LNAPL decrease

Hot water injection via injection wells heats the soil and groundwater and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery. 

Residual hydrocarbon saturation will exist within both the unsaturated and capillary zone through which the LNAPL phase migrated. As might be expected, residual hydrocarbon saturation tends to be higher as the grain size decreases and... 

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 recoverability of hydrocarbon from the subsurface refers to the amount of mobile hydrocarbon available. Hydrocarbon that is retained in the unsaturated zone is not typically recoverable by conventional means. Additional amounts of hydrocarbon that are unrecoverable by conventional methods include the immobile hydrocarbons associated with the water table capillary zone. Residual hydrocarbon is pellicular or insular, and is retained in the aquifer matrix. With respect to recoverability, residual hydrocarbon entrapment can result in volume estimate discrepancies as well as decreases in recovery efficiency. With increasing water saturation, such as when the water table rises via recharge or product removal, hydrocarbons essentially become occluded by a continuous water phase. This results in a reduction of LNAPL and product thickness as measured in the well at constant volume. When water saturation is decreased by lowering the water table (as during recovery operations), trapped hydrocarbons can remobilize, leading to increased recoverability. 

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 number of wells manifolded to an individual pump primarily reflects the depth to groundwater, but is also in part dependent on the pump size. In low-permeability formations, wells are typically pumped dry. As long as the pumps sustain sufficient vacuum to all wells at the intake depth, LNAPL eventually enters the well and is evacuated. In higher-permeability formations, fluid is pumped consistently from each well at a maximum rate. In both low- and high-permeability formations, individual pumps work harder to pump from greater depths. Decreasing... 

The primary disadvantage is that if these bailers are adjusted to remove only the LNAPL layer, they do not create a drawdown cone. Therefore, unless the product layer is relatively thick and quite fluid, the recovery rate tends to be slow. Also, use of bailers in deeper wells requires significant bailer travel time, which can decrease the effectiveness. 

Effective operation of the bioslurp system manages the problem of diminishing returns. When liquid recovery declines due to decreased saturation, the percentage of the recovered flow is vapor. The airflow to the well can be continued until water and/or LNAPL again enters the well. Or the dip tube can be raised or lowered to... 

FIGURE 12.3 Apparent LNAPL thickness decreases with time upon initiation of recovery operations. 

The lateral extent of the dissolved hydrocarbon plume has continued to decrease, although no significant decrease in dissolved benzene has occurred since January 1992, nor is anticipated to occur in the future as a result of continuation of conventional pump-and-treat (Figure 13.11). Aerobic biodegradation has also been documented to exist since 1992, and is ongoing as indicated by the inverse relationship between dissolved BTEX and DO. The dissolved plume is also situated hydraulically upgra-dient from a relatively larger LNAPL pool and dissolved hydrocarbon plume separation of plumes has historically been maintained and has continued to be maintained. 

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