LNAPL Hydrocarbon Recovery

The overall efficiency and effectiveness of LNAPL hydrocarbon recovery programs have also been impacted by other factors. These factors include limitations associated with LNAPL recovery from low-yielding formations, inability to gain access to optimal recovery and off-site locations, coproduced water-handling constraints, and economic constraints. 

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Summary of Conventional LNAPL Hydrocarbon Recovery Alternatives... 

At the time this order was issued, approximately six refineries had already commenced LNAPL hydrocarbon recovery programs, although such programs were limited in scope. In these cases, the large volumes of LNAPL present and the initial ease of hydrocarbon recovery proved economically favorable, since the recovered LNAPL product could be easily recycled and sold. 

LNAPL Recovery Conditions3 Depth to LNAPL Hydrocarbon ... 

LNAPL recovery operations in the southwestern portion of the refinery have been conducted using a two-pump recovery system. This system currently includes up to four two-pump hydrocarbon recovery wells. Each two-pump system uses a 16-in.-diameter recovery well that is designed to accommodate two independently operated pumps placed at different levels within the well. 

Two of the larger LNAPL hydrocarbon occurrences, site No. 1 and 4 (see Ligure 12.23), formerly reinjected coproduced groundwater into generally the same hydros-tratigraphic zone from which it is withdrawn site No. 1 reinjected without treatment into the Gage aquifer, whereas site No. 4 reinjected into the Old Dune Sand aquifer. Because of the presence of dissolved hydrocarbons, notably benzene, in the coproduced water that is typically returned to the aquifer during LNAPL recovery operations, immediate application of the EPA toxicity characteristic rule may result in classification of the reinjected water as disposal of a hazardous waste. This, in turn, would terminate use of UIC Class V wells (which many of these operations currently... 

A more in-depth discussion of the subsurface hydrogeologic setting, areal extent of LNAPL and dissolved hydrocarbons in groundwater, remedial strategy, and current status is presented in Chapter 12 (LNAPL Recovery Case Histories). 

Additionally, vertical fluctuations in the water table due to recovery operations or seasonal variation in precipitation have a direct effect upon the apparent or measured LNAPL thickness (Figure 6.6d). As the water table elevation declines gradually due to seasonal variations, for example, an exaggerated apparent thickness occurs, reflecting the additional hydrocarbon that accumulated in the monitoring well. The same is true for an area undergoing recovery operations where the... 

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 progress of recovery efforts cannot be based confidently on LNAPL product thickness maps. Although these maps provide quantification of overall trends, the numerous factors that impact hydrocarbon thicknesses make accurate quantification difficult. An estimate of effectiveness thus is based on volume recovered to date divided by the total volume that is considered recoverable. Furthermore, as the recovery project progresses and new data are introduced, the volume and time frame for recovery should be continually reevaluated and revised. 

Recovery of DNAPL is a very slow process that is alfected by those factors encountered with LNAPL (i.e., relative permeability, viscosity, residual hydrocarbon pool distribution, site-specific factors, etc ). Dissolution of a DNAPL pool is dependent upon the vertical dispersivity, groundwater velocity, solubility, and pool dimension. Dispersivities for chamolid solvent are estimated for a medium to coarse sand under laboratory conditions on the order of 1(L3 to 1(H m. Thus, limited dispersion at typical groundwater velocities is anticipated to be slow and may take up to decades... 

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. 

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. 

Where LNAPL is present, its recovery is an essential and typically immediate part of the cleanup effort. Removal of the floating LNAPL layer is almost always required prior to the initiation of other restoration- and remediation-related activities (enhanced bioactivity, vapor extraction, etc.) and response to other environmental issues (i.e., hydrocarbon-affected soils, vapors, or dissolved hydrocarbon in groundwater). 

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. 

Reductions in apparent LNAPL thickness in the subsurface have been effected over most of the site. In general, apparent thickness after 1 year were reduced 30 to 40% to those apparent thicknesses measured prior to recovery. The reduction in apparent thickness in a representative monitoring well with time is shown in Figure 12.3. However, in one area of the site, hydrocarbon thicknesses actually increased... 

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. 

The main refinery LNAPL recovery system consists of 11 single-pump 4- and 6-in.-diameter production wells. Recovery wells are constructed of slotted PVC screens and casing. For the recovery of LNAPL, 4-in.-diameter submersible pumps have been installed. The submersible pumps installed within the recovery wells are of stainless steel construction. These pumps were not adversely affected by water, hydrocarbon, or minor amounts of fine sand and silt produced by the recovery wells. 

Recovery of LNAPL via trenches was specified in the ROD but found to be unsuccessful due to its discontinuous and relatively thin apparent thickness of less than 1 in. A backhoe test pit program revealed the hydrocarbons to be distributed over a distinct 3-ft-thick smear zone. Observation of the hydrocarbons in the smear zone indicated significant weathering may have occurred as evidenced by a silver-gray staining of petroleum on the sand and gravel. 

Source recovery operations have been completed (i.e., hydrocarbon-affected soil and LNAPL have been removed). 

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. 

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