LNAPL free-phase

Since hydrocarbon and water are immiscible fluids, free-phase recoverable LNAPL can simplistically be viewed as being perched on the capillary fringe above the actual water table with the understanding that what is being referred to as actual NAPL thickness is what one could perceive as being equivalent to the approximate thickness of that portion of the zone of hydrocarbon saturation that is considered mobile. The physical relationships that exist are illustrated in Figure 6.5. This discrepancy can be a result of one or a combination of factors or phenomena. Some of the more common factors or phenomena are schematically shown in Figure 6.6 and include ... 

Construction of a subsurface structure that penetrates the water table only a short distance can be an effective LNAPL retention technique. Most underflow structures function in the same way as a surface baffle in an oil water separator or a septic tank. The structure must be carefully installed perpendicular to groundwater flow, and have some arrangement to collect the free-phase LNAPL. Often, a simple French drain (constructed parallel to the retaining wall) leading to a recovery well (with skimmer) is effective. A schematic diagram of a hydraulic underflow structure with a skimming unit is presented in Figure 7.3. 

Open trenches with free-phase LNAPL can be a fire hazard. 

In addition, disposal options are presented with a discussion on the reinjection of untreated groundwater during free-phase LNAPL recovery. 

In water-NAPL systems, NAPL may exist as a continuous mass (mobile phase or free phase), which can flow under a hydraulic gradient, or as individual ganglia or blobs (residual phase), which are more difficult to mobilize hydraulically. The residual saturation, S, defines the NAPL saturation (VNAPL/I voids) below which NAPL distribution is discontinuous. Typical residual saturation values for soils and sands range from 26% to 75% (Thibodeaux 1996), but have been reported as low as 15-25% (Bedient et al., 1997). In separate field studies, free-phase DNAPL (Pankow and Cherry, 1996) and LNAPL (Huntley et al., 1994) were detected at saturation values as low as 15% and 25%, respectively. In unsaturated soil, typical residual saturation values are between 5% and 20% (Bedient et al., 1997), whereas in saturated soil, residual saturation may be 15-50% of the total pore space (Mercer and Cohen, 1990). 

In summary, when undertaking a project such as the recovery of LNAPL, treatment of the coproduced water, prior to reinjection, may not be beneficial or technically necessary. A large percentage of the spilled or leaked petroleum hydrocarbon (40 to 60%) will be retained in the unsaturated zone as residual saturation. This residual hydrocarbon cannot be recovered by conventional withdrawal techniques. Without removing this continual source of contamination to the groundwater system, dissolved contamination will continue. Therefore, in most cases, it may be pointless and extremely costly to treat the coproduced groundwater prior to reinjection while the free- and residual-phase hydrocarbon contamination exists. 

In situ oil skimmers are commercially available for the recovery of free product [i.e., light non-aqueous-phase liquids (LNAPLs) such as oil, grease, or other hydrocarbons] floating on the water table. Oil skimmers can be used alone or in conjunction with other remediation technologies, such as (in situ) soil vapor extraction, bioventing, or bioremediation, or (ex sim) membrane filters, coalescers, or chemical processes. The technology is implemented in sim by lowering the skimmers into wells located in the zone of contamination. 

---