LNAPL migration

Apparent/actual LNAPL thickness ratios can be very high at the perimeter of LNAPL pools, notably, under low permeability conditions. Once a well is installed, it can take several months before the LNAPL migrates from the formation into the well reflecting the presence of low-permeability soils in the zone of LNAPL occurrence. As clearly shown in Figure 6.8, a well screened at the perimeter of a known LNAPL pool initially had no detectable LNAPL until 4 months after installation, whereas upon detection the apparent thickness slowly increased with time up to 15.71 ft. 

All passive systems rely on the natural hydraulic gradient to transport LNAPL to the recovery location. Under most circumstances, the flow of LNAPL into this type of system is very slow. At open surface recovery sites (trenches and ponds) constructed in low-permeability soils, the LNAPL migrates in so slowly that free volatile product often evaporates before it accumulates sufficiently to be collected. High-permeability soils typically are subject to a low hydraulic gradient, which limits the rate of flow into the system. Conditions that are more favorable to passive recovery, shown schematically in Figure 7.1, include ... 

LNAPL occurrence was determined to reflect several factors, including, but not limited to, lithofacies control over LNAPL migration and volume, timing, rate, and... 

The problems associated with LNAPLs are well documented in the literature, ranging from small releases where just enough LNAPL is present to be a nuisance, to pools ranging up to millions of barrels of LNAPL and encompassing hundreds of acres in lateral extent. Subsurface migration of LNAPL (and DNAPL) are affected by several mechanisms depending upon the vapor pressure of the liquid, the density of the liquid, the solubility of the liquid (how much dissolves in water at equilibrium), and the polar nature of the NAPL. 

Where the source is limited or ceases, capillary spreading eventually slows until further migration is limited and equilibrium is reached. This stable condition is attained when the leading edge of the laterally spreading light LNAPL fails to be... 

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 location of the reinjection wells with respect to the recovery well is also an important consideration. Locating the wells too closely can create counterproductive effects. Locating the reinjection wells too far away from the recovery well may significantly reduce the desired effect or eliminate the benefits altogether. If the latter is the case, dead spots may result and may cause short-circuiting of the LNAPL pool hydrocarbon plume. Once this happens, off-site migration of the plume is possible. In fact, reinjection wells placed and spaced improperly can, in some instances, accelerate off-site migration. 

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. 

A case study is presented that demonstrates the importance of evaluating lithofacies distribution, or lateral and vertical heterogeneities, and depositional environment as a control on LNAPL occurrence and migration, and implementation of an effective and efficient remediation strategy. This is followed by a case history on the development of a long-term remedial strategy for LNAPL recovery and aquifer restoration from a regional perspective. 

Near-shore facilities are typically characterized by shallow groundwater conditions. The occurrence of LNAPL product on the water table presents the need for immediate containment and continued recovery of the product to abate degradation of ground-water quality, hydrocarbon vapor migration, and discharge of hydrocarbon product to surface waters. A pneumatically operated, double-diaphragm, suction-lift pump has been frequently used in such circumstances to contain and recover LNAPL. 

Fig. 4.7 Simplified conceptual model for Light nonaqueous phase liquid (LNAPL) release and migration. Reprinted from Mercer JW, Cohen RM (1990) A review of immiscible fluids in the subsurface Properties, models, characterization, and remediation. J Contam Hydrol 6 107-163. Copyright 1990 with permission of Elsevier...

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. 

MTBE and TBA with densities of < 1 g/cm belong to the so called LNAPL (light non aqueous phase liquids) whereas CAH with densities of > 1 g/cm belong to the DNAPL (dense non-aqueous phase liquids). While DNAPL (heavy phase) tend to migrate as an independent phase in greater depths, LNAPL (light phase) float on the groundwater table. 

Once the NAPLs (light and dense LNAPLs and DNAPLs, respectively) penetrate into the subsurface, their migration in the soil may occur via various mechanisms, ultimately resulting in either accumulated bulk, or trapped NAPL in the aquifer s unsaturated zones, as well as above or below its saturated zones. The conceptnalization of this penetration/migration process is illustrated in Figures 15.1 and 15.2 concerning LNAPLs and DNAPLs, respectively [8]. 

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