Cone of depression

The figure also show how pumping of groundwater will create a cone of depression around the well. The size and shape of this cone is mainly related to the pumping rate and the hydraulic conductivity of the aquifer. 

If the test has proceeded till a steady state (cone of depression fully developed), the leakage factor as well as the storage coefficient can be determined. These factors are less important but may be critical when it comes to modelling... 

The hydraulic impact shown as cones of depression and uplift around the wells. 

The pumping well method is more suitable for a water table that is too deep for the trench method. Pumps draw water, forming a cone of depression in the water table to control the movement of floating gasoline. The gasoline is then pumped out. The pumps can be either single- or a dual-pump... 

The dual-pump system is used when a large amount of gasoline is to be recovered. Separate gasoline and water pumps are used. The dual-pump system significantly reduces the amount of water that must be treated. Water pumps are placed at a depth lower than the water table to be able to establish a cone of depression, and the gasoline pumps draw out the gasoline that floats into the depression on the top of distorted water table for product recovery (Figure 18.6b). 

Dual-pump systems are better able to control a constant cone of depression than the singlepump system. It is important to maintain a nearly constant cone of depression to prevent the migration of the gasoline plume. If a constant cone of depression is not maintained, the water table and... 

The cone of depression in a dual-pump system is controlled by a detection probe. Initially, the probe is set in the well at the depth of the proposed cone of depression of the gasoline-water interface. The water pump draws the water table down, reaching the pump probe. The water pump ceases... 

Installation of the pumping well is more time consuming than digging a trench. There is a lag period between the start of pumping, the formation of the depression cone, and containment of the plume. This limits its use as a rapid containment measure. The water table depression must be kept constant otherwise, if the water table is allowed to fluctuate, gasoline droplets may adhere to soil particles and get trapped below the water table, especially when the depth of the cone of depression gets lower. 

FIGURE 3.20 Trace of half a cone of depression showing variations in drawdown with distances from a pumping well. 

The recovery most often used at sites with significant quantities of recoverable LNAPL is the two-pump system. A submersible water pump installed below the lowest possible probable interface level is used to create a drawdown cone of depression, while a second pump is suspended with its intake port located at the oil-water interface. A typical two-pump system installation is shown in Figure 7.15. 

The pumping rate of the lower pump is adjusted to produce sufficient water to cause a cone of depression extending outward to intercept and retrieve the LNAPL. While the water pump operates continually, the upper product pump cycles on and off as necessary to recover the product as it accumulates. 

Extraction wells should be located where hydraulic control is achieved at the boundary of the contaminant plume. Cones of depression created by recovery wells should intersect so that the overall hydraulic gradients ensure capture and recovery. 

Determination of initial recovery well (or trench) locations is an important design parameter. Floating LNAPL product tends to move in the direction of overall ground-water flow, as determined by the water table gradient. As a well or trench is pumped, the fluids (water and/or oil) migrate toward the area of lower pressure to fill the void. A cone of depression develops that extends outward. The fluid surface exhibits a rapid slope near the well, diminishing to a very low gradient at a distance. 

Recovers VOCs from within the cone of depression created by pumping of the aquifer, where pump-and-treat technologies are normally ineffective. 

When water is pumped from a production or remediation well, water from the surrounding aquifer enters the well in response to the head gradient created by the water removal. This leads to a lowering of the hydraulic head around the well, forming a cone of depression (Fig. 3-10). It takes some time for this cone of depression to fully develop while it is developing, the flow of water must be analyzed by transient techniques that account for flow changes over time (see Section 3.2.4). Ultimately, the water removed by a well must be replaced—in an unconhned aquifer, this is usually by rainwater percolation or by inflow from a river—or else the well will go dry. Wells often are... 

In designing a production or remediation well for an aquifer, it is important to be able to predict total drawdown in the aquifer. Theoretically, in an idealized aquifer having unlimited extent and no recharge, the cone of depression advances outward to infinity. Thus, steady-state analysis is approximate and useful only after pumping has occurred for some time. Steady-state drawdown at any given radius rl from the well, relative to drawdown at another radius r2, can be determined by integrating Eq. [3-7b],... 

FIGURE 3-27 Typical schemes to recover a floating NAPL, such as gasoline, from a phreatic aquifer. In each case a cone of depression is created by pumping to encourage the NAPL to flow toward the collection point. Either a NAPL/water separator or two pumps are required. 

Damage water resources by cutting down water quantity because of cone of depression s presentence and contaminating surface and underground water resources by liquid (solid) permeation and leaching pollution... 

Cone of depression or exhaustion deveioped around a pumped weii in an unconfined aquifer. Q=nk hP-h )l nRlr). Q=quantity /<=coefficient of permeabiiity. 

Critical Mineral depletion, formation of guUies, occurrence of cones of depression, karst, suffosion, dewatering of soils, ground removal, humus decay, soil erosion, chemical pollution, changes of water level, projective cover fragmentation Industrial pollution, chemical pollution... 

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