes diffuse much deeper into the rock matrix under performance assessment flow conditions than under site characterisation flow conditions. The modelling approach applied in this study, associated with the Äspö Task Force, enables evaluation of the contribution of the individual immobile layers to the overall retention. This makes it possible to determine the influence of the immobile zone heterogeneity on solute retention under different flow conditions. It appears that there is a significant difference between the dominating immobile retention zones on site characterisation and performance assessment scales. Fractured rock is characterised by heterogeneity and in particular a large spread of hydraulic properties. This favours formation of the preferential flow paths by leading to a few dominating transport paths. Large hydraulic features are, on average, better hydraulic conductors than smaller ones. This causes spatial scale effects for the solute retention properties. In particular, the hydraulic properties at the early parts of flow paths are more favourable to retention than those at the later parts of the flow paths.     

非均匀土壤中溶质运移的两区模型及其解析解

给出了非均匀土壤中考虑水动力弥散尺度效应的一维溶质运移两区模型。然后在初始浓度为零,半无限一维空间内常数通量边界条件下,推导出了可动区和不动区溶质浓度分布的解析表达式。并用Mathematica软件包作了数值计算。计算结果表明,该模型的模拟结果符合溶质运移的一般规律,因而该模型在土壤溶质运移的研究和应用中有一定的价值。     

围岩蚀变过程中地球化学组份质量迁移计算——以湖南沃溪Au-Sb-W矿床为例

在以往围岩蚀变的质量迁移研究基础上,本文提出了一种判定惰性元素的方法,同时结合等浓度线和加权最小二乘法,给出研究热液围岩蚀变中组分质量迁移的计算方法。在判定惰性元素时,更多地应用了线性回归分析中的方差分析与回归诊断参数与计算方法。这一系列的参数与方法被用于评述沃溪金锑钨矿床褪色化围岩蚀变质量迁移,取得了较好的效果。研究表明,在沃溪矿床围岩蚀变过程中,Ｔｉ表出现了一定的活动性。     

围岩蚀变过程中地球化学组份质量迁移计算

在以往围岩蚀变的质量迁移研究基础上,本文提出了一种判定惰性元素的方法,同时结合等浓度线和加权最小二乘法,给出研究热液围岩蚀变中组分质量迁移的计算方法。在判定惰性元素时,更多地应用了线性回归分析中的方差分析与回归诊断参数与计算方法。这一系列的参数与方法被用于评述沃溪金锑钨矿床褪色化围岩蚀变质量迁移,取得了较好的效果。研究表明,在沃溪矿床围岩蚀变过程中,Ｔｉ表出现了一定的活动性。     

Mass change calculations in altered rock series

A technique is presented whereby precursors of altered rocks in a fractionated rock series can be identified, and changes in chemical components, mass and volume accurately calculated. The procedure utilizes elements with high degrees of immobility in hydrothermal and associated interactive water-rock systems. Igneous rock series, and sedimentary and metamorphic rocks with primary continuity of chemical composition, are amenable to this treatment. In the procedure for igneous rocks, best-fit fractionation lines are established for an immobile igneous compatible-incompatible element pair, the latter element functioning as the fractionation monitor. Alteration paths are linear and pass through the origin. Intersections of the fractionation and alteration lines yield the immobile element concentrations in the precursors to the altered rocks; mobile components are determined from fractionation lines and monitor values. Total mass changes are proportional to the displacement of altered samples from the fractionation line. Masses of other elements in an altered sample (reconstructed values) are recalculated to the concentration of the monitor element in the precursor. Chemical changes are the differences between the calculated precursor compositions and the reconstructed altered rock compositions. Errors are mainly a function of the positioning of the fractionation lines.     

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.     

改进时间分数阶模型模拟非Fick溶质运移

通过将经典时间分数阶对流-弥散方程的等待时间分布函数的尾部修改为指数型,推导出了改进时间分数阶对流-弥散方程,并提出有效的时空算子分裂数值求解方法。对两个理想算例和一个实际算例进行计算,结果表明,改进的时间分数阶对流-弥散方程继承了时间分数阶对流-弥散方程能模拟穿透曲线幂率型拖尾分布的优点,还可模拟穿透曲线尾部由幂率型转换到指数型的过程;特征时间λ、分数阶指数γ和两相容量比例系数β共同决定了运移行为。改进的新模型可以区分非均质介质中流动相和非流动相中的溶质浓度, 更细微地模拟非Fick溶质运移行为。     

围岩蚀变过程中地球化学组分质量迁移计算：以湖南沃溪金锑钨矿床为例

在以往围岩蚀变的质量迁移研究基础上,本文提出了一种判定惰性元素的方法,同时结合等浓度线和加权最小二乘法,给出研究热液围岩蚀变中组分质量迁移的计算方法。在判定惰性元素时,更多地应用了线性回归分析中的方差分析和回归诊断参数与计算方法。这一系列的参数与方法用于评述湖南沃溪金锑钨矿床裉色化围岩蚀变质量迁移,取得了较好的效果。研究表明,在沃溪矿床围岩蚀变过程中,Ｔｉ表现出了一定的活动性。     

An experimental study into the geometry of supply-limited dunes

The relationship between dune geometry and the volume of mobile sediment was studied in flume experiments. In these flume experiments, the volume of mobile sediment on top of an immobile coarse sediment layer was increased stepwise and the bedform characteristics were observed. A strong relationship was found between the volume that is mobile – and therefore available for bedform formation – and the dune dimensions and regularity. If the sediment supply is limited, dunes are smaller and more regular. Series of experiments with a decreasing supply limitation were conducted for different flow velocities and water depths. The relationship between the dune dimensions and the available volume is different for each series. The observed relationships between dune dimensions and layer thickness collapse to one relationship for height and one for length if scaling parameters are introduced. Current models for bedform dimensions under alluvial conditions can be extended to partial transport conditions using this relationship.     

Modeling reactive transport in non-ideal aqueous–solid solution system

The numerical simulation of reactive mass transport processes in complex geochemical environments is an important tool for the performance assessment of future waste repositories. A new combination of the multi-component mass transport code GeoSys/RockFlow and the Gibbs Energy Minimization (GEM) equilibrium solver GEM-Selektor is used to calculate the accurate equilibrium of multiple non-ideal solid solutions which are important for the immobilization of radionuclides such as Ra. The coupled code is verified by a widely used benchmark of dissolution–precipitation in a calcite–dolomite system. A more complex application shown in this paper is the transport of Ra in the near-field of a nuclear waste repository. Depending on the initial inventories of Sr, Ba and sulfate, non-ideal sulfate and carbonate solid solutions can fix mobile Ra cations. Due to the complex geochemical interactions, the reactive transport simulations can describe the migration of Ra in a much more realistic way than using the traditional linear K_D approach only.     

A self-adapting model for assessing hazardous environmental releases

Simulation results are presented by using an h-adaptive mass consistent finite element method (FEM) coupled with a Lagrangian particle transport technique (LPT) for dispersion associated with hazardous atmospheric releases. A three-dimensional wind field is first constructed from the adaptive FEM model. Lagrangian particles that define the contaminant dispersion are then produced with the LPT scheme, employing a random walk/stochastic approach. The application of FEM permits flow patterns with irregular geometries to be easily simulated, while the LPT permits contaminant particle dispersion patterns to be quickly depicted. The hybrid model is fast, runs on PCs, and appears well suited for emergency response dispersion predictions and assessment.     

Contaminant transport with non‐equilibrium processes in unsaturated soils and implicit characteristic Galerkin scheme

A non‐equilibrium sorption—advection—diffusion model to simulate miscible pollutant transport in saturated–unsaturated soils is presented. The governing phenomena modelled in the present simulation are: convection, molecular diffusion, mechanical dispersion, sorption, immobile water effect and degradation, including both physical and chemical non‐equilibrium processes. A finite element procedure, based on the characteristic Galerkin method with an implicit algorithm is developed to numerically solve the model equations. The implicit algorithm is formulated by means of a combination of both the precise and the traditional numerical integration procedures. The stability analysis of the algorithm shows that the unconditional stability of the present implicit algorithm is enhanced as compared with that of the traditional implicit numerical integration procedure. The numerical results illustrate good performance of the present algorithm in stability and accuracy, and in simulating the effects of all the mentioned phenomena governing the contaminant transport and the concentration distribution. Copyright © 2000 John Wiley & Sons, Ltd.     

A new quantitative interpretation of the long-tail and plateau-like breakthrough curves from tracer tests in the artesian karst aquifer of Stuttgart, Germany

In 1998 and 1999, two multi-tracer experiments were conducted in the artesian karst aquifer of the mineral springs of Stuttgart, Germany. The breakthrough curves (BTCs) monitored at the springs showed very long tails or developed plateau-like concentration levels for more than 200 days. Initially, this observation was qualitatively explained by exchange between cavities with stagnant water and the active conduits. Since then, a new analytical solution for tracer transport in karst aquifers has become available, the “two-region non-equilibrium model” (2RNE), which assumes the presence of mobile and immobile fluid regions, and mass transfer between these two regions. The experiments were thus revisited, and it was possible to provide a more quantitative explanation of the observed behaviour. The new model simulated all BTCs very well, thus confirming the earlier qualitative explanation. The prolonged BTCs can be attributed to intermediate storage in cavities containing quasi-immobile groundwater, and slow release into active fractures and conduits. The results also demonstrate that karst aquifers are not always fast-flushing systems, but contaminants can sometimes remain in immobile fluid regions for long periods.     

Transport of colloidal contaminants in groundwater: radionuclide migration at the Nevada test site

Large-volume groundwater samples were collected at the Nevada Test Site from within a nuclear detonation cavity and from approximately 300 m outside the cavity. The samples were filtered and ultrafiltered, and the filtrates and various particle size fractions were analyzed for chemical composition and radionuclide activity. In samples from both locations, approximately 100% of the transition element (Mn, Co) and lanthanide (Ce, Eu) radionuclides were associated with colloids. Their presence outside the cavity indicates transport in the colloidal form. Distribution coefficients calculated for Ru, Sb, and Cs nuclides from both field sample locations indicate equilibrium partitioning on the 0.05-0.003 μm colloids. Calculation of transport efficiencies relative to colloid mass concentrations and dissolved neutral or anionic nuclides indicates that both the cations and the radiolabelled colloids appear to experience capture by or exchange with immobile aquifer surfaces.     

三维溶质运移问题的分步广义迎风解法

对对流占优的三维溶质运移问题提出了分步广义迎风解法，首先利用Ｎ．Ｎ．Ｙａ－ｎｅｎｋｏ对水动力弥散方程分步求解的思想，将原来的一个定解问题分解为两个定解问题即对流定解问题和扩散定解问题，对对流定解问题采用广义迎风对偶单元均衡法求解，对扩散定解问题采用一般的Ｇａｌｅｒｋｉｎ有限元法求解，不仅避免了用一般有限元法和有限差分法求解对流占优的地下水水质数学模型时常出现数值弥散和过量问题，而且避免了求节点速度这一步，节省运算步骤，对井点的浓度变化给出了更合适的求解方法。     

Analysis of undisturbed layers of a waste deposit landfill – Insights into the transformation and transport of organic contaminants

Comprehensive GC/MS analysis was applied to both the mobile liquid phase (seepage water) and the immobile solid matter of discrete layers derived from a waste deposit landfill. The vertical distribution of organic compounds supports information on the transport, transfer and transformation processes with depth and, consequently, with time.Numerous low molecular weight organic contaminants of natural and xenobiotic origin were identified and partially quantified. Several were selected to act as molecular indicators for different processes. Interpretation of their occurrence and concentration profiles (considering possible waste sources) and their molecular properties allowed us to (i) differentiate immobile and mobile fractions, (ii) reveal restrictions in the vertical transport by transfer processes between particulate and water phase, (iii) identify dynamic accumulations of individual contaminants and (iv) estimate approximate residence times. In addition, intensive degradation processes were pointed out for the natural fraction of the organic matter by way of determination of specific transformation products. Besides the transformation of natural components, transformation of numerous xenobiotics was recognised. In particular, with respect to an important group of contaminants, the phthalate-based plasticisers, a detailed view of (i) the influence of transfer and transport phenomena on transformation processes as well as (ii) the consecutive appearance of different degradation steps in both seepage water and solid waste was pointed out. The information provides a valuable base for the prediction of the long term behaviour of organic contaminants in waste deposit landfills.