18.1 Introduction 688
18.2 Legislative and Regulatory Overview 689
18.2.1 Subtitle I of the RCRA 689
18.2.2 Subtitle J of the RCRA 691
18.2.3 State and Local UST Programs 691
18.2.4 USTs Containing Other Hazardous Chemicals 691
18.3 Causes of Leaks and Leak Identification Methods 691
18.3.1 Causes of Tank Failure 691
18.3.2 Leak Identification Methods 692
18.4 Underground Conditions and Factors Affecting Transport of Liquids in the Subsurface 694
18.4.1 Underground Formations 694
18.4.2 Gravitational Force Affecting Underground Liquid Movement 694
18.4.3 Atmosphere Pressure Affecting Underground Liquid Movement 695
18.4.4 Surface Tension and Capillary Potential Affecting Underground
Liquid Movement 695
18.4.5 Adsorptive Force Affecting Underground Liquid Movement 697
18.4.6 Combination of Capillary and Adsorptive Forces Affecting
Underground Liquid Movement 698
18.4.7 Energy Conservation Affecting Underground Liquid Movement 700
18.4.8 Water Movement in Saturated Zone of Soil Formation 701
18.4.9 Water Movement in Unsaturated Zone of Soil Formation 701
18.5 Properties of Gasoline and its Movement in Soil 702
18.5.1 Properties of Gasoline and the Forms of Release Underground 702
18.5.2 Fates of Gasoline Underground: Adsorption and Degradation of Gasoline and the Effect on Gasoline Movement 703
18.5.3 Multiphase Movement of Gasoline Compounds 705
18.6 Management of Tanks and the Environment as Remedial Actions 706
18.6.1 Tank Removal 707
18.6.2 Tank Repair 707
18.6.3 Tank Replacement 707
18.6.4 Alternatives for Tank Abandonment and Replacement 708
18.7 Control of Contaminants Migration as Remedial Actions 708
18.7.1 Gas Control 708
18.7.2 Control of Plume Migration 708
18.8 Removal of Contaminants as Remedial Actions 709
18.8.1 Removal and Recovery of Free Product 709
18.8.2 In Situ Biological Treatment of Groundwater Decontamination 713
18.8.3 Pump-and-Treat Processes for Groundwater Decontamination 719
18.8.4 Removal of Gasoline from Contaminated Soil 734
18.9 Phenomena Related to the Release of DNAPLs and Other Hazardous
18.9.1 Chemical and Physical Properties of DNAPLs 745
18.9.2 Fate of DNAPL Release Underground 745
18.9.3 Site Remediation 746
18.9.4 Practical Examples 750
Underground storage tanks (USTs) comprise one or a combination of tanks (including the associated underground piping) that are used to contain substances regulated under the RCRA1,2 (Resource Conservation and Recovery Act) or CERCLA3,4 (Comprehensive Environmental Response, Compensation, and Liability Act—Superfund), the volume of which include 10% or more located below ground surface (bgs). Generally, this term does not encompass residential and farm tanks holding 4164 L (1100 gal) or less of motor fuel used for noncommercial purposes, tanks storing heating oil to be used on the premises where it is stored, tanks on or above the floor of an underground area, such as basements or tunnels, septic tanks, and systems for collecting wastewater and stormwater, flow-through process tanks, emergency spill and overfill tanks, and related pipeline facilities.5-7
When the UST program began, there were approximately 2.1 million regulated tanks in the U.S. Today there are far fewer, because many substandard UST systems have been closed.8 According to the U.S. Environmental Protection Agency (U.S. EPA), less than 5% of the current number of UST tanks store hazardous substances.6 The majority of these tanks are used to store petroleum products for retail and industrial purposes. of the regulated tanks, 80% are believed to be made of bare steel, which can quickly corrode, allowing the contaminants to seep into the ground, posing a significant threat to the environment. The greatest potential hazard from a leaking UST is that the petroleum or other hazardous substance may seep into the soil and contaminate groundwater, the source of drinking water for nearly half of all Americans.8 A leaking UST can present other health and environmental risks, including the potential for fire and explosion.
Federal UST regulations9,10 promulgated in September 1988 established the minimum requirements for the design, installation, operation, and testing of USTs in the U.S. Through the implementation of the Clean Water Act11 (CWA) (including the regulations issued for oil pollution prevention) and the Occupational Safety and Health Administration12 (OSHA) (incorporating underground motor fuel storage tanks in its regulations dealing with flammable and combustible liquids), the control of USTs has helped in the minimization of the adverse environmental impact caused by the leakage of products from underground tanks.
This chapter will discuss those USTs storing petroleum products, such as gasoline, fuel oil, kerosene, and crude oil, and the problems related to petroleum release. In this context, the term "oil" or "gasoline" will be used in the text. Accordingly, the sections on underground release and transport remedial technologies mainly deal with petroleum products. Most petroleum products are nonaqueous-phase liquids (NAPLs) that are immiscible with water and have a lower specific gravity. The remainder of NAPLs with specific gravities greater than water are called the dense nonaqueous-phase liquids (DNAPLs). DNAPLs constitute only a small percentage of the petroleum products stored in USTs.
The consequences of the release of petroleum from leaking USTs include a loss of valuable fuel, contamination of drinking water supplies, and danger to human life, property, and the environment. The RCRA was enacted to regulate the generation, transportation, storage, treatment, and disposal of waste material that met the definition of hazardous waste. Subtitles I and J of the RCRA are specifically promulgated for the management of underground storage tanks.
Subtitle I of RCRA was enacted to control and prevent leaks from underground storage tanks.1,6 It regulates substances, including petroleum products and hazardous material. Tanks storing hazardous wastes, however, are regulated under Subtitle C, and are not the concern of this chapter.
On September 23, 1988, U.S. EPA issued the final technical performance standards and associated regulations for USTs.13 On October 26, 1988, U.S. EPA issued the final regulations for financial responsibility for those USTs related mainly with petroleum products. The technical standards for USTs comprise eight components, as described in the following sections13,14:
184.108.40.206 Program Scope and Interim Prohibition
Both the program scope and the interim prohibition must be clearly identified and documented.
220.127.116.11 Design, Construction, Installation, and Notification Requirements
U.S. EPA has established standards for tanks and piping tightness tests. In lieu of the standards specified in the regulations, new USTs may be constructed using alternative standards as long as they are equally protective of human health and the environment. The cathodic protection systems of new USTs must be designed and installed in accordance with industry codes. Tank installation includes securing the tank, obtaining clean backfill, and ensuring that the substances to be stored are compatible with the tank system. Tanks must be properly installed following manufacturer specifications and certified by the state regulatory agency when installation is satisfactorily completed. USTs must also be fitted with equipment to prevent the spills and overfills that are the common causes of tank leakage. Existing USTs had to comply with all requirements for new tanks by December 22, 1998. Any UST systems that were unable to meet the deadline were closed.
18.104.22.168 General Operating Requirements
Four steps must be taken to meet the general operating requirements to prevent spills or overfills:
1. Ensuring that the capacity of the tank is greater than the volume of product to be transferred
2. Having someone present at all times during the transfer
3. Incorporating equipment that can prevent or severely limit spills, such as automatic shutoff devices that act when the tank is almost full
4. Following manufacturer recommendations regarding proper maintenance, including inspections, record keeping, periodic maintenance, and corrosion protection.
Release detection is one of the most important requirements of the UST program. The detection system should be capable of detecting a release from any part of the UST system. Detection methods will be discussed under Section 18.3.
22.214.171.124 Release Reporting, Investigation, and Confirmation
Any spill or overfill of over 95 L (25 gal) petroleum must be reported within 24 h. An amount less than 95 L (25 gal), that cannot be cleaned up within 24 h should also be reported.
Following the immediate response activities (including release reporting, immediate containment, and monitoring of explosive hazards), the actions that the facility must implement as initial abatement measures include the following:
1. Further containing the regulated substance to prevent continued release
2. Preventing further migration of aboveground and underground release
3. Continuously monitoring and mitigating explosive hazards
4. Remedying hazards posed by excavated soils resulting from response activities
5. Performing a site check to evaluate the extent of the release
6. Determining the presence of free product on the water table
7. Compiling detailed corrective action plans if further corrective action is found to be required.
Unless permanently closed, all systems containing regulated substances must continue to comply with all the normal regulatory requirements. USTs closed for less than three months have no special requirements. USTs closed for between three and twelve months must leave vent lines open and cap all other lines. After 12 months out of service, USTs must be closed permanently. Before closing the UST system, the site must be assessed to ensure that no further release has occurred.
Under the new petroleum UST regulations, financial assurance (between ca. 0.5 and 1 million USD per occurrence or between 1 and 2 million USD for aggregate coverage) is required to cover both the cost of any required corrective action, and compensation for third-party liability from accidental release. State and federally owned facilities are exempt from these requirements.
As part of the amendments to Superfund, U.S. Congress created the Leaking Underground Storage Tank Trust Fund under RCRA Subtitle I. The Trust Fund is financed through a tax on gasoline, diesel, and aviation fuels and is used when the following conditions are met:
1. Cleanup costs exceed the coverage requirements of the financially responsible party.
2. The owner or operator refuses to comply with a corrective action order.
3. A solvent owner or operator cannot be found.
4. An emergency situation exists.
5. To cover the administrative and enforcement costs associated with a cleanup.
In order to regulate USTs storing hazardous substances and to provide a second means of containing the substance should the tank fail, U.S. EPA revised Subtitle J of the RCRA, which regulates secondary containment systems. This secondary containment system would have the following features:
1. It will prevent waste or liquid from escaping to the soil or water for the life of the tank.
2. It will collect waste or leakage until the material is removed.
3. It will be constructed or lined with material compatible with the waste and with sufficient strength to prevent failure from pressure, climate, traffic, and daily use.
4. It will have an adequate base or foundation capable of resisting settlement compression and uplift.
5. It will have a system capable of detecting leaks within 24 h of occurrence.
6. It will have a slope or drain system to permit removal of leaks, spills, and precipitation, and contain provisions for such accumulation to be removed.
7. It will have 110% of the design capacity of the largest tank within the containment boundary.
8. It will prevent run-on or infiltration of precipitation unless the collection system has excess capacity (beyond the 110%) to hold precipitation consistent with the 25-yr, 24-h rainstorm prediction.
Several states already have, or are developing, regulatory programs for USTs. Subtitle I of the RCRA is designed to avoid interfering with those state programs and to encourage other states to press ahead with control programs.
According to the state program approval regulations (promulgated on September 23, 1989) U.S. EPA will evaluate various elements of the state program against the corresponding Federal requirements. U.S. EPA must determine that the state's requirements are "no less stringent" than the Federal program, and that there is provision for "adequate enforcement."
The regulatory standards for leak detection in tank systems containing hazardous chemicals are more stringent than those for tanks containing petroleum motor fuels. Both above standards and those required in RCRA hazardous substances management should be met.
18.3 CAUSES OF LEAKS AND LEAK IDENTIFICATION METHODS 18.3.1 Causes of Tank Failure
USTs release contaminants into the environment as a result of (1) corrosion, (2) faulty installation, (3) piping failure, and (4) overfills.15-17
Corrosion and poor installation are by far the most common causes of storage system leaks. The most common causes of release from bare-steel UST systems are galvanic corrosion and the breakdown of hard refined steel to its natural soft ore. Because older USTs are usually constructed from bare steel, corrosion is believed to be the leading factor contributing to release. The speed and severity of corrosion varies depending on site characteristics, such as soil conductivity, groundwater or soil water chemistry, and weather. Most commonly, part of a tank becomes negatively charged with respect to the surrounding area and acts as a battery. The negatively charged part of the steel UST starts to corrode at a rate proportional to the intensity of the current. Corrosion rate can be reduced significantly or eliminated if cathodic protection or other protection methods are used.
Faulty installation of USTs encompasses a wide variety of problems, for example, accidents from vehicles colliding with the storage system, or faulty installation arising from inadequate compaction of backfills and unsealing of joints. Therefore, precautions should be taken to ensure that poor construction or installation do not degrade the performance of the USTs.
Piping failure can be caused in several ways. A study by U.S. EPA16 has shown that piping failure accounted for a substantial portion of releases at USTs. Spills and overfills are usually caused by human error. Repeated spill can also increase the corrosive nature of soils.
Three basic actions can be considered to identify leakage from USTs518: (1) direct observation (visual observation of losses or environmental and mechanical signs of leaks), (2) checking (inventory monitoring), and (3) testing (instrumental testing of tanks and piping for leaks). These are described in the following sections.
Visual inspection may be carried out by entering the tank if it is large enough for a person to be able to enter and walk in the tank, or by inspection of the tank's outer walls following the removal of pads or backfill material.
126.96.36.199 Watching for Environmental Signs
There are at least five signs to look for:
1. The odor of motor fuel in the soil near the tank may be a sign of leakage.
2. The odor of motor fuel present in underground structures such as basements and sewers is also a sign of leakage.
3. Plants located on property near a UST may grow sluggishly, look sickly, or die.5
4. Motor fuels may be found in drinking water wells or rivers.
5. A higher than expected gain or loss of fuel in a tank may be caused by water infiltration or leakage of fuel through the tank wall.
188.8.131.52 Watching for Mechanical Signs
There are three phenomena to be monitored5:
1. Interruption in the delivery of motor fuel dispensed by the suction pump
2. A rattling sound and irregular fuel flow in the suction pumping system
3. Meter spin without motor fuel delivery
It should be noted that these can also be caused by other problems besides tank leakage, such as leaking valves, loose fittings, or other factors.
By carefully checking inventory records one is able to determine whether there is loss or gain of fuel in USTs. Inventory review is generally an inexpensive and relatively easy way to check for leakage.
This method is particularly useful for identifying large leaks, although small leaks may also be noticed, particularly in tanks with metered dispensing pumps. Interested readers can refer to U.S. EPA5 and API (American Petroleum Institute)20 for detailed procedures of inventory checking for tanks with metered or nonmetered pumps.
Another method to examine tank leakage uses instrumentation. An instrumental test should be conducted if there is the suggestion of a leakage from various environmental or mechanical indicators or from an inventory review.
When the leaked motor fuel is at a deeper level or flows away from underground strata, there may be no visual sign, and instrumentation may be necessary to detect the leak. Such an instrumental test on the tank environment is called an external test, and is the counterpart of visual observation.
There are a number of methods for detecting the sign of leakage from external tanks. The most common method uses monitoring wells. Typically, the monitoring well reaches 2 ft below the bottom of the underground storage tank. Detection sticks are placed in the well, and indicate the existence of motor fuel within the well. Other methods include soil sampling, fuel vapor testing, ground penetrating radar, seismic methods, electromagnetic induction, resistivity, magnetometers, and X-ray fluorescence.
Tracer methods can also be used, in which tracers such as freon, fluorescent materials, and isotope-fuel are added to a tank, and are then detected externally. An analogy of tracer methods includes pressurizing the tank with a noble gas, then detecting the gas if it escapes from the tank through cracks or holes.
Some tanks are installed with permanent leak identification sensors, which can check for leaked fuel vapor or liquid as it comes into contact with the sensors.21 However, these, as well as all the environmental sign tests (visual or instrumental) may be triggered by a spill instead of a leak. The success of external systems depends on the sensitivity of the sensor, the ability of the sensor to distinguish the stored chemical from other chemicals, the ambient background noise level of the stored chemical, the migration properties of the chemical, and the sampling network.
In-tank measurement uses equipment that is placed inside the tank or pipes. Some tests can qualitatively determine whether a tank is leaking; others can establish the leakage rate. Most of the work can be performed within a time of 2 to 4 h, excluding setup time.5,21 A common method measures the changes in the amount of fuel in the tank by measuring the fuel level or pressure. These tests may be influenced by several factors, including evaporation, condensation, and changes in temperature, changes in the shape of the tank due to changes in the fuel load, temperature air packets, vibrations from traffic, groundwater, or soil moisture.
Other devices and methods can also be applied, such as laser interferometry, which measures the change in the height of fuel in the tank using lasers, or acoustics methods that measure the sound of fluid escaping or entering the tank.
An instrument can be used to test tank walls directly, for example, by using acoustics or sound waves to identify holes or cracks in the tank walls.18
184.108.40.206 Release Detection Approaches for Modern Tank Systems
Release detection is an important aspect of the management of USTs. U.S. EPA regulations required an upgrade of release detection during the 10-yr period between 1988 and 1998. The external or internal detection systems should be in compliance with the requirements for modern tank systems.
There are three methods of release detection that are associated with modern tank systems.18,22 The first approach is to conduct an annual tank or line tightness test to detect small releases and to use more frequent monitoring by another method to detect large releases. All tank and line tightness tests must be performed at least once a year and must be able to detect leaks of 0.38 L/h (0.1 gal/h). In all cases where annual tightness tests are used, the regulation requires an additional form of leak detection in which tests on tanks are conducted at least monthly and those on pressurized lines at least hourly; this ensures the detection of excessively large releases. For tanks, daily inventory records must be reconciled monthly. for pressurized lines, leaks of up to 11.4 L/h (3 gal/h) must be reliably detected.
The second approach is to install an automatic tank gage or automatic line leak detector that is capable of detecting leaks of 0.76 L/h (0.2 gal/h); all monitoring tests must be done at least once a month. This option also requires that there be a system for detecting large leaks. The tank gage can be used to satisfy inventory control requirements, and most automatic line leak detectors are designed so as to be able to satisfy the 11.4 L/h (3 gal/h) test for pressurized piping.
The third approach is to install an external monitoring system that can detect the presence of the stored chemical in or on the groundwater or in the backfill and soil surrounding the tank system. In many instances both internal and external methods are used in conjunction as a way to increase the liability of detection.
18.4 UNDERGROUND CONDITIONS AND FACTORS AFFECTING TRANSPORT OF LIQUIDS IN THE SUBSURFACE
Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration.
In the lower region of the unsaturated zone, immediately above the water table, is the capillary fringe, where water is drawn upward by capillary attraction. Above the capillary fringe, moisture coats the solid surfaces of the soil or rock particles. If the liquid coating becomes too thick to be held by surface tension, a droplet will pull away and be drawn downward by gravity. The fluid can also evaporate and move through the air space in the pores as water vapor.
The moisture in the upper unsaturated zone can be affected by plant transpiration and atmospheric conditions. Some scholars classify the unsaturated zone into subzones such as the soil water zone and the intermediate zone.23
18.4.2 Gravitational Force Affecting Underground Liquid Movement
Soil water, like other bodies in nature, has two principal forms of energy, kinetic and potential. Kinetic energy is proportional to the square of velocity. As the velocity of groundwater is quite slow, the kinetic energy is usually negligible. Potential energy, due to position or internal conditions, determines the movement of water from a higher energy level to a lower energy level in soil formations. Accordingly, there are three forces related to potential energy:
1. Gravity (the weight of the fluid)
2. External pressure (atmosphere pressure)
3. Molecular attraction (surface tension, adsorptive, diffusive, and osmotic forces)
The forces resisting groundwater flow are shearing stress and normal stress due to viscosity, collision, and turbulence.
Gravity force can be measured by means of the mass of the water. The direction of the force is, obviously, downwards toward the Earth's center. The gravitational potential of soil water at each position is determined by the elevation of the position relative to some reference level. If we only consider the elevation potential and the related velocity energy, then a water body at a higher elevation will flow to a lower elevation, decreasing the elevation potential but increasing its velocity.
18.4.3 Atmosphere Pressure Affecting Underground Liquid Movement
Atmospheric pressure is not obvious, because it is balanced in opposite directions. The combination of atmospheric pressure and the weight of the overlying water create the total pressure in the saturation zone.24
18.4.4 Surface Tension and Capillary Potential Affecting Underground Liquid Movement
Tension in the free surface of a liquid is the cause of the tendency of a liquid surface to assume the form having a minimum area, as manifested in the shape of a bubble or a drop of liquid.25 The tendency to contract is a special case of the general principle that potential energy tends toward a minimum value.
When a drop of liquid is placed on a solid surface, it will displace the gas and spread over the surface. If the contact angle is <90°, the liquid wets the solid (wetting, Figure 18.1a); if the contact angle is >90°, the liquid does not wet the solid (nonwetting, Figure 18.1b).
Whether a liquid wets or does not wet a solid surface depends on the affinity between the liquid and the solid. In the case of wetting, the smaller contact angle enables the liquid to enlarge the liquid-solid interface area (which has a lower surface energy than the liquid surface energy) and shrinks the liquid surface area (which has a greater surface energy), thus reducing the total energy. In the case of nonwetting, the greater contact angle enables the reduction of the liquid-solid
Liquid — - ---
FIGURE 18.1 (a) Drop contact angle and (b) a sessile drop showing characteristic dimensions.
FIGURE 18.1 (a) Drop contact angle and (b) a sessile drop showing characteristic dimensions.
interface area (which has a higher surface energy than the liquid surface energy) and the enlargement of the liquid surface area (which has a lower surface energy), thus bringing about a reduction in the total energy.
A liquid-solid contact angle away from 90° induces the formation of a meniscus on the free surface of the liquid in a vertical tube (the solid phase). In the nonwetting case, the meniscus concaves upwards to the air. The upwards meniscus is the result of a downward surface tension at the liquidtube interface, causing a capillary depression. In the wetting case, the meniscus has a concave-downward configuration. The downwards meniscus is the result of an upward surface tension at the liquid-tube interface, causing a capillary rise.
A typical profile of the pressure potential of soil moisture tested by a tensometer across the free-water surface shows a negative pressure (lower than atmosphere pressure) in the capillary zone (Figure 18.2). The negative pressure in the capillary zone indicates that the capillary zone belongs to the unsaturated zone.
Surface tension is independent of tube size. However, the extent of capillary rise or depression by surface tension is dependent on tube size. This can be seen from Equation 18.1 in Section 220.127.116.11. For example, in the case of a capillary rise, the greater the tension, the higher the water rises above the free-water surface. For the same amount of water, the smaller the tube is, the higher the water rises.
18.104.22.168 Relative Soil Wettability of Two Liquid Phases
The predominant form of released petroleum products is a liquid that is immiscible with water; this is called the free product (in this section it will be referred to as oil). The behavior of water and oil in soil depends on the interaction of the three phases water, oil, and soil. The affinity of water or oil with the soil can be estimated by establishing the contact angle of oil/water/soil triple line.
Note that the contact angle of the fluid 1/fluid 2/solid triple line is still largely unpredictable, even though the material properties of the three phases, taken separately, are known. It is difficult to compare the wettabilities of a solid with respect to two fluids that wet it perfectly, or, in other terms, to measure the fluids' spreading parameters, even on an ideal surface. There are several methods used for wettability evaluation. The AMOTT-IFP (E. Amott - Institut Francais du Pétrole) test is probably the most widely accepted one. Other advanced methods for measuring wettability include the computerized automated tomographic X-ray scanner, magnetic resonance imagery,
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