The Fate and Transport of Oxygenates and Other Petroleum Constituents, and their Influence on Appropriate Underground Storage Tank Management

By Michael J. Day
Applied Hydrology Associates, Inc., Denver, Colo.
mday@appliedhydrology.com

Abstract

Fuel leaks from underground storage tanks (USTs) and piping have been a major source of groundwater contamination.  In the US and Europe, regulations requiring upgrading of USTs to meet specific standards have significantly reduced instances of fuel contamination.  However, complete containment of tank and piping systems is generally not required, and routine groundwater monitoring is not usually performed.  The limitations of UST leak detection systems result in a situation where contamination of groundwater by fuel components may occur due to undetected leaks from uncontained UST systems.  The fate and transport characteristics of fuel components significantly influence the potential risk to groundwater supplies and the methodologies to manage and remediate contamination resulting from UST leaks.  The recognition that oxygenates in fuels tend to be more mobile in groundwater systems has put an increased emphasis on early detection and response to fuel leaks and spills.

Introduction

Leaks of fuel from underground tanks and associated piping have been a major source of groundwater contamination.  The fate and transport characteristics of fuel components significantly influence the potential risk to groundwater supplies and the methodologies to manage and remediate contamination resulting from UST leaks.  Understanding the fate and transport of fuel components under various hydrogeologic and geochemical conditions is critical to developing an appropriate management plan.  The recognition that oxygenates in fuels tend to be more mobile in groundwater systems has put an increased emphasis on early detection and response to fuel leaks and spills.

Phase Transformation of Fuel Components

Fuel components may be converted from the fuel liquid phase to the air, water, and solid-phases by a variety of physical, chemical, and biological processes that can occur in the subsurface. The physical and chemical characteristics of organic compounds tend to govern the tendency for the compound to favor certain phases over others.  For example, highly soluble compounds will favor the water phase over the air or solid phases.

The volatilization process from the liquid phase to the vapor phase is driven by concentration gradients.  Given sufficient time, in a closed system, equilibrium will be established between concentrations of a compound in the air and in the released fuel.  The vapor pressure of a volatile fuel component, which is temperature dependent, is a measure of the tendency of that component to move from the liquid to the vapor phase.  The more volatile constituents of fuel will tend to be most readily vaporized.  The partial pressure of a volatile component in equilibrium with fuel is equal to its pure-phase vapor pressure multiplied by its fractional content in the fuel as described by Raoult’s Law (Barker and others, 1991).  Therefore, as a volatile component in the NAPL is reduced, both the contact efficiency and the partial vapor pressure is also reduced, making it more difficult for the component to volatilize.  The partial pressure of MTBE is higher than other fuel components and it will therefore volatilize more readily. 

Volatilization and condensation are complementary processes that describe the exchange of a volatile compound between water and air.  Volatilization refers to the movement of the compound from water into air whereas condensation refers to the movement from air into water.  The interchange between the soil vapor and the pore water will occur within the vadose zone and at the water table interface.  Equilibrium can be established between concentrations of a compound in air and water as described by the Henry's Law constant, which is temperature dependent.  If the same units are selected for the air and water concentrations, then the Henry's Law constant is dimensionless.  A compound with a value of 0.05 or larger is considered to volatilize easily from water.  In comparison with BTEX components, ethanol and MTBE both tend to partition strongly from the gas phase into the water phase.  Consequently, dissolved ethanol and MTBE tend to stay in the water phase. 

Solution describes the process of component exchange from the fuel phase into the water phase.  Solution of fuel components will occur within the pore water of the vadose zone as well as in the saturated groundwater zone below the water table.  Infiltrating water that passes through soils affected by fuel leaks will tend to dissolve the volatile components of residual fuel within the soils.  Water solubility is probably the most important chemical property affecting the partitioning of organic compounds between fuel and water.  Ethanol is infinitely soluble in water and MTBE is more water-soluble than the BTEX compounds in fuels.  The pure-phase solubility of an organic compound in fuel is reduced in proportion to its fractional content in the fuel because of partitioning between the organic mixture and water (Barker and others, 1991).  For example, a reformulated gasoline that is 10% by weight MTBE and 1% benzene, reduces the solubility of MTBE in water to about 4,300 mg/L and benzene to about 18 mg/L.  In contrast, for a nonoxygenated gasoline, the total hydrocarbon solubility in water is typically about 120 mg/L (Poulsen and others, 1992).  Because of their relatively high concentration in reformulated gasoline and their water solubilities, oxygenates such as ethanol and MTBE will tend to be found in higher concentrations than BTEX in shallow groundwater that is in contact with oxygenated gasoline released from UST leaks. 

Water solubility also affects the partitioning of organic compounds between water and subsurface solids.  Many organic compounds exhibit water solubilities in the low milligrams-per-liter to micrograms-per-liter range.  In general, these low solubilities indicate a strong partitioning to the organic carbon associated with the subsurface solids.  The relatively high solubilities of ethanol and MTBE result in a tendency for these constituents to stay in the water phase and not sorb to subsurface solids.  The tendency for compounds to sorb to the organic phase of soils is also reflected by the organic partition coefficient (K_oc).  Ethanol and MTBE have low K_oc values in comparison to BTEX components, reflecting their lesser affinity for soil adsorption. 

Degradation of an organic compound refers to its transformation by abiotic or biotic reactions.  Other organic compounds may be formed as byproducts of the degradation process.  Complete degradation (mineralization) of an organic compound to carbon dioxide and water is almost always associated with some form of microbial activity.  Biological transformations can involve many reactions and may require a long period of time to complete, but often provide the predominant decay pathways in water and soil.

It is well documented that BTEX compounds undergo biological transformations in groundwater.  Ethanol is readily biodegradable under aerobic and anaerobic conditions (Powers et al, 2001).  There is increasing evidence that MTBE also naturally degrades, particularly under aerobic (oxygenated) conditions, albeit at a rate generally slower than the BTEX compounds, and under methanogenic conditions (Wilson et al, 2000).  This indicates that some indigenous microorganisms are able to degrade MTBE (Thomas and others, 1988).  Ongoing research projects, such as at Vandenberg AFB in California, have demonstrated that adding oxygen may be extremely effective in stimulating indigenous biodegradation of MTBE (McKay, 2000).

Biodegradation in the vapor phase can also occur, particularly if atmospheric exchange allows the continued introduction of oxygen to promote aerobic respiration.  Barometric and temperature variations associated with diurnal fluctuations and storm events can produce air exchanges between the atmosphere and the soil vapor of the vadose zone (Massmann and Farrier, 1992).  Vapor phase biodegradation of organic compounds is usually much faster than aqueous phase biodegradation as long as there is sufficient water vapor present. 

Fate and Transport Processes at LUST Sites

Fate and transport of fuel components in the subsurface at leaking underground storage tank (LUST) sites are determined by their physical and chemical characteristics and by the hydrogeologic and geochemical conditions at the site. A leak in one or more tanks at an LUST site may provide a source of fuel to the subsurface.  As long as the source of the fuel release continues, the subsurface extent of fuel liquid will continue to expand until it reaches a physical barrier (such as the water table), or until natural processes remove the fuel components at the same rate as they are introduced. 

The fate and transport of fuel components released from a typical LUST site are discussed in terms of three distinct zones where different hydrogeological conditions exist. The zones considered are: (a) the tank backfill, (b) the unsaturated or “vadose” zone below the tank backfill and above the groundwater table where the soil pores are not entirely saturated with water, and (c) the saturated groundwater within a shallow unconfined aquifer below the LUST site.

Tank Backfill Zone

Typically, a fuel retail station has several USTs located in a common excavation that is backfilled with clean sand.  The depth to the base of the tank backfill is usually 8 to 10 feet (2.5 to 3 meters) below ground surface, depending on the size of the USTs.  The sandy backfill materials would be expected to have fairly high permeability.  The excavated area is usually capped with asphalt so that infiltration of surface water into the tank backfill is very limited.  Typically, the tank excavation is not lined prior to tank installation and sand backfilling, so that there is direct hydraulic connection between the backfill and the native soils.  In areas where the natural groundwater table is close to the ground surface, the base of the underground tanks may be in water-saturated backfill.  More typically the tank backfill is located well above the natural groundwater table and the backfill sands are unsaturated. 

In the case of a LUST in an unsaturated backfill, released fuel will tend to migrate by gravity to the base of the tank backfill.  If the native soils are less permeable than the tank backfill, fuel will tend to accumulate and spread out at the base of the backfill.  If the tank “pit” is unlined then fuel will eventually accumulate in the backfill to the point where it will overcome the air-entry pressure of the underlying soil pores and start to infiltrate into these soils. 

The major processes operating in the tank backfill are migration of the non-aqueous phase liquid (NAPL) fuel and volatilization of released fuel into the air within the backfill pore spaces.  Soil vapor transport of fuel constituents can occur through diffusion and advection processes in the tank backfill.  Biodegradation in the vapor phase can also occur, particularly if atmospheric exchange allows the continued introduction of oxygen to promote aerobic respiration. 

In cases where the tanks are located well above the natural groundwater table, the backfill provides an aerated zone that can be monitored for fuel vapors as a method of leak detection.  MTBE has the highest vapor pressure of all the volatile constituents within oxygenated fuels.  It also may constituent a relatively high percentage of the fuel (10 to 15% in reformulated and oxyfuels) and so will tend to be found in the highest concentrations in soil vapor.  Due to its high vapor pressure, monitoring of fuel vapors is a particularly effective method for detection of MTBE.  Low-volume soil vapor venting of tank and piping backfill can also be used to detect and remove the volatile components of fuel from small chronic leaks (Day et al, 2001). 

Vadose Zone

If the tank pit is unlined, accumulation of fuel in the tank backfill will eventually lead to infiltration of the underlying soils of the vadose zone.  Fuel would continue to migrate both vertically and laterally at a rate determined primarily by the stratification and permeability of the native soil materials.  Stratification of the native soils in the vadose zone beneath the tank backfill will tend to cause localized accumulation and lateral migration of fuel.  If the fuel reaches the water table, it will tend to “float” and spread out on top of the saturated water zone due to its lower density. 

The major processes that occur within the vadose zone are volatilization, condensation, solution, adsorption, and biodegradation.  Volatilization of released fuel will continue to occur into the air-filled pore spaces of the vadose zone.  Soil moisture in the vadose zone will interact with soil vapor and allow exchange of volatile fuel constituents from the air phase to the water phase. 

Once the fuel components are in the air or water phase they can continue to migrate under the prevailing geologic conditions.  Soil vapor transport of fuel constituents can occur through diffusion and advection processes in the soil vadose zone.  Mass loss of fuel components to the atmosphere can occur at the land surface interface if the constituent remains in the vapor phase.  In arid and semiarid environments, the vadose zone will have a relatively low moisture content over most of the year and there is limited opportunity for fuel constituents that have volatilized directly from leaked fuels to solubilize into the water phase as a result of vapor contact with soil moisture.  However, if the soil moisture content in the vadose zone is high, then relatively soluble compounds such as ethanol and MTBE will not tend to stay in the vapor phase.  

The overall downward velocity and travel times of infiltrating water containing dissolved gasoline constituents originating at a LUST site depends on many factors, such as recharge rate, horizontal and vertical hydraulic conductivity of the materials in the vadose zone, and the thickness of the vadose zone.  However, due to the different physical characteristics of the gasoline components, they tend to migrate in the dissolved phase at different rates.  Migration of dissolved ethanol and MTBE in the vadose zone does not tend to be retarded by sorption to soils and it therefore moves to the ground water at almost the same velocity as the recharge water.  The dissolved BTEX components of gasoline will tend to migrate at a slower rate due to their higher tendency to be sorbed to the soils. 

Aerobic biodegradation of all dissolved constituents that may occur within the vadose zone will reduce the overall mass of constituents reaching the groundwater table.  The more mobile constituents of oxygenated fuels, such as ethanol and MTBE, will tend to be at the leading edge of an advancing dissolved “front” and therefore have the most exposure to soil air (and oxygen) in the vadose zone.  Aerobic degradation tends to be most active at the front and edges of the dissolved plume of fuel constituents where oxygen contents are highest, so that ethanol and MTBE have the highest potential for degradation.  Consumption of oxygen by biological degradation processes can lead to anaerobic conditions behind the leading edge of the dissolved plume so that degradation of less mobile dissolved constituents is not as rapid.  Several researchers have speculated that the high biodegradability of ethanol could lead to increased mobility of BTEX constituents within a depleted oxygen “shadow” behind an advancing ethanol front (Powers et al, 2001). 

Saturated Aquifer Zone

If leakage of fuel is sufficient, it may eventually migrate to the water table, where it will tend to “float” and spread out on top of the saturated water zone due to its lower density.  The accumulation of fuel at the water table will provide a source for dissolved constituents in the saturated groundwater zone below the water table.  The major processes that occur within the saturated zone are dissolution, advection, dilution, dispersion, adsorption, diffusion, and biodegradation.  Volatilization of dissolved fuel constituents can also continue to occur at the air-water interface if the saturated groundwater is unconfined. 

Constituents from released fuels that dissolve into groundwater will migrate at a rate dependent on their tendency to sorb onto the soil matrix.  The ratio of the groundwater velocity to the velocity at which a compound is transported is frequently referred to as the retardation factor, R.  The actual values of R for a particular compound depend on the organic partition coefficient (K_oc) of the compound and aquifer properties, such as porosity, bulk density, and organic carbon content.  A compound that moves at one-half the velocity of the ground water has an R value of 2.  Once ethanol or MTBE is in the saturated zone groundwater, their low adsorption characteristics allow them to move at virtually the same velocity as the groundwater (i.e. their R values are fairly close to 1 for typical aquifers).  BTEX compounds have R values that can range from 1.1 to about 2.0 (Zogorski and others, 1997).  Ground-water flow velocities vary widely depending on several factors including the permeability, porosity, and hydraulic gradient of the aquifer.  Velocities under typical hydraulic gradients can range from a few millimeters per year to a meter per day.  Ground-water velocities near a pumping well tend to be higher due to increased hydraulic gradients existing near these wells.

Dispersion will tend to mix and dilute the concentrations of fuel constituents within the saturated groundwater zone.  Dispersion will tend to be more apparent in more heterogeneous aquifer systems.  Dilution of dissolved constituents by mixing with unaffected groundwater recharge is also commonly observed in unconfined aquifers.  Recharge that occurs along the flow path of an affected groundwater plume will tend to establish a downward vertical hydraulic gradient and cause the affected groundwater plume to migrate downwards.  This has been referred to as a “sinking” plume, and is most apparent in situations where an affected groundwater plume has migrated over a significant lateral distance. 

Diffusion of dissolved constituents into lower permeability lenses within the aquifer, or confining units above and below the aquifer, may also contribute to a decrease in constituent mass.  Diffusion effects will be most apparent in highly stratified aquifer units.

Due to their chemical characteristics, oxygenates such as ethanol and MTBE tend to be more mobile in groundwater than other fuel components.  Ethanol would be expected to degrade very quickly in the subsurface so that it would not tend to persist.  However, very few field studies of ethanol in groundwater have been performed so that this assumption has not been verified.  The high biodegradability of ethanol could lead to increased mobility of BTEX constituents due to a decreased rate of biodegradation of these constituents within a depleted oxygen “shadow” behind an advancing ethanol front (Powers et al, 2001). 

The detection of MTBE ahead of benzene in downgradient monitoring wells at many LUST sites would tend to support the expected higher mobility of MTBE.  However, several field studies have not shown a significant difference in MTBE and benzene plume lengths, suggesting that natural attenuation processes for MTBE have typically been underestimated.  Where MTBE plumes in groundwater persist or grow, this can usually be attributed to a continuing source, including residual fuel in soils below LUST sites.  This emphasizes the importance of early detection and complete source removal when dealing with releases of oxygenated fuels. 

Implications with Respect to Managing UST Sites

In the US and Europe, regulations requiring upgrading of underground storage tanks (USTs) to meet specific standards have significantly reduced instances of fuel contamination.  Leak detection systems at gasoline retail stations are primarily dependent on physical leak measurement systems rather than on indirect monitoring of backfill or shallow groundwater.  These systems are generally capable of detecting leaks as small as 0.05 gallons (0.2 liters) per hour.  Fuel leaks that are smaller than this detection threshold may remain undetected for long periods of time and result in releases of fuel to the subsurface.  Federal regulations in the US governing upgraded USTs do not include the requirement for containment of tank and piping systems, or for routine groundwater monitoring.  This situation may lead to contamination of groundwater by fuel components due to undetected leaks from uncontained UST systems. 

Most fuel components have relatively low solubility, sorb to the soils, and can be shown to naturally degrade in the subsurface.  As a consequence, natural attenuation processes largely prevent widespread fuel contamination from leaking USTs.  However, small leaks from USTs might result in gradual groundwater contamination unless the leaks are detected and appropriate response taken in a timely fashion.

Due to the physical and chemical characteristics of fuel oxygenates such as MTBE, it is clear that the key to managing oxygenated fuels is to be able to detect releases soon after they occur, and to respond rapidly with appropriate corrective action.  Undetected releases of fuel from leaking USTs have the potential to impact shallow groundwater, particularly if subsurface conditions are not conducive to natural attenuation processes, and secondary containment does not exist.  Monitoring of the tank and piping backfill for persistent fuel vapor concentrations under very low vapor extraction conditions may be the most definitive way to detect small chronic fuel leaks.  For underground tank systems that are above the groundwater table, this technique is particularly applicable for MTBE because of its high vapor pressure characteristic.  Venting systems can be installed in existing tank systems without the necessity of pulling the tanks.  This is a relatively low cost method of preventing groundwater contamination by volatile fuel components from small chronic leaks.  Routine monitoring of shallow groundwater should be a component of a leak detection program, particularly in high-risk areas.

References

Barker, J.F., Gillham, R.W., Lemon, L., Mayfield, C.I., Poulsen, M., and E. A. Sudicky, 1991, Chemical fate and impact of oxygenates in groundwater--Solubility of BTEX from gasoline--Oxygenate Compounds: Washington, D.C., American Petroleum Institute Publication 4531, 90p.

Day, M.J., R. Reinke, and J.A.M. Thomson, 2001, Fate and Transport of Fuel Components below Slightly Leaking Underground Storage Tanks.  International Journal of Environmental Forensics, 3(1), March 2001.

Mackay, D, M. D. Einarson, R. D. Wilson, B. Fowler, K. Scow, M. Hyman, C. Naas, M. Schirmer, and G. C. Durrant, 1999, Field Studies of In-Situ Remediation of an MTBE Plume at Site 60, Vandenberg Air Force Base, California.  Proceedings of the 1999 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation. Houston, TX, Nov. 17-19, 1999, 178-188.

Massmann, J., and D.F. Farrier, 1992, Effects of atmospheric pressures on gas transport in the vadose zone. Water Resources Research, 28(3), 777-791.

Poulsen, Mette, Lemon, Lloyd, and Barker, J.F., 1992, Dissolution of monoaromatic hydrocarbons into groundwater from gasoline-oxygenate mixtures: Environmental Science & Technology, v. 26, no. 12, p. 2483-2489.

Powers, S.E., D. Rice, B. Dooher, P. Alvarez, 2001, Will Ethanol-Blended Gasoline Affect Groundwater Quality?  Environmental Science & Technology, v. 35, no. 1, p. 24A-30A.

Squillace, P.J., J.F. Pankow, N.E. Korte, and J.S. Zogorski, 1998, Environmental Behavior and Fate of Methyl tert-Butyl Ether (MTBE).    U.S. Department of the Interior - U.S. Geological Survey, National Water Quality Assessment Program (NAWQA), Fact Sheet FS-203-96.

Thomas, J.M., Clark, G.L., Tomson, M.B., Bedient, P.B., Rifai, H.S., and C. H. Ward, 1988, Environmental fate and attenuation of gasoline components in the subsurface: Washington, D.C., American Petroleum Institute, final report, p. 111.

Wilson, J.T., J.S. Cho, B.H. Wilson, and J.A. Vardy, 2000, Natural Attenuation of MTBE in the Subsurface under Methanogenic Conditions; National Risk Management Research Laboratory, Ada, OK.  EPA/600/R-00/006, 49p.

Zogorski, J.S., and others, 1997, Fuel oxygenates and water quality, in Interagency Assessment of Oxygenated Fuels: Washington D.C., Office of Science and Technology Policy, Executive Office of the President, chap. 2, 80 p.

Table 1:  Comparison of Fate and Transport Properties for Fuel Constituents

*    Based on gasoline containing 10% mtbe or ethanol, 5% toluene, 8% xylenes, 1% ethylbenzene, and 1% benzene
**  At a temperature of 25^oC
NA Not Available

1         Urie Environmental, Chemical Substance Hazard Assessment & Protection Guide, 1994.

2         Fuel Oxygenates and Water Quality: Current Understanding of Sources, Occurrence in Natural Waters, Environmental Behavior, Fate, and Significance. Chapter 2 in Interagency Assessment of Oxygenated Fuel, Office of Science & Technology Policy, Executive Office of the President, Washington, D.C., Zogorski, J.S., A. Morduchowitz, A.L. Baehr, B.J.Bauman, D.L. Conrad, R.T. Drew, N.E. Korte, W. W. Lapham, J. F. Pankow, and E.R Washington., 1997

3         Malcolm Pirnie, Inc., 1998, Evaluation of the fate and transport of ethanol in the environment:  Report prepared for the American Methanol Institute, November  1998.

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