Permeability groundwater flow

Subsurface drains are essentially permeable barriers designed to intercept the groundwater flow. The water must be collected at a low point and pumped or drained by gravity to the treatment system (Figure 8). Subsurface drains can also be used to isolate a waste disposal area by intercepting the flow of uncontaminated groundwater before it enters into a contaminated site. 

Figure 6-7 illustrates the runoff paths for HOF and SOF, as well as for subsurface stormflow and groundwater flow. Subsurface stormflow is a moderately rapid runoff process in which water flows to a stream through highly permeable surface soil layers (without reaching the water table). Note in Fig. 6-7 that while HOF and subsurface stormflow may occur over a large fraction of an infiltration-limited hillslope, SOF occurs over a smaller portion adjacent to the stream. 

Pumping during well development performs two important functions. First, pumping removes the materials from the borehole left behind by drilling. Second, as the water in the well is removed, groundwater flow velocity from the surrounding formation increases when it reaches the higher permeability filter pack around the well... 

Subsurface barriers, low-permeability cutoff walls or diversions below ground are used to contain, capture, or redirect groundwater flow. The most common method uses bentonite slurry... 

Grouted barriers use a variety of fluids injected into a rock or soil mass, which is set in place to reduce water flow and strengthen the formation. Grouted barriers are seldom used for containing groundwater flow in unconsolidated materials around hazardous waste sites because they cost more and have lower permeability than bentonite slurry walls. Nevertheless, they are suited to sealing voids in rock for waste sites remediation. 

The wastes are injected into the lower part of the carbonate Floridan aquifer, which is extremely permeable and cavernous. The natural direction of groundwater flow is to the southeast. The confining layer is 45 m (150 ft) of dense carbonate rocks. The chloride concentration in the upper part of the injection zone is 1650 mg/L, increasing to 15,800 mg/L near the bottom of the formation.172 The sources used for this case study did not provide any data on the current injection zone. The native fluid was basically a sodium-chloride solution but also included significant quantities of sulfate (1500 mg/L), magnesium (625 mg/L), and calcium (477 mg/L). 

Even where it is not occluded, the mineral surface may not be reactive. In the va-dose zone, the surface may not be fully in contact with water or may contact water only intermittently. In the saturated zone, a mineral may touch virtually immobile water within isolated portions of the sediment s pore structure. Fluid chemistry in such microenvironments may bear little relationship to the bulk chemistry of the pore water. Since groundwater flow tends to be channeled through the most permeable portions of the subsurface, furthermore, fluids may bypass many or most of the mineral grains in a sediment or rock. The latter phenomenon is especially pronounced in fractured rocks, where only the mineral surfaces lining the fracture may be reactive. 

Carbonate rocks consist mostly of calcite and dolomite with minor amounts of clay. The porosity of carbonate rocks ranges from 20 to 50%, but in contrast to sandstone, it tends to decrease with depth. Often, carbonate rocks are fractured, providing a permeability that is much greater than the primary one. In some cases, initial small-scale fractures in calcite and dolomite are enlarged by dissolution during groundwater flow, leading to an increase in rock permeability with time. 

According to information from the U.S. Department of Energy (DOE), the main limitations of synthetic barriers are the potential for the barrier to leak at the seams, depth limitations, and increased costs in areas with high concentrations of boulders and cobbles. Another limitation is that the use of barriers often relies on the presence of an aquitard. For the purposes of this discussion, an aquitard is a region of low permeability that acts as a barrier to groundwater flow. If no aquitard is present at the base of the installed barrier, groundwater can simply flow under it. 

Creates both vertical and horizontal groundwater flow, allowing penetration of low-permeability horizontal layers. 

A Waterloo Barrier was installed to a depth of 32 ft at Canadian Forces Base Borden in Ontario, Canada. The sheet piles interlocked to form a cell that was 18 ft long and 5 ft wide. The joints were sealed with a bentonite-base sealant. The barrier was used to control groundwater flow to allow for the installation of a permeable reactive barrier (PRB). After the PRB was installed, the Waterloo Barrier was removed and treatment began. The installation costs for the PRB were 30,000. This total included the installation and removal costs for the Waterloo Barrier but excluded the costs for labor and the reactive material used in the PRB (D21297F, p. 33). 

Groundwater Flow. 2. Effect of Water-Table Configuration and Subsurface Permeability Variation, Water Resources Res. (1967) 3 (2), 623-634. 

Flow and diffusion transport dissolved and mobile particulate arsenic in groundwater. The flow velocity (speed and direction) of groundwater is largely controlled by changes in the elevation of the water table with lateral distance, water pressure and density, and the permeability and other properties of the aquifer. In some circumstances, temperature gradients may also affect groundwater flow (Freeze and Cherry, 1979), 25. 

Permeable reactive barrier An in situ remediation wall, membrane, or layer, which is installed in the subsurface to intercept and remove contaminants from groundwaters flowing out of landfills, mines, or other sites. Once contaminants come into contact with the barriers, they may be sorbed, undergo ion exchange, biodegrade, precipitate, coprecipitate, or filter out. 

A closer look at the zone of lateral base flow (overflow). Overflowing water flows laterally by the critical angle. This angle is determined by the water viscosity, which in turn is dependent on the temperature and concentration of dissolved ions. Groundwater flows laterally toward the terminal base of drainage at a critical angle that is determined by the hydraulic conductivity, or permeability, of the rocks (k) the water viscosity, which depends on the temperature (7), and the concentration of dissolved ions (i) ... 

Fig. 3.8 A fault has placed a high conducting aquifer against an igneous rock of low permeability. Water ascends along the fault zone, forming a line of springs. Only part of the faults are open and conduct groundwater flow others are clogged by compression and/or mineralization.

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. 

Characterization of the natural setting is usually a major portion of the field investigation. At most sites, permeability of the local soil and rock types, the depth of the water table, and the direction of groundwater flow will strongly influence movement of contaminants from the point of disposal. The anomalies which occur naturally within the geohydrologic section must be taken into consideration. Surface drainage, sewers, and buried utilities can affect surface and groundwater flow around a hazardous waste site. 

A further demonstration of the importance of fundamental properties of both pollutants and water bodies is provided by the behaviour of chemicals upon reaching a groundwater aquifer. Soluble chemicals, such as nitrate, move in the same direction as groundwater flow. A poorly soluble liquid which is less dense than water, such as petrol, spreads out over the surface of the water table and flows in the direction of the groundwater. Poorly soluble liquids which are denser than water, such as various chlorinated solvents, sink below the water table and may flow separately along low permeability layers encountered at depth in the aquifer and not necessarily in the same direction as that of the overlying groundwater. ... 

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