LNAPL recovery

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). 

In principle, the recovery of LNAPL is similar in mechanical operation to production of a low-pressure, water-driven reservoir. Almost all documented petroleum remediations have been characterized by subsurface conditions under water table conditions (i.e., the top surface of the fluids are at atmospheric pressure). Few cases of confined aquifer situations have been reported in the literature, and although the mechanical recovery procedures are slightly different, the economic considerations are similar. 

Detailed planning is necessary to ensure that the fieldwork meets predetermined goals with regard to administrative requirements, the development of recovery facil- 

Meeting regulatory compliance. In many situations, recovery efforts are driven by statutory forces, which are result oriented and not particularly oriented toward any specific site. Most projects of this variety have specified quality time constraints, and the economic considerations are secondary. 

Maintenance of current commercial status quo. At a few locations, the decision has been made to continue operation of the facility without disruption, so long as NAPL product and dissolved chemicals do not exit the boundary of the facility or provide a risk to workers or the environment at the site. Hydraulic containment of the aquifer is the procedure that is usually selected for these sites. A system including recovery wells and injection wells can often be operated to balance the subsurface flux so that product loss equals product recovery at a minimal cost. 

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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). 

During the later phases of remediation of an aquifer, it has often been suggested that the expense of continuing LNAPL recovery is not worth the effort, considering the remaining small quantities of recoverable product. The implication of this argument has been that the product will eventually go into solution and therefore will be recovered as part of the dissolved contaminant recovery. Experience has demonstrated that this is not usually a viable option, because the time required and treatment costs to accomplish the task are much greater than with respect to LNAPL product recovery efforts. 

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. 

Determination of transmissivity can be useful in designing LNAPL recovery strategies because the rate of LNAPL recovery is a function of LNAPL transmissivity (Tn). 

LNAPL Recovery Conditions3 Depth to LNAPL Hydrocarbon ... 

Where LNAPL is situated at shallow depth (<20 ft or 6 m), in moderate to high permeability soil (> 10 s cm/s), a well-point system may be the appropriate recovery technology. The use of well points for LNAPL recovery is similar to construction dewatering. A major advantage to this type of system is that it is possible to lower... 

The double-diaphragm suction-lift pump LNAPL recovery system is patterned after the concept of a shallow well-point dewatering system commonly used in the con-... 

For effective LNAPL recovery, the suction-lift pumping system requires routine monitoring and operation and maintenance of the equipment. Since the pumps have a minimum of moving parts, and control is accomplished by valving of the air supply, the maintenance of the equipment is minimal. 

FIGURE 7.13 Photograph showing aboveground wellhead and control box for one-pump LNAPL recovery system. 

The use of traditional rod and piston pumps continues at many LNAPL recovery locations, particularly at refineries and distribution terminals. These units are usually powered by single-speed electric motors and have adjustable stroke lengths to control the pumping rate. When installed with the intakes set at the optimum pumping depth, they function fairly well. The primary advantage of rod and piston pumps is that the smooth slow stroke rate can pump mixtures of product and water without creating a significant emulsion. 

Despite its humorous name, this technology is a fairly efficient procedure to combine the benefits of vacuum-enhanced recovery and bioventing to promote vapor recovery and in situ biodegradation. Integration of these technologies into a single step results in LNAPL recovery and remediation of residual soil contamination in the vadose zone. 

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

The following discussion is directed toward evaluating the costs of operating LNAPL recovery systems. The principles presented are equally applicable to all types of LNAPL recovery situations. 

Debits associated with LNAPL recovery projects are the costs for ... 

FIGURE 11.1 Example project of LNAPL recovery, cumulative costs, and production vs. time. 

A simplified analysis of production decline curves was initially intended for evaluation of individual oil well production however, this type of analysis can also provide reasonable estimates when applied to multiple-well LNAPL recovery systems. This analytical method is applicable to most types of decline curves, whether they tend to follow exponential, hyperbolic, or harmonic forms. The following general differential equation is applicable to all forms of decline curves ... 

LNAPL recovery projects require careful planning, operation, and management. Some additional factors become important at unforeseen times. Recovery of free product often produces flammable vapors at concentrations that can violate air quality standards if freely released. Vapor treatment should be considered early in the design process. Storage of flammable liquids in appropriate containers is necessary. 

Selection of approach or combination of approaches to LNAPL recovery and ultimately aquifer rehabilitation and restoration is dependent on numerous factors as discussed in the previous chapters. The case histories presented below reflect different remediation approaches and strategies in response to varying geologic and hydrogeologic conditions, site-specific constraints, and regulatory environment. A variety of LNAPL recovery systems are first discussed, including ... 

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. 

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. 

TABLE 12.2 LNAPL Recovery LNAPL Pool No. Media System Effectiveness Estimated Original Volume Recoverable (barrels) Recovered to Date (barrels) Estimated Present Volume Recoverable Estimated Volume Removed (%)... 

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. 

The LNAPL recovery program in the southwestern portion of the site was initiated in 1982. To date, the volume of LNAPL recovered is approximately 185,000 barrels, which includes about 2000 barrels from the perched pool and 183,000 barrels from the water table pool. About 28,000 barrels have been recovered to date from beneath the main portion of the refinery. 

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