Behavior Of Petroleum Hydrocarbons In The Subsurface

Because of their low water solubilities, most of the compounds classified as petroleum hydrocarbons are generally considered as nonaqueous phase liquids (NAPL). If mixed into water, NAPLs separate into a distinct liquid phase with a well-defined boundary between the NAPL and the water, like oil and water or milk and cream.

NAPLs are further subdivided into light nonaqueous phase liquids (LNAPL) and dense nonaqueous phase liquids (DNAPL). LNAPLs are liquid hydrocarbon compounds or mixtures that are less dense than water, such as gasoline and diesel fuels and their individual components. DNAPLs

figure 5.1 The BTEX group of aromatic hydrocarbons. Heavier Fuel Oils and Lubricating Oils

FIGURE 5.2 Hydrocarbon ranges, corresponding uses, and analytical methods.

are liquid hydrocarbon compounds or mixtures that are more dense than water, such as creosote, PCBs, coal tars, and most chlorinated solvents (chloroform, methylene dichloride, etc.).

The distinction between LNAPLs and DNAPLs is important because of their different behavior in the subsurface. LNAPL spills travel downward through soils only to the water table, where they remain "floating" on the water table surface. DNAPLs sink through the water-saturated zone to impermeable bedrock, where they collect in bottom pools. Obviously, remediation methods are different for LNAPLs and DNAPLs.

Soil Zones and Pore Space

As illustrated in Figure 5.3, the subsurface soil may be divided into a water-unsaturated zone, from the soil surface down to just above the water table (also called the vadose zone), and a water-saturated zone, from the water table down to bedrock. Capillary action extends the saturated zone somewhat above the water table with a region of transition between the unsaturated and saturated zones. The capillary fringe can vary from a fraction of an inch in coarse-grained sediments to several feet in finegrained sediments such as clay.

Each zone contains soil particles with pore spaces between them. In normally permeable soils, most of the pore spaces are continuous, allowing movement of water and liquid contaminants through them. In the absence of contaminants, pore spaces in the unsaturated zone contain air with some water adsorbed to the soil particles. Pore spaces in the saturated zone contain mainly water. When contaminants enter the subsurface region as spilled liquid petroleum (free product),

Volatile compounds vaporize from the free product mixture into the atmosphere and into air in the soil pore spaces.

FIGURE 5.3 Soil zones and partitioning behavior of free product pollutant. All the Ks are partition coefficients. They quantitatively describe how the pollutant distributes itself among water, soil, air, and free product.

• Many compounds in the free product partially dissolve into water contained on soil particle surfaces, into water percolating down from the ground surface, and into ground-water in the saturated zone.

• A small fraction of the free product is taken up by microbiota.

• The remaining free product adsorbs to soil particles and, where free product is abundant, fills the pore spaces.

Partitioning of Light Nonaqueous Phase Liquids (LNAPLs) in the Subsurface

Before a petroleum release occurs, the voids of vadose zone earth materials are filled with air and water. After a release, some voids contain immobile petroleum held by capillary forces and sorbed to soil surfaces. There may also be liquid petroleum moving downward through the pore interstices under gravity. If LNAPL reaches the water table, its buoyancy will prevent further downward movement and it will spread out horizontally over the water table to form a layer of free product, "floating" above the saturated zone. The individual components of the petroleum become partitioned into air, water and solid phases that come in contact with the free product.3

Oil Mobility Through Soils

Oil pollutants moving through soil, dissolved in water, or migrating as liquid free product leave a trail of contamination sorbed on soil particles and trapped in soil pore spaces. This trapped contamination is not easily removed by water flushing or air sparging. In the subsurface environment, a significant portion of oil contamination must be regarded as "permanent," with a lifetime of well over 25 years, unless deliberate efforts are made to mobilize, degrade, or remove it.24 Immobilized oil contaminants in the subsurface act as a long-term source of groundwater contamination, as the more soluble components continue to diffuse to the oil-water interface and dissolve into the water.

This is nothing new to oil field workers. Liquid petroleum fields are found in rock formations of 10% to 30% porosity. Up to half of the pore space contains water. Primary recovery of oil, which relies on pumping out the portion of oil that is mobile and will accumulate in a well, collects only 15% to 30% of the oil in the formation. Secondary recovery techniques force water under pressure into the oil-bearing rocks to drive out more oil. Primary and secondary techniques together extract somewhat less than 50% of the oil from a formation. Tertiary recovery techniques use pressurized carbon dioxide to lower oil viscosity along with detergents to solubilize the oil. Even with using tertiary techniques, producers expect 40% of the oil to remain immobile and unrecoverable.

Processes of Subsurface Migration

After part of the spilled petroleum has partitioned from the free product into other phases, hydrocarbons (HCs) are present in solid, liquid, dissolved, and vapor phases.

1) Solid phase HCs are sorbed on soil surfaces or diffused into micropores and mineral grain lattices. They are immobile and degrade very slowly.

2) Liquid phase HCs exist in the subsurface as

• Immobile residual liquids held by capillary forces and as a thin layer sorbed to sediments in the unsaturated zone and capillary fringe.

• Free mobile liquids in the unsaturated zone above the capillary fringe.

• Immobile residual liquids trapped below the water table in the saturated zone.

3) Dissolved phase HCs are found in

• Water infiltrating through the unsaturated zone.

• The residual films of groundwater sorbed to sediments in the capillary fringe and elsewhere in the HC plume.

• The groundwater of the saturated zone.

4) Vapor phase HCs are found

• Mostly in void spaces of the unsaturated zone not occupied by water or liquid HCs. Here, they are mobile.

• As small bubbles trapped in the HC plume and in the water-bearing zone below the plume. Here, they are immobile.

• Dissolved in the groundwater of the saturated zone, where they move with the ground-water.

Behavior of LNAPL in Soils and Groundwater

LNAPL movement in the subsurface is a continual process of partitioning different components among different phases that are present in the subsurface matrix. Spilled LNAPL at or near the soil surface penetrates and thoroughly saturates the soil because there is little trapped water or air to block its movement. Under the influence of gravity, the LNAPL sinks vertically downward, leaving behind in the soil a trail of residual LNAPL trapped by sorption and capillary forces. Capillary forces cause the LNAPL to spread horizontally as well as vertically downward, creating an inverted funnel-shaped zone of soil contamination.

As liquid free product moves downward through soil, a significant portion becomes immobilized by sorption to soil particle surfaces and capillary entrapment in soil pore space. This continually reduces the amount of mobile contaminant. If the liquid free product is not replenished by a continuing leak, it eventually is completely depleted by entrapment in the soil and becomes essentially immobilized. However, even when immobilized, the trapped free product continues to lose mass into the dissolved and vapor phases during biodegradation.

figure 5.4 LNAPL fuel leaking from underground storage tanks migrates downward under gravity. Enough fuel free product has leaked from the left tank to reach the saturated zone and spread out above the water table, moving in the direction of groundwater flow. The smaller spill from the right tank is insufficient to reach the water table and has become immobilized within the unsaturated zone by sorption and capillary forces. The more soluble components of the free product are present in the dissolved plume, which extends beyond the free product plume into the saturated zone. There also is a vapor plume in the unsaturated zone of the most volatile components. The vapor plume extends in all directions independent of gravity. It may enter underground cavities such as sewers and basements, and may escape through the ground surface into the atmosphere.

figure 5.4 LNAPL fuel leaking from underground storage tanks migrates downward under gravity. Enough fuel free product has leaked from the left tank to reach the saturated zone and spread out above the water table, moving in the direction of groundwater flow. The smaller spill from the right tank is insufficient to reach the water table and has become immobilized within the unsaturated zone by sorption and capillary forces. The more soluble components of the free product are present in the dissolved plume, which extends beyond the free product plume into the saturated zone. There also is a vapor plume in the unsaturated zone of the most volatile components. The vapor plume extends in all directions independent of gravity. It may enter underground cavities such as sewers and basements, and may escape through the ground surface into the atmosphere.

When all the spilled oil has entered the subsurface, the LNAPL "front" continues downward, leaving behind an ever-widening "inverted funnel" of contaminated soil containing residual "immobile" LNAPL sorbed to soil surfaces and trapped in pore spaces. The term "immobile" is used loosely and really means that, although some mobility may still occur, it will be very slow compared to the remediation time frame of interest.

A spill may or may not reach the water table. If the groundwater table is deep enough or if the amount of spilled LNAPL is small enough, the mobile LNAPL can be completely depleted by entrapment in the soil before it reaches the groundwater. If the spill is large enough or the groundwater table is shallow, mobile LNAPL, commonly called free product, will contact the groundwater. The weight of the free product depresses the water table locally below the free product column (see Figure 5.4).

Free product will continue to spread laterally as a layer over the water table, leaving a trail of residual LNAPL entrapped in the soil, until it spreads out to a saturation level so low that it all becomes immobile. The lateral spreading of the free product is influenced by a viscous "frictional" interaction at the water-LNAPL interface, which tends to move the free product preferentially in the direction of groundwater movement, along the hydraulic gradient. The relative downgradient velocities of water and free product depend on their relative viscosities and the soil conductivities for the different liquids.

When the water table rises and falls, the "floating" free product is moved vertically, "smearing" LNAPL into a region thicker than the free product thickness. Still more residual LNAPL becomes immobilized in this "smear zone." The end result is that the smear zone of entrapped LNAPL extends above and below the average level of the water table.1

Summary of LNAPL Behavior

The following summarizes the behavior of spilled LNAPL:

1. Spilled liquid hydrocarbons move downward through the unsaturated zone under gravity.

2. A large fraction sorbs to the subsoil surfaces as trapped residual free product.

3. Some horizontal spreading occurs in the unsaturated zone because of attractive forces to mineral surfaces and capillary attractions.

4. Free product tends to accumulate and spread horizontally above layers of low permeability (low hydraulic conductivity).

5. At the water-bearing region of the capillary fringe, the free liquid phase floats on the water and begins to move laterally.

6. If the spill is small enough, LNAPL may not reach the water table. However, a portion that dissolves in downward percolating water will be carried to the water table and will contaminate it.

7. The vapor phase spreads widely in the unsaturated zone and can escape to the atmosphere and accumulate in cellars, sewers, and other underground air spaces.

"Weathering" of Subsurface Contaminants

With time, the composition of immobilized oil changes in the following ways:

• Less viscous components move downgradient through the soils.

• Volatile components are lost into the atmosphere.

• Soluble components are lost into the groundwater.

• Biodegradable components are lost to bacterial activity.

However, the total mass of immobilized oil decreases slowly because the loss processes are usually slow unless they are artificially enhanced as part of a remediation program. The natural rate of depletion becomes progressively slower with time, as the remaining contaminants are increasingly rich in those components that resist the loss mechanisms. The remaining oil becomes more and more firmly fixed in the subsurface soil, continually releasing its more soluble components in slowly decreasing concentrations to the groundwater.

Rules of Thumb

1. Less than 1% of the total mass of a gasoline spill will dissolve into water in the vadose and saturated zones.

2. Since more than 99% of a fuel spill remains as adsorbed or free product, it is impossible to clean up groundwater fuel contamination simply by "pump-and-treat" without eliminating the source residual and free product remaining in the soil.

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