Ground water velocity

Typically, steeper slopes signify higher ground-water velocity. 

Nonequilibrium transport of solutes through porous media occurs when ground-water velocities are sufficiently fast to prevent attainment of chemical and physical equilibrium. Chemical reactions in porous media often require days or weeks to reach equilibrium. For example. Fuller and Davis Q) reported that cadmium sorption by a calcareous sand was characterized by multiple reactions, including a recrystallization reaction that continued for a period of days. Sorption of oxyanions by metal oxyhydroxides often occurs at an initially rapid rate the rate then decreases until steady-state is achieved (2-4). Unless ground-water velocity in such a situation is extremely slow, nonequilibrium transport will occur. 

Information about rate-controlling mechanisms that could occur in the field often can be obtained from carefully performed laboratory column experiments. The asymmetric shape of breakthrough curves from column experiments done using field ground-water velocities can indi-... 

Vjj average linear ground-water velocity pardlel to ground-water flow... 

The velocity of contaminants may also be influenced by the diffusion of contaminants into dead space pores or fractures (where the ground water velocity is essentially negligible) or within grains. These processes may become significant in attenuating the movement of contaminants at sites where soil and rocks have been exposed to contamination for long periods. 

Sugisaki, R., Measurement of effective flow velocity of ground water by means of dissolved qases, American J. Sci.. 259. 144-153 (1961). 

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). 

Dispersivity is a property of the porous medium that has been shown to be proportional to the scale of the system under consideration [4]. Thus, the dispersivity of a porous medium in a laboratory column will be considerably smaller than the dispersivity of an aquifer through which contaminated ground-water is flowing over distances of hundreds of meters. Presumably, this scale-effect is due to the increased spreading caused by variations in velocities due to larger scale heterogeneities [2]. 

The intimate mixing of oxidizer and fuel in emulsions give these explosives much higher detonation velocities when compared to ANFO. For example, in 150 mm diameter PVC ANFO has a velocity of about 4000 m/sec, and a sensitized emulsion would have a velocity closer to 6000 m/sec at a density of 1.20-1.25 g/cc. Also, the layer of oil surrounding each oxidizer solution droplet protects the emulsion from extraneous water intrusion and subsequent deterioration of the explosive. Many studies have shown that when mining operations use emulsion explosives rather than ANFO, which has basically no water resistance, the amount of nitrate salts in mine ground water is reduced considerably. This can be a very important factor in today s environmentally conscious mining and explosives industry. 

Example 9.11. Retention of Hydrophobic Organic Pollutants in Ground-water Aquifer Because of a leak in a waste container, traces of tetrachlor-ethylene and 1,2,4,5-tetrachlorobenzene escape in a groundwater. Provide an estimate of how long it takes to find these pollutants at a distance (direction of flow) of 25 m. The flow velocity of the groundwater is 2 m day . The following characteristics of the aquifer are available ... 

The concentration of Fe in pore water from core 1 was 50 mg/L, and the concentration in pore water from core 2 was 32 mg/L (Table 2). Chemical analyses measured only Fe(II) Fe(III) was below the detection limit. The concentration of Fe(II) in leachate from the contaminated cores rapidly decreased as Fe-free uncontaminated ground water displaced the contaminated pore water (Figs. 8a-8b). Within a few pore volumes, Fe(II) concentrations were less than 5 mg/L. For the remainder of the experiments, Fe(II) concentrations deaeased at a much slower rate. The low concentrations (<5 mg/L) measured in the first 25 pore volumes of leachate coincided with higher flow velocities and measurable O2 concentrations. Based on the mass of Fe(II) in leachate from core 1, the concentration of reactive organic carbon necessary to reduce Fe(III) to Fe(II) was about 2% of the total organic carbon. 

If the maximum difference in chloride concentration between ground-waters from bedrock and drift aquifers is equal to 200 mg/1 = 5.6310 mol/1 (Table I), and the distance travelled is the mean saturated thickness of the drift, or 1500 cm, and the appropriate diffusion coefficient of NaCl at 25°C is 1.576 10" cm s (Robinson and Stokes, 1955), is calculated to be 5.91 10 molcm" s . This calculation, indicates that molecular diffusion is insignificant in magnitude when compared with the velocity of groundwater. It therefore appears that mechanical mixing in the lower drift-upper bedrock aquifer is the most important cause of dispersion. 

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