Dispersion groundwater

Recovery of DNAPL is a very slow process that is alfected by those factors encountered with LNAPL (i.e., relative permeability, viscosity, residual hydrocarbon pool distribution, site-specific factors, etc ). Dissolution of a DNAPL pool is dependent upon the vertical dispersivity, groundwater velocity, solubility, and pool dimension. Dispersivities for chamolid solvent are estimated for a medium to coarse sand under laboratory conditions on the order of 1(L3 to 1(H m. Thus, limited dispersion at typical groundwater velocities is anticipated to be slow and may take up to decades... 

Anderson, M.P. 1984. Movement of contaminants in groundwater groundwater transport—advection and dispersion. Groundwat. Contain. 6 37-45. 

Once the source modeling is complete, the quantitative result is used in a consequence analysis to determine the impact of the release. This typically includes dispersion modeling to describe the movement of materials through the air, or a fire and explosion model to describe the consequences of a fire or explosion. Other consequence models are available to describe the spread of material through rivers and lakes, groundwater, and other media. 

Spill Anatomy and Remediation. Contrary to past arguments that leaks or spills from aboveground tanks would stay near the surface, they go straight down into the aquifer and spread out. Various obstacles, such as clay lenses, rock, or impermeable layers of sod, simply divert the downward path. Slow leaks from tank bottoms tend to form a narrow plume, whereas larger spills cover much wider areas. When the contaminant reaches groundwater, it tends to be dispersed in the direction of the groundwater current and movement. 

Several high-rate air flotation clarifiers (both DAF and dispersed air flotation) with less than 15 min of detention times have been developed for groundwater decontamination, industrial effluent treatment, resources recovery, and water reclamation. Both insoluble and soluble impurities such as... 

Flush models can also be configured to simulate the effects of dispersive mixing. Dispersion is the physical process by which groundwaters mix in the subsurface (Freeze and Cherry, 1979). With mixing, the groundwaters react with each other... 

Chemical mass is redistributed within a groundwater flow regime as a result of three principal transport processes advection, hydrodynamic dispersion, and molecular diffusion (e.g., Bear, 1972 Freeze and Cherry, 1979). Collectively, they are referred to as mass transport. The nature of these processes and how each can be accommodated within a transport model for a multicomponent chemical system are described in the following sections. 

Where groundwater is allowed to flow in an arbitrary direction instead of along one of the coordinate axes, the dispersion coefficient assumes a tensor rather than vector form. In this case, the dispersive fluxes are given by,... 

Molecular diffusion (or self-diffusion) is the process by which molecules show a net migration, most commonly from areas of high to low concentration, as a result of their thermal vibration, or Brownian motion. The majority of reactive transport models are designed to simulate the distribution of reactions in groundwater flows and, as such, the accounting for molecular diffusion is lumped with hydrodynamic dispersion, in the definition of the dispersivity. 

The accounting for diffusion in these models, in fact, is in many cases a formality. This is because, as can be seen from Equations 20.19 and 20.21, the contribution of the diffusion coefficient D to the coefficient of hydrodynamic dispersion D is likely to be small, compared to the effect of dispersion. If we assume a dispersivity a of 100 cm, for example, then the product av representing dispersion will be larger than a diffusion coefficient of 10-7-10-6 cm2 s-1 wherever groundwater velocity v exceeds 10 9-10-8 cm s 1, or just 0.03-0.3 cm yr-1. 

The effect of advection and dispersion on the distribution of a chemical component within flowing groundwater is described concisely by the advection-dispersion equation. This partial differential equation can be solved subject to boundary and initial conditions to give the component s concentration as a function of position and time. 

Substituting the transport laws for advection and dispersion (Eqns. 20.11 and 20.20), and noting that groundwater velocity v is related to specific discharge q according to Equation 20.7, gives... 

Where the Peclet number has a value near one, advection and dispersion are of comparable importance. Values much greater than one signify the dominance of advection, and those less than one indicate that diffusion or dispersion dominates. In the presence of groundwater flowing at any appreciable rate, D (X],vx (from Eqn. 20.19), as already discussed, and the Peclet number becomes,... 

Now, Pe depends only on the magnitude of the dispersivity relative to the scale of observation. The Peclet number of flowing groundwater is generally greater than one, reflecting the dominance of advection, since dispersivity is invariably found to be smaller than the scale on which it is observed (e.g., Neuman, 1990). 

To see why numerical dispersion arises, consider solute passing into a nodal block, across its upstream face. Over a time step, the solute might traverse only a fraction of the block s length. In the numerical solution, however, solute is distributed evenly within the block. At the end of the time step, some of it has in effect flowed across the entire nodal block and is in position to be carried into the next block downstream, in the subsequent time step. In this way, the numerical procedure advances some of the solute relative to the mean groundwater flow, much as hydrodynamic dispersion does. 

Here, Rj is reaction rate (mol cm-3 s-1), the net rate at which chemical reactions add component i to solution, expressed per unit volume of water. As before, Q is the component s dissolved concentration (Eqns. 20.14—20.17), Dxx and so on are the entries in the dispersion tensor, and (vx, vy) is the groundwater velocity vector. For transport in a single direction, v, the equation simplifies to,... 

A possible source of groundwater contamination, which has up to now almost been neglected, is associated with the introduction of surfactants into soils as pesticide additives (Table 6.7.3). Non-ionic surfactants composed of alcohols and fatty acids are most widely recommended as adjuvants to facilitate and enhance the absorbing, emulsifying, dispersing, wetting and penetrating properties of pesticides. Other pesticide adjuvants are silicone-based surfactants,... 

Aqueous phase migration — Dissolved in groundwater and soil moisture, advection, dispersion, and diffusion and... 

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