Estimating The Amount Of Residual Lnapl Immobilized In The subsurface

Residual LNAPL in the subsurface is the portion that will not flow into a well. It is the part of an LNAPL spill that cannot be removed by pumping to the surface. Residual LNAPL must be remediated by biodegradation, soil flushing, or excavation. Residual LNAPL is retained in the unsaturated zone by adsorption and capillary forces. Therefore, small soil particles and large surface area both increase the amount of residual LNAPL retained. The soil retention factor (volume of LNAPL per volume of soil) depends mainly on the soil pore size distribution, soil wettability, LNAPL viscosity, and LNAPL density.

Usually more LNAPL is immobilized in the saturated zone than in the unsaturated zone because part of the residual LNAPL in the unsaturated zone eventually drains down to the water table. In the saturated zone, water is the wetting fluid and LNAPL the nonwetting fluid. LNAPL becomes trapped in larger pores of the saturated zone by immobile water.

In the unsaturated zone, LNAPL is the wetting fluid and tends to spread into smaller pores and drain from the larger pores. Figure 5.11 shows soil retention factors for several kinds of LNAPL

FIGURE 5.11 Soil retention factors for LNAPL fuels in different soils, plotted from data in Reference 9. Calculations assume a soil bulk density of 1.85 g/cm3 and LNAPL densities of 0.7, 0.8, and 0.9 g/cm3 for gasolines, diesel fuel, and fuel oils, respectively.

in soils of different textures. The retention of LNAPL in soils above the water table usually ranges between about 80 L per cubic meter of soil, for fuel oil in silt, to 2.5 L per cubic meter, for gasoline in coarse gravel. LNAPL in the unsaturated zone can often be remediated without excavation by some combination of soil washing, volatilization, or bioremediation.

Example 5.2: Using Soil Retention Factors

One thousand gallons of fuel oil were spilled on a soil consisting mostly of medium to coarse sand. How much soil is required to immobilize 1000 gallons? If the spill area was confined by a berm to 100 ft2, how deep into the soil will the oil penetrate? Could it endanger a shallow aquifer 35 ft below the surface?

Answer: From Figure 5.11, the soil retention factor is about 30 L/m3 for fuel oil. The volume of soil needed to contain the entire spill is

1000 gal 30 L/m3

1 gal

1 m3

Assume the oil plume travels downward without spreading, so that its cross-section is 100 ft2. Then a volume of 4454 ft3 will extend downward by

4454 ft3

Depth of oil penetration until it all is retained and immobilized = = 44.5 ft.

100 ft2

Oil is likely to reach the aquifer at 35 ft.


Relative Importance of Different Subsurface Loci in Sandy Soils for Retention of Gasoline Contamination

Loci of Subsurface LNAPL Retention

1. Gasoline vapors in soil pores in the unsaturated zone.

2. Liquid gasoline sorbed to dry soil particles in the unsaturated zone. Locus

2 is especially important in the soil volume immediately below a spill, but not downgradient of the spill.

3. Gasoline dissolved in water on wet soil particles in the unsaturated zone.

4. Liquid gasoline sorbed to wet soil particles in the saturated and unsaturated zones.

5. Liquid gasoline in soil pore spaces within the saturated zone. Locus 5

contaminants may generally be regarded as immobile.

6. Liquid gasoline in soil pore spaces in the unsaturated zone. Contaminants enter locus 6 mainly from free product floating on the groundwater table when the table rises and then falls.

7. LNAPL gasoline free product floating on top of the groundwater table.

The most important loss mechanism from locus 7 occurs when a fluctuating water table moves contaminant into loci 5 and 6, where some of it remains trapped.

8. Gasoline dissolved in groundwater.

9. Gasoline sorbed to colloidal particles in water in the saturated and unsaturated zones.

10. Liquid gasoline diffused into mineral grains in the saturated and unsaturated zones.

11. Gasoline sorbed onto or into microbiota in the saturated and unsaturated

Average Gasoline Retention in Sandy Soils

0.095 36

0.0010 0.076

0.020 0.00013

0.000060 0.010

Percent of Total Retention in Sandy Soils




12. Gasoline dissolved into the mobile pore water of the unsaturated zone. 0.030

13. Liquid gasoline in rock fractures in the saturated and unsaturated zones. 0.21

Source: Calculated using data from Reference 10.

Subsurface Partitioning Loci of LNAPL Fuels

As part of an ongoing effort to provide an authoritative, defensible engineering basis for predicting contaminant behavior in soils, the EPA has identified 13 soil loci among which petroleum contaminants become partitioned.10 Contaminants may move within a given locus under the influence of pressure and concentration gradients or from one locus to another (for example, when soluble components in the free product layer dissolve into the groundwater).

In Table 5.4, the partitioning behavior of gasoline in sandy soils has been calculated, using the methods and data recommended by EPA.10 The table shows how much gasoline LNAPL is expected to be retained in the 13 different partitioning loci.

It is clear from Table 5.4 that when free product is present in locus 7 (gasoline LNAPL floating on top of the groundwater table), this condition is the controlling factor for the distribution of contaminants in other zones. With free product present above the water table, loci 2, 5, 6, and 7 are by far the most important in terms of mass and account for 99.9% of the total soil and groundwater contamination.

The mass of gasoline in the vapor and dissolved states (loci 1, 3, 8, and 12) is insignificant compared to that in the free product above the water table and in the smear zone created by a rising and falling water table (loci 2, 5, 6, and 7). Of course for remediation purposes, all loci are important and those with relatively small amounts of contaminant may be the most difficult to remediate to a regulated level.

Example 5.3: Calculation of Contaminant Plume Volume Required to Immobilize 1 Million Gallons of Gasoline

For a point source of contamination, we can assume that only loci 5 and 6, the smear zone caused by a fluctuating water table, are effective for immobilizing contaminant. Locus 2 lies primarily under the area of the initial spill or leak and probably is small compared to the total free product plume volume. LNAPL in locus 7 may be regarded as mobile. As LNAPL in locus 7 flows downgradient and is subjected to vertical movement caused by a fluctuating water table, it continually loses mass into loci 5 and 6.

Assume an average sandy soil with a hydraulic gradient of 0.009 ft/ft. Using the methods followed in U.S. EPA,10 the estimated flow velocity for LNAPL gasoline in locus 7 is about 1.3 ft/day.* This may be compared with a groundwater velocity of about 10 ft/day for the same conditions. Further assume that the seasonal groundwater table fluctuations average around ± 1.5 ft, giving a free product smear zone 3 ft in thickness. Taking the smear zone to be of uniform thickness over the plume volume and assuming that, on average, half of the smear zone is in the saturated zone and half in the unsaturated zone, we can say loci 5 and 6 are each 1.5 ft thick everywhere over the free product plume.

Locus 5 retention

Take the density of aged gasoline LNAPL to be 0.74 g/cm3, or 2800 g/gal. From Table 5.4, locus 5 retains 38 mg of gasoline per cm3 of soil, or about 16,500 gal/acre-ft. For a locus 5 thickness of 1.5 ft, 24,750 gallons of gasoline LNAPL will be immobilized per acre of contamination plume cross-sectional area.

Locus 6 retention

Similarly, retention in locus 6 will be about 73,000 gal/acre. The total amount of fuel retained in loci 5 and 6 is

(24,750 + 73,000) gal/acre = 97,750 gal/acre, or close to 100,000 gal/acre.

For a 1-million-gallon gasoline spill in sandy soil, where the water table fluctuates ± 1.5 ft annually, the soil contaminant plume could become immobilized after it had spread into 30 acre-ft, or a horizontal cross-section of about 10 acres.

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