LNAPL occurrence

The ubiquitous occurrence, areal extent, and estimated volume of major LNAPL hydrocarbon pools beneath these facilities situated on the Los Angeles coastal plain are shown in Ligure 12.25. Lor purposes of this discussion, a pool is defined as an areally continuous accumulation of LNAPL. Two or more pools that have distinct differences in their respective physical and chemical properties are referred to as coalesced pools. Individual accumulations of relatively uniform product are referred to as subpools, since it is inferred that they have coalesced to form areally continuous occurrences. The occurrence of several pools and subpools at a particular site reflects releases from multiple sources at various times. The combined areal extent of these LNAPL pools is on the order of 1500 acres. The estimated cumulative minimum volume is on the order of approximately 1.5 million barrels an estimated cumulative maximum volume is on the order of 7.5 million barrels. The discrepancy in LNAPL hydrocarbon volume reflects varying methodologies involved in the estimates. 

Source identification of dissolved hydrocarbons in groundwater and refined LNAPL is difficult due to several factors. These factors include numerous microbiological, chemical, and physical processes, the complex historical industrial development of the area, numerous property ownership transfers, and the close proximity of several crude and petroleum-handling facilities including clusterings of refineries, 

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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. 

Coproduced groundwater can be reinjected within, above, or below the LNAPL zone depending on subsurface conditions, LNAPL occurrence and removal strategies, and site-specific constraints. These options and their respective advantages and limitations are discussed below. 

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. 

For over a decade, LNAPL occurrence has been investigated beneath an active refinery site in southern California. Numerous monitoring wells along with LNAPL samples have been used to evaluate the extent and character of LNAPL occurrence. LNAPL was found to occur as five pools. The main pools each consist of individual accumulations of distinct product types occurring under both perched and water table conditions. Two different recovery and mitigation strategies have been utilized. In relatively high permeability zones, a system of two-pump recovery wells was used to recover fluids recovered water is reinjected without treatment. In relatively low permeability zones, a system of one-pump recovery wells was used. In the latter case, recovered water is treated prior to disposal. 

FIGU RE 12.19 Generalized LNAPL occurrence map showing individual accumulations that have coalesced to form distinct LNAPL pools. 

Beneath the facility, LNAPL product occurs as three main pools (Pool No. I, II, and III) and two smaller pools (Pool No. IV and V) of localized occurrence. The known extent of Pool No. I, HI, IV, and V is entirely beneath the main portion of the refinery. Pool No. II is located beneath the southwestern portion of the site. LNAPL occurrence beneath the facility is shown in Figure 12.19. For purposes of this discussion, a pool is defined as an aerially continuous accumulation of LNAPL. The three main pools consist of individual accumulations of differing product character and are therefore referred to as coalesced pools. Individual product accumulations within these coalesced pools were delineated on the basis of physical and chemical properties characteristic of free hydrocarbon samples retrieved from the wells. Individual accumulations of relatively uniform product are referred to as subpools since it is inferred that they coalesced to form areally continuous occurrences or pools. These data suggest multiple sources over time for coalesced pools. The numbering of individual product accumulations is based on product type and... 

IMPORTANCE OF LITHOFACIES CONTROL TO LNAPL OCCURRENCE AND RECOVERY STRATEGY... 

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

DNAPL once in the subsurface can occur in a variety of geologic scenarios making it difficult to fully assess the occurrences, as well as the lateral and vertical extent of the DNAPL (see Figure 5.2). DNAPL thickness when measured in a monitoring well, as with LNAPL, is not representative of the actual formation thickness. As with LNAPLs, the DNAPL thickness will be exaggerated as illustrated in Figure 6.9a. Limited laboratory studies have shown that the apparent DNAPL thickness is... 

More sophisticated methods for estimating LNAPL volumes have recently been proposed and are based on the general approach that the specific volume of LNAPL can be determined by integrating the hydrocarbon saturation over the vertical distance of its occurrence ... 

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. 

FIGURE 12.17 Generalized hydrogeologic cross section showing occurrence of perched and water table LNAPL. Approximate position of upper and lower informal units of the Lakewood Formation as shown on the left margin. Vertical exaggeration equals 25 1. (After Testa et al., 1989.)... 

Exploration for LNAPL beneath the main portion of the refinery commenced in October 1984 2-in. monitoring wells were initially installed followed by 4-in. test wells in areas of known occurrence. A one-pump recovery system was subsequently designed and installed in the less permeable material encountered beneath this area. The first well was activated in June 1987. 

Summary of LNAPL Hydrocarbon Occurrence at Refinery Site... 

Site No. 3, situated in the southern portion of the West Coast Basin, is underlain by, in descending stratigraphic position, the Semi-perched, Gage, Lynwood, and Silverado aquifers (see Figure 12.25). Most pertinent to LNAPL hydrocarbon occurrence is the Semi-perched aquifer, which is encountered under water table conditions. 

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... 

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