Groundwater retardation factor

The migration rate of a groundwater constituent, relative to the groundwater flow rate, is controlled by the retardation factor, where Ri = 1 + Ki. Where Ki 1 (e.g., for Th and Ra), Ri Ki, and Iads + Iw = IwRi- Note that ki and k-i are element-specific but not isotope-specific. All isotopes that decay slower than desorption, so that k-i have a value of Ki that is equal to that of a stable isotope (Eqn. 3). The value of Ki may be lower for the shortest-lived nuclides (see Fig. 2b), and so a series of equations derived from Equation (3) applied to different isotopes of the same element may be used to obtain absolute values for the separate rate constants. 

Behavior of Ra in groundwater. The general behavior of Ra has been examined under laboratory conditions and in various environments (see Osmond and Cowart 1992). A major goal of field studies of Ra isotopes have aimed at obtaining bulk, in situ values of adsorption rates and so the retardation factors. Note that Ba serves as a very close chemical analogue to Ra but is typically 10 times more abundant, and so its behavior is related to that of Ra. 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

Krishnaswami S, Graustein WC, Turekian KK, Dowd F (1982) Radium, thorium, and radioactive lead isotopes in groundwaters application to the in-situ determination of adsorption-desorption rate constants and retardation factors. Water Resour Res 6 1663-1675 Krishnaswami S, Bhushan R, Baskaran M (1991) Radium isotopes and Rn in shallow brines, Kharaghoda (India). Chem Geol (Isot Geosci) 87 125-136 Kronfeld J, Vogel JC, Talma AS (1994) A new explanation for extreme " U/ U disequilibria in a dolomitic aquifer. Earth Planet Sci Lett 123 81-93... 

Lesser retardation factor (slowing of migration with groundwater due to sorption to aquifer matrix). 

In simple cases, the mobility in the subsurface of a sorbing contaminant can be described by a retardation factor. Where contaminated water passes into a clean aquifer, a reaction front develops. The front separates clean, or nearly clean water downstream from fully contaminated water upstream. Along the front, the sorption reaction removes the contaminant from solution. The retardation factor describes how rapidly the front moves through the aquifer, relative to the groundwater. A retardation factor of two means the front, and hence the contamination, will take twice as long as the groundwater to traverse a given distance. 

Table 4.7 Estimated mean Retardation Factors for Organic Substances in the River-Groundwater Infiltration System of the Glatt River (Modified from Schwarzenbach et al., 1983)...

Nonionizable chemicals (e.g., hydrocarbons, ethers, alcohol) that sorb onto organic materials in an aquifer (i.e., organic carbon) are retarded in their movement in groundwater. The sorbing solute travels at linear velocity that is lower than the groundwater flow velocity by a factor of Ra, the retardation factor. If the Koc of a compound is known, the retardation factor may be calculated using the following equation from Freeze and Cherry (1974) for unconsolidated sediments ... 

So we deduce that only one DMB molecule out of 11 will be in the moving ground-water at any instant (Fig. 9.6). This result has implications for the fate of the DMB in that subsurface environment. If DMB sorptive exchange between the aquifer solids and the water is fast relative to the groundwater flow and if sorption is reversible, we can conclude that the whole population of DMB molecules moves at one-eleventh the rate of the water. The phenomenon of diminished chemical transport speed relative to the water seepage velocity is referred to as retardation. It is commonly discussed using the retardation factor, Rfi, which is simply equal to the reciprocal of the fraction of molecules capable of moving with the flow at any instant, ff (see Chapter 25). 

Consider the transport of phenanthrene in an aquifer exhibiting a porosity, 0, of 0.2, and an average density, ps, of the aquifer material of 2.5 kg solid L l. Furthermore, assume that the average organic carbon content of the aquifer material is 0.5% (i.e.,foc = 0.005 kg oc -kg-1 solid). Calculate the retardation factor Rn. see Section 9.2) of phenanthrene in this aquifer if the groundwater... 

This result implies that the TNT will move through the subsurface at a rate that is 1/30 the rate of the groundwater velocity. You also note that if the TNT concentrations anywhere in the plume are above 1 x 10 6 M, then the AiXNTd llll[e would be smaller (second term in the denominator of Eq. 11-20 won t be negligible) and the retardation factor will correspondingly decrease. 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

All chemical reactions comprise at least two species. For models of transport processes in groundwater or in the unsaturated zone reactions are frequently simplified by a basic sorption or desorption concept. Hereby, only one species is considered and its increase or decrease is calculated using a Ks or Kd value. The Kd value allows a transformation into a retardation factor that is introduced as a correction term into the general mass transport equation (chapter 1.1.4.2.3). 

Measured batch values can be used to calculate a retardation factor (R), which describes the ratio of the groundwater velocity to the velocity of radionuclide movement vp. 

System assumptions that should be valid for such applications include fluid flow in the porous media is isotropic and adsorption is fast, reversible, and linear (cf. Freeze and Cherry 1979). Given these constraints, the comparative transport of a conserved (nonadsorbed) tracer, such as Br , and an adsorbed or retarded species, such as Am, can be described as shown in Fig. 10.29. A comparison of migration distances of the two species after time t, is made at concentrations where C(measured)/Co(initial) = 0.5 for the conserved and adsorbed species. The migration distance X of the conserved species after time r is a measure of the average groundwater velocity (U), or X = vt. Similarly, the migration distance of the adsorbed species (X,) i related to its velocity of movement (v ) by Xf = vj. The retardation factor (/tj for the adsorbed species is then given by... 

Retardation Factor. Reported distribution coefficients for aldicarb range from 0.16 - 0.073 ml/gm (20), or are effectively equal to zero, suggesting that aldicarb moves at the same speed as the groundwater. In order to eliminate the distribution coefficient as a variable subject to sensitivity analysis, it was set equal to zero for all simulations. A distribution coefficient of zero is equal to a retardation factor of one. 

The relationship between the groundwater flow velocity and the transport speed of the compound is described by the retardation factor R. A compound which is not held back at all, moves with the same speed as water and there-... 

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