Formation Of Petroleum Contamination Plumes

In the subsurface soil environment, petroleum compounds are present in four phases and four plumes. The four phases are

1. Liquid petroleum free product

2. Petroleum compounds adsorbed to soil particles

3. Dissolved petroleum components

4. Vaporized petroleum components

TABLE 5.3

Solubility Variability of Gasoline Components from Different Mixtures

Concentration Dissolved in Water (mg/L)

TABLE 5.3

Solubility Variability of Gasoline Components from Different Mixtures

Concentration Dissolved in Water (mg/L)

Compound

Regular

Regular

Super

Pure

Leaded

Unleaded

Unleaded

Compound

Benzene

30.5

28.1

67.0

1740-1860

Toluene

31.4

31.1

107.0

500-627

Ethylbenzene

4.0

2.4

7.4

131-208

1,2-dichloroethane

1.3

8524

Methyl-f-butyl ether (MTBE)

43.7

35.1

966.0

48,000

f-butyl alcohol

22.3

15.9

933.0

miscible

m-xylene

13.9

10.9

11.5

134-196

o-, p-xylene

6.1

4.8

5.7

157-213

1,2-dibromoethane

0.58

4300

Each phase behaves differently and poses different remediation problems. The liquid free product originates directly from the contamination source and initially has the same composition. The adsorbed, dissolved, and vapor phases are extracted from the liquid free product as it contacts water, soil, and air in the soil pore space. Each phase moves independently in its own distinct contaminant plume.

In general, the vapor contaminant plume moves the most rapidly. The dissolved plume moves more slowly at groundwater velocity or less, depending on its retardation factor. The free product plume moves slower than the dissolved plume. The adsorbed plume may be immobilized, or in the saturated zone part of it can be sorbed to mobile colloids and move at approximately the groundwater velocity.

Rules of Thumb

1. With respect to the total mass of fuel contaminant in the subsurface soil environment, the floating free product and immobilized oil (trapped by capillary forces and adsorbed to soil particles) are generally more than 99% of the total mass.

2. When free product is present, the dissolved phase in the groundwater is generally less than 1% of the total mass. The dissolved plume is just the tip of the contamination iceberg.

Example 5.1: Comparing Dissolved and LNAPL Free Product Masses

A leaking underground storage tank (UST) released 1000 gallons of gasoline (density about 0.7 g/mL) to the subsurface. After 1 year the resulting dissolved-phase plume is about 1000 ft long, 100 ft wide, and 10 ft deep. The average concentration of hydrocarbons in the plume is 0.002 mg/L (estimated by measuring and adding the total volatile hydrocarbon [TVH] and total extractable hydrocarbon [TEH] concentrations). The porosity of the aquifer is 0.30. If no hydrocarbon is lost due to volatilization or biodegradation, how much of the original release is in the dissolved phase and how much is in the LNAPL phase?

Answer:

Total mass released = (1000 gal)(3.78 L/gal)(1000 mL/L)(0.7 g/mL)(1 kg/1000 mg) = 2646 kg.

Volume of contaminated groundwater = (1000 ft)(100 ft)(10 ft)(0.30)(28.3 L/ft3) = 8.49 x 106 L.

Mass of dissolved hydrocarbons = (8.49 x 106 L)(0.002 mg/L)(1 kg/1000 mg) = 17.0 kg.

Mass of LNAPL free product = 2646 - 17.0 = 2629 kg.

Percent of total mass that is dissolved = (17 kg/2646 kg) x 100 = 0.64%.

Percent of total mass that is LNAPL = (2629/2646) x 100 = 99.36%.

Dissolved Contaminant Plume

Water solubility is the most important chemical property for assessing the impact of a contaminant on the environment. Dissolved contaminants arise when the free product comes in contact with water. The water may be in the form of moisture retained in the soil, precipitation percolating downward through the soil, groundwater flowing through contaminated soil or groundwater lying under a layer of free product. Both crude and refined petroleum products contain hundreds of different components with different water solubilities, ranging from slightly soluble to insoluble.

Rules of Thumb

1. In general, the lightweight aromatics such as the BTEX group (benzene, toluene, ethylbenzene, and xylene) are the most soluble components of fuel mixtures. If MTBE additive is present, it is the most soluble component by far.

2. The overall water solubility of commercial gasoline without additives ranges between 50 mg/L and 150 mg/L, depending on its exact composition. When free product gasoline is present, the dissolved portion generally accounts for less than 1% of the total contaminant mass present in the subsurface.

3. The overall solubility of fresh No. 2 diesel fuel in water is around 0.4-8.0 mg/L, again depending on its composition. When free product diesel fuel is present, the dissolved portion generally accounts for less than 0.1% of the total contaminant mass present in the subsurface.

4. Nevertheless, dissolved contaminants can greatly exceed concentration levels where water is regarded as seriously polluted.

The composition of the free product and dissolved fractions is very different, as indicated in Figure 5.5, because the more soluble compounds become concentrated in the water-soluble fraction.

Dissolved contaminants become a part of the water system and move with the groundwater but they usually move at a lower velocity because of their retardation by sorption processes. Sorption to soil and desorption back into the dissolved phase is a continual process that retards the movement of the dissolved phase. The amount of retardation depends mainly on the organic content of the soil. Retardation is greater in soils with more organic matter. Because their water solubilities are low, dissolved fuel contaminants continue to partition between the dissolved phase and soil particle surfaces, especially in soils with a high organic content.

Rule of Thumb

Typical retardation factors for BTEX in sandy soil range from 2.4 for dissolved benzene (groundwater moves 2.4 times faster than benzene) to 6.2 for dissolved xylene.

Vapor Contaminant Plume

Vapor phase contaminants arise from the volatile components of the free product escaping into adjacent air. Lower mass hydrocarbon components commonly associated with the gasoline fraction

FIGURE 5.5 GC/FID chromatograms of gasoline, diesel, and JP5 fuels and their respective water-soluble fractions. Time of elution, which corresponds roughly to the number of carbons in the eluted compound, increases from left to right. Thus, peaks corresponding to heavier compounds appear farther to the right in each figure. The composition of free product and dissolved fractions are very different in each case because the more soluble compounds become concentrated in the water-soluble fraction. The water-soluble fractions are composed mainly of 1-, 2-, and 3-ring aromatic hydrocarbons. Using calibration standards for the water-soluble fractions improves the accuracy of identifying water sample contaminants.

FIGURE 5.5 GC/FID chromatograms of gasoline, diesel, and JP5 fuels and their respective water-soluble fractions. Time of elution, which corresponds roughly to the number of carbons in the eluted compound, increases from left to right. Thus, peaks corresponding to heavier compounds appear farther to the right in each figure. The composition of free product and dissolved fractions are very different in each case because the more soluble compounds become concentrated in the water-soluble fraction. The water-soluble fractions are composed mainly of 1-, 2-, and 3-ring aromatic hydrocarbons. Using calibration standards for the water-soluble fractions improves the accuracy of identifying water sample contaminants.

are the most volatile. Vapor movement is not influenced by groundwater motion and only weakly by gravity. It follows the most conductive pathways through the subsurface, from regions of higher to lower pressure. Much of the vapor remains trapped in soil near its origin, slowly escaping to the surface atmosphere. A small portion of vapor phase contaminants may dissolve into soil-water, but it is generally insignificant.

Rules of Thumb

1. A measurable vapor concentration will be produced if either

Henry's constant (KH = Ca/Cw) > 0.0005 atm m3 mol-1 (this produces significant partitioning from water to air), or

Vapor pressure > 1.0 torr at 20° C (this results in significant diffusion upward through the vadose zone).

2. Characteristic vapor pressures for gasoline:

Fresh gasoline: 260 torr (0.34 atm). Weathered gasoline (2-5 years old): 15 to 40 torr (0.02 to 0.05 atm).

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