xvEPA United States Environmental Protection Agency Environmental Monitoring Systems Laboratory P.O. Box 15027 Las Vegas NV 89114-5027 June 1987 600S87005 Research and Development Processes Affecting Subsurface Transport of Leaking Underground Tank Fluids ------- June 1987 PROCESSES AFFECTING SUBSURFACE TRANSPORT OF LEAKING UNDERGROUND TANK FLUIDS by Scott W. Tyler Michael R. Whitbeck Marcia W. Kirk John W. Hess Water Resources Center Desert Research Institute Reno, Nevada 89506 Lome G. Everett Kaman Tempo Santa Barbara, California 93102 David K. Kreamer Department of Civil Engineering Arizona State University Tempe, Arizona 85287 Barbara H. Wilson University of Oklahoma R.S. Kerr Laboratory Ada, Oklahoma 74820 CR 810052 Project Officer Jeff van Ee Advanced Monitoring Systems Division Environmental Monitoring Systems Laboratory Las Vegas, Nevada 89114 ENVIRONMENTAL MONITORING SYSTEMS LABORATORY OFFICE OF RESEARCH AND DEVELOPMENT U.S. ENVIRONMENTAL PROTECTION AGENCY LAS VEGAS, NEVADA 89114 ------- PREFACE This report documents the primary fate and transport processes affecting fluids leaking from underground tanks. Since these studies span a broad range of scientific disciplines, the Desert Research Institute, under its cooperative agreement with EPA/EMSL-LV, has solicited input from its own staff as well as from experts throughout the country. Each of the sections describing the processes has been separately written by individual experts in the fields of fluid flow, vapor transport, surface chemistry, and the microbiology of subsurface environments. Each author was given the task of describing the state of knowledge in his or her field and how this knowledge is applicable to the detection of leaks from underground storage tanks. Since this document had to be completed in a very short time period, literary freedom was given to each author as to section organization and con- clusions. It is suggested that the reader consider this report, therefore, as an edited collection of treatises whose sole purpose is to describe the fate and trans- port processes of fluids leaking from underground storage tanks. Scott W. Tyler John W. Hess ------- ABSTRACT As a result of increased public awareness concerning the ground-water contamination potential of leaking underground storage tanks, attention is being given to developing monitoring strategies for these facilities. The most common strategies include tank material inventories, tank integrity testing, and monitoring of the soils and water adjacent to the tank. Research sponsored by the U.S. Environmental Protection Agency is presently being conducted to determine the feasibility of these strategies. This document focuses solely on the processes affecting migration of fluids from a leaking tank and their effects on monitoring methodologies. Based upon the reviews presented, soil heterogeneities and the potential for multiphase flow will lead to high monitoring uncertainties if leak detection systems rely on liquid sampling alone. Vapor transport is also affected by these properties although to a lesser degree. More research is needed, however, to better understand the physics of vapor transport. Vapor transport of contaminants to the monitoring sensors will also be strongly affected by the volatility of the fluid. Difficulties in detection and monitoring systems may also be generated from fluid interactions with the soil and microbes. The processes of adsorption, partitioning, and microbial alteration of fluids in the subsurface may have strong effects on the uncertainty of monitoring systems. These fate processes have received less attention than liquid and vapor transport processes and will require significantly more research before their effects are fully understood. Present research indicates that high uncertainties in monitoring reliability will be associated with systems placed within the native, heterogeneous subsoil. Monitoring systems placed in engineered (and, therefore, more controlled and homogeneous) near- tank environments may have much higher certainty of detecting a leaking tank or leaking distribution system. ACKNOWLEDGMENTS The editors of this document wish to thank Dr. Lome Everett, Dr. David Kreamer, Dr. Michael Whitbeck, and Ms. Barbara Wilson for their significant contributions to this report. Without their timely efforts and expertise, this report could not have been completed. We would also like to thank Ms. Marcia Kirk for her background research and review comments. We also thank Glenn Broughton and Barry Hibbs of Arizona State University for their assistance. The editors also wish to thank Jeff van Ee and Leslie McMillion of the US EPA Environmental Monitoring Systems Laboratory - Las Vegas for their support and helpful review comments. Special appreciation is also given to Carolyn Hagopian, Barbara Salmon, and Deborah Wilson because without their expert clerical and word processing support, this report would not have been possible. IV ------- CONTENTS Preface '" Abstract |v Acknowledgments iv Figures vii Tables viii SECTION 1 Introduction (S.W. Tyler and J.W. Hess) 1-1 Scope of Report 1-1 Underground Tank Environments 1-2 Tank Failure Mechanisms 1-4 References 1-5 SECTION 2 Liquid Transport From Underground Storage Tanks (L.G. Everett)' 2-1 Introduction 2-1 Significant Physical-Chemical Properties 2-1 Vadose Description 2-2 Saturated Zone 2-6 Flow Regimes 2-6 Liquid Monitoring Difficulties and Future Research 2-12 References 2-17 SECTION 3 Vapor Transport and Its Implications to Underground Tanks (O.K. Kreamer) 3-1 Introduction 3-1 Sources of Gases in the Unsaturated Zone 3-1 Factors Which Effect Movement of Gases Near Underground Storage Tanks 3-3 Importance of Vapor Transport Surrounding Underground Storage Tanks 3-5 Vapor Transport Processes 3-6 Vapor Transport Proceses in the Underground Storage Tank Environment 3-8 Existing Gaseous Measurement Methodologies 3-10 Limits of Present Knowledge and Future Directions 3-14 References 3-15 SECTION 4 Soil Surface and Interfacial Effects in the Underground Storage Tank Environment (M.R. Whitbeck and M.W. Kirk) 4-1 Introduction 4-1 Partitioning and Adsorption of Chemicals in the Underground Storage Tank Environment 4-1 Leak Detection 4-3 Differences Between Laboratory and Underground Storage Tank Environment 4-4 Theoretical Deficiencies 4-4 Data Needs 4-5 Future Directions and Strategies 4-5 References 4-6 ------- SECTION 5 Implications of Subsurface Biological Activity for Monitoring Underground Storage Tanks (B.H. Wilson) 5-1 Organic Pollutants in Ground Water 5-1 Conclusions 5-3 References 5-4 SECTION 6 Conclusions and Recommendations 6-1 Process Parameters: A Synopsis 6-1 Process Impacts on Monitoring 6-1 Indirect Techniques 6-4 Process Matrices 6-4 Recommendations 6-4 VI ------- FIGURES Number Page 1.1 Elements of an Underground Storage Tank Installation (New York State, 1983). 1-3 2.1 Migration of Leaked Material through the Soil Zone. 2-3 2.2 Effect of Clay Lens in Soil on Hydrocarbon Migration Path (API, 1980). 2-4 2.3 Seepage of Oil through the Soil Zone (after Schwille, 1967). 2-4 2.4 Trapped Product Droplets (API, 1980). 2-5 2.5 Hydrocarbon Leakage Flow Paths (API, 1972). 2-5 2.6 Hydrocarbon Migration Pattern (Schwille, 1984). 2-7 2.7 Composition of a Region of Macropore-Mesopore Media. 2-7 2.7 Variation of Porosity, Specific Yield, and Specific Retention with Grain Size (after Bear, 1972). 2-10 2.9 Two-phase Flow Relative Permeability (and Dam, 1967). 2-10 2.10 A. Relative Permeabilities Three-phase Flow (after van Dam, 1967). 2-11 B. Three-phase Relative Permeability (after van Dam, 1967). 2.11 Spreading of Spill on Water Table Surface (after Schwille, 1967). 2-12 2.12 Model Experiment: Influence of Changing the Water Level on the Oil Distribution 2-14 (after Schwille, 1967). 2.13 A. Impregnation Body (Petroleum Product) having Reached Ground Water 2-15 (Zillion and Nutzer, 1975). B. Thickness of Layer of Oil in the Ground and in a Strainer Tube (Influence of Capillary Pressures in the Case of a Continuous Layer of Oil). 2.14 Displacement of Oil Envelope by Water - Considered Microscopically (Schwille, 1967). 2-16 3.1 Volatized Product Vapor Migration Opposite Ground-water Flow. 3-2 3.2 False Positives due to Background Vapor Concentrations. 3-4 3.3 Three-dimensional Diffusion or Advection of Product Vapor. 3-11 VII ------- TABLES Number Page 6.1 Fluid Transport Parameters 6-2 6.2 Vapor Transport Parameters 6-2 6.3 Surface Chemistry Parameters 6-2 6.4 Microbiological Parameters 6-3 6.5 Advantage Matrix for Liquid Monitoring Technologies 6-5 6.6 Complications Matrix for Liquid Monitoring Technologies 6-6 6.7 Advantage Matrix for Vapor Monitoring Technologies 6-6 6.8 Complications Matrix for Vapor Monitoring Technologies 6-7 6.9 Advantage Matrix for Geophysical Technologies 6-7 6.10 Complications Matrix for Geophysical Technologies 6-8 viii ------- SECTION 1 Introduction As a result of an increased awareness towards the sensitivity of the ground-water reserves of our nation to anthropogenic sources of pollution, research has begun to mitigate these effects. These sources of pollution are a result of waste disposal practices, withdrawal of mineral and petroleum resources, land management practices, agriculture, construction, and leaks and spills. Although initial Federal and State water protection legislation focused on waste treatment and disposal practices (Water Pollution Control Act, Toxic Sub- stances Control Act, Resource Conservation and Recovery Act), recent legislation has focused on control and monitoring of accidental releases of con- taminants from underground storage tanks. The impetus for this legislation lies in the magnitude of the potential source of pollution. It has been estimated that there are between 3 to 5 million underground storage facilities containing potentially hazardous substances in the United States. Even under ideal installation and operating con- ditions, a percentage of tanks will fail within their expected service life. It is estimated that 100,000 tanks are presently leaking while another 350,000 are expected to leak within the next five years (U.S. EPA, 1985). Since these tanks are located below ground, immediate visual detection of tank failure is unlikely. Minute amounts of many of the stored materials can pollute large amounts of ground water, and simple tank inventory monitoring may be insensitive to the detection of a potentially serious leak. Based on these scenarios, amendments to the Re- source Conservation and Recovery Act (RCRA) of 1984 authorized a federal underground tank program for regulated products. The U.S. Environmental Protection Agency (EPA) has defined an underground tank as a tank with 10 percent or more of its volume (including piping) underground. The term regulated products includes petroleum products and hazardous substances addressed in the Comprehensive Envi- ronmental Response, Compensation, and Liability Act (CERCLA). The tank regulations mandated by RCRA are of two kinds. Interim standards have been identified for new tanks. New tanks must be cathodically protected, constructed of noncorrosive material, clad in non- corrosive material or designed to prevent leakage in the event of corrosion or structural failure, if a new tank is placed in soil having a resistivity of greater than 12,000 ohm/cm, then it is exempt from the above standard. The second set of regulations will be the final standards for new and existing facilities. EPA has indicated that it plans to release final rules for all the above standards at the same time in February 1987. In addition, many states have passed or are in the planning stages of legislation regulating underground tanks (Askenaizer et al., 1985). Scope of Report The RCRA regulations have mandated prompt action by EPA for all phases of underground tank man- agement. To develop the standards required by RCRA, clear input from a wide range of scientific disciplines will be needed. The complexities of tank and piping construction and the reliable monitoring systems needed to insure rapid leak detection span many fields of research. The purpose of this doc- ument is to describe the processes and scenarios pertinent to one phase of underground tank man- agement: leak detection monitoring in the subsoil. To accomplish this goal, experts in the fields of liquid flow, vapor flow, adsorption, and microbiology have each written a section on these subjects. The discussions concentrate on organic contaminants because of their greater potential for contamination and because of the sheer numbers of the tanks; however, the processes described are also applicable to many inorganic fluids. Other studies sponsored by EPA, but not covered in this report, include in-tank monitoring, sensor development, and clean-up technologies. To design leak detection monitoring systems, it is critical that the factors affecting the fate and transport of leaking fluids be well understood. For example, many sensors require vapors to migrate from the leak to the sensor; however, if vapors are quickly adsorbed by soil materials, the monitoring system is improperly designed. It is the goal of this document to provide a sound background for the EPA to build its research program for leak detection monitoring systems. By documenting the deficiencies in our understanding of 1-1 ------- each of the fate and transport processes, research programs can be developed to address these issues. Concurrently, those processes which may enhance sensor efficiencies may be explored in research programs in much greater detail and may provide adequate monitoring systems immediately. Each of the next four sections discusses the present state of understanding of processes controlling mi- gration of leaking fluids and assesses the current state of subsurface monitoring of those processes. The final section describes in matrix form the effects of each process on existing monitoring techniques. Such techniques include liquid monitoring, vapor sampling, and borehole and surface geophysical methods. These generic classifications of monitoring technologies cover most of the presently available technology. Because of the recent growth of the monitoring industry, these categories may need to be expanded in the near future. This document is not intended as a "how to" manual. As the reader will see, the current understanding of the processes is well based, but the complexities of tank environments indicate that much more research is needed before reliable monitoring systems become the industry standard. Since each section was written by respected experts in their fields, the conclusions reached may not always agree. This diversity in opinion, however, can lead to a healthy and viable research program. It is hoped that this document will provide valuable input towards tank legislation and will guide future research. Underground Tank Environments Since each of the sections deals with one physical, chemical, or biological process in the subsoil sur- rounding underground tanks, a brief description of the overall tank environment is necessary. Not only does this set the stage for the reader, but it also points out many of the complexities involved. Figure 1.1 shows one installation of underground storage tanks and a hypothetical associated leak detection scheme. It is obvious from the figure that a wide range of both constructed and geologic con- ditions are possible in the underground tank envi- ronment. Underground tanks are utilized in a very wide spec- trum of environments ranging from drinking water storage to radioactive waste disposal. The majority of tanks, however, contain petroleum products. Tank sizes range from tens of liters as in the case of gasoline station waste oil storage to millions of liters of highly radioactive waste. Tanks are constructed of a myriad of materials; however, most fall into the category of steel, fiberglass, concrete, or a composite of these materials. Steel tanks are used quite extensively for storing a wide range of products. Corrosion of steel tanks in an underground environment can drastically reduce the effective life of a storage tank. A protective coating, cathodic protection, or electrical isolation are gen- erally used to extend tank life. Fiberglass reinforced plastic (FRP) tanks are con- structed of a plastic resin reinforced with fiberglass. The plastic resin provides chemical resistance while fiberglass gives the tank its structural strength. It is essential that the stored product be compatible with the tank material. Several resins and glass materials may be used in the fabrication of FRP tanks. Relatively new and effective designs include com- posite tanks and double wall tanks. Composite tanks incorporate steel tanks that are clad with a coating of fiberglass reinforced plastic. These tanks combine the corrosion resistance of FRP with the strength of steel. Tanks may be single or double wall. Double walled tanks are essentially a tank within a tank. This type of storage tank may be fabricated from coated steel or fiberglass. The space between the tanks may be pressurized or evacuated, and leaks due to internal or external corrosion may be detected by a loss of pressure or vacuum. The internal space may also be sampled. Improvements in piping and piping design for product handling have been initiated because of increased concern for leak prevention. Fiberglass-reinforced plastic piping is often used for handling petroleum products. Flexibility and corrosion resistance are the principal reasons for its wide acceptance. Expansion and swing joints are also used to reduce stresses which are due to thermal expansion or misalignment or both of piping systems. Misalignment of product handling systems may be caused by several factors, for example, improper installation or differential settlement of the storage tank and piping. Double walled piping is also used when very hazardous or toxic materials are to be handled. Prior to installation of a storage facility, an evaluation of the site conditions must be made. Clays, wet soils, cinders, and acid soils tend to be highly corrosive. Abandoned piping and tanks in the area may accelerate corrosion of unprotected steel tanks. Improperly abandoned storage tanks may collapse and may cause excessive stress on recently installed tanks. 1-2 ------- TANK TRUCK ©OVERFILL PREVENTION DEVICE (A * ^Sr .VAPOR RECOVERY LINE -FILL LINE VENT UME PRODUCT DISPENSER CORROSION-RESISTANT STORAGE TANK EXCAVATION CAP OBSERVATION WELL PFA GRAVEL OR BAND FILL EXCAVATION WALLS AND FLOORS OF IMPERVIOUS MATERIAL AUTOMATIC SHUTOFF VALVE PRODUCT DELIVERY LINE LEAK DETECTOR JBMEROED PUMP ASSEMBLY Well designed underground storage systems usually contain the following: (1) corrosion resistant tank; (2) striker plate under tank fill line; (3) submerged pump with leak detector on product delivery line; (4) float vent valve in tank vent line; (5) excavation walls and floor of impervious material; (6) asphalt or concrete excavation cap; (7) automatic shutoff valve on delivery line at pump island; (8) overfill prevention device at fill line on tank truck; (9) vapor recovery in tank truck during filling operation; (10) observation wells located inside excavation boundaries; (11) pea gravel or sand fill for excavation. Figure 1.1 Elements of an Underground Storage Tank Installation (New York State, 1983) Depth to the local ground-water table must also be investigated. In areas of high water table or in a flood plain, a method of anchoring the tank must be devised, since tanks that are low in stored product may float to the surface in an area of high water table. The resulting accident could have a significant en- vironmental effect. Once the site investigation is accomplished, excavation can begin. A secondary containment system may be installed after the excavation is complete. In addition to any secondary containment, fill material may be placed and compacted on the bottom of the excavation to isolate the tank from the surrounding soil. Steel tanks require sand fill material while pea gravel (9.5 mm) is recommended by manufacturers for fiberglass tanks. In an area where tank buoyancy may be a problem, a 0.3 meter reinforced concrete pad is typically placed at the bottom of the excavation, and the tank is anchored to prevent its displacement. The tank is placed in the excavation and is backfilled to about 3/4 of the tank height. Compaction of the backfill material is critical, providing most of the lateral support for the storage tank. Excessive void space may cause stresses beyond the design limits of the tank. This is especially true for fiberglass tanks since approximately 90 percent of the lateral support is given by the surrounding soil. Product lines and fill pipe are then installed. When galvanized pipe is used, swing joints are required at all locations where the direction of flow changes. Standard screw con- nections are used with fiberglass piping due to the flexible nature of the material. Fill material is placed above the top of tank and compacted. Coarse gravel is then placed to grade, even with the land surface. In many facilities, such as those used for retail gasoline distribution, an asphalt or concrete cover is also added. If no structures or cover are placed over the tanks, the surface can be mounded to discourage ponding of surface water and resultant infiltration. 1-3 ------- Tank Failure Mechanisms In general, there are three primary causes of leaks from underground storage tanks: corrosion, improper installation, and poor operating practices. Corrosion results from an interaction between tank or piping materials and the surrounding environment, both internal and external. Galvanic and electrolytic corrosion are the two principal forms of electrical corrosion. Electrolytic corrosion is the result of stray electric currents from outside sources entering and leaving by way of an electrolyte. Soil takes the form of the electrolyte in the case of underground structures. Galvanic corrosion is self-generated from electrical potential differences between dissimilar metals immersed in an electrolyte (soil). A current is gen- erated when two dissimilar metals are connected and placed in an electrolyte. Corrosion will occur in one of the metals. Current from the corroding metal (anode) will flow into the electrolyte to the other noncorroding metal (cathode) to complete the circuit. Methods currently employed to protect against corrosion are cathodic protection, electrical isolation, protective coatings, or a combination of these. Cathodic protection works by reversing the elec- trochemical action of corrosion. The current is forced to flow into the tank thereby protecting the structure. There are two basic types of cathodic protection: sacrificial anode (galvanic) protection and an impressed current method. Galvanic protection uses a sacrificial metal (zinc or magnesium) in contact with the structure to be protected. The impressed current cathodic protection relies on an outside direct current source to cause current flow in the proper direction. Electrical isolation involves the use of dielectric fittings and bushings that electrically isolate metal com- ponents of the system. This minimizes the generation of currents which are due to contact of dissimilar metals. Improper installation practices for underground stor- age tanks and components are a common source of leaks. Structural damage to piping and tank coatings may cause localized corrosion. This can lead to leakage once the tank is in place. Failure to use proper bedding or backfill-compaction procedures or both may result in a leaking storage tank. Another source of product leaks results from piping joints that fail because of improper fittings or joints that loosen or crack over time. Operating practices can also contribute to a loss of product. Over-filling tanks and spilling during transfer operations are probably the two most common problems. Also, the puncturing of a tank during inventory measurement is possible. Many tank operation scenarios add to the difficulties of subsurface leak detection monitoring. Spills or leaks above the tanks during filling or dispensing may move downward through the subsoil either as a result of the spill or through infiltration of precipitation. This source of contamination may allow for false positive signals from subsurface monitors designed to monitor tank performance. Tank and piping monitoring systems, therefore, must be designed to be insensitive to non- tank sources of contamination. Tanks may be located in single or multiple units. In the case of multiple units, pinpointing a leaking tank using subsurface methods may be difficult if several tanks contain the same material. As can be seen, the subsoil environment surrounding underground tanks is complicated by many sources. Methods to monitor tank performance must be chosen carefully for proper performance. Additionally, results of monitoring must be carefully analyzed and performance limits must be understood if monitoring systems are to be successful. The following sections outline the current understanding of the processes controlling contaminant migration in the subsurface and how these processes may influence leak detec- tion monitoring strategy. References Askenaizer, D.J., W. Barcikowski, K.V.B. Jennings, J.E. Sarna. 1985. Development of a Compliance Program for Underground Tanks Containing Hazardous Substances. University of California at Los Angeles, Report #85-81. U.S. EPA. 1985. Leaking Underground Storage Tanks, EPA/530-SW-85-009. 1-4 ------- SECTION 2 Liquid Transport From Underground Storage Tanks Introduction Although the Underground Storage Tank Permit System of the EPA covers liquid inorganic chemicals, natural mineral organics, and synthesized chemical organic liquids stored in hazardous waste tanks, the largest number of tanks by far are fuel storage tanks. It is estimated that California alone has over 220,000 underground storage tanks in place today. By January 1986, each of these tanks in California, as mandated by the Sher Act, will require some combination of soil-core monitoring, soil pore-liquid monitoring, soil-gas monitoring, and ground-water monitoring in addition to tank testing and tank monitoring programs. When viewed across the nation, the magnitude of the monitoring requirements is considerable. In response to this need, this section is directed towards a descriptive understanding of organic and inorganic liquid flow related to monitoring in the subsurface. The alternative to early alert monitoring, which is ex post facto liquid hydrocarbon removal by hydraulic sweeping, biodegradation, or excavation, can still result in enormous environmental liability. The discussion of liquid hazardous waste migration, which occurs as a continuous multiphase flow under the influence of capillary, viscous, and gravity forces, will include an appreciation of both unsatu rated and saturated flow regimes. A natural subset of these flow regimes includes both uniform Darcian flow and fracture flow. The hazardous wastes discussed will include both inorganic chemicals, such as from plating factories, and organic chemicals, which include both natural mineral products such as natural crude oils and synthetic chemical products such as liquid pesticides. The movement of a separate liquid hydrocarbon phase in a water and sometimes in air-filled porous soil and the movement of organically active dissolved hydrocarbon components that are subject to bio- degradation, absorption onto soil particles, and volatilization are very complex. Neither of these major transport mechanisms is well understood today. The dissolved component mechanism is the subject of intense study at Stanford University, MIT, University of Illinois, EPA, and several other research institutions and universities. The liquid phase transport mech- anism has been virtually ignored by the research community in the United States, although it has been the subject of empirical studies in Europe (e.g., Schwille, 1967, 1981, 1985). Recently, however, American researchers have been developing some laboratory results (Convery, 1979; Eames, 1981) and some simple numerical simulations of multiphase transport which focus on immiscible transport of continuous phases. A few researchers, notably at Delaware (Baehr and Corapciaglu, 1984), Princeton, and the New Mexico Institute of Mining and Technology, are looking into interphase transfer, including the volatilization and solution of hydrocarbon components. Significant Physical- Chemical Properties Superimposed on the unknowns associated with porous media and fracture flow under unsaturated and saturated conditions are the physical-chemical prop- erties of concern in establishing a monitoring program. The fundamental question is whether the fluid is miscible or immiscible in water. Fluids which are mis- cible with water are fully dissolved by water and as such can be handled through existing hydrodynamic equations and principles. Fluids which are immiscible in water, however, are held in the soil matrix at residual saturation. Residual saturation refers to that volume of fluid which is immobile with soil matrix. Density of the fluid is important because this parameter determines the level to which migration will take place. The density factor is further complicated by the fact that at certain temperatures, crude oil components are dissolved within one another. For example, if crude oils are cooled, substances such as paraffins and others can crystallize out and form agglomerates (micro-crystalline precipitation) which can interfere with fluid flow through the fine pores. In addition, fresh crude oils with volatile components become increasingly viscous as they evaporate. Viscosity is a major factor controlling the velocity of the flow process. For example, certain chlorinated hydrocarbons having relatively high vapor pressures have been shown to evaporate 50 percent in less than 0.5 hours. This substantial loss in the volatile phase results in significant changes in density and viscosity of the bulk fluid. Surface tension is a property that is responsible for capillary effects and for spreading on 2-1 ------- top of the water table. Although typical hazardous waste hydrocarbons in soils are treated as a non- wetting fluid, some of the hazardous waste may have wetting properties. Although we recognize that typical hazardous waste is wetting relative to air, our ap- preciation for field capacity and residual saturation may change if hazardous waste hydrocarbons exhibit some wetting properties relative to soil. Since hy- drocarbons can exist as an immiscible phase, as part of the water phase, and as part of the soil-gas phase, a determination of their various permeabilities is extremely complex. Vadose Zone Description Leakage from underground storage tanks typically occurs into the vadose zone (Figure 2.1) which is the geological profile extending from ground surface to the upper surface of the principal water-bearing formation. As pointed out by Bouwer (1978), the term "vadose zone" is preferable to the often-used term "unsat- urated zone" because saturated regions are frequently present in the vadose zone. Davis and DeWiest (1966) subdivided the unsaturated zone into three regions designated as the soil zone, the intermediate unsaturated zone, and the capillary fringe. Hydro- carbon movement is different in each of these zones. Soil Zone The surface soil zone is generally recognized as that region that manifests the effects of weathering of native geological material. The movement of water and contaminants in the soil zone occurs mainly as unsaturated flow caused by infiltration, percolation, redistribution, and evaporation (Klute, 1965). In some soils, primarily those containing horizons of low permeability, saturated regions may develop during infiltration and may create shallow perched water tables (Everett, 1980) and free product zones. The effects of soil layering on fluid migration are shown in Figure 2.2. Loss of volatile organics and micro- biological activity are highest in the soil zone. The physics of unsaturated soil-water movement has been intensively studied by soil physicists, agricultural engineers, and microclimatologists. In fact, copious literature is available on the subject in periodicals (Journal of the Soil Science Society of America, Soil Science) and books (Kirkham and Powers, 1972; Hillel, 1971). Similarly, a number of published references on the theory of flow in shallow perched water tables are available (Luthin, 1957; van Schilfgaarde, 1970). Soil chemists and soil micro- biologists have also attempted to quantify chemical- microbiological transformations during soil-water movement (Bonn et al., 1979; Rhoades and Bernstein, 1971; Dunlap and McNabb, 1973). Leakage from underground tanks, however, typically occurs below the soil zone. As shown in Figure 2.3, during the seepage period through the soil zone and into the intermediate un- saturated zone, the oil moves under the influence not only of gravity but also of capillary forces in all directions. Therefore, a zone develops around the core of the infiltration body which can be compared with the capillary fringe of the aquifer. Schwille (1967) calls it the "oil wetting zone." In it, as in the capillary fringe, the oil saturation decreases in an outward direction. In the "oil percolation zone," gravitational forces are dominant. Below a certain degree of saturation, the oil is retained in an immobile state by capillary forces. The oil saturation corresponding to this state is called "residual oil saturation," and the oil present in the pore structure under these conditions is called "residual oil." If the saturation of a non-wetting oil is reduced any further, the available flow channels become discontinuous and leave behind only isolated islands of the non-wetting fluid (Figure 2.4). For all pressure gradients which occur in laminar flow, these islands are largely stable. Intermediate Unsaturated Zone Weathered materials of the soil zone may gradually merge with underlying deposits which are generally unweathered and which comprise the intermediate unsaturated zone. In some regions, this zone may be practically nonexistent as the soil zone merges directly with bedrock. In alluvial deposits of western valleys, however, this zone may be hundreds of meters thick. If the hydraulic properties of the intermediate unsat- urated zone are uniform with depth, the migration (both vertically and horizontally) will be controlled by the leak rate and the soil texture. If the materials are layered (as is often the case), horizonal spreading of the contaminant at larger interfaces may dominate. These two processes are graphically shown in Figure 2.5. Water in the intermediate unsaturated zone may exist primarily in the unsaturated state, and in regions receiving little inflow from above, flow velocities may be negligible. Perched ground water and free product lenses, however, may develop in the interfacial de- posits of regions containing varying textures. Alter- natively, saturated conditions may develop as a result of deep percolation of water and oil from the soil zone during prolonged leakage. Studies by McWhorter and Brookman (1972) and Wilson (1971) have shown that perching layers intercepting downward-moving water may transmit the water laterally at substantial rates. Thus, these layers serve as underground spreading regions transmitting water and oil laterally away from the overlying source area. Water and oil may move downward from these layers and intercept a sub- 2-2 ------- Proper Location of a Sampling Point Siting Considerations Organic Liquid Monitoring Point Chemical Adsorption Biological \ Dilution and Degradation \ Dispersion Cultural and Chemical °ther Sitin9 Transformation Considerations Figure 2.1 Migration of Leaked Material Through the Soil Zone ------- Figure 2.2 Effect of Clay Lens In Soil on Hydrocarbon Migration Path (API, 1980) free groundwater level wetting zone percolation zone SEEPAGE Figure 2.3 Seepage of OH Through the Soil Zone (after Schwille, 1967) 2-4 ------- WATER FLOW 8AND GRAINS TRAPPED OIL FLUSHING WILL NOT REMOVE ALL OF THE TRAPPED PRODUCT BECAUSE OF CAPILLARY ATTRACTION Figure 2.4 Trapped Product Droplets (API, 1980) LAND SURFACE SLOW SEEPAGE INTO PERMEABLE SOIL HIGH VOLUME SEEPAGE INTO PERMEABLE SOIL SEEPAGE INTO STRATIFIED SOIL WITH VARYING PERMEABILITY Figure 2.5 Hydrocarbon Leakage Flow Paths (API, 1972) 2-5 ------- stantial area of the water table. Because of dilution and mixing below the water table, the effects of leak- age may not be noticeable until a large volume of the aquifer has been affected. The number of studies on water movement in the soil zone greatly exceeds the studies in the intermediate zone. Reasoning from Carey's equation, Hall (1955) developed a number of equations to characterize mound (perched ground water) development in the intermediate zone. Hall also discusses the hydraulic energy relationships during lateral flow in perched ground water. Bear et al. (1968) described the req- uisite conditions for perched ground-water formation when a region of higher permeability overlies a region of lower permeability in the unsaturated zone. Capillary Fringe At the base of the intermediate unsaturated zone is the capillary fringe. The capillary fringe merges with underlying saturated deposits of the principal water- bearing formation. This zone is not characterized as much by the nature of geological materials as by the presence of water or oil or both under conditions of saturation or near saturation. Studies by Luthin and Day (1955) and Kraijenhoff Van DeLeur (1962) have shown that both the hydraulic conductivity and flux may remain high for some vertical distance in the capillary fringe, depending on the nature of the materials. In general, the thickness of the capillary fringe is greater in fine materials than in coarse deposits. Apparently, few studies have been con- ducted on flow and chemical transformations in this zone. Taylor and Luthin (1969) reported on a com- puter model to characterize transient flow in this zone and compared results with data from a sand tank model. Freeze and Cherry (1979) indicated that oil reaching the water table following leakage from a surface source flows in a lateral direction within the capillary fringe in close proximity to the water table. Because oil and water are immiscible, the oil phase does not penetrate below the water table. Although many components of hydrocarbons are only very slightly soluble, the solubility levels greatly exceed the concentrations deemed safe for consumption. This can result in a large amount of contaminated ground water. Saturated Zone Two processes control the rate of spreading of im- miscible hydrocarbons in the saturated zone. The fluid density, if less than that of water, stabilizes the plume on the capillary fringe. Lateral spreading of the plume ceases when the residual oil saturation is reached behind the spreading plume. If the spilled material is equal or more dense than the ground water, the plume will migrate vertically through the aquifer. This vertical migration may have some horizontal component which is due to the lateral motion of the ground-water flow. The second and potentially more serious component of spreading is a result of the slight solubility of many hydrocarbons in water. Although slight, the equi- librium concentration levels frequently exceed those deemed safe for human consumption. These dis- solved constituents move with the ground water and are affected by the processes of diffusion, mechanical dispersion, adsorption and retardation, and microbial degradation. Each of these processes tend to de- crease the concentration of the original contaminant. Flowing ground water can dissolve certain com- ponents from the oil and move them away from the spill site. Around the oil infiltration zone proper, a diffusion corona is formed if the flow direction of the ground water changes often, and a diffusion trail is formed if the direction of the flow is mostly the same, leeward of the oil body. This downgradient transport is shown in Figure 2.6 (the substances are no longer bound to the capillary fringe but can migrate to lower parts of the aquifer). Presumably, the boundary between the oil zone and the diffusion zone is blurred by a transition zone of minute droplets of oil which are pulled out of the oil core by escaping surface active substances and which move ahead of the oil front. Flow Regimes Recent studies have demonstrated that soil water movement in the unsaturated zone is considerably more complex than the classical concept and that rapid infiltration to soil depths not predicted by Darcian flow can occur in soils with continuous or dis- continuous, or both, structural macropores. Macro- pores are large channels or fractures through which fluids may rapidly flow downward with little resistance to flow. From these macropores, the diffusion of fluid into the surrounding rock or soil matrix may be very stow, allowing water or hydrocarbons to travel great distances downward. Figure 2.7 shows a series of discontinuous macropores in an otherwise uniform soil. The macropore flow phenomena, which is primarily restricted to highly structured soils, or fractured rocks, involves the rapid transmission of free water through large, continuous pores or channels to depths greater than predicted by Darcian flow. The observation that a significant amount of water movement can occur in soil macropores was first reported by Lawes et al. (1882). Reviews of subsequent work are provided by 2-6 ------- \ Rain \ Ground surface \ Unsaturated zone Capillary fringe Water flow Oil dissolved In water Saturated zone Impermeable Figure 2.6 Hydrocarbon Migration Pattern (Schwille, 1984) DISCRETE MACROPORE O SOLID MATERIAL MESOPORES Figure 2.7 Composition of a Region of Macropore-mesapore Media 2-7 ------- Whipkey (1967) and Thomas and Phillips (1979). Macropore flow can occur in soils at moisture contents less than field capacity (Thomas et al., 1978). The depth of macropore flow penetration is a function of initial water content, the intensity and duration of the precipitation event, and the nature of the macropores (Aubertin, 1971; Quisenberry and Phillips, 1976). Macropores need not extend to the soil surface for flow to occur, nor need they be very large or cylindrical (Thomas and Phillips, 1979). Exemplifying the role of macropores, Bouma et al. (1979) reported that planar pores with an effective width of 90 m occupying a volume of 2.4 percent were primarily responsible for a relatively high hydraulic conductivity of 60 cm/day in a clay soil. Aubertin (1971) found that water can move through macropores very quickly to depths of 10 m or more in sloping forested soils. Some researchers feel that liquid moving in the macropore flow regime may bypass the soil solution in entrapped or matrix pores surrounding the macropores and may result in only partial displacement or dispersion of dissolved con- stituents (Quisenberry and Phillips, 1978; Wild, 1972; Shuford et al., 1977; Kissel et al., 1973; Bouma and Wosten, 1979; Anderson and Bouma, 1977). The current concept of infiltration in well-structured soils combines both classical wetting front movement and macropore flow. Aubertin (1971) found that the bulk of the soil surrounding the macropores was wetted by radial movement from the macropores sometime after macropore flow occurred. A number of researchers have presented mathematical models in an attempt to explain the macropore flow phenomena (Seven and Germann, 1981; Edwards et al., 1979; Hoogmoed and Bouma, 1980; Skopp et al., 1981). The occurrence of macropore flow poses implications for unsaturated zone monitoring and the protection of ground water. The first implication is that leakage may flow more rapidly through structured soils or fractured rock than would be predicted by porous media theory. Under this short circuit scenario ground-water con- tamination is probable when a shallow, well-structured soil is underlain by creviced bedrock (e.g., limestone solution channels, Shaffer et al., 1979) or a high water table or both (Anderson and Bouma, 1977). The second implication is that hazardous constituents moving with the rapid macropore flow may not be detected by using traditional soil-monitoring tech- nology. Current literature on soil-water movement in the un- saturated zone describes two flow regimes, the classical wetting front infiltration of Bodman and Colman (1944) and macropore flow. The classical concept of infiltration depicts a distinct, somewhat uniform, wetting front slowly advancing in a Darcian flow regime after a precipitation event. Contemporary models of water flow have this classical concept combined with the macropore flow phenomena. Darcian Flow The fundamental principle of unsaturated and sat- urated flow is contained in Darcy's law. In 1856, Henry Darcy, in a treatise on water supply, reported on experiments of the flow of water through sands. He found that the flow of water was proportional to the head loss and inversely proportional to the thickness of sand traversed by the water. Considering a generalized sand column with a flow rate, Q, through a cylinder of cross-sectional area, A, Darcy's law can be expressed as: Ah Q = KA AL (2.1) More generally, the velocity is given by: Q A dh K (2.2) where dh/dL is the hydraulic gradient. The quantity, K, is a proportionality constant known as the hydraulic conductivity. The velocity in Equation (2.2) is an apparent one defined in terms of the discharge and the gross cross-sectional area of the porous medium. The actual velocity varies from point to point throughout the column. Darcy's law is applicable only within the laminar range of flow where resistive forces govern flow. As ve- locities increase, turbulent flows occur and cause deviations from the linear relation of Equation (2.2) to become dominant. Fortunately, for most natural ground-water motion, Darcy's law can be applied. Unsaturated Flow When a porous medium becomes less than fully sat- urated, the hydraulic conductivity sharply decreases. This is caused by two phenomena. When the medium begins to desaturate, capillary theory suggests that the large diameter pores will be the first to drain. Since these pores can most easily conduct a large percentage of flow when saturated, their desaturation significantly reduces the fluid-transmitting properties of the medium. The second cause for the loss in con- ductivity is the increased tortuous path length that the fluid must flow through as a result of the dewatering or drainage of large pores. 2-8 ------- Another very important concept of multiphase flow is specific retention. Specific retention stems from the agricultural literature and loosely refers to the vol- umetric fluid content retained in a porous media that has been allowed to drain by gravity. The term is closely related to the unsaturated hydraulic con- ductivity. When the media drains, the conductivity is reduced, and drainage slows. At some point, the continued drainage is insignificant (from an agricultural perspective) when compared with the initial drainage. The specific yield is defined as the volume of water drained in this process and is equal to the difference between the saturated porosity and the specific retention. Figure 2.8 shows the values of porosity, specific yield, and specific retention for various soils. From these simple concepts of specific retention, the theory of multiphase flow may be developed. Given that little or no flow occurs when the soil moisture is below a critical level, it can be assumed that the conductivity or permeability is equal to zero below this water content. If the second phase in the medium is oil, it too will have a specific retention or residual saturation below which the permeability is also zero. Figure 2.9 graphically shows these results in the form of a relative permeability plot. The horizontal axis represents the percent saturation of each fluid while the vertical axis represents the relative permeability (Kr) of the medium. The relative permeability is cal- culated as the ratio of the permeability at a given sat- uration to the permeability of the medium at 100 percent saturation. Two-Phase Flow The flow of two immiscible fluids simultaneously in a porous medium has been described by van Dam (1967). van Dam's experimental results showed that: 1. The permeability of a given porous medium to one fluid in the presence of another fluid is reduced with respect to single-phase permeability. 2. The reduction in permeability is dependent on the wetting of the porous medium by one of the two fluids. 3. There must be a minimum saturation for each fluid before the medium is permeable to the fluid. Figure 2.9 shows that the flow of oil (non-wetting fluid) is not possible before an oil saturation of approx- imately 10 percent is obtained. For water, a minimum saturation of approximately 20 percent is necessary before the flow can take place, van Dam's equations are equally applicable to a water-air system and an oil- air system. The latter system will apply after the introduction of a quantity of oil into a permeable substratum before the oil has penetrated as far as the ground-water level. In this case, a minimum oil saturation of approximately 10 to 20 percent (de- pending on the porous medium) must be achieved before migration of oil in the substratum can take place. This minimum saturation is called the residual oil saturation and represents the quantity of oil which is held permanently by the porous medium. This phenomenon limits the migration distance of a given quantity of oil introduced into a permeable soil. Where the infiltrated volume of oil is large enough to reach the ground water, an analogous situation could arise in the aquifer. The contaminated zone could be re- stricted to a porous volume large enough to contain the infiltrated oil at a saturation equal to the residual saturation. Three-Phase Flow The mathematical description of three-phase flow, although far more difficult than two-phase flow, has been developed by van Dam (1967). The three-phase relative permeabilities for air, oil, and water systems are shown schematically in Figure 2.10-A. Each point within the triangle corresponds to a different degree of saturation for air, oil, and water as indicated on the scales along the sides of the triangle. Equal values for the relative permeabilities for each of the three phases are indicated by "isoperms" drawn inside the triangle. From the diagram it appears that there are large regions where at least one of the three fluids present is immobile and that only in a very limited "saturation region" is simultaneous flow of all three phases possible. This is more clearly indicated in Figure 2.10-B. Another significant observation is that residual water saturation is almost the same irre- spective of the magnitude of oil and air saturations, while the residual air saturation is largest in the region where oil and water saturation are the same order of magnitude. Finally, the residual oil saturation is larger if no water saturation is present but becomes nearly constant with increasing water saturations approach- ing the residual value. The capillary forces prevailing on introduction of oil into a partially water-filled porous medium are governed by very complicated conditions. For the sake of simplicity, it will be assumed that everywhere in the porous medium there will be at least a residual water saturation and that the grains are always enclosed by a thin water film such as found in wet soils with a shallow ground-water system. When oil or any other third phase is introduced into the soil, three- phased conditions will exist. In this case, water is the wetting fluid with respect to oil, but oil will, by pref- 2-9 ------- Figure 2.8 Variation of Porosity, Specific Yield, and Specific Retention with Grain Size (after Bear, 1972) 0%---*-WATER SATURATION 100% 100% OIL SATURATION * 0% Figure 2.9 Two-phase Flow Relative Permeability (van Dam, 1967) 2-10 ------- too% ao AO 40 30 WATER Figure 2.10 A Relative Permeablilities Three-phase Flow (after van Dam, 1967) WATER 100% Figure 2.10 B Three-phase Relative Permeability (after van Dam, 1967) 2-11 ------- erence, wet the water surface exposed to air. Con- ditions may arise where the ground-water level is depressed and where oil comes in contact with water without the presence of air. In this case, two-phased conditions would prevail. The following conclusions may be drawn from van Dam's work concerning the migration of hydrocarbons in isotropic homogeneous media. Oil will first migrate downwards in a vertical direction towards the ground- water table and will then spread in a horizontal direction parallel to the ground-water table (Figure 2.11). The latter phenomenon is caused by capillary forces in the air-water capillary zone which prevents oil from entering the aquifer proper. The presence of air and water in residual saturations in the porous medium will reduce the migration velocity of the oil. The volume of the porous medium which is invaded by a limited quantity of oil will be restricted for these two reasons: (a) in all regions through which the oil passes, a minimum residual saturation must be es- tablished before the oil can continue to flow, (b) and if the oil spreads on top of the capillary zone above the water table, a minimum thickness must be established before lateral migration can occur. It follows that large quantities of oil must be introduced into a permeable substratum before the presence of oil can be observed at some distance from the source of infiltration. In the case of an inclined ground-water table, the oil infil- trated zone will move in the direction of the ground- water gradient. Oil migration in a direction opposite to the direction of the ground-water gradient will be limited to relatively short distances. In a hetero- geneous porous medium, the migration of oil in the presence of ground water will, by preference, follow the most permeable layers. Strong capillary forces in a porous medium with a low permeability will prevent the entry of oil into these zones. This is particularly so in clay environments. The existence of oil infiltrations in the subsurface can effectively be detected by ob- servation wells that are perforated throughout the section covering the capillary zone above the ground- water table. Liquid Monitoring Difficulties and Future Research The fundamental problem of leaking underground tanks is that there are many accidental surface spills over time which create a complex background condition of spatially and temporarily variable contam- ination. The residual hydrocarbons in the soil can occupy from 15 percent to 40 percent or more of the pore space. The residual hydrocarbons act as a continual source of contaminants as water comes into contact with the trapped immiscible phase and leaches soluble components. Consequently, back- ground conditions and spillage can result in significant soil and water contamination. Qround surface Oil phase Unsaturated zone Capilary fringe Oil components dissolved in water Saturated zone" Figure 2.11 Spreading of Oil Spill on Water Table Surface (after Schwille, 1967) 2-12 ------- For the case of a miscible inorganic hazardous waste moving in the unsaturated zone as two-phase flow or in the saturated zone as single-phase flow, the problems are seemingly straightforward. From a predictive standpoint, one can feel relatively com- fortable with Darcy's Law and existing ground-water flow models. The area of concern is that unsaturated zone models to date have not been able to adequately predict flow conditions to the satisfaction of most hydrologists. The reason principally is the site specific variability of the hydrogeology both vertically and horizontally in these upper materials. In addition, one must recognize that the presence of discontinuous macropores, fractures or fissures, wormholes, and deep roots, etc., offer pathways for transporting con- siderable volumes of water. Various techniques such as neutron probes and tensiometers (Everett and McMillion, 1985), can be used to monitor the vertical and horizontal distribution of soil moisture. One of the fundamental problems of sampling, however, is related to the sphere of influence of each of the sampling devices. The sphere of influence of various unsaturated zone sampling devices has not been determined, and, consequently, the number and depths of sampling devices beneath underground tanks is dependent upon professional judgment. When dealing with an increase in soil moisture which results in a decrease in soil suction, one can monitor leakage of immiscible and dissolved fractions of immiscible solution with tensiometers, neutron probes, or other vadose zone monitoring devices (Everett and McMillion, 1985). The halo of dissolved hydrocarbon components which precedes the immiscible phase offers the opportunity for monitoring in the subsurface. A critical milestone in unsaturated zone monitoring is to determine when significant unsaturated flow is expected to begin. When dealing with residual oil saturation, however, devices have not been developed which will monitor the build-up of residual oil in the unsaturated zone. High success in collecting soil pore liquid sample containing volatile hydrocarbons has been obtained by using ceramic suction lysimeters. The ceramic has a pore size of approximately 2.3 microns and, as such, does not allow air to enter. The suction lysimeters are installed wet (Everett and Wilson, 1984), and, as a result, all the pore spaces in the ceramic are initially filled with water. The miscible components of hydrocarbons, including 1, 1-dichloroethane, trichlo- roethene, benzene, ethylbenzene, toluene, and meta, ortho, and para zylenes, have been successfully sampled by using suction lysimeters. As a result, a strong case can be made for using suction lysimeters to detect the dissolved fractions within hydrocarbons as they migrate in the unsaturated zone. Unfor- tunately, direct sampling of the immiscible phase is hampered by the limited oil wettability of the ceramic material. From the analysis of van Dam (1967), the water table acts as a barrier for the downward migration of immiscible low density organic hazardous wastes. Based on the specific densities, the hydrocarbons are pushed up by the water below the oil core, and this causes the hydrocarbon to spread along the surface of the water table and ultimately the capillary fringe. One problem associated with ground-water monitoring occurs when obtaining a pumped sample. If the cone of depression lowers the water table in the area of the leakage (see Figure 2.12), oil is spread over a greater thickness of aquifer, and, hence the zone of residual saturation is increased in thickness. A second problem associated with monitoring hydrocarbons in the saturated zone is related to the interpretation of the physical thickness of the hydrocarbon lense. As demonstrated in Figure 2.12, the hydrocarbon lense may ride on top of the capillary fringe downgradient from the leakage area. Although one can perforate an observation well through the capillary fringe into the intermediate zone to pick up a hydrocarbon lense, it is difficult to interpret the thickness of the lense itself. A mathematical interpretation of this phenomenon is presented by Zillion and Nutzer (1975) and is repre- sented in Figure 2.13. A rule-of-thumb has emerged from their interpretation which indicates that one typically observes a four-to-one ratio of oil observed in an observation well to what is actually present as a hydrocarbon lense. While the problem of two-phase flow through sat- urated porous media is complex, several case histories are available to provide insights into mi- gration rates. The problem of immiscible two-phase flow in fractured media is considerably more complex but has been recently defined by Schwille (1984). The problem of three-phase flow in the unsaturated zone is technically, mathematically, and conceptually ex- tremely challenging. If one superimposes precipitation events on top of a hydrocarbon leak, the residual saturation concepts (see Figure 2.14) become even more complex as the water phase displaces the oil envelopes. In isolated instances involving dense hydrocarbons, the oil moves downward through the water table under gravity forces and offers a more complex situation. Physical models including the capillary fringe should be developed (Schwille, 1984) to achieve a basic representation of the migration process for chosen representatives of each hazardous waste group of substances. Using these models, one could stand- ardize the determination of retention capacity including 2-13 ------- oil K = 3.0x10 ~3m/»/. '.J J = 0.01 10O cm Figure 2.12 Model Experiment: Influence of Changing the Water Level on the Oil Distribution (after Schwille, 1967) instrumentation, soil type, water content, and methods. In addition, the need exists to develop typical diagrams for relative permeability for air, water, and immiscible fluid phases. It is very clear that the basic understanding of hy- drology, including the phenomenology and associated mathematics, needs to be expanded to include liquid hydrocarbon transport. There is a need for geo- chemists, petroleum engineers, microbiologists, and hydrogeologists to work in concert in expanding hydrogeologic principles to hydrocarbon transport and monitoring. 2-14 ------- ... WATER FLOW.... Figure 2.13 A Impregnation Body (Petroleum Product) Having Reached Ground Water (Zillion And Nutzer, 1975) Figure 2.13 B Thickness of Layer of Oil in the Ground and in a Strainer Tube (Influence of Capillary Pressures in the Case of a Continuous Layer of Oil) 2-15 ------- Figure 2.14 Displacement of Oil Envelope by Water - Considered Microscopically (Schwllle, 1967) 2-16 ------- References American Petroleum Institute. 1972. The Migration of Petroleum Products in Soil and Ground Water. Washington, D.C. American Petroleum Institute. 1980. Underground, Spill Clean-Up Manual. Washington, D.C. Anderson, J.L., and J. Bouma. 1977. Water Movement Through Pedal Soils: I. Saturated Flow. Soil Sci. Soc. Am. J. 41:413-418. Aubertin, G.M. 1971. Nature and Extent of Macropores in Forest Soils and Their Influence on Subsurface Water Movement. USDA Forest Serv. Res. Paper NE-912. Northeast Forest Exp. Stn., Upper Darby, PA. Baehr, A., and M.Y. Corapciaglu. 1984. A Predictive Model for Pollution from Gasoline in Soils and Ground Water. In: Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA, Houston, TX, November. Bear, J. 1972. Dynamics of Fluids in Porous Media. American Elsevier, New York, New York. Bear, J., D. Zaslavsky, and S. Irmay. 1968. Physical Principles of Water Percolation and Seepage. UN Educational, Scientific and Cultural Organization. Seven, K., and P. Germann. 1981. Water Flow in Soil Macropores. 2. A Combined Flow Model. J. Soil Sci. 32:15-29. Bodman, G.B., and E.A. Coleman. 1944. Moisture and Energy Conditions During Downward Entry of Water into Soils. Soil Sci. Soc. Amer. Proc., 8: 116-122. Bohn, H.L, B.L. McNeal, and G.A. O'Connor. 1979. Soil Chemistry. Wiley Interscience, New York. Bouma, J., A. Jongerius, and D. Schoondebeek. 1979. Calculation of Hydraulic Conductivity of Some Saturated Clay Soils Using Micromorphometric Data. Soil Sci. Soc. Am. J. 43:261-265. Bouma, J., and J.H.M. Wosten. 1979. Flow Patterns During Extended Saturated Flow in Two Undisturbed Swelling Clay Soils with Different Macrostructures. Soil Sci. Soc. Am. J. 43:16-22. Bouwer, H. 1978. Groundwater Hydrology. McGraw- Hill, New York. Convery, M.P. 1979. The Behavior and Movement of Petroleum Products in Unconsolidated Surficial Deposits. M.S. Thesis, U. of Minn. Davis, S.N., and R.J.M. De Wiest. 1966. Hydrogeology. John Wiley and Sons, New York. Dunlap, W.J., and J.F. McNabb. 1973. Subsurface Biological Activity in Relation to Ground Water Pollution. EPA-660/2-73-014. U.S. Environmental Protection Agency, Corvallis, Oregon. Eames, V. 1981. Influence of Water Saturation on Oil Retention Under Field and Laboratory Conditions. M.S. Thesis, U. of Minn. Edwards, W.M., R.R. Van Der Ploeg, and W. Ehlers. 1979. A Numerical Study of the Effects of Non- Capillary Sized Pores Upon Infiltration. Soil Sci. Soc. Am. J. 43:851-856. Everett, L.G. 1980. Groundwater Monitoring. General Electric Co. Technology Marketing Operations, Schenectady, New York. Everett, L.G., and L.G. McMillion. 1985. Operational Ranges for Suction Lysimeters. Groundwater Monitoring Review 5(3): 51-60. Everett, L.G., and L.G. Wilson. 1984. Unsaturated Zone Monitoring for Hazardous Waste Land Treatment Units. EPA/530-SW-84-016. Environmental Monitoring Systems Lab., Office of Research and Development, U.S. Environmental Protection Agency, Las Vegas, NV. Freeze, R.A., and J.A. Cherry. 1979. Groundwater. Prentice-Hall, Englewood Cliffs, New Jersey. Hall, W.A. 1955. Theoretical Aspects of Water Spreading. Amer. Soc. Agric. Eng. 36(6): 394-397. Hillel, D. 1971. Soil and Water Physical Principles and Processes. Academic Press, New York. Hoogmoed, W.B., and J. Bouma. 1980. A Simulation of Model for Predicting Infiltration Into Cracked Clay Soil. Soil Sci. Soc. Am. J. 44:485-462. Kirkham, D., and W.L. Powers. 1972. Advanced Soil Physics. Wiley Interscience, New York. Kissel, D.E., J.T. Ritchie, and E. Burnett. 1973. Chloride Movement in Undisturbed Swelling Clay Soil. Soil Sci. Soc. Am. Proc. 37:21-24. 2-17 ------- Klute, A. 1965. The Movement of Water in Unsaturated Soils. In: The Progress of Hydrology, Proc. First Int. Seminar for the Hydrology Prof., National Science Foundation Science Seminar, Dept. of Civil Eng., U. of Illinois, July 13-25. Kraijenhoff van deLeur, D.A. 1962. Some Effects of the Unsaturated Zone on Nonsteady Free-Surface Groundwater Flow as Studied in a Scaled Granular Model. J. Geophys. Res., 67(11): 4347- 4362. Lawes, J.B., J.H. Gilbert, and R. Warrington. 1882. On the Amount and Composition of the Rain and Drainage Waters Collected at Rothamsted. William Clowes and Sons, Ltd, London. Luthin, J.M. (ed). 1957. Drainage of Agricultural Lands. Amer. Soc. Agron., Madison, Wisconsin. Luthin, J.M., and P.R. Day. 1955. Lateral Flow Above a Sloping Water Table. Soil Sci. Soc. Amer. Proc., 19:406-410. McWhorter, D.B., and J.A. Brookman. 1972. Pit Recharge Influenced by Subsurface Spreading. Ground Water 10(5): 6-11. Quisenberry, V.L., and R.E. Phillips. 1976. Percolation of Surface Applied Water in the Field. Soil Sci. Soc. Am. J. 40:484-489. Rhoades, J.D., and L. Bernstein. 1971. Chemical, Physical and Biological Characteristics of Irrigation and Soil Water. In: Water and Water Pollution Handbook, Vol. 1, L.L. Ciaccio, (ed)., pp. 141-222. Marcel Dekker, Inc., New York. Schwille, F. 1967. Petroleum Contamination of the Subsoil - A Hydrogeological Problem. In: The Joint Problems of the Oil and Water Industries. Proceedings of a symposium held at the Hotel Metropole, Brighton, 18-20 January. Schwille, F. 1981. Groundwater Pollution in Porous Media by Fluids Immiscible with Water. In: Quality of Groundwater. Proceedings of a Symposia, Noordwijkerhout, the Netherlands. Elsevier, Amsterdam. Schwille, F. 1984. Migration of Organic Fluids Immiscible with Water in the Unsaturated Zone. In Pollutants in Porous Media. Ecological Studies 47, Springer -Verlag. Shaffer, K.A., D.D. Fritton, and D.E. Baker. 1979. Drainage Water Sampling in a Wet, Dual-Pore Soil System. J. Environ. Qual. 8:241-246. Shuford, J.W., D.D. Fritton, and D.E. Baker. 1977. Nitrate-Nitrogen and Chloride Movement Through Undisturbed Field Soil. J. Environ. Qual. 6:736- 739. Skopp, J., W.R. Gardner, and E.J. Tyler. 1981. Solute Movement in Structured Soils: Two-Region Model with Small Interaction. Soil Sci. Soc. Am. J. 45:837-842. Taylor, G.S., and J.N. Luthin. 1969. The Use of Electronic Computers to Solve Subsurface Drainage Problems. Water Resources Research 5(1): 144-152. Thomas, G.W., and R.E. Phillips. 1979. Consequence of Water Movement in Macropores. J. Environ. Qual. 8:149-152. van Dam, J. 1967. The Migration of Hydrocarbons in a Water Bearing Stratum. In: The Joint Problems of the Oil and Water Industries. Proceedings of the symposium held at the Hotel Metropole, Brighton, 18-20 January. van Schilfgaarde, J. 1970. Theory of Flow to Drains. Advances in Hydroscience. 6:43-106. Whipkey, R.Z. 1967. Theory and Mechanics of Subsurface Storm Flow. pp. 155-260. In: W.E. Sopper and H.W. Lull (ed). Int. Symp. on For. Hydrol., Natl. Sci. Found., 29 August - 10 September 1965, Penn. State U., University Park, PA. Pergamon Press, NY. Wild, A. 1972. Nitrate Leaching Under Bare Fallow at a Site in Northern Nigeria. J. Soil Sci. 23:315-324. Wilson, L.G. 1971. Observations on Water Content Changes in Stratified Sediments During Pit Recharge. Ground Water 9(3): 29-40. Zillon, F., and J. Nutzer. 1985. Protection of Groundwater from Oil Pollution. Prepared by CONCAWE, Special Task Force No. 11, April 1979. 2-18 ------- SECTION 3 Vapor Transport and Its Implications to Underground Tanks Introduction Up until recent times, there had been little scientific interest in vapor movement around buried storage tanks. Most studies of gaseous migration just below ground surface were carried out by agronomists or agricultural scientists interested primarily in properties related to soil aeration, fumigation, and denitrification. However, recently there has been a great increase in the attention directed to unsaturated zone vapor flux around buried tanks; this increased attention has been fueled by a growing realization of the importance and utility of the unsaturated zone flux in detection of leaks and contaminant plumes. In the underground storage tank environment, gas- eous movement can be a significant component of overall migration of leaked product, particularly if that product is highly volatile. Movement of compounds in the vapor phase through advection or diffusion or both processes can and often does occur in all directions from a leak source. Therefore, a portion of volatile, leaked product vapor could migrate in a direction opposite of the underlying ground-water flow (Figure 3.1). The vapor could then enter the ground-water system by redissolving across the capillary fringe and water table and could appear to be liquid pollutant moving up the hydraulic gradient. This outward flux of gases in the unsaturated zone has important implications to leak detection. A gaseous phase detector would be less likely to "miss" a gas moving radially outward, than a monitoring well attempting to detect a finite plume moving down the hydraulic gradient in the aqueous phase. Because vapor phase monitors are normally emplaced at shallower depths than aqueous phase samplers, there is some emplacement cost saving with vapor detection systems in the unsaturated zone. There are some situations where vapor detection devices would be more effective than their liquid detection counterparts in the saturated zone, particularly where the saturated zone is at great depth below the ground surface and leak source. Several factors, such as background contamination and geologic stratification, can have a negative effect on leak detection in the vapor phase, but monitoring of gases in unsaturated porous media, while not perfect, has been shown to be a useful tool if interpreted correctly. Recent legislation in the State of California has recognized the value of gaseous moni- toring and has opened the way for its use in situations of contamination. The following section will describe vapor transport surrounding underground storage tanks, and will discuss naturally occurring and man-made gases in the subsurface, mechanisms of transport with par- ticular attention to transport in the underground tank environment, existing types of vapor monitoring meth- odologies, requirements and shortcomings of present vapor detection systems and theory, and future direc- tions of vapor-leak monitoring. Sources of Gases in the Unsaturated Zone The most prevalent gas exchange process governing the composition of the soil atmosphere is respiration; the biological consumption of O2 and the concurrent production of CO2. Primary mechanisms of respira- tion include: a. The oxidation of organic materials used as an energy source by aerobic bacteria and the production of CO2 during their metabolic activity and b. The oxidation of synthesized organic compounds by plant roots and the corresponding production and liberation of CO2. An important ramification of respiration involves a gradual increase in the CO2 concentration in the unsaturated zone from about 0.03 percent at the soil surface to 1-5 percent beneath the plant rooting zone (Bolt and Bruggenwert, 1976). With depth, oxygen exhibits a decrease in concentration correlating with progressive increase in CO2 levels. Because of the formation of the CO2 /O2 gradient, the ambient soil gases continually move to equilibrate with the outer atmosphere (21 percent O2, 0.03 percent CO2). This produces replenishment of O2 into the subsurface with simultaneous emission of CO2 at the soil-air interface. Because the ability of CO2 and O2 to 3-1 ------- MONITORING WELL IMPERVIOUS CONFINING LAYER co to GASEOUS MIGRATION OPPOSITE GROUNDWATER FLOW WATER TABLE \ / VOLATIZATION t t i DIRECTION OF GROUNDWATER FLOW REDISSOLUTION Figure 3.1 Volatlzed Product Vapor Migration Opposite Ground-water Flow ------- move out of and into the unsaturated zone is largely a function of continuous porosity, soil media of low porosity (e.g., well graded soils) will seriously impede the aeration process, and oxygen concentrations will drop to near zero values, while there will be a cor- responding buildup in CO2 concentrations. In such oxygen deficient environments, processes of anaerobic reduction and gaseous formation can occur. Anaerobic bacteria use oxygen previously bound to nitrogen, carbon, or other elements to sustain life processes. Through a reductive process to support their metabolic activity, these bacteria strip oxygen from compounds such as carbon dioxide (CO2) and nitrate (NO3), and liberate gaseous end products such as methane and ammonia. While the soil atmosphere consists primarily of naturally occurring gases, the use of fumigants and pesticides has contributed significantly to the concentrations of subsurface gases where applied. Chloropicrin, ethylene dibromide, methyl bromide, carbon disulfide, and numerous other fumigants are frequently added to the soil in an effort to control parasitic fungi, nematodes, and weeds. These fumi- gants vary considerably in their diffusivity and inter- action with the soil environment and, therefore, have variable influences on the soil atmosphere where applied. Pesticides are usually applied to the soil in the solid or aqueous phase. The rate at which pesti- cides volatilize is affected by their chemical properties, soil characteristics, and environmental conditions. The vapor density of an applied pesticide therefore depends on the combination of these factors in the soil environment. Spills and dumping of chemical products onto the ground surface or into dry wells can also have a major effect on soil vapor concentrations. This problem is of particular importance in underground tank leak de- tection because tank product is often spilled on the ground surface during many filling and withdrawal operations, and a background of product vapors will exist in the vadose zone without a leak being present. Surface spills of fluids identical to those contained in an underlying tank constitute one of the most serious threats to accurate gaseous leak detection. Back- ground vapor concentrations in the unsaturated zone surrounding a tank could cause false positives and could reduce leak detectability. This is shown graph- ically in Figure 3.2. Bacterial degradation of pesticides and leak products should be mentioned as a secondary source of vapors in the unsaturated zone. One important process involves the decomposition of synthetic organic com- pounds by soil microorganisms which liberate simple gases such as NH3 and CO2. For example, Mckee et al. (1972) describe the utilization of gasoline and other petroleum products as a food source and consequent release of CO2 by the bacterial genera Pseudomonas and Arthrobacter. Finally, downward percolating rainfall can bring dis- solved gases, picked up in the atmosphere, into the unsaturated zone. For example, the fluorocarbons F- 11 and F-12 which are used as refrigerants or spray- can propellents are ubiquitous in most shallow vadose zone air that has been measured, probably as a result of coming out of solution from rainwater that has infiltrated. Thus, gases in the unsaturated zone can have many different sources and concentrations. Vapor from leaking tanks enters this variable en- vironment. Factors Which Effect Movement of Gases Near Underground Storage Tanks The mobilities of gases near underground storage tanks are affected by a great number of complex, sometimes interrelated parameters. Aspects of the local geology, such as the geometry, homogeneity, and bulk composition of the local stratigraphy, and tank design and tank installation are the most important factors that impact gaseous behavior, next to leak size and shape. (Leak source strength and configuration will be discussed later in this section.) Rate of gas exchange between the soil air and outer atmosphere depends greatly on the resistance local stratigraphy imposes on gas mobility. Stratigraphic factors influencing gaseous transport include: (a) differences in soil types, (b) soil and rock heteroge- neities, and (c) geological discontinuities. Differences in soil types can have significant impact on gas mobilities. A clean gravel, for instance, would act as a conduit for soil gases because of lesser resistance to vapor diffusion and advection than tightly packed silts, clays, or poorly sorted material. An air impermeable clay lens, on the other hand, will act as a vapor barrier and would therefore deflect gases into surrounding mediums. If water is removed from the same clay lens, however, shrinkage might occur, and the resultant microfissures would then act as a sec- ondary conduit for gases. Heterogeneities in soil and rock also influence gas mobilities. A gas slowly diffusing through a mod- erately permeable loam soil might, for example, encounter a buried stream channel of well-sorted sands and gravels. The porous sand and gravel material would then act as a vapor sink, and the gas would rapidly diffuse into the more porous medium. In 3-3 ------- VAPOR MONITOR a \\ FLUID SPILLS r-?!! //&/&/*//* VOLATIZED PRODUCT UNDERGROUND STORAGE TANK FALSE POSITIVE POSSIBLE WATER TABLE MONITORING WELL DIRECTION OF GROUNDWATER FLOW Figure 3.2 False Positives due to Background Vapor Concentrations ------- response to the diffusion increase, a concentration gradient would be established and would result in the movement of additional gas from the source toward the buried stream channel. A preferred orientation of gaseous flow would be established from gas source to vapor sink. Subsurface discontinuities such as faults can increase vapor mobilities by creating a conduit for gases along the fault face, fissure, or other conduit, or can de- crease potential movement of gases by the dis- placement of a gas impermeable layer adjacent to a zone where gases could move more easily. There- fore, vapor transport either could be enhanced or diminished in areas which have experienced sig- nificant rock displacement events. Clearly the potential for gaseous movement in an underground tank environment hinges significantly on the composition, geometrical nature, and physical properties of the stratigraphic medium. Another important factor influencing gaseous behavior in the subsurface is related to the design and installation of the underground tank. The following is a list of subsurface tank installation specifications outlined by the American Petroleum Institute which directly influence gas mobilities: 1. At least 15 cm, and preferably 30 cm of clean, well sorted sand or gravel should be placed under the tank. 2. Tanks should be surrounded with at least 15 cm of noncorrosive, inert, and uniform material such as clean, uniform sand or gravel. 3. In areas of little traffic, the tank should be covered by a minimum of 60 cm or not less than 30 cm of well tamped, uniform material plus a 10 cm reinforced concrete slab. 4. In areas of heavy traffic, the tank should be covered by at least 90 cm, or not less than 45 cm of well tamped, uniform material plus 15 cm of reinforced concrete or 20 cm of asphalt concrete. The use of uniform, well sorted sands and gravels around underground storage tanks, emplaced for proper structural support and corrosive resistance, creates a medium into which both naturally occurring and leaking product gases are diffusible. The movement of these gases, whether into the outer atmosphere or into adjacent soil or rock mediums, critically depends on the interrelationship between the amount of gas present, the air permeability or dif- fusivity of the adjacent soil medium, and the presence or absence of asphalt or reinforced concrete at the ground surface. For example, volatilized product leaking from a buried tank overlain by an extensive concrete cover would tend to have greater transport laterally and downward into the surrounding fill material than would a similar leaking tank without a concrete ground surface cover. In the latter case, the uncovered ground surface would act as a math- ematical sink, that is, volatilized product would move preferentially, though not exclusively, toward ground surface. If the soil material adjacent to the tank is essentially impermeable to air, the leaking gases would move along any existing fractures or other open conduits, particularly to ground surface if such frac- tures existed. The movements of gases are therefore partly dependent on the combination of surface and stratigraphic variables at a given site. Tank liners may also influence gaseous transport in the subsurface. Liners are commonly installed be- neath underground storage tanks to help prevent ground-water contamination if a leak occurs. The liner material used, such as synthetic membranes or clay, will restrict gaseous movements into adjacent soils if its integrity remains intact. Any volatile product would therefore preferentially move toward ground surface at uncovered buried tank sites where properly sealed liners are used. Importance of Vapor Transport Surrounding Underground Storage Tanks Pollutant Migration Because many of the substances in underground storage tanks are moderately to highly volatile, a leaking tank often involves the evaporation of some liquid into the gaseous phase, and resulting vapor transport in all directions into the soil. In some field studies, this vapor movement has been interpreted to comprise a significant component of overall con- taminant migration. Therefore, an understanding of the movement of leaking tank contents in the vapor phase may be required, in addition to understanding the liquid phase, to correctly predict the migration of potential contaminants. The gaseous behavior of tank products, such as petroleum hydrocarbons, is largely a function of the diffusivity and air permeability of the vapor in the porous media of concern, and its distribution co- efficients describing interactions with soil particles and moisture. The potential for contamination of soils and ground water in a particular tank location depends on 3-5 ------- the combination of these parameters at that site. Even so, the following general statements may be made outlining the contribution of the vapor phase to contamination near a underground tank. 1. Vaporization may remove some of the potential subsurface contamination as volatilization allows a portion of the leaked compound to be released into the outer atmosphere. This also implies that an above-ground surface problem could be created if dangerous gases were allowed to build up (in a building or other surface structure) by vapor flux upward across the ground surface. 2. Despite statement 1, additional contact with waters percolating in the unsaturated zone is made possible through vapor transport, in which larger areas of ground water and soil surfaces may be contaminated. 3. The areal extent of possible vapor phase transport and contamination is largely dependent upon several factors including: the gaseous transport properties of the soil, the volatile properties of the tank product, and the interaction of the resultant vapor with soil particles and water contained in the porous medium. Leak Detection Although the movement of vapors from leaking tanks in the unsaturated zone presents an additional threat of soil and ground-water contamination, beneficial aspects of vapor transport may be realized from economical leak detection utilizing vapor sensing methods. Vapor detection sensors adjacent to leaking underground tanks have some advantages over conventional static line and volumetric leak detection methodologies, e.g., vapor detection methods do not require the tanks to be topped off with product and may proceed without the disruption of operations associated with static line and volumetric testing methods. Models may be formulated to correlate the concen- tration of vapor detected with the amount of product leak by defining and calibrating a set of parameters (e.g., effective diffusion coefficient, air permeability, the vapor/liquid partitioning ratio of the particular tank components). Unfortunately, not only is precise calibration of these parameters difficult to achieve, but also the effect of many environmental factors is usually unknown. Important variables, such as the sorption of gases on the soil matrix, their solubility in water, and amount of vapors attributable to accidental spillage during tank filling would greatly affect not only quantitative correlation, but also impact qualitative assessment of leaks. For example, a tank leak of acetone would likely release appreciable vapor, because vapor pressure (184.5 Torr at 20°C) of ace- tone is high relative to many other solvents. However, acetone is miscible with water in all proportions, and if there is a high moisture content in the environment outside the leaking tank, the gaseous migration and subsequent sensing of this solvent could be impaired relative to other compounds less soluble in water. Therefore, although attempts can be made to estimate leak location and amount, meaningful interpretation of subsurface vapor concentration is often impossible. Vapor Transport Processes The following is a description of gaseous diffusion, advection (also called transpiration, forced diffusion, laminar viscous flow, convective flow, or bulk flow), and effusion (also called free-molecule or Knudsen flow) which are mechanisms of vapor transport originally discussed by Thomas Graham in the 19th Century (Mason and Evans, I969). Gaseous Diffusion Diffusion is a process whereby elements in a single phase equilibrate. This process arises because of random molecular motions; each molecule is con- stantly undergoing collision with other molecules, and the result is constant motion with many changes in direction and no preferred direction of motion. Al- though this motion is random, there is a net transfer of molecules of one compound from regions of high concentration to low concentration. The net transfer of molecules is predictable, whereas the direction of any individual molecule at any moment is not. Gaseous diffusion is a spreading out or scattering of gases and may be divided into the categories of ordinary gas- eous diffusion, thermal gaseous diffusion, and particle diffusion. Ordinary gaseous diffusion is a process in which the components of any gas-filled space will eventually become thoroughly mixed. Taylor and Ashcroft (1972) outline the rate potential of ordinary gaseous diffusion in the soil atmosphere, stating that no ordinary diffusion will take place if the density (concentration) of the diffusing substance is the same throughout a given region. Further, if the density of the diffusing sub- stance is different at different points, diffusion will take place from points of greater to lesser density, and will not cease until the density at all places is the same. Therefore it follows that ordinary gaseous diffusion of each gas component in the subsurface will occur 3-6 ------- whenever there is a difference in its concentration between (a) the soil and outer atmosphere, or (b) at points within the soil because of irregularities in the consumption and release of gases. The rate at which ordinary gaseous diffusion occurs is governed, in part, by the magnitude of the con- centration gradient at a point. In turn, the magnitude of the concentration gradient is dependent upon soil type, soil homogeneity, and amount of gas generated by a source. The interrelationships between these factors will govern the extent in which ordinary gaseous diffusion becomes a major source of unsat- urated zone vapor transport. Ordinary gaseous diffusion is normally described by an equation written by Pick in 1855 after the principles described by Graham and in analogy with Fourier's Law for heat conduction. Pick's Law, as the equation has become known, relates diffusive flux to a concentration gradient over distance multiplied by an empirically derived coefficient called the effective diffusion coefficient. The magnitude of the effective diffusion coefficient is dependent of the properties of the porous media (tortuosity, porosity, moisture content) and properties of the diffusing gas. Thermal gaseous diffusion occurs when a temperature gradient is established in a gaseous mixture. The non-uniformity of temperature results in a con- centration gradient as denser molecules move down temperature gradient while less dense molecules move in the direction of increasing temperature (Grew and Ibbs, 1952). Causes of thermal gradients near subsurface tanks may vary from heating of indoor facilities, to radiant energy absorbed by surface concrete, to proximity to a geothermal source. The greater the temperature gradient, the more effectively a gaseous mixture may be separated into its denser components. Thermal diffusion is a slow process, however, and is not usually considered to contribute significantly to vapor transport in an underground tank environment. Particle diffusion refers to diffusion of ions to or from exchange sites on soil particles. Hellfereich (1962) states the chemical potential for interactions between the gas molecules and a sorptive matrix depends on: (a) the generation of diffusion induced electric forces, (b) the adsorbent selectivity or preference for a particular gas, and (c) specific interactions such as Coulombic forces, Van der Waals forces, and hydro- gen bonding. Particle diffusion is significant when interactions between the exchange medium and diffusing gas molecules are great. Often an equilibrium condition is attained, however, in which the contribution of ions to the particle site are matched by those returning to the gas phase. Particle diffusion, in such instances, makes insignificant contributions to the concentration gradient and, thus, has little influence on vapor densities. Advectlon Advection (also called bulk flow, convective flow, laminar viscous flow, transpiration, and forced dif- fusion) is a transport process in which a gas moves in response to a pressure gradient. Unlike diffusion, advection occurs as a bulk flow, i.e., the mixture of gases has no tendency to migrate according to the concentrations of its separate gaseous components; it behaves as one gas. This is because any diffusive effects that would vary the movements of individual gases according to their concentration are minor in comparison to the overall pressure gradient flow. In advection, the gas acts as a fluid continuum driven by the pressure gradient. Movement of gases in the unsaturated zone through advection is governed partly by the air permeability of the porous medium and partly by the pressure created by the gaseous source. For example, if a gas were to form rapidly in a slightly permeable medium, a pressure gradient would be established forcing the gas to move through the medium. Other factors which could cause pressure gradients influencing advection include barometric pressure changes, rise and fall in water table, wind fluctuation, and rainfall percolation. Laminar viscous flow of a fluid through porous media is usually described by Darcy's Law (analogous to Fick's and Fourier's Laws) which is an empirical equation relating fluid flux to a pressure gradient over distance times a constant of proportionality called permeability or, in the case of water flow, hydraulic conductivity. Permeability, in turn, is dependent on properties of the flowing fluid and the porous media through which it flows. Klinkenberg (1941) pointed out that gases do not stick to pore walls as required in Darcy's Law and that a phenomenon known as "slip" occurs. This gives rise to an apparent dependence of permeability on pressure, referred to commonly as the Klinkenberg effect, and often must be considered in mathematical descriptions of advective flow. Thermal advection is another type of flow and is caused by a thermally induced pressure gradient. As temperature increases in unsaturated porous media, the resultant increase in molecular energies results in increased pressure exerted by the gaseous con- stituents. These heated gases then flow in an effort to reestablish a pressure equilibrium. 3-7 ------- The effectiveness of thermal advection depends somewhat on the thermal regime of the soil. A soil medium with low thermal conductivity can create a greater thermal gradient over a given distance because heat is contained. Air is a poor thermal conductor, therefore thermal advection is enhanced (because of high temperature gradient development) in soils with low bulk density (high air content). But the overall effect of thermal advection as a transport mechanism is considered to be limited. Currie (1971) cites Keen on the question of thermal advection as a soil aeration mechanism (Keen, 1931): Keen pointed out that because there is a phase lag in temperature at depth (it is 12 hours at 30 or 40 cm), there are periods during summer days when the surface is colder than the soil at depth. He calculated the amount of convective flow, and hence exchange, that might occur in those circumstances and decided that, in the absence of any suitable air entry point at depth, it is extremely unlikely that this form of ventilation will occur to any useful degree. However, if a heat generating source below ground surface exists, thermal advection could become significant. Effusion In porous media where pore spaces are extremely small, collisions between gas molecules can be ignored compared to collisions of molecules to the surrounding walls. If molecule-wall collisions dom- inate, the flux of molecules through any shape pore space is equal to the number of molecules entering the pore space times the probability that any one molecule will pass through and not be deflected back the way it entered. This forward flux is called effusion, free-molecule, or Knudsen flow and its mathematical description is different from diffusion or advection processes. Because underground storage tank ex- cavations are normally backfilled with coarse sand or gravel, effusion is unlikely to be a dominant driving force in the immediate tank surroundings. Combinations of Transport Processes The previously described vapor transport processes can and often do occur in combination. Effusion and advection in combination leads to the phenomenon, discovered experimentally in 1875 by Kundt and Warburg, known as "viscous slip." Advection and diffusion processes occurring in concert were called "diffusive slip" when first discussed by Kramers and Kistemaker in 1943. A third combination, that of effusion and diffusion, was researched extensively in the early to mid-1940's in support of work on isotope separation. And all three processes of diffusion, advection, and effusion could be significant in a given field situation and could be described by yet another combination. This variability in types of transport processes points out the need to clearly understand gaseous migration in any given field situation before accepting a mathematical representation of its mi- gration in that situation. Vapor Transport Processes in the Underground Storage Tank Environment The primary mechanisms of vapor transport in the underground tank environment are ordinary gaseous diffusion and isothermal advection. If a leak is small and there is no great pressure buildup, vapor transport of the volatile contents away from the tank will occur, in large part, through ordinary gaseous diffusion. The concentration gradient is established between the leaked volatile product and uncontaminated soil air results in the movement of the leaked vapor usually in all directions into the soil. Diffusion of the gaseous form of a leaked product will continue indefinitely into the soil atmosphere and outer atmosphere until the concentration gradient is eliminated. For cases of a high pressure gas leak or a quickly forming gas, sufficient pressure may build up in the soil gases to create advective flow. While the effective diffusion coefficient and air per- meability are substantively different coefficients of proportionality, both are dependent on properties of the porous media. Both coefficients are affected by differing media tortuosity, porosity, moisture content, texture, and other aspects of geometry. Both coef- ficients characterize the ease with which a fluid can be made to move (by diffusion or flow) through porous material and, as defined in Pick's or Darcy's Law, are macroscopic properties of the soil. Most underground storage tanks are surrounded with a uniform, coarse-grained backfill to reduce tank corrosion. In this situation, rapid diffusion and ad- vection will be promoted. There are other sources of pressure gradients which can influence advective motion in the unsaturated zone around tanks. These influences include both man-made and environmental factors. Some man- made factors are tank installation and air rotary/air percussion drilling, while environmental influences include barometric pressure changes, water table fluctuations, rainwater percolation, and the effects of wind. There are also several factors that modify vapor 3-8 ------- migration by changing the direction, speed, or nature of a moving gas. These include media heterog- eneities, sorption, and microbiological transformation. Most underground tank environments will experience significant changes in barometric pressure. Surface atmospheric pressure fluctuations are not immediately transmitted below ground surface, but arrive lagged in time. There is currently some debate over the extent to which barometric pressure variations augment gaseous exchange across the ground surface. It is thought by some that an increase in barometric pressure would compress the volume of the soil atmosphere and permit a volume of atmospheric air to penetrate the soil. A decrease in barometric pressure would have the opposite effect, causing some of the soil air to enter the outer atmosphere. This effect would permit a flux of naturally occurring and leaking product vapors to be aerated out of the storage tank environment. The significance of barometric pressure as an ad- vective flow mechanism is in question. Buckingham (1904) determined that barometric changes causing the penetration of atmospheric air will amount to only 3 to 5.6 mm of mercury in a 3 meter permeable soil column. From this estimate it would appear that barometric effects as an advective mechanism are minimal at best. However, experimentation in regions with deep water tables indicates that in these sit- uations, barometric fluctuations may have a significant effects on the near surface gaseous regime. Advective movements can be caused by fluctuations in the water table in an underground storage tank environment. An increase in water table height would result in the displacement of a similar volume of soil air once equilibrium is attained. The opposite effect accompanies a falling water table. Therefore, a leak product may move into the outer atmosphere or deeper into the soil, accompanying water table fluc- tuations. Surface rainfall can have an impact on vapor move- ments by creating an advancing wetting front which traps soil air between the front and the water table as it percolates downward. This creates greater pressure on the soil air. A large portion of the soil air remains entrapped in soil pores and is not affected by the advancing wetting front. Rommel (1922) showed that rainfall produces about 1/12 to 1/16 of the normal soil aeration. Therefore, rainwater percolation could become significant as an aeration mechanism in regions of high precipitation, given the right sub- surface conditions. The pressure and venting effects of wind as an advection mechanism exert little influence over the exchange of soil air with the outer atmosphere. Rommel (1922) concluded that wind action is not responsible for more than 1/1000 of the normal aeration of a vegetated soil. A more porous soil might experience significantly greater venting effects. The effects of wind over a leaking underground tank environment will depend on the areal extent of con- crete groundcover, if one exists, and the porosity of the tank backfill. Environmentally induced advection mechanisms gen- erally have only a minor effect on vapor transport. Given the right conditions, however, these mech- anisms may exert a significant influence on vapor transport in a leaking underground storage tank environment. More information is needed to accur- ately quantify these effects. Tank installation will completely upset the "normal" subsurface gaseous regime by allowing air from the atmosphere to mix with backfill material. If a steady- state vapor condition can possibly be achieved after excavation in a given area, the amount of time nec- essary to establish this equilibrium would be difficult to mathematically predict. Air drilling, whether rotary or air percussion, also perturbs the surrounding subsurface environment by large losses of circulating air from the borehole into the formation. Loss of circulating air from a drilled borehole can be at least 10 to 30 percent. If air is circulated at average rates of 20 m3/min, massive quantities of air would be lost, much into the formation, during the normal time it would take to drill. Again, recovery to any preexisting steady-state condition would be almost impossible to accurately predict mathematically. Probably the most important consideration in any process of mass transport through porous media is the effect of heterogeneities on movement. Although many tanks are backfilled uniformly after em- placement, unequal tamping or settling could modify the normally uniform material or perhaps its uniform moisture distribution. Many heterogeneities commonly exist in geologic materials, and areas outside of the region excavated for tank emplacement would very often be expected to contain heterogeneities that would affect vapor migration. Layers of fine materials, such as silt or clay, would act as barriers to move- ment, particularly if an accompanying high moisture content were present and pore spaces were occluded. Secondary permeabilities created by fis- sures, plant roots, animal burrows, or other agents would increase and direct the transport of vapors around underground tanks. Despite their importance, a full understanding of the distribution of heter- ogeneities at any given site is difficult to achieve. 3-9 ------- Sorptive effects are complex phenomena which depend on numerous interacting variables. Taylor and Ashcroft (1972) define adsorption as the concentration of a substance at the surface or interface of another material. Adsorption can result from chemical ad- sorption (Coulombic forces), physical adsorption (Van der Waals forces), or hydrogen bonding (Lyman et al, 1982). If a volume of a gas is adsorbed near the source of a leak, its vapor density in the soil air will be reduced. Consequently, its transport potential will be reduced because of a decrease in concentration gradient. Given a continuous input of vapor from any source, however, equilibrium may be attained between ions being adsorbed and those leaving adsorbent sites. In this situation, vapor transport would not be greatly affected. The potential for sorption processes to effectively reduce the vapor density of a gaseous component is largely determined by the volume input of the gaseous source in the leaking tank envi- ronment. The adsorption of any vapor product liberated from a leaking underground storage tank is primarily governed by the interactions between the chemical structure and properties of the gas; the chemical, biological, and physical characteristics of the soil; and general environmental conditions. The following general principles are often important in these sub- surface vapor-filled porous media: 1. The forces of adsorption increase with increasing polarity of a gaseous molecule. 2. The charge on the adsorbent may contrast with the polarity of a gaseous molecule and add an additional adsorptive force. 3. The efficiency of an adsorbent depends largely on its specific surface (area per unit mass). 4. The adsorption of gases generally increases with increasing organic content. 5. The volumetric water content may displace ions by competing for adsorbent sites. 6. Temperature increases in the soil will result in adsorption losses, due to higher kinetic energies of molecules at sorted sites. Microbes in the subsurface environment can entirely transform moving vapors. The effect of this microbial action will be discussed in Section 5 of this report. Existing Gaseous Measurements Methodologies Sampling gaseous vapors in shallow soils can provide an effective method of detecting underground tank leaks and a resultant contaminant plume. As stated earlier, a major advantage of gaseous leak detection is the radially outward movement of gases which usually fills the porous media more completely than liquid migration (Figure 3.3). According to Scheinfeld and Schwendeman (1985), characteristics that a leak- detection system should exhibit include the ability to: detect an unconfined fluid of concern, be main- tenance-free, screen out false alarms, be cost- effective, be tamper proof and secure, be applicable to existing tank systems, and be simple, safe, and reliable. They further state that, optionally, in environmentally sensitive areas, leak detection sys- tems might be desired to monitor continuously and automatically. The science of vapor measurement is, in some ways, in its infancy. New methods are appearing with great frequency. Therefore, the following review is meant to be a summarization of the different categories of measurement, not a detailed listing of each existing method. This review is in no way intended to constitute an endorsement of any of the methodologies. The methods of gaseous detection of leaks can be broken into the categories of quantitative meas- urement and non-quantitative or "red flag" methods. More accurately, at this writing, no method of out-of- tank vapor detection is quantitative to the point of allowing an accurate estimation of a volume of tank leakage; rather, what is called "quantitative" herein is meant to designate methods by which varied subsurface vapor concentrations can be measured. The term "non-quantitative" method is used to de- scribe a measurement technique which signals the presence of a given vapor at a certain level but which cannot distinguish varying concentration magnitudes. So-called quantitative measurement methods can be further grouped into: (a) analysis of withdrawn vapor samples (instantaneous collection), (b) surface sam- pling, (c) longer term, static emplacement of absorbent material which is subsequently removed and ana- lyzed, (d) in situ quantitative measurement, and (e) tracer techniques. One group of quantitative methods is based on the analysis of a vapor sample obtained by inserting a probe into the soil and by withdrawing subsurface gas with a metal bellows pump or similar device. Analysis of the sample is then performed on-site or at a 3-10 ------- PRODUCT VAPOR MIGRATION ' ':.'"**''>. . >» . ./;. v .'.;. ^ LIQUID PRODUCT LEAK WATER TABLE Figure 3.3 Three-dimensional Diffusion or Advectlon of Product Vapor ------- laboratory. Samples can be withdrawn from a driven probe or from a permanent installation of air pie- zometer-type probes. A driven probe is a small diameter pipe with a drive point below a screened region that allows sampling of vapors in an undisturbed part of the vadose zone. Driven probes can be shallow (a depth of 1 to 3 m), or can be driven in the center of a hollow stem auger in advance of the bit during breaks in drilling. As drilling proceeds, it is possible to obtain samples at various depths giving an indication of the vertical profile of vapor concentration at a given site. Soil gas is withdrawn from shallow drive point probes to clear other gases out of the evacuation line; the pumped volume is usually several times the internal volume of the evacuation line. A smaller sample is then ana- lyzed. Sample size depends on the composition and concentration of the suspected contaminant. There are many pitfalls for the inexperienced in- vestigator in this methodology. Great care must be exercised when driving a probe around buried tanks and associated piping, particularly because many tank owners are not certain of the exact position of all subsurface utilities, pipes, tanks, and other features below their property. In addition, steps must be taken to ensure that with each sample no atmospheric air is sucked downward from the ground surface (or borehole), around the outside of the driven probe. The material used for probes and evacuation lines must be free of contamination and must not adsorb critical compounds of interest. (Almost surprisingly, PTFE Teflon-type tubing can be a poor material for gaseous sampling because many subsurface con- taminant gases can move into and through its walls. This same property is very valuable in the use of the material as a permeation device - a controlled emis- sion source for gaseous tracer tests [Kreamer, 1982]). Probes placed in predrilled boreholes can be used as long-term sampling locations for monitoring a storage facility or an existing contamination plume. There are several different designs for long-term probes which can be resampled. Most are installed and backfilled with sand or gravel, and a seal is placed between vertically displaced sampling stations to prevent vertical vapor movement in the borehole. Air drilling, moisture changes in the borehole (particularly if cement is used as grout between stations), and advective perturbation of the subsurface gaseous regime by overpumping are but a few of the potential problems with long-term sampling. The latter potential problem can be reduced by design of a very low volume evacuation line (this will also reduce the absorptive surface area) and by pumping only a few internal line volumes before sampling. Surface sampling is carried out in a variety of ways. One of the most quantitative is a method that places a chamber, open at the bottom, on the ground surface. The chamber is allowed to fill with gases moving upward, and periodically analyzed for gases of interest. Quantitative aspects of this surface flux measurement suffer when concentrations or pressures build up in the chamber which inhibit further upward movement. The use of a flushing or carrier gas through the chamber reduces most of these effects. Once a vapor sample is brought to the surface, there are many ways of measuring its composition, each with varying degrees of sensitivity, precision, ac- curacy, size, and expense. For example, illustrating the wide variety of field methods of measurement available, a partial list could include: (a) portable gas chromatographs with direct or syringe injection and with any of a number of detectors including electron capture (ECD), flame ionization (FID), and photo- ionization (PID), (b) small, portable devices with just a detector, PID or FID, which gives an indication of bulk gases such as total hydrocarbons, (c) combustible gas indicators (CGI) used for general assessment of explosive potential by aspirating vapor over a heated filament which changes temperature and electrical resistance, (d) catalytic detectors which are similar to but have more sensitive response than CGIs, (e) metal oxide semiconductors and adsorption sensors which vary their internal resistance in response to certain vaporized compounds, and (f) many other types of devices and instrumentation. Collection and transportation of samples to the laboratory can have the benefit of the use of more powerful analytical chemistry techniques, such as gas chromato- graphy/mass spectrometry (GC/MS), but has the potential to suffer if there are changes in the sample during transportation. Again, it should be emphasized that some of the methods presented here are quan- titative only in the loosest sense; that is, they can only indicate relative increases or decreases in bulk concentrations. Static collection methods of sampling vapors in the vadose zone have the potential to produce quan- titative results. In these static methods, volatile pro- ducts within the soil are collected over time by an absorbent material placed below land surface near the tank. The absorbent material is removed after some time and is then analyzed for contaminants. Collec- tion of a sample occurs over a period of one to two weeks, and therefore is known as a static, as opposed to an instantaneous, collection method. Because static collection of vapor samples occurs over a longer period of time, short-term variations of gaseous concentration at a given site are less im- portant than in instantaneous collection methods. 3-12 ------- Fluctuations in barometric pressure, ground-water elevation, and wind can all influence an instantaneous collection method which is repeated in the same location over time. Potential problems with static collection include the premature desorption of col- lection material before analyses are conducted and inefficiency of the sorbent collection material to sorb particular compounds. A variety of in situ measurement techniques exist, and new methods are being developed and tested pres- ently. Some examples of in-place subsurface meas- urement technology, to illustrate this variety, include: metal oxide semiconductors, adsorption sensors, and remote fiber spectroscopy. Metal oxide semiconductors and adsorption sensors are devices which change their internal resistance in reaction to a number of gases. As mentioned earlier, they can be used to measure samples pumped to the surface, but they can also be placed in a monitor or vapor monitor well. The resistance change can be calibrated to ascertain the concentration of the gases. These sensors have a number of different config- urations for the detection of vapors. Remote fiber spectroscopy (RFS) utilizes two emer- ging technologies: development of intense light sources and efficient optical fibers in the UV part of the electromagnetic spectrum. RFS employs a laser light source, fiber optics for signal transmission, and an optrode for vapor detection. A laser light source focuses an input beam into the optical fiber which is coupled to an optrode in contact with the sample. Light that has interacted with the optrode returns on the same optical fiber and the resultant signal is then sorted by an optical spectrometer for analysis. Each optrode can measure a specific chemical or physical property. The intensity of the signal in a specific frequency is related to the chemical reaction or physical change. One limitation of RFS detection is optrode development, which has just begun and cannot yet provide the wide range of monitoring capabilities it is projected to have in the future. Any given sample site may vary in concentration of contaminants, and the optrode must be able to monitor specific organic or inorganic or both com- pounds consistently when there has been no sample preparation. Tracers have been used in leak detection technology, and several different uses have been found for gaseous tracers. One use is calculation of diffusive or permeable aspects of the porous media by tracer release in the vadose zone at a known, controlled rate and configuration. The arrival of the released tracer is measured at distance, and, subsequently, media properties can be calculated. In another method, tracer is introduced into a product storage tank at a concentration such that properties of the product remain unchanged. The most volatile component of a product leak is the tracer. As a leak occurs the tracer quickly diffuses into the surrounding soil. Gaseous samples can be taken and analyzed for the tracer. Still another method is geared toward distinguishing between a liquid product leak and vapor leakage from a tank. Recalling that vapor leaks of a product can occur around fittings at the top of a tank, it can be difficult if not impossible to distinguish between a product leak and vapor leak with vapor phase monitoring. In this situation two volatile tracers can be used to differentiate vapor and product leaks. One tracer (No. 1) is injected into the product while the other (No. 2) is introduced into the air space above the product. Sampling of the soil vapors is then per- formed. If only tracer No. 1 is discovered, then a pro- duct leak is occurring provided that the tracer introduced into the tank air space does not readily diffuse into the tank liquid product or is not pref- erentially detained in the soil medium. Sampling is completed before a significant amount of tracer No. 2 enters into the liquid phase in the tank. Detection of both tracers indicates a vapor leak or possibly a product and vapor leak occurring simultaneously. This procedure is relatively new, and supporting research is ongoing. All of the above "quantitative" measurement methods are most commonly used as non-quantitative or "red- flag" indicators of leaking tanks. Occasionally, these methods give helpful quantitative information, but their primary value is in locating leaks and in giving an early warning which is more of a qualitative site assess- ment. It is often very useful to use one or several of these out-of-tank vapor monitoring methods in concert with other techniques. There are many strictly non-quantitative vapor measurement methods. For example, one group of devices involves the degradation or dissolution of a weighted cord hooked to an alarm. These product- soluble compounds, such as styrene-butadiene copolymer, are primarily used for liquid phase warning but could be adapted to vapor warning as well. These same product-soluble materials can be used to insulate conductors of an electrical resistivity-type warning device. Another group of warning systems are product permeable devices which utilize materials that are hydrophobic (water repellent) but are oleophillic (permeable to selected organic material). Vapors from product leaks can penetrate these materials and set off an alarm. Several other types of non-quantitative measurement methods exist. If a leak has already occurred, gaseous monitoring methods can be extremely useful in contaminant 3-13 ------- plume delineation. Determining the areal extent of a contaminant is possible by analyzing overburden soil for chemical vapors. In an area of suspected contamination, it is possible to transect an area until boundaries of a plume are defined. Routine meas- urement of methane, benzene, toluene, and hydro- carbons provides information about ground-water plume movement. A comparison of contaminant concentration in the vadose zone and ground water is used to locate sources within the plume. Several final generalized statements can be made regarding present vapor detection technology in addition to what is presented above. First, gaseous sampling is not optimal for all compounds. Contam- inants that are most easily detected in gaseous sampling are low molecular weight petroleum and halogenated hydrocarbons. These products have a high vapor pressure, high liquid-gas partitioning coefficient, and low aqueous solubility. Second, con- cerning sampling techniques, precautions should be taken so that samples are not contaminated and so that false positives are not obtained. Third, calibration of field and lab equipment is required on a regular basis to provide consistent results. Fourth, very little information exists on the operable life and reliability of many devices, in part because many are so new. This fact will make evaluation of these leak detection systems difficult. Because the technology is new and because great pressure presently promotes the rapid marketing of new devices, quality control is essential. Finally, vapor leak detection cannot determine leak rate from an underground storage tank in most circumstances. The utility of vapor leak detection is in identifying the presence and nature of leaks, and in contaminant plume identification once major leaks have occurred. Limits of Present Knowledge and Future Directions The direction of future efforts in the arena of vapor transport and measurement surrounding underground storage tanks should be directed toward answering the shortcomings of our present knowledge. In particular, there is a need to construct workable, cal- ibratible models of contaminant flow based on theory; to develop usable, reliable vapor detection technology which will stand the test of time; and to establish better design criteria for the installation and operation of underground tanks. One of the greatest problems with any attempt to apply transport theory to actual field situations is the lack of field verification to this point in time. There has been relatively little on-site investigation to identify or determine the magnitude of field parameters, bound- ary conditions, or environmental effects on vapor transport. As a result, application of transport theory is done based on hypothetical conditions, unverified assumptions, or is carried out merely as a heuristic exercise. Careful field and laboratory measurement of the factors that affect transport is crucial to our understanding and interpretation of field results and presents a great challenge for the future. There is presently a great rush to develop leak de- tection technology based on gaseous movement in the unsaturated zone. The impetus comes from an in- dustry need to monitor their underground storage tanks inexpensively and reliably, and to assess the extent of existing pollution problems quickly. There is concern that rapid marketing of new products might leave aspects of device performance untested for years. Large-scale leak detection device failures could destroy the underpinnings of a regulatory struc- ture built on leak detection safeguards and could eventually impair the credibility of such a program. Development of uniform performance standards and testing criteria for leak detection devices is an area that needs much consideration. In the future, successful vapor leak detection might be able to reliably quantify rates of tank leakage. The optimal design or designs to accomplish this monitoring could eventually be included as one of several alternatives for overall new tank construction. In addition, new information on vapor migration and the factors that affect it in the subsurface tank envi- ronment could aid future decisions on tank installation, design, and liner or barrier placement. 3-14 ------- References Buckingham, E. 1904. Contributions to Our Knowl- edge of the Aeration of Soils. U.S. Dept. Agri., Bureau of Soil Bulletin 25. Currie, J.A. 1971. Movement of Gases in Soil Res- piration. SCI Monograph No. 37, pp. 153-159. Grew, K.E., and T.L. Ibbs. 1952. Thermal Diffusion in Gases. Cambridge University Press. Helfferich, F. 1962. Ion Exchange. McGraw-Hill, New York. Keen, B.A. 1931. The Physical Properties of the Soil. Longmans, Green & Co., London. Klinkenburg, L.J. 1941. The Permeability of Porous Media to Liquids and Gases. American Petroleum Institute Drilling Products Practices, pp. 200-213. Kreamer, D.K. 1982. In Situ Measurement of Gas Diffusion Characteristics in Unsaturated Porous Media by Means of Tracer Experiments. Ph.D. Dissertation. Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona. Lyman, W.J., F.W. Reehl, and D.H. Rosenblatt. 1982. Handbook of Chemical Property Estimation Methods. McGraw-Hill, New York. Mason, E.A., and R.B. Evans. 1969. Graham's Laws: Simple Demonstrations of Gases in Motion. Part 1, Theory. J. Chem. Ed. 6(6): 358-364. McKee, J.E., F.B. Laverty, and R.M. Hertel. 1972. Gasoline in Groundwater. J. WPCF 44(2): 293- 302. Rommel, L.G. 1922. Luftvaxlingen Marken Som Ekologisk Faktor. Medd. Statens Skogsfarcoks- anstalt, 19, no. 210. Scheinfeld, R.A., and T.G. Schwendemann. 1985. The Monitoring of Underground Storage Tanks. In: Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA, November 13-15, Houston, Texas. Taylor, A.T., and G.L. Ashcroft. 1972. Physical Edaphology. W.H. Freeman and Co., San Francisco. 3-15 ------- ------- SECTION 4 Soil Surface and Interfacial Effects in the Underground Storage Tank Environment Introduction The detection of contaminants leaking from underground storage tanks or monitoring such tanks for leaks in the underground environment is dependent on the physical and chemical properties of the contaminants that might be sensed by chemical instrumentation. Sensible properties of the leaking contaminant or its degradation products that may be of use include: refractive index, thermal conductivity, acoustic conductivity, electrical conductivity, ultraviolet, visible, or infrared absorbance spectra, fluorescence spectra, or electrochemical oxidation/reduction potentials. These properties may be profoundly affected by the physical chemistry of the contaminants at interfaces. Adsorption and partitioning between available phases are the more notable of these interfacial phenomena. The adsorption-desorption of contaminants with soil particles, distribution between immiscible phases, and emulsification constitute important parameters in: pollutant transport kinetics, bioavailability, chemical degradation, and the sensible properties given above. Additionally, the leaking chemicals may greatly modify the physical and chemical properties of the sur- rounding fill material. Adsorption of aromatic hydro- carbons, for example, can distort the interatomic distances in clays and, subsequently, can greatly affect the porosity and permeability. The evaluation of the myriad of possible effects in the environment of leaking underground storage tanks may seem at first to be a desirable undertaking. Evaluation of their individual and collective impact on those properties, such as transport, are important in leak detection strategies. The use of numerical models permits a convenient means to estimate the impact of these phenomena collectively and individually and also to estimate their relative importance under specified conditions. Partitioning and Adsorption of Chemicals in the Underground Storage Tank Environment The partitioning of chemicals among different available phases represents the most significant surface/ interfacial phenomena and is crucial to the under- standing and modeling of the transport and reactions of contaminants. In the underground storage tank environment, the partitioning of contaminants can be among many different phases: liquid to liquid liquid to gas liquid to solid gas to solid. The first two processes are usually referred to as partitioning while the latter two are referred to as adsorption. Adsorption processes may be physical or chemical. Chemisorption usually involves the formation of new bonds and is associated with large energy changes (»10 kcal/mole). This typically results in formation of monolayer coverage of reactive molecules at active sites on the adsorption surface. Physical adsorption, on the other hand, usually involves only relatively weak Van der Waals forces, rather small energy changes (<10 kcal/mole), and adsorption of many molecular layers (Jaycock and Parfitt, 1981; Oudar, 1975). Adsorbed molecules, particularly chemisorbed molecules, will have different physical properties than their free counterparts. They would not necessarily possess the same ultraviolet, visible, infrared, or fluorescent spectra and would typically have different oxidation-reduction potentials and different bio- availability and chemical reactivities. There are many factors in the adsorption and de- sorption processes. The moisture content or presence of other organics can either compete for adsorption 4-1 ------- sites or provide a second solute phase. The ad- sorption and desorption processes involve activation energies and are thus temperature dependent. The forces involved may be either electrostatic or dipole- dipole depending on the particular circumstances. The molecular polarizability (polarizability refers to the induration of a dipole moment by an external electric field because of the displacement of the average positions of the electrons relative to the nuclei of the molecule), dipole moment, charge distribution, size, and shape can affect the adsorption process (Browman and Chesters, 1975). The same considerations must also be given to the solid phase properties and the solvent phase as well (the solvent phase might be water or contaminant). Generally, the solubility of the contaminant is quite important in the adsorption equilibria. There is usually an inverse relation between the solubility and the extent of adsorption. This is Lundelius' rule; the stronger the solute-solvent bond, then the smaller the extent of adsorption (Jaycock and Parfitt, 1981). The equilibrium relationship between the contaminants concentration in solution and that adsorbed to soil particles can be described from a theoretical basis using the Langmuir adsorption isotherm: 0 = b[C] (4.1) where 6 is a fractional surface coverage, [C] is concentration in solution, and b is a parameter related to the energetics of the process (Oudar, 1975). This equation is derived assuming a constant temperature, monolayer coverage of contaminant molecules at active sites, and a homogeneous distribution of active adsorption sites. Actually b should also be a function of 6, and there typically would not be a homogeneous distribution of adsorption sites. The Langmuir adsorption isotherm may perform well when the con- taminant is strongly adsorbed but generally is not applicable to the underground storage tank environ- ment where sorption effects may be weak. The Freundlich isotherm is most commonly used to describe adsorption to solid surfaces in the soil en- vironment: X/M = K [C]n (4.2) where X is the mass of contaminant adsorbed to a mass M of soil, [C] again is the concentration of contaminant in solution at equilibrium, and K and n are fitting parameters. This is purely an empirical relation (Jaycock and Parfitt, 1981), but it can be derived from the Langmuir adsorption isotherm by assuming a heterogeneous surface with adsorption at each class of sites following Langmuir behavior. The principal drawback of the Freundlich isotherm is that it predicts that the amount of material adsorbed increases indefinitely as the concentration (or partial pressure) increases and is thus unsatisfactory for application at high contaminant concentrations (Oudar, 1975), the same conditions that might exist with a leaking storage tank. There are many other adsorption isotherms of in- creasing complexity (Oudar, 1975; Jaycock and Parfitt, 1981). However, because of the large number of parameters and general complexity of the soil environment, it may not be useful to apply more theoretical models to this system. Liquid-liquid partitioning can be described in terms of the solubility of a solute in the different immiscible phases. The interfacial tension between the two phases can be strongly affected by trace con- centrations of other dissolved species; the "salting out" of emulsions being a familiar example. Dissolved salts reduce the solubility of non-electrolytes in water and reduce the surface tension of electrolytes (Bikerman, 1958). The partitioning of contaminant between its bulk phase and aqueous phase or between aqueous phase and some other organic phase, such as humic material, can usually be presented as a simple linear distribution coefficient or equilibrium expression. Knowledge of these relationships is critical to the understanding of retardation of contaminants in soils and ground water. The partitioning might also be expected to obey a simple linear free-energy rela- tionship (if the effect of a substituent on a molecule is to only cause a small perturbation in the free energy change for a process or reaction, there will be a linear relationship between the substituents and the parent molecule for that process or reaction). Both aqueous solubility and partitioning between aqueous phase and a soil organic (humic) phase should correlate on a log scale (Chiou et a!., 1983; Miller et al., 1985). The octanol-water partition has been correlated with solubility and the soil organics - water partitioning of non-ionic organic contaminants. As with other linear free-energy relationships, this follows qualitatively from the similarity of the processes. A more rigorous attempt to justify the correlation has been offered by Chiou (1983) using the Flory-Huggins polymer solution theory. The Flory-Huggins theory is a direct gen- eralization of the Bragg-William approximation in the lattice model of binary solutions and provides for the marked differences in size for the solute and solvent when one is a polymer. This theory is useful in pre- 4-2 ------- dieting both solubility and the miscibility of the polymer with another chemical. While the Flory-Huggins treatment is only approximate and not strictly suited for very dilute solutions (Hill, 1960) (low humic content), it nevertheless serves rather well. Humic material is chemically quite complex and contains an abundance of polar substituents such as -COOH, - C=O, aliphatic-, phenolic-, and enolic-OH, -NH2, and - SH groups as well as aromatic rings (Browman and Chesters, 1975). These substituent groups provide the adsorption sites on the polymer. Both lattice statistical models of adsorption to polymers and the Flory-Huggins theory are derived with the assumptions that all the sites on the polymer are equivalent, which clearly is not the case for humic material. Comparatively little attention has been given the liquid-gas partitioning or gas transport in the soil environment. However, the forces involved are similar to those of liquid-liquid partitioning. Accordingly, there should be linear free-energy relationships for the liquid-gas partitioning. For a homologous series, the activity at the liquid-vapor surface for an aqueous solution of organic solutes should increase strongly and regularly as the series is ascended; this is Traube's rule. A similar relationship for a solid interface might be expected but is not realized; other factors such as pore diffusion may dominate (Jaycock and Parfitt, 1981). It should be noted that the retarded transport of an organic contaminant in the soil/ground-water en- vironment need not necessarily involve adsorption to active sites on particles but could also conceivably involve distribution between the bulk solvent and a second solvent, typically water or humic material, adsorbed on the particles. Similarly the bulk phase could be either water or the leaked contaminant depending on the particular soil conditions. Many of these processes have been studied indi- vidually under controlled laboratory conditions for idealized systems but few if any laboratory studies have quantitatively dealt with these processes as a system. Since all of these processes may occur simultaneously, numerical model calculations could be most advantageous in evaluating their effects. Such model calculations can be useful in utilizing the data from the diverse laboratory studies to estimate the importance of the various processes in concert with one another. All possible chemical processes and reactions in a subsurface regime are represented by the reaction term in the advection-dispersfon equation. Processes which can be described by a distribution coefficient, such as adsorption or ion-exchange, are represented by a retardation factor which is the ratio of the ground- water average linear velocity to the rate of the advancing contaminant front. Rate controlled reactions such as hydrolysis, oxidation/reduction, or biodegradation can be represented by a decay function. Movement and reaction of emulsions of organic chem- icals in water have not yet been addressed but may be important in biodegradation of chemicals. Non-linear adsorption can be accounted for by modifying the conceptual model to include kinetic processes. Pol- lutant concentrations in the soil-air are generally not computed in the commercially available, traditional differential equation type of models (Bonazountas, 1983). Bonazountas claims that compartmental type models can handle more complex geochemical pro- cesses and can include the estimation of pollutant concentration in the soil-air. Baehr and Corapcioghi (1984) have recently developed an unsaturated zone model which predicts hydrocarbon concentrations in all phases (water, air, immiscible, absorbed) in time and space. This type of model has the potential to assess the long-term impact of residual hydrocarbons trapped in pore spaces and to access the impacts to underlying ground water. The difficulty in simulating the trans-formations and reactions in a contaminant plume arises from a deficiency in being able to quantify the individual processes and mathematical complexity of computing the resultant solute distri- butions by using differential equations. Generally, all the processes are lumped together into one reaction parameter and are then calibrated with actual field data to match the observed results. The science of contaminant transport prediction via computer models is, therefore, only as accurate as the understanding of processes which govern their movement and data which are input into the model. Leak Detection The time interval in which a leak detection monitoring device located outside the tank should be able to detect a leak depends upon a number of variables which can be addressed by solute transport modeling. The velocity of a leaking organic chemical is a function of multiple variables including the properties of the porous media adjacent to the tank (including the tank backfill material), properties of the leaking chemical, vertical and area! position of the monitoring devices, distance to the water table, water table gradient, and soil moisture content. The consequences of the surface chemical effects are immediately apparent with respect to the transport of the pollutant to a sensor location. It should also be apparent that since, in general, chemical instrumen- tation does not detect the total amount of contaminant 4-3 ------- present but rather usually only the contaminant in a particular phase (such as gas or aqueous), these properties are important in selecting the most judicious form of the contaminant to sample. Perhaps less apparent is the potential application of the surface chemical properties to the actual instru- mentation itself. The adsorption of an organic substance to a metal electrode can be used to precon- centrate it for later sampling. This can be cyclic voltametry, simple amperometry, or in the form of a CHEM-FET (chemically sensitive field-effect tran- sistors which are sensitive analytical chemical tech- niques that can measure trace concentrations of contaminants by the voltage-current changes that occur at exposed electrodes when the analyte(s) are oxidized or reduced). The accumulation of the pollutant on a surface need not be restricted to metal electrodes. Polynuclear aromatic compounds may deposit on glass surfaces such as fiber optic probes where they may be detected by their spectroscopic properties such as laser-induced fluorescence. Chemical and biological degradation of contaminants may depend on the partitioning between phases or adsorption to soil particles. This may lead to the formation of new contaminants not being sensed by a particular instrument. For example, degradation of tetralkyl leads may produce lead(ll) halides, car- bonates, di- and tri-alkyl lead salts. A sensor specific for tetraethyl lead might, in general, respond equally or not at ail to mono-, di-, or tri- ethyl lead chloride or to lead(ll) chloride. In conclusion, there is a considerable uncertainty regarding the detailed chemical kinetics and reaction mechanisms of many toxic pollutants in the soil/ ground-water environment (Callahan et al., 1979). Differences Between Laboratory and Underground Storage Tank Environment In general, results from laboratory tests cannot be directly correlated to field situations. Knowledge of the assumptions and limitations under which laboratory experiments are conducted should be used to deter- mine initially the applicability of data to a natural hydrogeologic regime. Measurement of retardation parameters in the labora- tory by batch equilibrium tests or column breakthrough curve experiments is common but does not account for the heterogeneities or anisotropic conditions encountered in the hydrogeologic regime surrounding an underground storage tank. The properties of the porous media may be altered upon transfer of the sample to the laboratory. Disturbance of the original orientation or compaction of the geologic sample can cause a change in permeability or porosity of the sample and can have a significant affect on the speed and directional movement of the leaking chemical. The geochemical properties of the sample can also be altered by degassing of CO2 or invasion of O2. A small difference in mineral or organic content of the geologic medium can also dramatically affect the resultant contaminant dis-tribution. These subsurface parameters are difficult to accurately quantify in the laboratory or by field methods. Laboratory tests are usually run with low solute concentrations which may not be the case around a leaking tank. Non-linear adsorption may occur when large quantities of free product exist near the tank or when the adsorptive or permeable properties of the porous media may be altered. A permeability ex- periment demonstrated an average 10,000 percent increase in permeability of four clays to xylenes over that measured with water (Anderson, 1982). A possible explanation of this large increase in per- meability may be due to a structural change of the clays caused by the adsorption of xylenes. Theoretical Deficiencies There is a basic inconsistency between theoretical frame-works of numerical ground-water models, laboratory experiments, and field research (Bonazountas, 1983). A need exists to efficiently and simplistically model the transport of these chemicals from leaking underground storage tanks. The relative importance or sensitivity of those parameters controlling the transport processes of organic chem- icals can be determined by a numerical modeling sensitivity analysis. The most sensitive variables are those which need to be most accurately measured or calculated in the laboratory and field. Once these parameters are identified, future research can be directed towards more accurately determining these variables instead of those properties or coefficients which can simply be estimated. A better understanding of the adsorption and dis- tribution processes would significantly improve the predictive capability of models (Cherry et al., 1985). Prediction of the dilute yet often significant edges of a plume can be important in the early detection of leaks from leaking underground storage tanks. To meet these needs, it is necessary to have better models which describe adsorption and phase par- titioning of contaminants including non-equilibrium effects (Valocchi, 1985). The limitations of the Freundlich isotherm and the applicability of the Flory- 4-4 ------- Muggins solution theory to the transition region from low to high humic content need to be addressed. There is a need for theoretical or semi-empirical relationships describing adsorption of multicomponent systems. The possible occurrence and behavior of emulsions in the environment needs to be addressed as well. Data Needs 1. Most laboratory data have been collected over narrow temperature ranges or at a single (ambient) temperature. Both adsorption and solubility are temperature dependent, and, thus, more data at a range of temperatures need to be acquired. 2. The partitioning of contaminant mixtures and separate components onto various mineral and organic soil types needs to be defined so that a relatively quick estimate of contaminant migration from leaking tanks can be determined. 3. The role of inorganic mineral surfaces in hydrophobic adsorption should be further explored since many aquifers are low in organic matter. 4. Solute competition for adsorption sites at higher solute concentrations, such as those encountered near leaking underground tanks, should be investigated (Cherry et al., 1985). 5. The role of dissolved organic matter in the transport of trace level organics in ground water is another area of concern. This could be an important process in the migration of organic contaminants away from leaking underground tanks. A greater degree of characterization of naturally occurring organic material is necessary to establish the range of values for partition coefficients that may occur and to relate the properties of the dissolved organic carbon (DOC) to the partitioning processes. 6. The potential for oxidative or hydrolytic degradations in the underground storage tank environment should be investigated for specific contaminants which are susceptible to these types of reactions. Future Directions and Strategies A greater amount of laboratory data is needed both to more closely approximate realistic field conditions and to accommodate the development of better theories or models for the transport and reactions of organic contaminants in the underground storage tank en- vironment. Particular attention should be paid to tem- perature dependence and the role of dissolved organic matter. The improvement to such models would provide a valuable asset in evaluating leak detection and moni- toring strategies. Such models may prove invaluable in selecting the optimum sensor locations and in the selection of the preferred contaminant for multicom- ponent storage tanks. 4-5 ------- References Anderson, D. 1982. Does Landfill Leachate Make Clay Liners More Permeable? ASCE-Civil Eng., Sept., pp. 66-69. Bikerman, J.J. 1958. Surface Chemistry. Academic Press Inc., New York. Bonazountas, M. 1983. Soil and Groundwater Fate Modeling. In: Fate of Chemicals in the Environ- ment. R.L. Swann and A. Eschenroeder, (eds). ACS Symposium Series 225. pp. 41-66, ACS, Washington, D.C. Browman, M.G., and G. Chesters. 1975. Fate of Pol- lutants in the Air and Water Environments, Part 1. I.H. Suffet (ed). John Wiley and Sons, New York. Callahan, M.J., M.W. Slimak, N.W. Gabel, I.P. May, C.F. Fowler, J.R. Freed, P. Jennings, R.L. Durfee, F.C. Whitmore, B. Maestri, W.R. Mabey, B.R. Holt, and C. Gould. 1979. Water Related Environmental Fate of 129 Priority Pollutants, Vol. II. EPA-440/4- 79-029b. Cherry, J.A., R.W. Gillman, and J.F. Baker. 1985. Contaminants in Groundwater: Chemical Pro- cesses. In: Groundwater Contamination - Studies in Geophysics, pp. 46-64. National Academy Press, Washington, D.C., 1984. Chiou, C.T., P.E. Porter, and D.W. Schmedding. 1983. Partition Equilibria of Nonionic Organic Com- pounds between Soil Organic Matter and Water. Env. Sci. and Tech., 17:227-231. Hill, T.L. 1960. Statistical Thermodynamics, Addison- Wesley Pub. Co., Inc., Reading, Massachusetts. Jaycock, M.J., and G.D. Parfitt. 1981. Chemistry of Interfaces. Halstead Press, John Wiley and Sons, New York. Miller, M.M., S.P. Waslik, G.L. Huang, W.Y. Shiu, and D. Mackay. 1985. Relationships between Octanol- Water Partition Coefficient and Aqueous Solubility. Env. Sci. and Tech. 19:522-529. Oudar, J. 1975. Physics and Chemistry of Surfaces. Blackie and Son Ltd., Glasgow. Valocchi, A.J. 1985. Validity of the Local Equilibrium Assumption for Modeling Sorting Solute Transport through Homogeneous Soils. Water Resources Res. 21 (6):808-820. 4-6 ------- SECTION 5 Implications of Subsurface Biological Activity for Monitoring Underground Storage Tanks Predicting the behavior of organic compounds released into the subsurface environment is a chal- lenging task. Microorganisms in the subsurface can transform many of the organic contaminants that typically escape underground storage tanks. However, the rate and extent of transformation is controlled by the geochemical and hydrological properties of the subsurface. As a result, biological activity at a particular site must be described within a clearly defined chemical and physical context. In laboratory studies, microbiologists usually study rapidly growing cultures of organisms that are only limited by the capacity of their internal machinery to process nutrients. In the subsurface this situation is rare. More commonly the populations of metabolically capable organisms increase until they are limited by some requisite for metabolism. Once this point is reached, the rate of transformation of an organic material is controlled by transport processes that supply the limiting nutrient. The vast majority of microbes in the subsurface are firmly attached to soil particles. As a result, nutrients must be brought by advection or diffusion through the mobile phases, water and soil gas. In the most common case, the organic compound to be consumed for energy and cell synthesis is brought in aqueous solution in infiltrating water. At the same time oxygen, the electron acceptor used to oxidize the carbon source, is brought by diffusion through the soil gas. In the unsaturated zone, volatile organic compounds can also move readily as vapors in the soil gas. Below the water table all transport must be through liquid phases, and as a result the prospect for aerobic metabolism is severely limited by the very low solubility of oxygen in water. In the final analysis, predictions of biological activity encompass: 1. The stoichiometry of the metabolic process 2. The concentration of the required nutrients in the mobile phases 3. The advective flow of the mobile phases or the steepness of concentration gradients within the phases 4. The opportunity for colonization of the subsurface by metabolically capable organisms 5. The toxicity exhibited by the waste or a co-occurring material. Biological processes have two obvious implications for monitoring releases from underground storage tanks. Soil microbes can consume a small leak and can prevent the spread of organic contaminants; in this way they effectively mask the presence of the leak. In this role biological activity protects the quality of associated ground water, but it greatly complicates the task of monitoring for a leak by chemical analysis for compounds originally present in the tank. The second implication follows from the first. If release of organic materials elicits biological activity, it may be more convenient to monitor for some other consequence of metabolism, particularly if the concentration of the released material is reduced below the analytical detection limit. The most promising candidates are (1) reduction in oxygen concentration, (2) increase in sol- uble iron, (3) methane production, and (4) a reduction in electrode potential associated with changes in the redox status of the subsurface environment. Organic Pollutants in Ground Water Ground water pumped from aquifers is an important source of water in much of the industrialized world. Unfortunately, many ground-water supplies have been polluted with organic chemicals used in industry, agriculture, and the home. These chemicals find their way into aquifers from accidental spills, leaking underground storage tanks, landfills of industrial or municipal wastes, septic tanks, leaking industrial impoundments, agricultural pest control, and even through pollutants dissolved in rain water. Micro- organisms in aquifers can transform many organic pollutants into harmless substances. Occasionally the organic contaminants are transformed into new substances that are even greater threats. The biode- gradation of a particular class of organic contaminants depends on the physiological capabilities of the organisms present in the aquifer. These capabilities depend in turn on the geochemical environment of the organisms. As a result, the behavior of organic com- pounds released from underground storage tanks depends on the geochemical properties of the ground waters that receive them. 5-1 ------- Ground Water Containing Oxygen Many water-table aquifers contain oxygen; these aquifers can support aerobic microorganisms that can degrade a wide variety of organic contaminants. Examples include benzene, toluene, the xylenes, and other alkylbenzenes that leak into ground water from gasoline spills or solvent spills (Wilson et al., 1986; Lee et al., 1984); naphthalene, the methylnaph- thalenes, fluorene, acenaphthene, dibenzofuran, and a variety of other polynuclear aromatic hydrocarbons released from spilled diesel oil or heating oil (Wilson et al., 1985); acetone, isopropanol, methanol, ethanol, and t-butanol from solvent spills and gasoline (Novak et al., 1984; Lokke, 1984; Jhaveri and Mazzacca, 1983); and many methylated phenols and heterocyclic organic compounds seen in certain industrial waste waters. Many synthetic organic compounds can also be degraded. Examples include dichlorobenzenes (Kuhn et al., 1985), the mono-, di-, and trichloro- phenols (Sulflita and Miller, 1985), the detergent- builder nitrilotriacetic acid (NTA) (Ward, 1985), and some of the simpler chlorinated compounds such as methylene chloride (dichloromethane) (Jhaveri and Mazzacca, 1983). The extent of biodegradation of these compounds in ground water will depend on the concentration of oxygen. For the compounds discussed above, roughly two parts of oxygen are required to completely metabolize one part of the organic. For example, microorganisms in a well-oxygenated ground water containing 4 milligrams/liter (mg/L) of molecular oxygen can degrade only 2 mg/L of benzene. The solubility of benzene, 1780 mg/L, is much greater than the capacity for its aerobic degradation in ground water. Obviously the prospects for aerobic metabolism of these compounds will depend on their concentration as well as on the concentration of other degradable organic materials in the aquifer. Concentrated plumes of organic contaminants cannot be degraded aero- bically until dispersion or other processes dilute the plume with oxygenated water. Many of the commonly encountered organic pollutants in aquifers are synthetic organic solvents that do not ordinarily degrade in oxygenated waters. Examples include tetrachloroethylene (PCE), trichloroethylene (TCE), cis and trans 1,2-dichloroethylene, ethylene dichloride (1,2-dichloroethane), 1,1,1-trichloroethane (TCA), 1,1,2-trichloroethane, carbon tetrachloride, and chloroform. Ground Waters Producing Methane Organisms that produce methane, called methano- gens, are only active in highly reduced environments. Molecular oxygen is very toxic to them. Methane can be produced by fermentation of a few simple organic compounds such as acetate, formate, methanol, or methylamines. Molecular hydrogen can also support a form of respiration in which the hydrogen is used to reduce inorganic carbonate to methane. Although the microorganisms that actually produce the methane can use a very limited set of organic compounds, they can act in consort with other microorganisms which break more complex organic compounds down to substances that the methanogenic organisms can use. These partnerships or consortia can totally degrade a surprising variety of natural and synthetic organic compounds. The rates of reaction are usually slow and often require long lag periods before active transformation begins (Wilson, 1985). Microbiologists are accus- tomed to microorganisms that grow to high densities in only a few days, and rarely conduct experiments that last longer than a few weeks. However, the residence time of organic pollutants in aquifers is at least months or years and is frequently decades to centuries. As a result, much of what was learned in earlier laboratory studies cannot be applied to the subsurface environ- ment. Currently, microbiologists are re-examining the potential for biodegradation of organic contamination in ground waters that actively produce methane and are finding many unexpected reactions. It was previously thought that the metabolism of benzene, toluene, the xylenes, and other alkylben- zenes required molecular oxygen as a co-substrate for the enzyme that began the metabolism of this class of compounds (Young, 1984). Thus, their metabolism would not be expected in methanogenic environments. Recently the metabolism of these compounds was demonstrated in methanogenic river alluvium that has been contaminated with landfill leachate (Wilson and Rees, 1985). When radioactive toluene was added to this material at least half the carbon was metabolized completely to carbon dioxide. The same material also metabolized several methyl- and chlorophenols (Sulflita and Miller, 1985). The halogenated solvents that are persistent in oxygenated ground water can be transformed in methanogenic ground water. Examples include tri- chloroethylene, tetrachloroethylene, the dichloro- ethylenes, 1,1,1 dichloroethane, carbon tetrachloride, and chloroform (Parsons et al., 1984, 1985; Wood et al., 1985). The chlorinated ethylenes undergo a sequential reductive dehalogenation from tetrachloro- ethylene to trichloroethylene, then to the dichloro- ethylenes (primarily the cis isomer), and finally to vinyl chloride (Wood et al., 1985). In some materials appreciable quantities of vinyl chloride accumulate, and the accumulation is unfortunate because this compound is considerably more toxic and carcin- 5-2 ------- ogenic than its parent compound. In other materials the vinyl chloride is further metabolized. The factors that control the fate of vinyl chloride are at present entirely unknown (Wilson, 1985). The chloroalkanes follow a similar pattern (Wood et al., 1985); carbon tetrachloride is converted to chloroform, then to meth- ylene chloride, while 1,1,1-dichloroethane is converted to 1,1-dichloroethane which in turn goes to ethyl chloride. These reductive dehalogenations resemble respi- rations. In aerobic respiration, molecular oxygen accepts an electron and is reduced to the hydrogen- ated compound, water. The chlorinated compounds accept electrons and are reduced to the corre- sponding hydrogenated compound while the chlorine is released as a chloride ion. It is unknown whether these reductive dehalogenations benefit the micro- organisms that carry them out. However, the active microorganisms must have a source of hydrogen or some other organic compound to provide the electrons for the reduction of the chlorinated compounds. The source of electrons can be a co-occurring contam- inant, such as volatile fatty acids in landfill leachate, or it can be a geological material. Reductive dechlori- nation of trichloroethylene has been associated with flooded surface soil, buried soils in glaciated areas, buried layers of peat, and coal seams (J.T. Wilson, personal communication). Ground Waters Reducing Sulfate or Nitrate Once oxygen is depleted, certain classes of organic compounds can be degraded by bacteria that respire nitrate or sulfate. Ground waters recharged through soils that support intensive agriculture often have high concentrations of nitrate, and ground waters with appreciable concentrations of sulfate are widespread, particularly in arid regions. Microorganisms respiring nitrate can degrade a number of phenols and cresols (methylphenols). Recently, it has been shown that nitrate-respiring organisms in river alluvium could degrade all three xylenes (dimethylbenzenes) (Kuhn et al., 1985). However, the microorganisms could not degrade para-dichlorobenzene. Nitrate-respiring micro-organisms can also degrade carbon tetra- chloride and a variety of brominated methanes. However, they have not been shown to degrade chloroform or those chlorinated ethylenes or ethanes which are also stable in oxygenated ground water (Bouwer and McCarty, 1983). Like the methanogens, the sulfate-respiring bacteria can participate in consortia that degrade a wide variety of natural organic compounds. In contrast to the behavior of methanogenic subsurface material, chlorinated derivatives of naturally occurring aromatic compounds were not degraded in river alluvium containing appreciable sulfate concentrations (200 mg/L) and exhibiting active sulfate respiration (Sulflita and Miller, 1985; Sulflita and Gibson, 1985). As they did in methanogenic material, tetrachloroethylene and trichloroethylene underwent reductive dehalogena- tions. Conclusions Biological activity can complicate monitoring for materials released from underground storage tanks. Materials in solution in ground water as well as vapors in the unsaturated zone can be completely degraded, or they can be transformed to new compounds. The behavior of the released materials is controlled by the availability of oxygen or other electron acceptors required for microbial metabolism. It may be more convenient, therefore, to monitor for the conse- quences of biological activity, such as oxygen sag, methane production, or reduced electrode potential, than for the presence of the material reduced from the tank. 5-3 ------- References Bouwer, E.J.. and P.L. McCarty. 1983. Trans- formations of Halogenated Organic Compounds Under Denitrification Conditions. Applied and Environmental Microbiology 45(4):1295-1299. Jhaveri, V., and A.J. Mazzacca. 1983. Bioreclamation of Ground and Groundwater Case History. Presented at the 4th National Conference on Man- agement of Uncontrolled Hazardous Waste Sites, Washington, D.C., 31 October-2 November 1983. Kuhn, E.P., P.J. Colberg, J.L. Schnoor, O. Wannen, A. Zehnder, and R.P. Schwarzenbach. 1985. Micro- bial Transformation of Substituted Benzenes during Infiltration of River Water to Groundwater: Laboratory Column Studies. Environmental Science and Technology 19(10):961-968. Lee, M.D., J.T. Wilson, and C.H. Ward. 1984. Microbial Degradation of Selected Aromatics in a Hazardous Waste Site. Developments in Industrial Microbiology 25:557-565. Lokke, H. Leaching of Ethylene Glycol and Ethanol in Subsoils. 1984. Water, Air, and Soil Pollution 22:373-387. Novak, J.T., C.D. Goldsmith, R.E. Benoit, and J.H. O'Brien. Biodegradation of Alcohols in Subsurface Systems. 1984. In: Degradation, Retention, and Dispersion of Pollutants in Groundwater. Spe- cialized Seminar. Copenhagen, Denmark, 12-14 September, 1984, pp. 61-75. Parsons, F., G.B. Lage, and R. Rfce. 1985. Biotrans- formation of Chlorinated Organic Solvents in Static Microcosms. Environmental Toxicology and Chemistry 4:739-742. Parsons, F., P.R. Wood, and J. DeMarco. 1984. Transformations of Tetrachlorethene and Tri- chlorothene in Microcosms and Groundwater. Journal of the American Water Works Association 76(2):56-59. Sulflita, J.M., and S.A. Gibson. 1985. Biodegradation of Haloaromatic Substrates in a Shallow Anoxic Groundwater Aquifer. Proceedings of Second International Conference on Ground Water Quality Research, March 26-29,1984, Tulsa, Oklahoma. Sulflita, J.M., and G.D. Miller. 1985. Microbial Me- tabolism of Chlorophenolic Compounds in Ground Water Aquifers. Environmental Toxi-cology and Chemistry 4:751-758. Ward, T. 1985. Characterizing the Aerobic and Anaerobic Microbial Activities in Surface and Subsurface Soils. Environmental Toxicology and Chemistry 4:727-737. Wilson, B. 1985. Behavior of Trichloroethylene, 1,1- Dichloroethylene, cis-1,2-Dichloroethylene, and trans-1,2-Dichloroethylene in Anoxic Subsurface Environments. M.S. Thesis, University of Oklahoma. Wilson, B.H., and J.F. Rees. 1985. Biotransformation of Gasoline Hydrocarbons in Methanogenic Aquifer Material. In: Proceedings of the NWWA/ API Conference on Petroleum Hydro-carbons and Organic Chemicals in Groundwater, Houston, Texas, 13-15 November 1985. Wilson, J.T., J.F. McNabb, J.W. Cochran, T.H. Wang, M.B. Tomson, P.B. Bedient. 1985. Influence of Microbial Adaption on the Fate of Organic Pol- lutants in Ground Water. Environmental Toxi- cology and Chemistry 4:721-726. Wilson, J.T., G.D. Miller, W.C. Ghiorsc, and F.R. Leach. 1986. Relationship Between ATP Content of Subsurface Material and the Rate of Biode- gradation of Alkybenzenes and Chlorobenzene. Journal of Contaminant Hydrology, in press. Wood, P.R., R.F. Lang, and I.L. Payan. 1985. Anaerobic Transformation, Transport, and Re- moval of Volatile Chlorinated Organics in Ground Water. In: Ground Water Quality. C.H. Ward, W. Giger, and P.L. McCarty (eds). John Wiley & Sons, New York, pp. 493-511. Young, L.Y. 1984. Anaerobic Degradation of Aro-matic Compounds. In: Microbial Degradation of Aromatic Compounds. D.T. Gibson (ed). Marcel Dekker, New York, pp. 487-523. 5-4 ------- SECTION 6 Conclusions and Recommendations As the preceding sections have shown, there exist many gaps in our understanding of transport and fate processes of fluids from leaking underground storage tanks. The difficulties and uncertainties associated with each of the fate and transport processes described indicate that continued research is needed if monitoring is to be successful. To bridge some of these gaps, this section will restate the important parameters relating to each process, describe the monitoring approaches, and provide recommendations for future research. Process Parameters: A Synopsis In each of the preceding sections, detailed de- scriptions of two transport processes -- liquid and vapor migration - and two fate processes - surface effects and microbial degradation -- have been presented. This section briefly summarizes the important physical parameters of each process. A knowledge and understanding of these parameters will be necessary if monitoring systems are to be properly used. Table 6.1 shows the key parameters of the soil, contaminant, and the environment which affect fluid transport. Although some of the characteristics are easily measured, i.e., fluid density, others are difficult or impossible to measure and will require considerable research. Table 6.2 shows some of the important parameters controlling vapor transport of a volatile substance. Although the list may be slightly shorter than that for fluid transport, many of the parameters have received very limited research. Table 6.3 describes the parameters controlling surface chemical effects. Since this is not a transport process, those parameters mentioned in Tables 6.1 and 6.2 must also be examined to determine if the pollutant will reach the soil particle surfaces. Finally, Table 6.4 summarizes the key parameters controlling bioactivity in the subsurface. In summary, a great deal of data and research is required to adequately describe the fate and transport processes in the subsurface. Further research will be needed to quantify the most important parameters. Process Impacts on Monitoring At this time, no one monitoring strategy is applicable or standardized for monitoring subsurface contam- ination. This apparent lack of standardization is easily explained by the relative infancy of the science. It is only in the last ten years that significant national emphasis has been placed on monitoring ground- water quality. Subsurface monitoring approaches can be broken down into two major categories: direct and indirect measurements. Direct measurement includes methods such as analysis of water samples, vapor samples, and waste teachability as indicators of contamination. Indirect measurements do not meas- ure the contaminant per se but instead measure a property which can be related to the effects of con- tamination. Such indirect methods include surface geophysics and borehole geophysics. With regard to leak detection monitoring approaches, both methods may be used. Direct measurement techniques can be broken down into the following four categories: Active Liquid Sampling Passive Liquid Sampling Active Vapor Sampling Passive Vapor Sampling. Active liquid sampling implies that driving gradients are artificially induced to increase the zone of effective sampling. In the case of saturated zone monitoring, well(s) are pumped to generate a cone of depression, and fluid within the cone of depression flows by gravity (or fluid pressure in the case of a confined aquifer) into the well. Fluid samples from the well are analyzed for dissolved contaminants while immiscible contaminants may be monitored on the top or the bottom of the water level in the well. The range of active sampling is a function of the hydraulic parameters of the aquifer. Active liquid samplers in the vadose zone include soil suction lysimeters. These devices, acting similarly to pumped wells, draw surrounding soil water (and pre- sumably dissolved contaminants) into the sampler through capillary gradients. The range of influence of these devices, however, is limited to less than a meter. 6-1 ------- Soil Multiphase permeability Residual saturation Pore size distribution Fracture density Wettability Soil texture Porosity Variability of soil properties Table 6.1 Fluid Transport Parameters Contaminant Density Viscosity Solubility Surface tension Environmental Temperature Precipitation Depth to water table Water table gradient Soil Porosity Water content Soil structure and variability Permeability to air Soil texture Table 6.2 Vapor Transport Parameters Contaminant Volatility Vapor diffusivity Distribution coefficients Environmental Biological activity Recharge Temperature Barometric changes Water table fluctuations Soil Table 6.3 Surface Chemistry Parameters Contaminant Environmental Moisture content Organic content Clay content Soil surface area Pore water chemistry Solubility Concentration Temperature Pressure 6-2 ------- Soil Soil gas diffusion Colonization potential Oxygen concentration Methane concentration Contaminant velocity Table 6.4 Microbiological Parameters Contaminant Environmental Nutrient loading Toxicity Solubility PH Temperature Recharge and ground-water transport The advantage of active liquid samplers is to sig- nificantly increase the volume of the subsurface that is sampled by a monitoring device. These techniques also allow for the tentative quantification of the magnitude of the contaminant. In addition, such techniques tend to return to background levels quickly, which allows for the delineation of small leaks and spills versus massive contamination. On the other hand, operational costs of active methods tend to be rather high. Passive liquid samplers operate in the same areas as do active samplers. Instead of drawing potential contaminant towards the sampler, however, passive samplers assume that the contamination plume will be large enough that it will pass through the samplers. Passive liquid samplers include monitor-well bailing or infrequent pumping, soil core water extraction, or buried contaminant adsorption devices. These techniques have low operating costs; however, many samplers or samples are needed because of the very limited range of effective monitoring. Since convective transport occurs under slow, natural gradients in passive samplers, long time periods may be needed to return a sensor to a background con- dition after a small spill or leak. Active vapor samplers are useful for volatile contam- inant monitoring. As in active liquid sampling, a zone of lower air pressure is produced at a well point in the unsaturated zone. Because of the much higher con- ductivity of soils to air than water (except in nearly saturated soils), soil air containing vapors from volatile contaminants move rapidly by convection and dif- fusion to the well point. The soil air is then analyzed by various techniques for the dissolved contaminants of interest. This technique offers several advantages over liquid sampling. The first major advantage is the speed at which volatile contaminants may be detected. Vapor movement through partially saturated soils is con- trolled by concentration gradients, pressure gradients and the soil water content. Since the conductivity of soil to air is much higher than that of water, the vapor migration rates may be on the order of hours to days while the liquid migration rate may be on the order of months to years. This time factor may be very critical to prevent significant damage to existing water supplies. Both active liquid and active vapor sampling will produce similar results with respect to sample inte- gration, i.e., transient small spills and leaks should be distinguished from larger, continuous leaks by the concentration levels in the pumped sampler. Short duration "spikes" would therefore not be indicative of a continuous vapor source. Vapor samplers should have considerably lower operating costs than would active liquid samplers. Passive vapor samplers have much of the same advantages as do active vapor samplers and one major disadvantage. Because of the rapid convection and diffusion of vapors in soils, a passive vapor sampler will have a large radius of sample influence, and, therefore, few samplers would be needed. The technique may be seriously deficient, however, in its ability to discriminate between transient spills and leaks from more serious long-term leaks. If a spilled material is held up in the vadose zone, its removal by volatilization may be very slow. Passive vapor sam- plers near this source will continue to measure high vapor content even though the liquid source is small. A more major leak may never be detected by this sampler since there may be no increase in vapor density. 6-3 ------- Indirect Techniques Two major classes of indirect measurement tech- niques are currently available: surface geophysics and borehole geophysics. As the nomenclature implies, surface geophysical techniques involve measurement of subsurface properties from the surface. Within this category numerous techniques are available; each having a common thread. Each technique is designed to measure variations of some physical property relating to subsurface contamination. Variations in the hori- zontal aspect (profiling) or vertical aspect (sounding) are mapped, and anomalies can be used to delineate contamination. Surface geophysical methods may show promise for the detection of some types of migrating contamin- ants, particularly conductive materials. The methods are economical, easy to repeat, and non-destructive. On the negative side, techniques are not yet available to delineate areas of low level contamination deamed unsafe for drinking. Further clouding of the signal is derived from the subsurface environment adjacent to the tank which may be highly variable due to tank con- struction. Borehole geophysical methods allow for a detailed view of the subsurface in the vertical dimension. The techniques are quite similar to surface techniques; however, the technique is limited to boreholes. Since the technique is non-destructive, repeatable, and comes in contact with the formation, it may be quite useful in determining hydraulic properties and con- taminant locations as a function of depth. It is limited, however, to point location (wells) and has some of the same drawbacks as passive liquid sampling. Process Matrices To accurately assess the applicability of monitoring technologies to leak detection monitoring of under- ground storage tanks, it is necessary to understand both the advantages and limitations of each tech- nology. In the previous sections, data have been presented describing the effects of physical processes on leaking fluids. This section presents these effects versus the various monitoring technologies in a matrix format. In the first matrix, the advantages produced by each of the processes toward the adequacy and reliability of the monitoring technology are presented. For example, an advantageous process would be microbial production of a reduced dissolved oxygen "halo" in the saturated zone which would allow for early detection of potential contamination by either active or passive liquid sampling. The second matrix shows the disadvantages or complications produced by the process with respect to the monitoring tech- nology. An example of a disadvantage would be the impacts of adsorbtion and partitioning of the contam- inant on soil surfaces retarding or decreasing the contaminant to below detectable limits before it reaches a passive liquid sampler. Leaks would there- fore go undetected for a significant length of time, and a large part of the soil/aquifer system would be con- taminated. The matrices are presented in the following order: Table 6.5 Active and Passive Liquid Sampling - Advantages 6.6 Active and Passive Liquid Sampling - Disadvantages 6.7 Active and Passive Vapor Sampling - Advantages 6.8 Active and Passive Vapor Sampling - Disadvantages 6.9 Surface and Borehole Geophysics - Advantages 6.10 Surface and Borehole Geophysics - Disadvantages Recommendations for Future Work Based upon the preliminary discussions presented in this document, the following conclusions may be made: 1. There are many complicating factors for detection of leaks from underground tanks. No one monitoring approach will be applicable for all applications. 2. Monitoring in the near-field environment of the tank(s) holds less uncertainty than does far-field monitoring. 3. Active samplers appear to be less affected by transient spills; however, further research is needed to develop sensor criteria. In each of the processes described, the hetero- geneous nature of geologic materials indicated the need for a detailed sampling array. Uncertainty in flow directions because of variations in conductivity, water content, texture, etc., produced similar uncertainties in terms of whether or not a sensor would detect a leak. 6-4 ------- Since an increase in the distance from the leak to the sensor also increases the number of heterogeneities encountered and, hence, the increase in the un- certainty in flow direction, it is important to locate the sensor close to the leak source. To take this point a step further, tank installations may be engineered to reduce further the heterogeneity near the tank, and this would make the sensor location less sensitive. For example, in an engineered installation, i.e., gravel backfill, with sensors located within the backfill, the uncertainty in leak detection would be much less than if the sensors were located in the heterogeneous soils outside of the immediate tank site. Although additional research and field experience is required, it appears that criteria for monitoring at engineered installations could be developed fairly easily. Developing criteria for preexisting installations where engineering controls were not used will be much more difficult, and much higher uncertainties will be associated with these leak detection systems. In conclusion, the study of monitoring systems for underground storage tanks is in its infancy. This document presents the fundamental theories and an understanding of the processes controlling con- taminant migration and fate in the subsurface. It is hoped that future work will be based upon these foundations. Fate and Transport Process Table 6.5 Advantage Matrix for Liquid Monitoring Technologies Monitoring Technology Active Liquid Sampling Passive Liquid Sampling Liquid Flow Vapor Flow Physico/Chemical Transformations Microbiological Activity Control of gradient possible. Quantification of leak possible. Reduces effects of heterogeneity. Easy clean-up possible. Immiscible fluids easily monitored. Vapor migration may contaminate a large volume of aquifer because of resolubilization aiding in rapid detection. Short residence times may reduce sorption in kinetic-controlled reactions. Short residence times may reduce sorption in kinetic-controlled bio-reactions. Depressed D.O. halo may proceed actual contaminant plume. Increased soluble iron may occur because of biotransformations, which is easily monitored. Quantification of leak possible. Immiscible fluids easily monitored. Low operating cost. Vapor migration may contaminate a large volume of aquifer because of resolubilization aiding in rapid detection. If water levels do not fluctuate, a low density contaminant will float on the water surface and will only adsorb on a small volume of aquifer material. Depressed D.O. halo may proceed actual contaminant plume. Increased soluble iron may occur because of biotransformations. 6-5 ------- Fate and Transport Process Table 6.6 Complications Matrix for Liquid Monitoring Technologies Monitoring Technology Active Liquid Sampling Passive Liquid Sampling Liquid Flow Vapor Flow Physico/Chemical Transformations Microbiological Activity Uncertainty in flow direction. Large amount of water may be produced. If contaminant is immiscible, spreading of material may occur. Dilution at pumped well may reduce contaminant level to below detectable limits. Non-leak sources may lead to false positives. Large, explosive leaks may be present long before detection. Sorption may reduce contaminant levels below detectable limits. Selective microbial degradation may reduce monitored parameter. Pumping may stimulate activity adjacent to well, masking actual contamination. - Uncertainties in flow direction. - Difficulty to collect representative sample from bailing or intermittent pumping. - Non-leak sources may lead to false positives. - Large, explosive leaks may be long before detection. - Sorption may reduce contaminant levels below detectable limits. - Selective microbial degradation may reduce monitored parameter. Fate and Transport Process Table 6.7 Advantage Matrix for Vapor Monitoring Technologies Monitoring Technology Active Vapor Sampling Passive Vapor Sampling Liquid Flow Vapor Flow Physico/Chemical Transformations Microbiological Activity - Areas of low moisture content will enhance flow. - Rapid migration leading to early detection. - Active pumping will lessen impacts of transient spills. - Concentration gradients may be discernible. Sorption effects may be of secondary importance. Degradation effects may be of secondary importance. If degradation is occurring, monitoring for depressed O2 or methane may be possible. - Areas of bw moisture content will enhance flow. - Rapid migration leading to early detection. - Large, explosive leaks may be present long before detection. - Concentration gradients may be discernible. - Sorption effects may be of secondary importance. - Degradation effects may be of secondary importance. - If degradation is occurring, monitoring for depressed O2 or methane may be possible. 6-6 ------- Fate and Transport Process Table 6.8 Complications Matrix for Vapor Monitoring Technologies Monitoring Technology Active Vapor Sampling Passive Vapor Sampling Liquid Flow Vapor Flow Physico/Chemical Transformations Microbiological Activity - Variations in soil-water content affect vapor migration. - High water table precludes techniques. - Requires volatile contaminant. - Non-leak sources may lead to false positives. - Variation in temp, pressure, water content affect transport. - Vapor migration paths variable. - Leaking fluid must be available. - Selective sorption may reduce vapor levels. High O2 levels may enhance biological activity, and hence reduce contaminant levels below detectable limits. - Variations insoil-water content affect vapor migration. - High water table precludes techniques. - Requires volatile contaminant. - Non-leak sources may lead to false positives. - Variation in temp, pressure, water content affect transport. - Vapor migration paths variable. - Leaking fluid must be available. - Selective sorption may reduce vapor levels. - High O2 levels may enhance biological activity, and hence reduce contaminant levels below detectable limits. Fate and Transport Process Table 6.9 Advantage Matrix for Geophysical Technologies Monitoring Technology Surface Geophysics Borehole Geophysics Liquid Flow Vapor Flow Physico/Chemical Transformations Microbiological Activity Conductive plumes may be discerned easily. Spreading of plume may be advantageous to detection. - No advantages. - Transformation or sorption may produce more "visible" plumes by alteration of aquifer properties. - Transformation or sorption may produce more "visible" plumes by alteration of aquifer properties. - Vertical delineation possible. - Spreading of plume advantageous as it increases the range of detection of the tools. - No advantages. - Transformation or sorption may produce more "visible" plumes by alteration of aquifer. - Transformation or sorption may produce more "visible" plumes by alteration of aquifer properties. 6-7 ------- Table 6.10 Complications Matrix for Geophysical Technologies Monitoring Technology Fate and Transport Process Surface Geophysics Borehole Geophysics Liquid Flow - Techniques may be insensitive to leaking fluid. - Heterogeneities will require - Variations in water content and aquifer large sampling network. properties may obscure results. - Technique may be insensitive to leaking fluid. Vapor Flow - Present technologies not applicable to vapor - Present technologies not applicable flow. to vapor flow. Physico/Chemical - Adsorption may reduce sensitivity. - Adsorption may reduce sensitivity. Transformations Microbiological - Biodegradation may reduce sensitivity. - Biodegradation may reduce sensitivity. Activity 6-8 *U.S. Government Printing Office : 1988 - 516-002/80048 ------- |