Velocity of groundwater flow,

An often quoted case study (Vogel, 1970) is that of an artesian aquifer in an area near the south coast of South Africa (Fig. 11.8). The decrease in 14C downslope from the aquifer has been taken by the researcher to indicate continuity. One can even calculate the velocity of groundwater flow in the aquifer by selecting two points on the lines of Fig. 11.8, for example, 2 km-4000 years and 18 km-28,000 years. The average flow velocity in the aquifer is... 

Application of equation 5 requires caution. In this simplistic form, the equation can be used to find only one component of fluid velocity, namely that defined by the direction over which the gradient is measured, ie, the line between two monitoring wells. In general, however, the direction of groundwater flow at a point is fully characterized by assignment of values in three mutually orthogonal directions. Figure 3 provides an example of such a situation. 

Advection is the transport of dissolved contaminant mass due to the bulk flow of groundwater, and is by far the most dominant mass transport process [2]. Thus, if one understands the groundwater flow system, one can predict how advection will transport dissolved contaminant mass. The speed and direction of groundwater flow may be characterized by the average linear velocity vector (v). The average linear velocity of a fluid flowing in a porous medium is determined using Darcy s Law [2] ... 

Conclusion. Different ions travel at different velocities, the conservative ions moving fastest. The conservative ions are most helpful for the understanding of groundwater flow. 

Mazor, E. and Nativ, R. (1992) Hydraulic calculation of groundwater flow velocity and age examination of the basic premises. J. of Hydrology 138, 211-222. 

For the barrier to prevent the movement of heavy metal cations, it follows that the hydraulic flow velocity of groundwater through the soil (vHyd) should not be greater than the sum of the electroosmotic velocity (veof) plus the electromigration velocity (vion), assuming diffusion is neglected ... 

When a tracer is injected via a drillhole into groundwater, it is subject to diffusion, dispersion, dilution and adsorption. Dispersion is the result of very small variations in the velocity of laminar flow through porous media. Molecular diffusion is negligible, unless the velocity of flow is unusually low. Even if these processes are not significant, flow through an aquifer may be stratified or concentrated along discontinuities. Therefore, a tracer may remain undetected unless the observation drillholes intersect these discontinuities. 

Th = Thickness of aquifer where groundwater flow is assumed to completely mix with recharge (ft) Vj, = Darcy velocity of groundwater (ft/yr)... 

Locational considerations include both surficial location and screened interval, ie, the sampling depth. The surficial location is selected based on whether the sample is to represent background quaUty or quaUty at the location of contamination, or potential leak location. In selecting the surficial location, the groundwater flow parameters, velocity and direction, are assumed to be known from other monitoring wells or borings already completed. 

Extrapolation of laboratory results to the field scale is at best qualitative because of the complexity of the geochemical and hydrological systems in the Bengal Basin. Movement of As-contaminated groundwater towards a well screened in the uncontaminated aquifer is likely to contain both vertical and horizontal components of flow. For modeling purposes presented here, a relatively slow interstitial groundwater velocity of 3 m/yr (Stollenwerk et al. 2007) is compared with a more rapid interstitial velocity of 30 m/yr (McArthur et al. 2008). 

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 ... 

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