Groundwater flow rates

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. 

Dyes or tracers may be introduced into the aquifer to measure the actual groundwater flow rate, or indicate the dispersivity that can be expected. 

High groundwater content increases processing costs, and high groundwater flow rates may prevent the technology from operating efficiently. 

In the simplest case, groundwater-flow rates for lakes at isotopic steady state (or those with relatively long hydraulic-residence times) can be estimated from data on average annual precipitation rates average annual evaporation rates the isotopic compositions of precipitation, lake water, and inflowing ground-water and relative humidity and lake temperature. 

The isotope mass-balance method is not as useful for estimating groundwater-flow rates for groundwater-poor lakes as it is for lakes that receive substantial quantities of groundwater. Solute tracers, such as dissolved calcium, may be useful in assessing... 

At high-groundwater flow rates (kf k+), these expressions reduce to C = k CJkf, and R = k+Q, so that a maximum solution rate is reached, independent of flow rate. At the opposite extreme of slow groundwater flow kf 0), C = C, and R = kfC,. Saturation is attained and the rate of dissolution is controlled by the groundwater flow rate. In other words, at high flow the dissolution rate is surface-reaction controlled. The slower process is rate limiting. 

It is important to note that Equations (10.2) and (10.3) are based on the local equilibrium condition which assumes that reaction rates are fast in relation to groundwater flow rates (Cherry et al., 1984). Other assumptions in Equation (10.2) include the reversibility of reaction and the absence of competing species for the same surface sites. 

The residual inventory, or excess , is then divided by the concentration of the tracer in the discharging groundwater to calculate the groundwater flow rate. 

Fig. 6. Equilibration distances calculated for albite at different groundwater flow rates and fracture apertures in a hypothetical parallel plate fracture. Distances were calculated at 25°C and neutral pH in pure water using a transition-state theory rate equation and experimental data from Knauss Wolery (1986).

Finally, it is of note that the most recent Swiss HLW safety assessment (Kristallin-1) assumes that the near-field will provide the main constraint on radionuclide release and transport and the main role of the geosphere is to provide a suitable environment for near-field longevity and performance. The three most important safety features provided by the geosphere are ... mechanical protection, adequate geochemical conditions and sufficiently low groundwater flow rates (Nagra 1994a). Precisely how these features impact on the chemical containment of the waste will be discussed below. 

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