Advection, groundwater

Bolviken and Logn (1975) and Smee (1983) include groundwater transport as a possible mechanism for element dispersion from mineralisation. Webber (1975) pointed to the much higher potential migration rate of groundwater as compared with that of diffusion or electrochemical transport and concluded that advective groundwater transport is likely to be the most important dispersal mechanism. 

Multiple pathways are a major concern since depostion of PIC would have occurred. Specific soil conditions determine attenuation rates of penta PIC leachate. Once penta reaches the water table, other transport and fate processes become important. Penta exists in two forms ionized and non-ionized. The ionized form is soluble in water, while the non-ionized form is not. The ratio of the two forms in water is dependent on the pH of the aquifer. In alkaline environments penta PIC tend to be more soluble and more susceptible to advective transport and biological decay. Half-lives of penta leachate in groundwater have been estimated ranging from 27 days to 58 years. 

Processes controlling nuclide distributions. The general equations for onedimensional advective transport along a groundwater flow path of groundwater constituents, and the incorporation of water/rock interactions, are given in such texts as Freeze and Cherry (1979). The equations can be applied to the distribution in groundwater of each isotope I with a molar concentration Iw and parent with Pw to obtain... 

Figure 2. (A.) The radionuclides in an aquifer are divided into three reservoirs groundwater, the host aquifer minerals, and adsorbed onto active surfaces. Also shown are the processes adding to a daughter nuclide (closed circles) in the groundwater of weathering, advection, recoil from decay of parent atoms ( P ) in the aquifer minerals, and production by parent decay, the processes of losses of a radionuclide of advection and decay, and exchange between dissolved and adsorbed atoms.

Figure 4. Systematics of radionuclides along the series. The major and minor fluxes to each nuclide can be readily seen from the arrows shown. The behavior of each nuclide can be evaluated by considering the surface and groundwater populations individually, or together as the mobile pool. As with the Th series (Fig. 3), there is addition to the mobile pool of nuclides produced by a decay along the series. Note, however, that steady state abundances may not be achieved for which is long-lived and may also be sufficiently mobile in the groundwater for advection to be important.

Figure 7. Sediment eontains derived both seavenged from the water eolumn during particle settling and contained in solid material. Ra produced in the sediments is highly soluble in pore waters and diffuses into the overlying water or is advected across the sediment-water interface by discharging groundwater. Rn is produced within the water column from dissolved Ra and within the underlying sediments.

MMT (32) is a 1- or 2-dimensional solute transport numerical groundwater model, to be driven off-line by a flow transport, such as VTT (Variable Thickness Transport). MMT employs the random-walk numerical method and was originally developed for radionuclide transport. The model accounts for advection, sorption and decay. 

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. 

Advective transport, or simply advection, refers to movement of chemical mass within a flowing fluid or gas. For our purposes, it is most commonly migration of aqueous species along with groundwater. In constructing a transport model, we prefer to consider how much of the thermodynamic components - the total masses of the basis entries Aw, A(, A, and Am - move, rather than track migration of the free masses of each individual aqueous species. 

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). 

Now, if the system is observed over a distance scale L, the characteristic time frrans associated with advective transport is simply the time L/vx required for groundwater to traverse distance L. The Damkohler number, the ratio of the two characteristic times,... 

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

Organic chemicals dissolved in groundwater originate from a release source, which may be point source (defined leak) or it may emanate from an area source, such as a mass of contaminated soil. Once dissolved, the chemicals disperse into the ground-water by molecular diffusion and advection (combined as dispersion), both of which are influenced by equilibrium distribution relationships with the aquifer materials. 

Illustrative Example 25.1 Darcy s Law in Groundwater Systems S Illustrative Example 25.2 Dispersion and Advection in Groundwater System S... 

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