IntroductionGasoline CompositionFate and TransportContaminant
CharacteristicsContaminant PropertiesRisk AssessmentRemediation TechniquesPerspectives Organic compounds can be a major pollution problem in groundwater. Their presence in water can create a hazard to public health and the environment. This page will focus on the BTEX's which makes up one of the main groups of soluble organic compounds that find their way into our soil and groundwater. One of the most common sources for BTEX-contamination of soil and groundwater are spills involving the release of petroleum products such as gasoline, diesel fuel and lubricating and heating oil from leaking oil tanks. Because of their polarity and very soluble characteristics, the organic chemicals (BTEX's) of petroleum products will be able to enter the soil and groundwater systems and cause serious pollution problems.
The BTEX group of contaminants consists of benzene, ethyl benzene, toluene, and three isomers of xylene. These organic chemicals make up a significant percentage of petroleum products. The reason why the BTEX's, entering our soil and groundwater system, are considered such a serious problem is that they all have some acute and long term toxic effects. In addition to the toxicity's benzene is known to be a carcinogen.
The purpose of this page is to provide information and a visual understanding of BTEX contaminants and their characteristics. This page will try to provide information on what BTEX's are, how their characteristics determines their ability to move through groundwater systems, why they are a risk to humans and how they might be removed from the groundwater.
One of the most frequently encountered sources for BTEX contamination of soil or groundwater is spills that originates from leaking oil tanks. The Office of Technology Assessment estimates the number of underground storage tanks in The United States, that makes up a potential for soil and groundwater pollution, to be 2.5 million [2].
In order to describe the composition of a petroleum product we would like to use gasoline as a representative. As already described gasoline is one of the frequently encountered contributaters of BTEX contamination. Typically gasoline is composed of various hydrocarbons - the main compounds are alkanes, monocycloalkanes, dicycloalkanes, akylbenzenes, indanes, tetralins, naphthalenes and some oxygenated alcohol additives [7]. The BTEX's make up the alkylbenzenes and as shown in the pie chart below (left) they make up about 18% (w/w) in a standard gasoline blend. When stating this it's of importance to mention that the composition of gasoline can vary a lot depending on the manufacturer, the refining process and the time of production. Of the different components that make up gasoline the BTEX's is the largest group that's related to any health effects. Naphthalenes, which are also of health concern, only make up 1% (w/w) of gasoline [7]. Looking at the BTEX's in gasoline this fraction can be broken down into the specific fractions shown in the pie chart below (right).
The release of BTEX's to the environment is influenced by their fate and transport mechanisms. The BTEX appearance in soil and groundwater and it's ability to be remediated is affected by volatilization, dissolution, sorption and degradation by microorganisms.
Volatilization will affect the actual concentration of BTEX's. When analyzing a gasoline contaminated site the first thing that will happen is that some of the gasoline will volatilize due to the high solubility, the relatively low molecular weight and the high vapor pressure [2]. Looking at the characteristics for the different components benzene is the compound that will volatilize the easiest.
Concurrently with volatilization the gasoline will begin to dissolve into the groundwater. Compared to the other main group of components in gasoline, such as the aliphatics, BTEX's are very soluble, and the solubility's of the different BTEX components will have a great effect on the concentrations that may appear in the groundwater. When gasoline is dissolved in a aqueous phase, it will be able to move with the groundwater.
Instead of dissolution, sorption might take place between the organics and the soil particles. Sorption is defined either as the attachment of the organics to the soil surface or as the intake of the organics into the soil matrix. The sorption is controlled by contaminant characteristics such as the solubility, polarity and the octanol-water partition coefficient (Kow). In addition sorption is influenced by the characteristics of the soil matrix and the fluid media [2]. The BTEX's are not sorbed to the soil matrix as strong as the aliphatic components (the different alkanes), and they are more likely to contaminate larger water volumes. Sorption will be a factor in determining the movement of contaminants with the groundwater flow. If one is dealing with a small gasoline contaminated area sorption might be favorable to trap the pollutant for remediation and by that way avoiding the spreading of the contaminant. In that case it will be fairly easy to dig up and remove the contaminated soil. On the other hand, if one is dealing with a site where gasoline has dissolved and now is present in a larger area sorption will be unwanted/favorable because it will be more difficult to dig up the much larger amount of soil, and one would rather deal with a nonsorbing contaminant that's readily available for degradation techniques .
The degradation of BTEX's is another aspect that affects the concentration in soil and groundwater. The natural bacterial flora in soil has an ability to aerobicly degrade the BTEX's. This degradation will reduce the concentration in soil and groundwater. The degradation techniques are sometimes recommended as a natural means of remediation (intrinsic bioremediation). By adding nutrients and oxygen its possible to enhance the degradation process. Anaerobic biodegradation has been repeatedly established when dealing with toluene, and some would mention the possibility of an anaerobic biodegradation of xylenes too.
Thus the fate and transport mechanisms are affected by the contaminant characteristics, which vary with the different BTEX compounds. In general the processes volatilization, dissolution and degradation determines the concentrations of BTEX's and the sorption (and dissolution) determines the transport in soil and groundwater systems.
This drawing links together the different processes relatated to soil and groundwater that are discribed in the section above.
This table is to summarize the characteristics for the six
organic components in context.
| Benzene | Toluene | m-Xylene | o-Xylene | p-Xylene | Ethyl benzene | |
|---|---|---|---|---|---|---|
| Chemical structure |
 |
 |
 |
 |
 |
 |
| Chemical formula |
C6H6 | C7H8 | C8H10 | C8H10 | C8H10 | C8H10 |
| Molecular weight [g/mole] |
78 | 92 | 106 | 106 | 106 | 106 |
| Water solubility [mg/l] |
1700 | 515 | - | 175 | 198 | 152 |
| Vapor pressure (at 20
C) [mm Hg] |
95.2 | 28.4 | - | 6.6 | - | 9.5 |
| Specific density (at
20 C) |
0.8787 | 0.8669 | 0.8642 | 0.8802 | 0.8610 | 0.8670 |
| Octanol-water partition
coeff. (at 20 C) [log Kow] |
2.13 | 2.69 | 3.20 | 2.77 | 3.15 | 3.15 |
| Henry's law constant
(at 25 C) [kPa*m3/mole] |
0.55 | 0.67 | 0.70 | 0.50 | 0.71 | 0.80 |
| Polarity |
Non-polar | Non-polar | Non-polar | Non-polar | Non-polar | Non-polar |
| Biodegradability |
Aerobic | Anaerobic/Aerobic | Aerobic | Aerobic | Aerobic | Aerobic |
| Maximum Contaminant Level(MCL) [mg/l] |
0.005 | 1 | 10 * | 10 * | 10 * | 0.7 |
* The Environmental Protection Agency (EPA) lists an
MCL of total xylenes of 10 mg/l [8].
The molecular weight of compound is measured in
g/mole. Generally, the higher the molecular weight the
less soluble in water. Molecular weight also effects the
density of a compound.
Solubility is the measurement of the maximum
concentration of a chemical that will dissolve in pure
water at a specific temperature, measured in mg/L. Water
solubility plays a large role in a chemicals movement and
distribution through soil and groundwater.
Polarity is associated with a compounds charge. The
polarity arises from the existence of a slightly negative
charge on one part of a compound and a slightly positive
charge on the other, which will cause the formation of a
dipole. Water for instance, is considered a dipole
because of its offset of positive and negative charges.
Non-polar compounds are hydrophobic, meaning they
don’t want to be attached to water molecules, and
will be more likely to adsorb to the organic portion of a
soil or they will volatalize. Polar compounds have a
affinity for the liquid. Benzene is non-polar because of
its almost neutral charge. It is not as non-polar as the
other contaminants in the BTEX group and has the ability
to dissolve in water.
The specific density is the ratio between the density
of the actual component and the density of water. The
density is measured as dry mass per volume [kg/m3].
The density of the contaminants effects the organic
compound's ability to float on water.
This a ratio of the concentration of a dissolved
substance in a two-phase system at equilibrium. After a
chemical has been mixed in a octanol and water solution
the system is allowed to reach equilibrium. The two
phases will partition and a ratio of the chemicals
concentration in the octanol phase and water phase is
taken. This ratio gives a relation of a chemicals
accumulation in water. More polar compounds will tend to
have a low Kow. This is also a measurement of the
hydrophobicity of an organic. The more hydrophobic the
more the contaminant will adsorb to soil and have a low
solubility.
The vapor pressure of a liquid is the pressure of the
gas in equilibrium with respect to the liquid or solid at
a given temperature. Vapor pressure represents a
compounds tendency to evaporate and is essentially the
solubility of an organic solvent in a gas. High vapor
pressures mean that the compound is more likely to
volatilize out of solution.
The Henry’s Law constant is a property of a
chemical that expresses its partition between the air and
water phases. It helps to predict the behavior of a
organic compound in the environment and in remediation
procedures such as air stripping processes. These values
also describe a chemicals movement from water to air and
vise versa. High values mean that the chemical will move
more toward the gas phase where as low values will stay
in the aqueous phase.
All of these organic chemicals under go aerobic
biodegradation by microorganisms. This meaning that the
degradation takes place in the presence of oxygen(O2).
Toluene can undergo degradation in aerobic conditions.
Biodegradation in subsurface environments, such as in
soil and groundwater systems, is important in determining
the fate of these BTEX contaminants. Knowing how well
these compounds will degrade can help with treatment
procedures, such as in-situ treatments.
Maximum contaminant levels are used to regulate the
levels of BTEX's in drinking water. These levels are set
by the Environmental Protection Agency (EPA) in order to
prevent health effects (see 6.
Risk Assessment). The specific levels are determined
from analyzing animal laboratory experiments and an
imposed safety factor [11].
What make BTEX's a topic of concern, are the toxic effects that they can have on human health. All the compounds are acutely toxic and have noticeable health effects at high concentrations. Exposure to these compounds from groundwater systems is usually minimal but the exposures can be persistent over a long period of time (long time effects). For that reason it's more the long term effects of BTEX exposure and intake that needs to be considered.
The BTEX compounds can enter the body through ingestion of contaminated crops, inhalation of vapor from the soil, intake of contaminated drinking water and skin exposure. Drinking and bathing in water containing the organic compounds can put one at risk of exposure. Since BTEX's will evaporate out of water one can also be exposed by inhaling the vapors that come off from drinking water [5].
Most studies have been done on benzene which is the only proven carcinogen. Benzene is suspected to be link to leukemia which is a disease of the blood making system in our bones. Strong evidence between long term exposure to benzene and leukemia have been found. Upon exposure to benzene, the benzene will move into the blood stream. From the blood stream it can get into fatty tissues where it can undergo reactions that produce phenol, which is an even more serious carcinogen than benzene [5], [6].
The inhalation of toluene and xylenes in concentrations of 0.4 mg/l causes headache, dizziness and irritation of the mucous membranes. In higher concentrations toluene and xylenes can led to a reduced ability of co-ordination. Long term exposure of toluene and xylenes have been proven to cause brain damage, but neither of them are carcinigens [5].
Biodegradation in a technique to remediate
contaminated soil and groundwater. Using this technique
microorganisms are degrading the organic components to CO2
and water. Oxygen and nutrients might be injected to
promote the degradation rate. If nothing is being added
the biodegradation is called intrinsic. The degradation
may occur under the use of different electron acceptors
than oxygen. For instance toluene may degrade via an
anaerobic pathway using nitrate as an electron acceptor.
Vapor phase extraction is used for the removal of
volatile organics from the unsaturated zone. By
implementing vapor phase extraction, slotted pipes are
placed in the soil and a vacuum is applied to volatilize
the organic compounds. When volatilized a pressure
gradient will ensure that the organic compounds are
transported to the extraction wells. As a side effect the
increased air flow through the soil might be able to
increase the natural biological degradation of the
organics [10]. The two pictures below shows (left) the
vapor phase extractor and (right) a site where slotted
pipes are installed in the contaminated soil.
Bioventing is a microbiological degradation process.
Like in soil vapor phase extraction (SVE) both injection
and extraction wells are installed, but in addition to
the SVE technique air, moisture and nutrients may be
added to stimulate a biological degradation of the
organics [10]. The process is able to remove both the
volatile organics such as BTEX's and the more heavy-end
contaminants such as diesel and crude oil. By going to
this site you'll see a schematic outline of the bioventing process.
Air sparging is a physical and/or microbiological
degradation process. By driving large quantities of air
into the saturated zone this technique enables an aerobic
degradation of organics. The injection of air to the
saturated zone serves two purposes. First it drives the
volatile organics into the unsaturated zone, from where
they may be removed by vapor phase extraction. Secondly
it increases the microbiological activity in the
saturated zone [10]. By jumping to this site you'll see a
schematic outline of the combined air sparging
and vapor phase extraction process.
This treatment is used after groundwater has been
pumped out of the aquifer. The contaminated water is
passed through the activated carbon unit where the
organics are adsorbed and collected. This is accomplished
through the adsorption of the chemical substance onto a
carbon matrix. The effectiveness of this processes is
related to the amount of surface area on the activated
carbon and the properties of the chemical substance. This
process is often used to remove relatively low
concentrations. By going to this site you'll see a
schematic outline of the activated carbon
treatment process.
Air stripping is a pump and treat method that uses a
countercurrent flow of upward air-solvent gas and
downward water-solvent liquid to physically strip the
contaminant out of the water. Often activated carbon
systems are used as a secondary step to this procedure.
Go to this site to see a schematic outline of how the
process of air
stripping works.
.
Because of the health concerns surrounding soil and groundwater contamination from petroleum products, there is a high interest in researching this area. The continual release of petroleum products into the subsurface environment, requires the need for monitoring and remediation techniques. Even though there has been a lot of research going on within this area the problem of how to set up actual remediation processes is still very complex.
What on the technical side is being done to limit the problem
with leaking underground storage tanks ?
Click here for sound answer. An alternative to the steel tanks that corrode more easily is the use of fiberglass tanks which are more resistant to corrosion. A disadvantage to the use of fiberglass tanks is that they are very fragile and have a tendency to crack, especially during installation. Because of this tendency to break, steel tanks are most commonly used.
Send comments or suggestions to:
Student Authors: Jesper Steen Christensen, jechrist@vt.edu and Jason
Elton, jelton@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
