LNAPL water interface

The recharge test has the advantage of yielding results regardless of the location of the mobile LNAPL-water interface in relation to the potentiometric surface. Although more accurate than a baildown test, it is also more complicated to conduct. 

A typical result of conventional pumping in low conductivity and transmissivity formations is increased, and sometimes rapid, drawdown with steep gradients, with corresponding low recovery rates. This condition limits the influence of the conventional pumping well. MPE overcomes this limiting factor with the application of a vacuum. The vacuum enhancement of MPE also can overcome the capillary forces that can trap contaminants within the capillary zone. This allows better recovery of LNAPL, which tends to accumulate in the capillary zone at the air-water interface. 

Baildown testing is a widely used field method to evaluate the actual thickness of LNAPL product in a monitoring well. Baildown testing involves the rapid removal of fluids from the well, and subsequent monitoring of fluid levels, both the water level or potentiometric surface (oil-water interface) and NAPL level (oil-air interface), with time. Such testing was originally used as a preliminary field method to evaluate recoverability of NAPLs and thus to determine potential locations for recovery wells. All monitoring wells at a site that exhibited a measurable thickness of LNAPL were typically tested. Whether or not all the LNAPL product could be... 

Baildown tests have been used for decades during the initial or preliminary phases of LNAPL recovery system design to determine adequate locations for recovery wells and to evaluate recovery rates. Baildown tests involve the rapid removal of fluids from a well with subsequent monitoring of fluid levels, both the LNAPL-water (or oil-water) interface and LNAPL-air (or oil-air) interface, in the well with time. Hydrocarbon saturation is typically less than 1, and commonly below 0.5, due to the presence of other phases in the formation (i.e., air and water). Since the relative permeability decreases as hydrocarbon saturation decreases, the effective conductivity and mobility of the LNAPL is much less than that of water, regardless of the effects induced by increased viscosity and decreased density of the LNAPL. 

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. 

As more LNAPL enters the soils, it increases the pressure on the moving front, which allows increasing displacement of water. If a sufficient quantity of sufficiently mobile oil is available, it can produce a pressure head large enough to displace water through the funicular zone and to form a distinct interface at the top of the capillary fringe. 

The value of S0 decreases with increasing elevation. Zao, the interface between air and the LNAPL phase, may or may not coincide with Zu, the upper boundary of the aquifer. Typically, the saturation of the LNAPL phase extends over two distinct regions (see Figure 5.10). These are (1) water and LNAPL phase zone, and (2) water, LNAPL phase, and air zone. When a single homogeneous stratum is considered, O can be assumed constant. In a stratified medium, however, saturation discontinuities generally exist due to the variation in soil characteristics, and the determination of LNAPL volume based on Equation 6.22 may become much more involved. 

The two pumps within each recovery well are controlled by a series of electrodes that are positioned at predetermined levels within the well. The water pump utilizes a power interrupter probe to detect free hydrocarbon. This probe is positioned above the water intake and is adjusted to turn off the pump automatically when the hydrocarbon interface approaches the pump intake. This prevents the lower pump from accidentally pumping LNAPL to the injection wells. 

When the amount of product released is large relative to the volume of available soil, the downward migration of bulk product ceases as water-saturated pore spaces are encountered. If the density of the bulk product is less than that of water, the product tends to "float" along the interface between the water saturated and unsaturated zones and spread horizontally in a pancake-like layer, usually in the direction of groundwater flow. Almost all motor and heating oils are less dense than water (Knox 1993 Mackay 1988) and are referred to as LNAPLs. 

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