Groundwater table

There are two kinds of aquifers. If the groundwater stands in direct contact with the atmosphere as in the figure above, the aquifer is regarded as unconfined. If, on the other hand a permeable formation below the groundwater table is covered by a less permeable layer, the aquifer is regarded as confined, see Figure 33. 

Figure 32. The saturated zone defines the groundwater table...

Figure 36. Variations of the groundwater table in UK, compared to normal values...

The principal variation of the undisturbed groundwater table over a series of years is shown in Figure 36. This natural variation is mainly climate dependent. Most important is the precipitation and under what season the precipitation occurs. Normally, most of the recharge take place under non growing seasons in west European climate zones. In arid climate zones the recharge take place more occasionally and then combined with temporary cold and rainy conditions. 

Groundwater table gradient (natural flow direction). 

During the test the groundwater table is monitored and the drawdown cone around the pumping well is established. From these drawdown data the hydraulic properties of the aquifer can be analyzed as shown in Figure 37. 

The figure illustration shows occurrence of clogging by plotting data from an observation well (OW) and compare that to the production well (PW). In this case the production well shows a decreased specific capacity while the observation well shows a steady level versus time. The only explanation is then that the resistance for water to enter the production well is increasing. The increased resistance will lower the drawdown inside the well, while the groundwater table outside the well is kept constant. This will increase the hydraulic gradient (the driving force) between the well and the aquifer and hence maintain a constant flow rate. 

False clogging sometimes occurs. Such events are either explained by a general lowering of the groundwater table or by failure of the submersible... 

Lower groundwater table than assumed in the design. 

Subsurface drains function like an infinite line of extraction wells, and can be used to contain and remove a plume or to lower the groundwater table (Figure 16.12). They are more cost-effective than pumping for shallow contamination problems at depths of less than 12 m (40 ft). Depths may be increased if the site is stable, if the soil has a low permeability, and if no rock excavations are encountered. 

Shallow depths to groundwater or fluctuations in groundwater table can cause upwelling and interference with airflow Off-gas typically requires treatment... 

MPE provides several advantages when compared with the use of SVE or pump-and-treat alone. MPE provides for an increase in groundwater recovery rates, an increase in ROI in individual ground-water recovery wells, and recovery of shallow free product. By depressing the groundwater table in the vicinity of the extraction wells, MPE provides for remediation of the capillary fringe and smear zone, and remediation of volatile, residual contaminants located above and below the water table.46... 

In some configurations, the vacuum used in MPE increases the effective drawdown of ground-water (i.e., the increase or lowering of the depth of the groundwater table) locally near the pumped well. This has the effect of increasing exposed soil in the saturated zone and the removal of volatile contaminants located above and below the original water table. 

We can compare the Eh measured for the Morro do Ferro groundwater (Table 7.2) with the Nernst Eh values (Eqn. 7.1) given by the reactions for dissolved oxygen and iron oxidation, as reported in the program output ... 

Harrison (1990) proposed that the diagenetic alteration observed in the Latrobe group resulted from the mixing within the formation of two types of groundwaters. Table 25.1 shows analyses of waters sampled from two oil wells, which she took to be representative of the two water types as they exist in the producing areas of the basin. 

The results of indirect and direct investigations were constantly validated against one another and reinterpreted where necessary on the basis of reciprocal results, and field / laboratory data, regarding both information in the sub-surface (e.g., state and degree of fracturing of bedrock, lithological variations, groundwater table level, salinity, seawater intrusion) and its contamination (e.g., presence of LNAPL). 

Plume No. 1 (areas 1, 3 and 6 in Figure 1) is located in low permeable silty clay deposits to 26m below ground surface (bgs). The groundwater table is 6m bgs and the hydraulic conductivity, k, is 3 x 10 m/s. Concentrations of chlorinated solvents vary from 1 Omg/1 to free phase. 

Plume No. 3 (areas 4 and 5) is located in low permeable, partially fractured clay from 0 - 10m bgs (k = 4 x 10" m/s), over a layer of sand and gravel (k = 5 x 10" m/s) down to 18m bgs, and underlain by clay. The groundwater table is at 1 Om bgs in the sand and gravel. In areas where this layer is absent, only perched groundwater occurs, at depths of between 2 and 12m bgs. Contamination was found throughout the whole profile, in concentrations between 15mg/l to free phase, with most of the free phase found in the upper clay layer. 

For example, if an excavation needs to be performed, ground water is always an issue. In order to remove the most concentrated mass of soils, one may have to dig below the groundwater table. If you are continually dealing with groundwater in an excavation, more often than not, your excavation bottom below the surface may be a highly liquid soup of... 

Aqua-Sparg is an in sitn technology for the treatment of gronndwater contaminated with volatile organic componnds (VOCs). The technology introduces pressurized air below the groundwater table to volatilize contaminants. 

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