Permeable zones aquifer

Increases in permeability caused by limestone dissolution approximately doubled the injection index (the amount of waste that can be injected at a specified pressure). As of 1974, the effects of the pressure created by the injection were calculated to extend more than 40 miles radially from the injection site.167 An updip movement of the freshwater/saltwater interface in the injection-zone aquifer, which lies less than 32 km (20 miles) from the injection wells, was also observed. 

Above the water table, groundwater can also occur in perched aquifer conditions. In these instances, groundwater occurs in relatively permeable soil that is suspended over a relatively low permeability layer of limited lateral extent and thickness at some elevation above the water table. Perched groundwater occurrences are common within the vadose zone high-permeability zones overlie low-permeability zones of limited lateral extent in unconsolidated deposits. However, perched conditions can also occur within low-permeability units overlying zones of higher permeability in both unconsolidated and consolidated deposits. In the latter case, for example, a siltstone or clay stone overlies jointed and fractured bedrock such that groundwater presence reflects the inability of the water to drain at a rate that exceeds replenishment from above. 

The saturated zone usually contains strata of varying permeabilities, forming aquifers and aquicludes, described in the following sections. 

Table 1 shows water-level measurements made at the same time in the water-table aquifer, the upper permeable zone, and the lower permeable zone. 

Table 1.—Water levels and head differences between the water table and the two permeable zones in the Upper Floridan aquifer [WT = water table UPZ upper permeable zone ...

Based on a comparison of the head differences shown in Table 1, the degree of hydraulic separation is much greater between the upper and lower permeable zones in the Upper Floridan aquifer than between the upper permeable zone and the water-table aquifer. 

The small head differential, exhibited throughout the year, between the water-table aquifer and the upper permeable zone indicates that although the residuum acts as a confining unit for the Upper Floridan aquifer, vertical leakage does occur through the base of the residuum, and there is some degree of hydraulic connection between the two hydrologic zones. 

Because the residuum water-table aquifer and the upper permeable zone are hydraulically connected, they respond rapidly (within hours) to seasonal fluctuations in precipitation. During winter and spring of 1983, the water levels in the residuum and the upper permeable zone rose in response to increased precipitation and decreased evapotranspiration. Water levels in the residuum and the upper permeable zone declined during the summer and fall as precipitation decreased and evapotranspiration increased. During late fall, decreased evapotranspiration and increased precipitation caused the water levels to rise. 

The water-table aquifer is hydraulically connected to the upper permeable zone of the Upper Floridan aquifer, and water infiltrates through the residuum to recharge the upper permeable zone. 

The effect of lower oil saturation and location of injection wells can be demonstrated by the next example of some wells in Oerrel (Figs. 11 and 14). In this unit, polymer injection started down-dip at the oil water contact. The injected solution was partly lost in the aquifer. The distance to up-dip wells was too large. It is assumed that the polymer bank was also diluted by high-salinity reservoir brine channeling through permeable zones in the middle part of the reservoir. Oil banking could only be achieved by additional injection in Well O.S. 57. The small oil banks correspond to the low oil saturation in this region. 

This system consists of an in situ polyethylene tank, an application system, and a bottom water recovery system.65 An underlying, permeable, water-bearing zone facilitates the creation of ingradient water flow conditions. The tank defines the treatment area, minimizes the potential for release of bacterial cultures to the aquifer, and maintains contaminant concentration levels that facilitate treatment. The ingradient conditions facilitate reverse leaching or soil washing and minimize the potential for outmigration of contaminants. 

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). 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

Are any pesticides being leached in significant quantities from permeable agricultural soils into the unsaturated zone of British aquifers in a similar manner to that observed for nitrate ... 

The produced water tram the oil field, alter retention in skimmer tanks, was directly disposed of into a shallow aquifer zone located at a depth of about 700 ft [210 m]. This aquifer, which has excellent permeability, is a potential source of water for future drinking-water projects in Bahrain. 

The permeability of the reactive material in the gated zone must be equal to or greater than the aquifer permeability to minimize flow restrictions. 

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