Diffusive seepage

The movement of groundwater to and from surface water can range from slow, diffusive seepage across large areas of sediment beds to rapid, concentrated flows at specific locales. Four methods are commonly used to determine these seepage rates, they are (1) water balance, (2) hydrographic analysis, (3) hydraulic-conductivity and... 

The primary characteristic necessary for a liner, cover, or cutoff wall is low permeability, which essentially enables them to slow down the seepage or diffusion of chemicals. Clay is therefore the main material used to construct these containment systems. The thickness and chemical compatibility of containment systems are of concern in assessing the performance of a system. For example, clay liners are constructed as a simple liner that is 2 to 5 ft thick. In composite and double liners, the compacted clay layers are usually between 2 and 5 ft thick, depending on the characteristics of the underlying geology and the type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 1.2 in./yr. However, the effectiveness of clay liners can be reduced by fractures induced by freeze-thaw cycles, drying out, and the presence of some chemicals. 

Comparison of profundal diffusion rates with observed increases in the hypolimnion (Table III) indicated that pore-water diffusion calculated from these profiles was probably not an important transport mechanism for Hg in this seepage lake. For the June-July period, pore-water diffusion accounted for only 13% of the hypolimnetic increase. For the July-August interval, pore-water diffusion could account for only 7% of the observed increase. Therefore, we can assume that the buildup in the hypolimnion is more likely a result of redissolution of recently fallen particulate matter at the sediment surface than of direct pore-water diffusion. Our present sampling scheme (2-cm intervals) precludes evaluation of dissolution in the uppermost sediments and would require much more detail (<1 cm) in the sediment-water interfacial zone. 

Although lakes are distinguished from rivers in part by the relative absence of a pronounced downstream flow of water, the waters of a lake are by no means stationary. Water currents, typically driven by wind instead of gravity, are a major feature of these water bodies. Water currents not only provide advective transport of chemicals in lakes but also cause transport by eddy diffusion because the water currents are almost always turbulent. In a lake, the average amount of time that water remains in the lake is called the hydraulic residence time, which can be estimated by the ratio of the lake volume to the rate at which water is lost through all processes (e.g., outflow, seepage, and evaporation). 

In an aquifer, the total Fickian transport coefficient of a chemical is the sum of the dispersion coefficient and the effective molecular diffusion coefficient. For use in the groundwater regime, the molecular diffusion coefficient of a chemical in free water must be corrected to account for tortuosity and porosity. Commonly, the free-water molecular diffusion coefficient is divided by an estimate of tortuosity (sometimes taken as the square root of two) and multiplied by porosity to estimate an effective molecular diffusion coefficient in groundwater. Millington (1959) and Millington and Quirk (1961) provide a review of several approaches to the estimation of effective molecular diffusion coefficients in porous media. Note that mixing by molecular diffusion of chemicals dissolved in pore waters always occurs, even if mechanical dispersion becomes zero as a consequence of no seepage velocity. 

Equation [3-17] does not hold at very low seepage velocities because mechanical dispersion no longer dominates Fickian mass transport. When the mechanical dispersion coefficient becomes less than the effective molecular diffusion coefficient, the longer travel times associated with lower velocities do not result in further decreases in Fickian mass transport. 

The diffusion in a porous media is affected by the flow field, so we treat here the seepage flow problem in the bentonite starting with the Stokes equation and applying the multiscale HA method. 

We here showed that for bentonite clay, we can determine the nano-scale material properties such as diffusion coefficient and viscosity by molecular dynamics (MD) simulation and extend the microscale characteristics to the macroscale behavior by the multiscale homogenization analysis (HA) method. A seepage flow and diffusion problem is treated. The micro/macro problem can be simulated well by this procedure if we know the microscale geometrical characteristics. 

It turned out that the mechanical dispersion may be formally described by the same Pick s laws if the diffusion coefficient is replaced with mechanical dispersion coefficient This is a proportionality coefficient between value of a deflection of component i migration rate from the average seepage velocity on the one hand, and the component concentration gradient between mixed waters, on the other. In a case of unidimensional and bidimensional fluxes this correlation by analogy with the first Pick s law has the following format... 

Besides, the water flow causes mechanical mixing, which together with the diffuse one determines the value of hydrodynamic dispersion, the value dependent on the direction and seepage velocity of flow. The coefficient of longwise hydrodynamic dispersion by analogy with the diffusion coefficient may be associated with the dispersion of statistical distribution by equation... 

There are mainly two types of Slurry seepage paths named cylindrical and surface-shaped, during the slurry diffusion process, cylindrical diffusion refers to slurry along the approximate space of the cylinder movement. The planar diffusion model of the slurry, is refers to the slurry along the approximate plane of the spatial movement... 

The model ignores flow of airflow diffusion term in the momentum equation, just considers goaf Merry Darcy seepage. Meanwhile, when the Reynolds number exceeds a second slope Merry Renault value R > 2300) (Jiang Wang 1995), the slope air current steps into the automatic modeling. Models face section size is 0.04 x 0.03 m, then the critical velocity is ... 

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