Velocity, rate groundwater

Observations the mass-balance models can help the user to determine and quantify the geochemical processes that are most important to the chemical evolution of a water, whether natural or contaminated. With sufficient information (e.g., isotopic data) the models can provide information on the hydrogeology, including groundwater flow paths and velocities. If groundwater velocities are known, the models can determine reaction rates. Use of these models can help to identify critically needed data or measurements, such as identification of the mineralogy or isotopic compositions, etc. 

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,... 

Retardation also arises when a fluid undersaturated or supersaturated with respect to a mineral invades an aquifer, if the mineral dissolves or precipitates according to a kinetic rate law. When the fluid enters the aquifer, a reaction front, which may be sharp or diffuse, develops and passes along the aquifer at a rate less than the average groundwater velocity. Lichtner (1988) has derived equations describing the retardation arising from dissolution and precipitation for a variety of reactive transport problems of this sort. 

A notable aspect of this equation is that L appears within it as prominently as the rate constant k+ or the groundwater velocity vx, indicating the balance between the effects of reaction and transport depends on the scale at which it is observed. Transport might control fluid composition where unreacted water enters the aquifer, in the immediate vicinity of the inlet. The small scale of observation L would lead to a small Damkohler number, reflecting the lack of contact time there between fluid and aquifer. Observed in its entirety, on the other hand, the aquifer might be reaction controlled, if the fluid within it has sufficient time to react toward equilibrium. In this case, L and hence Da take on larger values than they do near the inlet. 

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). 

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. 

This expression describes the fastest and most important mode of transport in groundwater. In fact, an important task of the hydrologist is to develop models to predict the effective velocity u (or the specific flow rate q). Like the Darcy-Weis-bach equation for rivers (Eq. 24-4), for this purpose there is an important equation for groundwater flow, Darcy s Law. In its original version, formulated by Darcy in 1856, the equation describes the one-dimensional flow through a vertical filter column. The characteristic properties of the column (i.e., of the aquifer) are described by the so-called hydraulic conductivity, Kq (units m s"1). Based on Darcy s Law, Dupuit derived an approximate equation for quasi-horizontal flow ... 

The reactive transport of contaminants in FePRBs has been modeled using several approaches [179,184,186,205-208]. The simplest approach treats the FePRB as an ideal plug-flow reactor (PFR), which is a steady-state flow reactor in which mixing (i.e., dispersion) and sorption are negligible. Removal rates (and therefore required wall widths, W) can be estimated based on first-order contaminant degradation and residence times calculated from the average linear groundwater velocity [Eq. (27)]. The usefulness of... 

One of the simplest forms of the ADE that has been applied to an FePRB includes both dispersion and sorption [205]. A one-dimensional steady-state ADE was used to estimate W for 1000-fold reduction in contaminant concentrations at a groundwater velocity of 1 ft day-1. Applying the model to chlorinated aliphatic compounds (using rate coefficients summarized in Ref. 86) gave the results shown in Fig. 11. These estimates,... 

The physical properties of soils and sediments are particularly important in determining infiltration rates in soils as well as groundwater flow velocities. One feature of particular importance is the void space or porosity of soils, rocks, and sediments. Water is capable of moving from one void space to another in these materials thereby allowing the flow of water. Total porosity is defined mathematically by the equation ... 

Mathematical models for mass transfer at the NAPL-water interface often adopt the assumption that thermodynamic equilibrium is instantaneously approached when mass transfer rates at the NAPL-water interface are much faster than the advective-dispersive transport of the dissolved NAPLs away from the interface [28,36]. Therefore, the solubility concentration is often employed as an appropriate concentration boundary condition specified at the interface. Several experimental column and field studies at typical groundwater velocities in homogeneous porous media justified the above equilibrium assumption for residual NAPL dissolution [9,37-39]. 

Numerous empirical correlations for the prediction of residual NAPL dissolution have been presented in the literature and have been compiled by Khachikian and Harmon [68]. On the other hand, just a few correlations for the rate of interface mass transfer from single-component NAPL pools in saturated, homogeneous porous media have been established, and they are based on numerically determined mass transfer coefficients [69, 70]. These correlations relate a dimensionless mass transfer coefficient, i.e., Sherwood number, to appropriate Peclet numbers, as dictated by dimensional analysis with application of the Buckingham Pi theorem [71,72], and they have been developed under the assumption that the thickness of the concentration boundary layer originating from a dissolving NAPL pool is mainly controlled by the contact time of groundwater with the NAPL-water interface that is directly affected by the interstitial groundwater velocity, hydrodynamic dispersion, and pool size. For uniform... 

As stated in section 2.10, the velocity by which groundwater flows is commonly calculated from the water table gradient and the coefficient of permeability (k, or the related parameter of transmissivity). The k value is determined by a pumping test. During such a test a studied well is intensively pumped and the water table is monitored in it as well as in available adjacent observation wells. The change in water table level as a function of the pumping rate serves to compute the aquifer permeability. 

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