Rorech, Gregory J. "Vapor Extraction and Bioventing" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 3 Vapor Extraction and Bioventing Gregory J. Rorech CONTENTS Introduction Contaminant Partitioning in the Subsurface Airflow Requirements and Capabilities Airflow Capability Airflow Requirements Evaluation of Conditions Where VES is Applicable Contaminant Properties Vapor Pressure Solubility Henry’s Law Other Molecular Properties Summary Properties of the Soil Bulk Density/Soil Porosity Soil Adsorption Soil Moisture Site Surface Topography Depth to Water Table Site Homogeneity Modeling Tools for Vapor Extraction System Design Engineering Design Model Flow Models Multiphase Transport Models Pilot Studies Laboratory Studies ©2001 CRC Press LLC Field Pilot Studies Vapor Extraction Testing Well Vapor Extraction Monitoring Wells Stage 1: Pilot Test Planning Stage 2: Conducting the Pilot Test Stage 3: Evaluating the Data Evaluation of Pneumatic Permeability Extraction Well Placement System Design Bioventing Introduction Advantages of Vapor Phase Biotreatment Performance Criteria and Bioventing Plan Protocols Laboratory Testing Field Respirometry Testing Soil Gas Permeability Testing Bioventing System Configurations Cleanup Goals/Costs Case Study Design/Operating Criteria Design Example Problem Solution Pilot Test Planning Conducting the Pilot Test Evaluating the Data References INTRODUCTION The vapor extraction and bioventing technologies induce airflow in the subsur- face using an above-ground vacuum blower/pump system. Adequate air movement within the contaminated zones is of primary importance to the success of the VES. The induced airflow brings clean air in contact with the contaminated soil, NAPL, and soil moisture. The contaminated soil gas is drawn off by the VES and the air in the soil matrix becomes recharged with new vapor phase contamination as the soil/pore water/soil gas/NAPL partitioning is re-established. Bioventing, or bioenhanced vapor extraction, is a remedial method similar to vapor extraction in that it relies upon an increase in the flow of air through the vadose zone. Vapor extraction is performed to volatilize the volatile organic constit- uents in situ . In bioventing, the increase in the flow of air is to provide oxygen in the subsurface to optimize natural aerobic biodegradation, and this becomes the dominant remedial process. While the design criteria for vapor extraction and bio- venting are different, once the physical system is in operation both processes occur. Compounds that are volatile move with the air, and compounds that are degradable ©2001 CRC Press LLC have an increased rate of degradation. The design sections of this chapter will show how to set up the process so that one process is favored over the other. Figure 1 shows the basic components of a VES. Subsurface vapors are withdrawn through an extraction well that may be vertically or laterally constructed. Recovered vapors are routed to an above-ground vapor treatment unit, if required. The key to a successful design is to place the wells and equipment so that when the system is in operation, an airflow pattern is created across the entire section of the unsaturated zone that is contaminated. The designer must also be careful that the air does not move through a small percentage of the area due to porous geology (short circuiting). The distribution of contaminants within the four phases (NAPL/vapor phase/soil moisture/adsorbed to soil) can be represented by mathematical equations, and com- puter models can simulate the distribution. For large projects where accurate pre- diction of system performance is required, pilot testing of the VES is often per- formed. The design engineer must gather information during the pilot testing activities or the predesign phase of the project to calibrate the simulation models. This will, in turn, aid in ascertaining achievable closure criteria to be compared with risk assessment goals, closure timeframes, and optimal system operating parameters (vacuum, flow rates, moisture content, well location, well screening, number of wells). Projects for which this level of sophistication is conducted may include litigation-oriented Superfund projects or projects where the site owner desires accu- rate prediction of the lifetime of the remedial system. For small projects, such as a gasoline service stations with shallow vadose contamination in a high permeability formation, modeling is often not conducted. If simulation models are not utilized, the design engineer must choose the required subsurface airflow velocities to achieve cleanup goals using published literature Figure 1 Basic VES. ©2001 CRC Press LLC values or other empirical design concepts. At a typical service station type of VES, the engineer is often constrained by subsurface structures for well locations. Due to these constraints, the systems may not be optimally designed and are often overde- signed to overcome site access limitations. A vacuum pump or blower is the tool that is used to create subsurface airflow (Figure 1). The vacuum created at the extraction well head is an indication of the subsurface soil resistance to airflow. If the subsurface is very porous there will be very little vacuum at the extraction well head regardless of vacuum pump that is utilized (i.e., low vacuum capability regenerative blower or high vacuum liquid-ring pump). If there is little resistance, there will be little resultant vacuum. If one increases the flow from a given well, a higher vacuum will be required at the extraction well head because of increased subsurface resistance created by the higher flow. Vacuum is an indication of subsurface resistance to flow and is not the variable that is critical to VES success. Sufficient airflow is the critical variable. The vacuum application is a system operational parameter that allows creation of the desired airflow. This chapter will discuss the theory of vapor extraction, site and chemical parameters that are used to predict its applicability, modeling of VES, pilot testing, system design criteria, and the biological enhancement that results from vapor flow. Vapor phase treatment options will be discussed in Chapter 6. CONTAMINANT PARTITIONING IN THE SUBSURFACE Contaminants that are released to the environment will be distributed in the subsurface in a manner consistent with their physical properties (vapor pressure, solubility, etc.) as well as related to properties of the soil (type, organic content, moisture extent, etc.). This subsurface distribution (in pore water, vapor, adsorbed to soil, or NAPL) is termed partitioning. Transfer of the contaminants between phases (dissolved, vapor, adsorbed, and free phase) is affected by the relative affinity of the contaminant to each phase. These affinities can be evaluated using the constituent partitioning coefficients to the various phases. The interphase partitioning coefficients can be expressed as the concentration ratio of the constituents in each phase, and this ratio is dictated by the equilibrium relations in the subsurface (Equations (1)-(3)). Since, to a large extent, the interphase transfer is governed by these equilibrium partitioning relation- ships, the most effective remediation will create the subsurface conditions that will drive the interphase transfer towards the phases that allow for the most efficient mass removal. Site remediation can therefore be viewed as implementing changes or perturbations to the subsurface that will drive chemical and/or biological processes towards the site remediation goals (Sims 1990). The subsurface change that is effected during vapor extraction is the replenishment of the subsurface soil vapor utilizing air as the carrier, and therefore, driving the contamination to the vapor phase where it is collected for above-ground treatment. Figure 2 is a schematic of the environmental compartment model showing the goals of vapor extraction. ©2001 CRC Press LLC When a volatile NAPL is present on the soil, the bulk of the mass removal by VES will come from direct volatilization of the NAPL. This would be similar to a fan blowing past a pool of gasoline. Research workers (Hoag et al. 1989) have shown that the bulk (more than 95 percent) of the NAPL can be removed within passage of several hundred pore volumes of air through the experimental soil columns (NAPL within the dry soil void space). In field applications, where airflow is usually over the NAPL layer rather than through it, VES still often recovers the bulk of the NAPL with passage of several hundred pore volumes of air. Under conditions of NAPL presence, mass removal rates are often linearly correlated with airflow rates. When NAPL is not present in the subsurface, airflow requirements become very different, and are often governed by nonequilibrium rate limiting conditions. The following section describes the partitioning of contaminants in the subsur- face. These equations are the basis of numerical simulation models that attempt to predict the remediation process of vapor extraction. Models achieve this prediction by repeatedly evaluating the new partitioning relationships after passage of clean air past the contaminated soil. Modeling is discussed later in this chapter. Under moist soil conditions, contaminant partitioning in the vadose zone (no residual NAPL) can be described by the following equation (1). Figure 3 is an illustration of the equation (1) where C T = Total quantity of chemical per unit soil volume; C A = Adsorbed chemical concentration; C L = Dissolved chemical concentration; C G = Vapor concentration; p b = Soil bulk density; θ L = Volumetric water content; and θ G = Volumetric air content. Figure 2 Environmental compartment model for VES. C T p b C A θ L C L θ G C G ++= ©2001 CRC Press LLC The equilibrium relationship between vapor concentration (C G ) and the associ- ated pore water concentration (C L ) is given by Henry’s Law (2) where K H = Henry’s Law constant. Henry’s Law is often thought of in relation to air stripping. In air stripping, air removes dissolved VOCs from the water stream. The efficiency of this removal process is related to the compound’s Henry’s constant. Under moist soil conditions, the extracted vapors during vapor extraction similarly remove VOCs from water. The success of this removal process is similarly related to the compound’s Henry’s constant. Likewise, the relationship between equilibrium solution concentration and adsorbed concentration is given by (3) where K d (L 3 /M) = the distribution coefficient expressed as K d = f oc K oc , and where f oc = the mass fraction of organic carbon, and K oc = the organic carbon partitioning coefficient. Equation (3) is generally considered to be valid for soils with high organic content (f oc >0.1 percent solids). For soils with lower organic carbon content (f oc <0.1 percent solids), sorption to mineral grains may be dominant (Piwoni and Bannerjee 1989 and Brusseau, Jessup, and Rao 1991). Adsorption is further discussed later in the chapter. There are several good sources for organic carbon partitioning coefficient Figure 3 Equation schematic. C G K H C L = C A K d C L = ©2001 CRC Press LLC (K oc ) values (U.S. EPA 1996 and Fetter 1994). The EPA document checked various literature sources and then calculated the geometric mean of what they considered the most reliable sources. This document also provided some relationships for various types of compounds to estimate K oc , if K ow is known, as presented below in Equations (4) and (5). Other relationships for these and other types of compounds are also available in the literature with some slight variation in each source. For polyaromatic hydrocarbons, polychlorinated biphenols and phthalates (4) For volatile organic compounds and chlorinated pesticides (5) Mathematical relationships can also be developed to quantify biological and/or other reaction transformations of the chemical contaminants. These relationships can subsequently be used to develop mathematical models of the subsurface condi- tions under advective air movement conditions. Mathematical models can be used to simulate subsurface changes caused by the VES (perturbation) and allow the user to select the most efficient perturbation that leads to the most contaminant mass removal. The above equations have defined partitioning of contaminants in the subsurface without air movement. Partitioning without advective air movement occurs via diffusion. The VES induces airflow (advective) past the contaminated zone; there- fore, under VES operating conditions, both diffusive and advective transport are occurring. Under the assumptions of uniform moisture distribution across the soil and incompressible air phase, the advective-dispersive transport equation in cartesian coordinates can be written as (Armstrong, Frind, and McClennan 1994, and Gierke, Hutzler, and McKenzie 1989) (6) where subscripts G, L, and A designate the gaseous, dissolved, and sorbed phases of the contaminant. C A , C L , C G , P b. , θ G , and θ L are as defined above. The air velocity component is derived from the air continuity equation and the subsequent application of the Darcy equation to pressure. The continuity equation states that the same mass of material entering a unit volume of space must also exit that volume space without biodegradability. The Darcy equation relates groundwater velocity to hydraulic gra- dient. In this application, the Darcy equation ( ) is applied for air movement rather than groundwater. The term D ij is the dispersion tensor, defined in terms of longitudinal and transverse dispersivity and the diffusion coefficient. Equa- K oc K ow log 0.094–=log K oc 0.78 K ow log 0.151+=log LC G () θ G D ij ∂ ∂x i ∂C G () ∂x j () θ G v i ∂C G ∂x i – θ G ∂C G ∂t θ L ∂C L ∂t P b ∂C A ∂t ++== ij, xz,= Vkφ • ⁄ dh dl⁄= ©2001 CRC Press LLC tions (1), (2), and (3) can be substituted into Equation (6) in order to represent the transport equation in terms of the gaseous phase only to yield (7) (8) where R is the retardation factor Equations (7) and (8) therefore, are a mathematical representation of partition- ing under diffusive and advective conditions. Actual field observations, however, indicate that the equations are only valid for diffusion dominated or weakly advec- tive conditions. Under strongly advective conditions that can be found while operating a VES, the above equations do not account for the asymptote that is observed in field applications. Tailing is the phenomenon that is often observed in VES applications whereby the contaminant mass removal rates are slower and the residual mass of contaminant adsorbed to the soil after vapor extraction is greater than what would be predicted by the equilibrium equation, Equation (8). Equilibrium predicted non- tailing and nonequilibrium type tailing effects are shown schematically in Figure 4. Figure 4 Equilibrium asymptotic tailing effect. K d f oc K oc = θ a D ij ∂ ∂x i ∂C G () ∂x j () θ G – v i ∂C G ∂x i Rθ G ∂C G ∂t = R 1 θ L θ G H P b K d θ G H ++= ©2001 CRC Press LLC As discussed in Chapter 2, the performance of a system in removing contami- nants changes with time. In most VES applications, the initial stages of the project yield the highest mass removal. The tailing effect implies that efficiency is decreasing with system lifetime and that closure goals or system operational modifications should account for this temporal change. Several authors (Armstrong, Frind, and McClennan 1994; Gierke, Hutzler, and McKenzie 1989; Sleep and Syikey 1989; and Brusseau, Jessup, and Rao 1991) have shown that the tailing effects can be represented by first order mass transfer, non- equilibrium, physical, and/or chemical processes. These nonequilibrium modifica- tions to the equilibrium relationships (Equations (1)-(3), (6)-(8)) will not be pre- sented in this chapter; however, the reader is referenced to the appropriate research publications for details. In brief, the nonequilibrium notions relate to the existence of a rate limiting criteria governing the mass transfer process for VES. Under fully wetted soil conditions without NAPL, the rate limiting step may be transfer across the air/water interphase, the soil/water interphase, dead end micropore effects, and/or a combination of all effects. Simply stated, these nonequilibrium effects slow down the vapor extraction process once the bulk of the contamination has been removed. (Armstrong, Frind, and McClennan 1994) have conducted a sensitivity analysis on several of these nonequilibrium rate limiting conditions. A sensitivity analysis or demonstration of the physical limitation of the VES ability is a powerful tool in ascertaining the system’s capability and achievable closure goals. This can be utilized to negotiate reasonable closure criteria with regulatory agencies and/or modify system operation during the project’s lifetime to minimize expenses while maximizing mass removal. VES is a very powerful mass removal technique. However, as the historic research and long-term operations have shown, VES can not overcome the natural limitations of the geology. Once most of the mass has been removed by the VES, an alternative treatment technique may have to finish the removal. AIRFLOW REQUIREMENTS AND CAPABILITIES The need to understand and predict the subsurface mass transfer relationships relates to the practical need to deliver the required airflow to achieve the remedial goals. Often the designer will only want the minimum subsurface air movement to achieve the remedial goals, since excessive airflow results in larger, expensive off gas treatment equipment as well as higher operating costs. In instances where NAPL is present in pockets, pools, or as a layer atop the groundwater, mass removal will often be linearly or semi-linearly rated to the airflow. This does not imply that if NAPL is present, high airflow is required, since often the NAPL is removed rapidly, leading to site conditions that may not require further high airflow. Airflow generation capability (airflow that can be generated based upon subsurface soil conditions) and airflow requirement (to achieve remediation) needs must be met in order to appropriately install a VES. [...]... Methyl tert-butyl-ether Naphthalene Pentachlorophenol Phenol Tetrachloroethylene Toluene 1,1,1-Trichloroethane 1,1,2-Trichloroethane Trichloroethylene Vinyl chloride o-Xylene Henry’s Law Constanta atm Reference 0 230 127 35 1282 145 171 0. 93 104 240 51 1841 160 429 35 9 37 .8 89 1.16 3. 2 32 .6 20 0.15 0.017 1 035 217 39 0 41 544 35 5000 266 1 1 1 3 1 2 1 2 4 1 1 1 1 1 1 2 1 2 2 1 4 2 2 1 1 1 2 1 3 1 At water... Press LLC Table 2 Henry’s Law Constants Compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 a Acetone Benzene Bromodichloromethane Bromoform Carbon tetrachloride Chlorobenzene Chloroform 2-Chlorophenol p-Dichlorobenzene (1,4) 1,1-Dichloroethane 1,2-Dichloroethane 1,1-Dichloroethylene Cis-1,2-Dichloroethylene Trans-1,2-Dichloroethylene Ethylbenzene Hexachlorobenzene... transport A secondary in uence of decreasing soil porosity is the increase in soil surface areas available for contaminant binding ©2001 CRC Press LLC The permeability will also have an economic affect on the VES The less permeable the soil, the higher the vacuum that is required to maintain the same airflow rate The zone of in uence will also be affected, requiring an increasing number of wells to... vapor probes can be used) Driving points in silty conditions may lead to clogging of the drive point screen The pilot test can be subdivided into three stages: pilot test planning, conducting the pilot test, and data evaluation These stages are described below Stage 1: Pilot Test Planning In addition to considering the contaminant and site characteristics prior to conducting the VES pilot test, there... point further away from the extraction lateral where the vault can be safely installed ©2001 CRC Press LLC Figure 16 Typical VES well Vapor Extraction Monitoring Wells A pilot test will generally require three to four monitoring points in a homogeneous setting to assess the zone of airflow in uence The number of required monitoring points increases if the site is nonhomogeneous and nonisotropic The increased... classes of modeling tools for use in VES design are described This is a broad, crude model classification, but is useful in presenting the basic requirements for VES design The following three different types of models will be discussed 1 Engineering design model 2 Airflow models 3 Multiphase transport models Engineering Design Model Models are available that simulate vacuum losses in piping networks,... the contaminants are biodegradable (petroleum contamination), since it is the general belief that the inducement of airflow will eventually lead to biological breakdown of the contaminants Reliance on minimally acceptable vacuum readings (0.5 inches to one inch of water) or minimally acceptable percentage of applied well head vacuum (1 percent of applied vacuum) to determine the VES radius of in uence... contaminants and soil conditions (discussed later in the chapter) ; or (3) by conducting computer simulation modeling (discussed later in the chapter) EVALUATION OF CONDITIONS WHERE VES IS APPLICABLE Vapor extraction system efficiency is affected by parameters relating to the contaminants to be removed and by variables relating to the site to be remediated Contaminant properties that affect VES are vapor pressure,... operated either in series or parallel This type of arrangement maximizes flexibility during the test and enables one to collect various operational data that can be evaluated to optimize VES system design 5 Type of monitoring points: Existing groundwater monitoring wells can be utilized, if they are screened in vadose zone Alternatively, new monitoring points can be installed Soil vapor probes (5/8 inch) can... of points depends on the complexity of the site It is best to locate at least one point close to the VES well in order to ensure one positive result point The monitoring points should be radially distributed at various distances from the extraction well Other points are located at increasing horizontal (and vertical if required) distances Often one tries to locate the test in an area where existing . and Bioventing" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 3 Vapor Extraction and Bioventing Gregory J. Rorech CONTENTS Introduction Contaminant. upon an increase in the flow of air through the vadose zone. Vapor extraction is performed to volatilize the volatile organic constit- uents in situ . In bioventing, the increase in the flow. phase treatment options will be discussed in Chapter 6. CONTAMINANT PARTITIONING IN THE SUBSURFACE Contaminants that are released to the environment will be distributed in the subsurface in