RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 

# Chapter 2 Estimation of Chemical Releases and Media Concentrations What’s covered in Chapter 2: � Source Release Systems � Releases to Groundwater and Estimation of Groundwater Waste Concentrations � Releases to Surface Pathways and Estimation of Waste Concentrations in Receiving Media

This chapter describes the methodology and assumptions used in the Delisting Risk Assessment Software (DRAS) program to compute releases of waste constituents from petitioned wastes and estimate waste constituent concentrations in media at the POEs. Source release systems are discussed in Section 2.1, releases to groundwater are discussed in Section 2.2, and releases to surface pathways are discussed in Section 2.3.
2.1 SOURCE RELEASE SYSTEMS The risk-based delisting process, as incorporated in DRAS, involves performing a risk assessment for petitioned wastes that are disposed of in two waste management units of concern to the U.S. EPA Delisting Program: surface impoundments and landfills. The process determines whether a waste that is petitioned for an exclusion (delisting) is not characteristically toxic and is thus exempt from Subtitle C disposal requirements, assuming that the petitioned waste meets all other criteria for delisting (see U.S. EPA 1996e for other delisting criteria). Once delisted, the petitioned waste may be disposed of in any municipal or industrial nonhazardous waste Subtitle D disposal unit. In preparing DRAS, U.S. EPA Region 6 focused its delisting petition evaluations on liquid and solid wastes disposed of in surface impoundments and landfills, respectively. Wastes disposed of in other waste management units will be evaluated on a case-by-case basis. In DRAS, U.S. EPA Region 6 assumed that petitioned liquid wastes will be disposed of in surface impoundments and that petitioned solid wastes will be disposed of in landfills. Whether a waste is liquid or solid is determined using methods specified in U.S. EPA’s “Test Methods for Evaluating Solid Waste”

2-1RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 (SW-846) (U.S. EPA 1997g). U.S. EPA’s SW-846 also is available on the Internet at “http://www.epa.gov/epaoswer/hazwaste/test/main.htm”. The assumptions used to quantify releases of chemicals from liquid-phase wastes in surface impoundments and solid-phase wastes in landfills are described in Sections 2.1.1 and 2.1.2, respectively.
2.1.1 Liquid-Phase Waste (Surface Impoundment) The method used to compute releases from liquid-phase wastes assumes that liquid industrial wastes are disposed of in an unlined surface impoundment with a sludge or sediment layer at the base of the impoundment. The determination of whether a waste is a liquid waste is made using U.S. EPA-approved Test Method 9095, referred to as the Paint Filter Test (U.S. EPA 1997g). The four parameters used to characterize a surface impoundment are (1) the area of the impoundment, (2) the ponding depth of the liquid in the impoundment, (3) the thickness of a relatively low-permeability sludge or sediment layer at the base of the surface impoundment, and (4) the hydraulic conductivity of this sludge or sediment layer. Additional information regarding characterization and modeling of liquid wastes disposed of in surface impoundments is provided in the “EPACMTP Technical Background Document” and the “EPACMTP Parameters/ Data Background Document” (U.S. EPA 2003a and 2003b).
2.1.2 Solid-Phase Waste (Landfill) The method used to compute releases from solid-phase wastes assumes that solid wastes are disposed of in a Subtitle D landfill and are covered with a 2-foot-thick native soil layer. It is assumed that the Subtitle D landfill is unlined or that any liner at the base of the landfill will eventually completely fail. The two parameters used to characterize landfills are (1) area and (2) depth (the thickness of the waste layer). Data to characterize landfills were obtained from a nationwide survey of industrial Subtitle D landfills (Westat 1987 and U.S. EPA 1997h). Parameters and assumptions used to estimate infiltration of leachate from a landfill are provided in the “EPACMTP Technical Background Document” and the “EPACMTP Parameters/ Data Background Document” (U.S. EPA 2003a and 2003b).

2-2RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.2 RELEASES TO GROUNDWATER (EPACMTP) This section describes the method used to compute the release and transport of chemicals from a waste management unit to the subsurface and their subsequent transport through the unsaturated and saturated zones to a theoretical downgradient receptor well.
2.2.1 Methodology for Estimation of Waste Constituent Concentration in Groundwater This section summarizes the method used to calculate groundwater exposure concentrations resulting from release of waste constituents into the subsurface from two waste management units: surfaceimpoundments and landfills. The exposure concentration is evaluated at a hypothetical groundwaterdrinking water well located a specific distance from the downgradient edge of the waste management unit. This well is referred to hereafter as the receptor well, and the exposure concentration measured at that well is referred to as the groundwater receptor well concentration ( C gw ). The groundwater fate and transport model used was the EPACMTP (U.S. EPA 2003a). Receptor well concentrations for both carcinogens and noncarcinogens, and for both degraders and nondegraders were determined using the finite source option within the EPACMTP.
2.2.2 Overview of EPACMTP The EPACMTP is a fate and transport model that simulates one-dimensional, vertically downward flowand transport of contaminants in the unsaturated zone beneath a waste disposal unit as well as twodimensional or three-dimensional groundwater flow and contaminant transport in the underlying saturated zone. The model accounts for the following processes affecting contaminant fate and transport: advection, hydrodynamic dispersion, linear or nonlinear equilibrium sorption, chained first-order decay reactions, and dilution from recharge in the saturated zone. The EPACMTP incorporates a Monte Carlo module that allows assessment of the uncertainty associated with receptor well concentrations that result from both uncertainty and variability in the model input parameter values. The EPACMTP consists of four major components:
• A module that performs one-dimensional analytical and numerical solutions for water flow and contaminant transport in the unsaturated zone beneath a waste management unit

• A numerical module for steady-state groundwater flow subject to recharge from the unsaturated zone

2-3RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

• A module of analytical and numerical solutions for contaminant transport in the saturated zone
• A Monte Carlo module for assessing the effect of the uncertainty resulting from uncertainty and variability in model parameter values on predicted receptor well concentrations The subsurface as modeled with the EPACMTP consists of an unsaturated (vadose) zone beneath a waste unit and an underlying water table aquifer. Contaminants move vertically downward through the unsaturated zone to the water table. The EPACMTP allows simulation of flow and transport in the unsaturated zone and in the saturated zone, either separately or combined. However, for the purposes of the delisting analysis, both the vadose zone and saturated zone were modeled. The EPACMTP is capable of simulating the fate and transport of dissolved contaminants from a point of release at the base of a waste disposal unit, through the unsaturated zone and underlying groundwater, to a receptor well at an arbitrary downstream location in the aquifer. The model accounts for the following mechanisms affecting contaminant migration: transport by advection and dispersion, retardation resulting from reversible linear or nonlinear equilibrium adsorption onto the soil and aquifer solid phase, and biochemical degradation processes. The latter may involve chain decay reactions if the contaminant or contaminants of concern form toxic daughter products that are of concern as well. As is true of any model, the EPACMTP is based on a number of simplifying assumptions that make the model easier to use and that ensure its computational efficiency. The major simplifying assumptions used in the EPACMTP are summarized below.

1. Soil and Aquifer Medium Properties . It is assumed that the soil and aquifer are uniform,porous media and that flow and transport are described by Darcy’s law and the advectiondispersion equation, respectively. The EPACMTP does not account for the presence of preferential pathways such as fractures and macropores. Although the aquifer properties are assumed to be uniform, the model does allow for anisotropy in hydraulic conductivity.
2. Flow in the Unsaturated Zone. Flow in the unsaturated zone is assumed to be steady-state, onedimensional, vertical flow from beneath the source toward the water table. The lower boundary of the unsaturated zone is assumed to be the water table. The flow in the unsaturated zone is assumed to be predominantly gravity-driven, and, therefore, the vertical flow component accounts for most of the fluid flux between the source and the water table. The flow rate is assumed to be determined by the long-term average infiltration rate through the waste management unit. In surface impoundments, this infiltration rate is assumed to be determined by the average depth of ponding. 3. Flow in the Saturated Zone. The saturated zone module of the EPACMTP is designed to simulate flow in an unconfined aquifer with constant saturated thickness. The model assumes regional flow in a horizontal direction with vertical disturbance resulting from recharge and 2-4RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 infiltration from the overlying unsaturated zone and waste disposal facility, respectively. The lower boundary of the aquifer is assumed to be impermeable. Flow in the saturated zone is assumed to be steady-state. The EPACMTP accounts for different recharge rates beneath and outside the source area. Groundwater mounding beneath the source is represented in the flow system by increased head values at the top of the aquifer. This approach is reasonable as long as the height of the mound is small relative to the thickness of the saturated zone.
4. Transport in the Unsaturated Zone . Contaminant transport in the unsaturated zone is assumed to occur by advection and dispersion. The unsaturated zone is assumed to be initially contaminant-free, and contaminants are assumed to migrate vertically downward from the disposal facility. The EPACMTP can simulate both steady-state and transient transport in the unsaturated zone with single-species or multiple-species chain decay reactions and with linear or nonlinear sorption.
5. Transport in the Saturated Zone . Contaminant transport in the saturated zone is assumed to be a result of advection and dispersion. The aquifer is assumed to be initially contaminant-free, and contaminants are assumed to enter the aquifer only from the unsaturated zone immediately beneath the waste disposal facility, which is modeled as a rectangular, horizontal plane source. The EPACMTP can simulate both steady-state and transient three-dimensional transport in the aquifer. For steady-state transport, the contaminant mass flux entering at the water table must be constant with time; for the transient case, the flux at the water table may be constant or may vary as a function of time. The EPACMTP can simulate the transport of a single species or multiple species, chain decay reactions, and linear sorption.
6. Contaminant Phases . The EPACMTP assumes that the dissolved phase is the only mobile phase and disregards interphase mass transfer processes other than adsorption onto the solid phase. The model does not account for volatilization in the unsaturated zone; this is a conservative approach for volatile chemicals. The model also does not account for the presence of a nonaqueous-phase liquid (such as oil) or for transport in the gas phase. When a mobile oil phase is present, significant contaminant migration may occur within it, and the EPACMTP may underestimate the movement of hydrophobic chemicals.
7. Adsorption Reactions . The EPACMTP computes chemical reactions involving adsorption. The EPACMTP assumes that sorption of organic compounds in the subsurface can be represented by linear adsorption isotherms in both the unsaturated and saturated zones. It is assumed that adsorption of contaminants onto the soil or aquifer solid phase occurs instantaneously and is entirely reversible. The effect of geochemical interactions is especially important in fate and transport analyses of metals. For simulation of metals, the EPACMTP uses one of two types of adsorption isotherms: 1) pH-dependent, empirically-derived isotherms (U.S. EPA 1990g); or 2) sorption isotherms generated by MINTEQA2 (Allison and others 1991). MINTEQA2 generates concentration-dependent effective partition coefficients for various combinations of geochemical conditions. This procedure is described in the background document for modeling of metal transport (U.S. EPA 1991d, 1996d, and 1999). The EPACMTP also accounts for chemical and biological transformation processes. All transformation reactions are represented by first-order decay processes. An overall decay rate is specified for the model; therefore, the model cannot explicitly consider the separate effects of multiple degradation processes such as oxidation, hydrolysis, and biodegradation. The user must determine the overall, effective decay rate 2-5RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 when multiple decay processes are to be represented. To maximize its flexibility, the EPACMTP has the capability of determining the overall decay rate from chemical-specific hydrolysis rate constants using soil and aquifer temperature and pH values (U.S. EPA, 1993e and 1996c). The EPACMTP assumes that reaction stoichiometry is prescribed for scenarios involving chain decay reactions. The speciation factors are specified as constants by the user (see the “EPACMTP Technical Background Document” and the “EPACMTP Parameters/ Data Background Document” (U.S. EPA 2003a and 2003b)). In reality, these coefficients may change as functions of aquifer conditions (for example, temperature and pH), concentration levels of other chemical components, or both.
2.2.2.1 Contaminant Release and Transport Scenario
Two source release scenarios are considered in the EPACMTP: continuous (infinite) and finite-source. Only the finite-source scenario is considered for delisting. For finite-source scenarios, the release of contaminants occurs over a finite period of time, after which the leachate concentration becomes zero (that is, all the contaminants in the waste disposed of in the waste management unit have leached out). Each type of waste management scenario is described by a relatively small number of parameters. The differences between waste management units are represented by different values or frequency distributions of the source-specific parameters. Source-specific stochastic parameters used by the EPACMTP for landfills include the capacity and dimensions of the waste management unit, infiltration and recharge rates, pulse duration, the fraction of hazardous waste in the waste management unit, and the density of the waste. The source-specific stochastic parameters used for surface impoundments include the area, the ponding depth (such as the depth of liquid in the impoundment), and the thickness and hydraulic conductivity of the sludge or sediment layer at the bottom of the impoundment. Data on the areas, volumes, and locations of landfills were obtained from the 1987 U.S. EPA survey of industrial Subtitle D waste facilities in the United States (Westat 1987 and U.S. EPA 1997h). Relevant information for surface impoundments was obtained from the 2001 EPA’s industrial Surface Impoundments Study (US EPA 2001). Derivation of the parameters for each type of waste management unit is described in the “EPACMTP Technical Background Document” and the “EPACMTP Parameters/ Data Background Document” (U.S. EPA 2003a and 2003b). For finite-source scenarios, simulations are performed for transient conditions, and the source is assumed to be a pulse of finite duration. In the case of landfills, the pulse duration is based on the initial mass of contaminant in the landfill, infiltration rate, landfill dimensions, and leachate concentration (U.S. EPA 1996c). For surface impoundments, the duration of the leaching period is determined by the waste 2-6RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 management unit’s lifetime (the default value is 50 years for surface impoundments). For a finite-source scenario, the model can calculate either the peak receptor well concentration for noncarcinogens or the highest average concentration over a specified period for carcinogens. The finite-source methodology in the EPACMTP is discussed in detail in the finite-source background document (U.S. EPA 1996c).
2.2.2.2 EPACMTP Modeling Assumptions and Input Parameters Specific EPACMTP modeling assumptions (in addition to the simplifying assumptions discussed in Section 2.2.1) are summarized in Table 2-1. This table also provides information on important input parameters as well as on their data sources. Overall, EPACMTP input parameters can be organized in the following four groups:

C Source-specific parameters

C Chemical-specific parameters

C Unsaturated zone-specific parameters

C Saturated zone-specific parameters For delisting, the EPACMTP is run in probabilistic (Monte Carlo) mode, and many of the source-, chemical-, unsaturated zone-, and saturated-zone specific parameters are represented by probability distributions reflecting variations on a national or a regional level. Specific capabilities and requirements associated with running the EPACMTP in the Monte Carlo mode are presented in Chapter 5 of the “EPACMTP Technical Background Document” (U.S. EPA 2003a). The Monte Carlo analysis enables parametric uncertainty and/or variability to be quantitatively accounted for. The flow and transport modules of EPACMTP are linked to a Monte Carlo driver that permits a probabilistic evaluation of variability and/or uncertainty in model input parameters, as described by specified (joint) probability distributions. The resulting sequence of receptor well concentrations (one for each iteration) are sorted and ranked from highest to lowest in order to obtain a probabilistic distribution of receptor well concentrations. The different groups of input parameters are summarized in Table 2-1 below.

2-7RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 TABLE 2-1 EPACMTP MODELING ASSUMPTIONS AND INPUT PARAMETERS Modeling Assumptions Modeling Element Description or Value Management Scenario Landfill Surface impoundment Modeling Scenario Finite-source Monte Carlo; LF: depleting source for organics, constant concentration pulse source for metals SI: constant concentration pulse for all constituents Exposure Evaluation Downgradient groundwater receptor well; maximum well concentration; 10,000-year modeling period Regulatory Protection Level 90 percent Source-Specific Parameters Parameter Description or Value Waste Unit Depth Waste Unit Area Waste Unit Volume Infiltration Rate Landfill Surface Impoundment Derived (for landfills), and site-based randomly selected from the EPA industrial SI data base for (surface impoundments) Site-based, randomly selected from OPPI database (for landfills), and from the EPA industrial SI data base (for surface impoundments) User-specified Site-based, derived from water balance using HELP model Site-based, derived from impoundment depth using Darcy’s law Leaching Duration Landfill Surface Impoundment Derived, continues until all constituents have leached out 50 years (operational life of unit) Chemical-Specific Parameters Parameter Description and Source Decay Rate Organic Constituents Metals Hydrolysis rate constants compiled by U.S. EPA ORD (U.S. EPA 1993e) No decay Sorption Organic Constituents Metals oc K constants compiled by U.S. EPA ORD (U.S. EPA 1993e) d MINTEQA2 sorption isotherm coefficients ( K ) for Ba, Cd, Cr (III), Hg, Ni, Pb, Ag, Zn, Cu, V, Be, Mo, As(III), Cr(VI), Se(VI), Tl, Sb(V), Mn, F, As(V), and Se(IV) (U.S. EPA 2003b); d empirical K distributions for Fe and Sn

2-8RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Unsaturated Zone-Specific Parameters Parameter Description and Source Depth to Groundwater Site-based, from API and USGS hydrogeologic database (Newell and others 1989 and 1990) Soil and Unsaturated Zone U.S. EPA ORD data, based on national distribution of three soil types (sandy loam, silt loam, Properties: silty clay loam) Bulk Density Carsel and others 1992 Saturated Conductivity Carsel and Parrish 1992 Moisture Retention Carsel and Parrish 1992 Parameters (alpha and beta) Residual Water Content Carsel and Parrish 1992 Saturated Water Content Carsel and Parrish 1992 Percent Organic Matter Carsel and others 1992 Dispersivity Gelhar’s analysis of EPRI field data (US EPA 2003b and EPRI 1985) Saturated Zone-Specific Parameters Parameter Description and Source Recharge Rate Site-based, derived from regional precipitation and evaporation data and soil type Aquifer Thickness Site-based, from API and USGS hydrogeologic database (Newell and others 1989 and 1990) Hydraulic Conductivity Site-based, from API and USGS hydrogeologic database (Newell and others 1989 and 1990) Hydraulic Gradient Site-based, from API and USGS hydrogeologic database (Newell and others 1989 and 1990) Porosity Effective porosity derived from national distribution of aquifer particle diameter Bulk Density Derived from porosity Dispersivity Derived from distance to receptor well (Gelhar and others 1992 and U.S. EPA 2003b) Groundwater Temperature Site-based, from USGS regional temperature map (Todd 1980 and US EPA 2003b) Fraction Organic Carbon National distribution, from U.S. EPA STORET database pH National distribution, from U.S. EPA STORET database Receptor Well Parameters Well Element Description and Source Radial Distance from Waste Nationwide distribution, from U.S. EPA OSW database (US EPA 1993) Management Unit Angle Off-Center Uniform within ± 90 E from plume center line (no restriction to always lie within plume) Depth of Intake Point Uniform throughout saturated thickness of aquifer Notes: API = American Petroleum Institute HELP = Hydrologic Evaluation of Landfill Performance ORD = U.S. EPA Office of Research and Development STORET = Database Utility for STORage and RETrieval of Chemical, Physical, and Biological Data for Water Quality USGS = U.S. Geological Survey

Source-Specific Parameters . Source-specific parameters give information about the waste management unit. The most sensitive parameters in this group include the waste management unit’s area and depth

2-9RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 and the infiltration rate (U.S. EPA 1996b and 1996c). It is important to note the difference in leaching duration for each waste management unit type (see Table 2-1). Chemical-Specific Parameters . Chemical-specific parameters describe the degradation (decay), adsorptive, and diffusive characteristics of each of the chemical species being simulated. Thus, the most important variables accounting for chemical characterization are the hydrolysis rate ( ë ) (for organic constituents), the normalized distribution coefficient for organic carbon ( K oc ) (for sorption estimation for organics), and the adsorption isotherm coefficient ( K d ) (for linear and nonlinear sorption estimation for metals). K oc is also referred to as the organic carbon partition coefficient, and K dis also referred to as the effective partition coefficient. For details on metals isotherms from MINTEQA2, see Appendix A-6, Application of CMTP and MINTEQA2 for determining DAFs
. Unsaturated Zone-Specific Parameters . The unsaturated zone is the first subsurface level below the waste management unit. It is unsaturated with groundwater, and, therefore, it does not have a water table. The available soil moisture in this zone enables contaminants to move in the subsurface. The simulated process of fate and transport in this zone is based on the following assumptions:

• The flow of the fluid phase is isothermal and is governed by Darcy’s law.

• The flow is one-dimensional, vertically downward, and steady-state.

• The fluid is slightly compressible and homogeneous.

• The dynamics of a second phase (such as a vapor phase or nonaqueous liquid) can be disregarded. The solutions for transient and steady-state transport in the unsaturated zone are based on the following assumptions:

• The leachate concentration entering the soil is either constant (with a finite or infinite duration) or decreasing with time following a first-order decay process.
• Sorption of contaminants onto the soil solid phase is described by a linear or nonlinear (Freundlich) equilibrium isotherm. The required input values for this parameter group include depth to groundwater and soil hydraulic parameters and other soil properties, including fraction organic carbon ( f

oc ) and bulk density.

2-10RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Saturated Zone-Specific Parameters . The saturated zone is located below the unsaturated zone and beneath the water table. Groundwater flow in the saturated zone is simulated using a steady-state solution for predicting hydraulic head and Darcy velocities in a constant-thickness groundwater system subject to infiltration and recharge along the top of the aquifer and a regional (ambient) gradient defined by upstream and downstream head boundary conditions. Simplifying assumptions used to simulate contaminant transport in the saturated zone are as follows:

• The flow field is steady-state.

• The aquifer is homogeneous and initially contaminant-free.

• Adsorption onto the solid phase is described by a linear or nonlinear equilibrium isotherm.

• Chemical and/or biochemical degradation of the contaminant can be described as a firstorder process.

• For a multicomponent decay chain, the number of component species (parent and daughters) does not exceed seven.

• The mass flux of contaminants through the source is either constant or controlled by firstorder decay until all mass has been released from the source.

• The chemical concentration in groundwater is dilute, and the chemical is present in the aqueous and aquifer solid phases only. The required input parameters for this group include recharge rate, saturated thickness, hydraulic conductivity, hydraulic gradient, and groundwater temperature and pH. The receptor well location is also a saturated zone-specific parameter. As the location at which potential exposure to groundwater is measured, it can be anywhere downgradient of the waste management unit, within the areal and/or vertical extent of the contaminant plume, and/or along the contaminant plume center line. The receptor well location is determined using the following parameters:

• Radial distance from the waste management unit (X-dimension)

• Angle off of the plume centerline (Y-dimension)

• Depth of the intake point (Z-dimension; defined as well depth within the saturated zone)

2-11DAF = dilution attenuation factor (unitless) calculated C L = leachate concentration (that is, TCLP) waste- and chemical-specific (milligram per liter [mg/L]) Default

gw C = waste constituent concentration in groundwater computed with RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 The EPACMTP fate and transport model was used to determine the degree of dilution and attenuation that a chemical will undergo as it leaches from a waste management unit and is transported in the subsurface, into the saturated zone, and to a theoretical downgradient receptor well. The decrease in chemical concentration during the chemical’s transport from the leachate to the receptor well is the DAF. The EPACMTP used Monte Carlo probability analyses to compute a DAF for each chemical in the DRAS chemical library. The EPACMTP determines the groundwater concentration at the theoretical drinking water well that is the 90 thpercentile of all predicted concentrations (C gw ) resulting from the Monte Carlo analyses (that is, 90 percent of the resulting receptor well concentrations are less than this concentration). The DAF is then calculated by dividing the initial (C ) waste concentration by the predicted groundwater concentration at the 90 thpercentile as shown in Equation 2-1. L

(2-1)

where:

( mg/L) EPACMTP If the maximum allowable concentration of a chemical at the groundwater receptor well (C gw ) is assumed to be an HBN, Equation 2-1 can be rearranged, and the HBN (or a maximum contaminant level [MCL], if available) can be substituted for the receptor well concentration ( C gw ) to back-calculate a maximum allowable leachate concentration ( C LMAX ) for the ingestion exposure pathway. Equation 2-2 reflects this approach.

(2-2)

where: Default C LMAX = maximum allowable leachate concentration calculated (TCLP concentration) (mg/L) HBN = health-based number (or MCL) (mg/L) chemical-specific DAF = dilution attenuation factor (unitless) computed with EPACMTP
2-12RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 For further details about using the CMTP in conjunction with MINTEQA2 to derive DAFs, see Appendix A-6, Application of CMTP and MINTEQA2 for determining DAFs
.
2.2.2.3 Derivation Waste Volume-Specific DAFs Application of the EPACMTP to the U.S. EPA Delisting Program allows evaluation of specific waste volumes. To generate waste volume-specific DAFs, a number of EPACMTP runs were conducted, each run with a user-specified waste volume. For each Monte-Carlo realization, the user-specified waste volume was divided by a waste management unit area randomly drawn from a site in a regional site database, while retaining the site’s original area/depth ratio. The regional site database contains waste management unit depth and area data obtained from a national survey of Subtitle D industrial landfill facilities performed by the U.S. EPA Office of Policy Planning and Implementation (OPPI) (hereinafter referred to as the OPPI survey data) (Westat 1987; U.S. EPA 1988b), and from the 2001 EPA’s Industrial Surface Impoundment (SI) Study (US EPA 2001). The OPPI and SI survey data were compiled along with hydrogeologic data in the regional site database. The data for each waste management scenario are presented in “EPACMTP Parameters/ Data Technical Background Document” (U.S. EPA 2003b). For a complete description of the regional site-based modeling approach, refer to the “EPACMTP Technical Background Document” (U.S. EPA 2003a). Rather than compute DAFs for each of 326 chemicals for a range of waste volumes for landfills and surface impoundments, a method was developed to scale the DAFs based on a specific waste volume to the DAFs computed using the OPPI and SI data bases (U.S. EPA 1988b, 2001). The EPACMTP fate and transport model was used to compute waste volume-specific DAF scaling factors for the landfill and surface impoundment waste management scenarios (U.S. EPA 2003a, b). The DAF scaling factors and the regression equations developed for the landfill and surface impoundment waste management scenarios are presented in the following sections.
2.2.2.3.1 Landfills DAF scaling factors were developed for lifetime landfill waste volumes ranging from 10,000 to 1,000,000 3 cubic yards (yd ), based on the range of waste volumes encountered in the OPPI survey (U.S. EPA 1988b). The landfill DAF scaling factors are plotted in Figure 2-1. Figure 2-1 indicates that the DAF 3 scaling factor is approximately 1.0 for landfill lifetime waste volumes greater than 154,000 yd . U.S. EPA did not consider DAF scaling factors less than 1.0. Therefore, for lifetime landfill waste volumes
2-13RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

3 greater than 154,000 yd , the waste volume-specific DAF is equal to the DAF computed based on the OPPI data base. Equation 2-3 can be used to determine the DAF scaling factor (DAF ) as a function of lifetime landfill sf waste volume for landfilled wastes.
(2-3)

where: Default DAF sf = DAF scaling factor (unitless) computed 3 V = landfill lifetime volume of waste (yd ) delisting petition-specific The correlation coefficient of this regression equation is 0.99, indicating that changes in the DAF scaling factor are explained by changes in the waste volume and that the DAF scaling factor can be predicted with confidence as a function of waste volume.

2-14RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.2.2.3.2 Surface Impoundments DAF scaling factors were developed for lifetime surface impoundment waste volumes ranging from 2,000 3 to 1,000,000 yd based on the OPPI survey data (U.S. EPA 1988b). The surface impoundment DAF scaling factors are plotted in Figure 2-2. For surface impoundment lifetime waste volumes greater than 3 about 40,000 yd , the DAF scaling factor is equal to 1.0. Equation 2-4 can be used to determine the DAF sf as a function of waste volume for surface impoundments.
(2-4)
where: Default DAF sf = DAF scaling factor (unitless) computed 3 V = impoundment lifetime volume of waste (yd ) delisting petition-specific The correlation coefficient of this regression equation is 0.99, indicating a good fit of this line to the data points and that the DAF scaling factor for surface impoundments can be predicted as a function of waste volume with a high level of confidence.
2.2.3 Calculation of Groundwater Waste Constituent Concentration at the POE As discussed above, the EPACMTP model estimates fate and transport of chemicals in groundwater. Within this medium, three human health exposure pathways are evaluated by the U.S. EPA Delisting Program: direct ingestion, dermal absorption, and shower inhalation. This section describes how the waste constituent concentration at the groundwater POE is calculated.

2-15RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

Figure 2-1. DAF Scaling Factors for Landfills

2-16RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

Figure 2-2. DAF Scaling Factors for Surface Impoundments

2-17RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 (2-5) To adjust for the petitioned waste volume, the EPACMTP-modeled DAF is multiplied by the DAF sf determined as described above. The product is defined as the waste volume-adjusted DAF ( DAF va ), which is specific for each waste constituent. Equation 2-5 is used for this calculation.
where: Default

va DAF = waste volume-adjusted DAF (unitless) calculated DAF sf = DAF scaling factor (unitless) Equation 2-3 (landfills) or 2-4 (surface impoundments) DAF = dilution attenuation factor (unitless) Computed with EPACMTP Using Equation 2-6, the TCLP leachate concentration of a petitioned waste constituent is divided by the waste volume-adjusted DAF to obtain the predicted groundwater receptor well concentration of the constituent. (2-6)

where: Default
gw C = waste constituent concentration in groundwater calculated (mg/L) TCLP = TCLP concentration of waste constituent (mg/L) waste constituent-specific

va DAF = waste volume-adjusted DAF (unitless) Equation 2-5 The waste constituent concentration in groundwater calculated using Equation 2-6 is used to determine exposure point concentrations for the three groundwater exposure pathways identified above: direct ingestion, dermal absorption, and shower inhalation. The exposures, in turn, are used to compute the risk and hazard associated with exposure to the waste constituent via each of these three pathways (see Chapter 4). It should be noted here that the EPACMTP DAFs may be very high (that is, greater than 1,000,000) for some waste constituents.

2-18RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.3 RELEASES TO SURFACE PATHWAYS This section describes the equations used to predict releases of chemicals from landfill and surface impoundment waste management units via the air and surface water pathways. The equations predict medium-specific (soil and air) concentrations of the waste constituent or constituents at the POE. The release of chemicals to air via particulate matter from landfills is discussed in Section 2.3.1, the release of volatile chemicals to air from landfills and surface impoundments is discussed in Section 2.3.2, and the methodology for calculating surface water concentrations for waste constituents eroded from landfills is presented in Section 2.3.3.
2.3.1 Calculation of Waste Constituent Concentration in Air — Particles
U.S. EPA Region 6 considers exposure to airborne particulate hazardous constituents released from wastes disposed of in landfills to be a function of (1) inhalation of particles and their absorption into the lungs at the POE and (2) air deposition of particles and subsequent ingestion of the soil-waste mixture at the POE. To address inhalation and ingestion of particles, U.S. EPA Region 6 calculates particulate emissions resulting from wind erosion of soil-waste surfaces and from vehicular traffic over the waste and waste loading and unloading. To estimate the respirable particulate emissions resulting from wind erosion of surfaces with an infinite source of erodible particles, U.S. EPA Region 6 used the methodology documented in “Rapid Assessment of Exposure to Particulate Emissions from Surface Contamination Sites (RAEPE)” (U.S. EPA 1985a). To calculate the dust and particulate emissions resulting both from vehicular traffic and from waste loading and unloading operations at a facility, U.S. EPA Region 6 used the methodologies documented in “Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources” (AP-42) (U.S. EPA 1985b). Particulate emission rates computed using these methodologies were summed and entered in the Ambient Air Dispersion Model (AADM), a steady-state, Gaussian plume dispersion model developed by U.S. EPA to predict the concentrations of constituents 1,000 feet downwind of a hypothetical land disposal facility (U.S. EPA 1985c). When evaluating delisting petitions, the DRAS assumes conservative values for all variables that are likely to influence the potential for soil erosion, including wind velocity and vegetative cover. U.S. EPA Region 6, however, modified the AADM assumptions regarding unit dimensions to more closely resemble a landfill’s dimensions. The method used to calculate total respirable particulate emissions from a landfill is described in
2-19RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Section 2.3.1.1. The methods used to calculate the average downwind concentrations of particles and the downwind concentrations of respirable particulate emissions are described in Section 2.3.1.2. Calculation of air deposition rates and resulting soil concentrations is described in Section 2.3.1.3.
2.3.1.1 Estimation of Particulate Emissions
U.S. EPA Region 6 calculates the mass flux (the source term or amount of waste material that becomes airborne) associated with particulate emissions using the methodology presented in RAEPE (U.S. EPA 1985a). The mass flux can be calculated using Equations 2-7 and 2-8.
(2-7)
where: Default

p Q = emission rate of waste constituent particles calculated (milligrams per second [mg/s]) á = mass fraction of constituent in waste (unitless) waste-specific (equal to total concentration in waste; mg/mg) E T = total emission rate of particles that may be Equation 2-8 inhaled (gram/hour [g/hr]) and:
(2-8)
where: Default E T = total emission rate of particles that may be calculated inhaled (g/hr)

w E = particulate emission rate from wind erosion (g/hr) Equation 2-9

v E = particulate emission rate from vehicular traffic (g/hr) Equation 2-17 E l = particulate emission rate from waste loading and Equation 2-21 unloading operations (g/hr) The equations used to compute emissions resulting from wind erosion ( E w ), vehicular traffic ( E v ), and waste loading and unloading ( E l ) are presented in the following sections.
Wind Erosion Emissions

2-20RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Wind erosion emissions of respirable particles (those smaller than 10 microns [ ì m] in diameter) acan be calculated using Equation 2-9. (2-9)

where: Default E w10 = wind erosion emission rate for particles up to 10 ì m calculated (g/hr) V

f = fraction of disposal site covered with vegetation (unitless) 0 (U.S. EPA 1994a) U = mean annual wind speed (meters/second [m/s]) 4 (discussed below) U t = threshold value of wind speed at 7 m (m/s) 5.44 (Equation 2-10) F(X) = dimensionless function obtained from a plot in RAEPE 1.33 (Appendix B, (U.S. EPA 1985a) Figure B-6) 2 A exposed = area of the waste management unit exposed (m ) Equation 2-15 For the landfill waste disposal scenario, U.S. EPA assumes that no vegetative cover is present, thereby assuming enhanced erodibility of soil or waste. Therefore, the fraction of the disposal site covered with vegetation ( Vf ) is equal to 0. The mean annual wind speed is assumed to be 4 m/s. This value represents the average of the wind speeds registered at U.S. climatological stations as documented in Table 4-1 of RAEPE (U.S. EPA 1985a). This value is also assumed to be associated with climate stability class D. b The threshold value of wind speed, U t, can be derived in the following manner as described in RAEPE (U.S. EPA 1985a) using Equation 2-10:

• The waste is assumed to exhibit a particle size of 0.2 millimeter (mm) (a typical size for fine sand). The wind erosion threshold friction velocity ( U *t ) used in Equation 2-10 is derived from RAEPE (U.S. EPA 1985a) plots (see Appendix B, Figure B-6 of this DTSD) and is equal to 33 centimeter per second (cm/s).

• A roughness height ( Z o ) of 1.0 cm (for a plowed field) is obtained from Appendix B.

• Based on Z , a ratio of wind speed at 7 m ( U ) to friction velocity ( U ) can be obtained o t * t from Appendix B. In this case, U /U = 16.5. t *t Therefore:

aParticles less than or equal to 10 : m in aerodynamic diameter are defined as the respirable fraction. Refer to discussions in RAEPE (U.S. EPA 1985a), AP-42 (U.S. EPA 1985b), and the “Draft Superfund Exposure Assessment Manual” (U.S. EPA 1986b).
bStability class is a meteorological classification of atmospheric properties as those properties relate to dispersion of airborne materials. The classes range from A, extremely unstable, to F, moderately stable. The coefficients defined by the stability class are used in Equation 2-9 to calculate the downwind dispersion. 2-21RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

(2-10)

Using the values identified above, Xis calculated as shown in Equation 2-11.

(2-11)

Given the value of Xcomputed using Equation 2-11, an F(X) value of 1.33 is obtained from the graph provided in RAEPE (U.S. EPA 1985a) (see Appendix B, Figure B-6 of this DTSD). The landfill area is determined using an equation developed for the U.S. EPA OSW Hazardous Waste Delisting Program (U.S. EPA 1991b). Waste volume is correlated with disposal unit area by means of a regression equation developed from national data on disposal unit dimensions obtained in the OPPI survey. A method to convert waste volume to disposal unit area was developed for both landfills and surface impoundments (U.S. EPA 1991b). Equation 2-12 below presents the regression equation to determine the disposal unit area for a one-time delisting of landfill waste or surface impoundment waste for a RCRA delisting.
(2-12)
where: Default A = area of the waste management unit (acres) calculated 3 V = total volume of waste (yd ) delisting petition-specific The area of the waste management unit is based on the volume of waste reported in the petition. U.S. EPA delisting program assumes that a petitioned waste is disposed of in one Subtitle D facility over a 20-year period (U.S. EPA 1988b). Based on maximum annual waste volumes and assuming waste disposal into the same landfill or surface impoundment over a 20 year period, Equation 2-13 below is

2-22RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 used to determine the disposal unit area for multi-year RCRA delistings to a landfill or surface impoundment.

where: Default A = area of the waste management unit (acres) calculated 3 V = annual volume of waste (yd /year) delisting petition-specific
U.S. EPA delisting program believes that, at most, a month’s (30 days’) worth of waste would be uncovered at any one time. Therefore, the fraction of the area in the landfill that would be exposed and available as a source for particulate emissions ( F exposed ) is calculated as shown in Equation 2-14. (2-13)

(2-14)

This fraction exposed is used to compute the exposed area (A exposed ) and the site width as shown in Equation 2-15. (2-15)

where:
A exposed F exposed A = = = area of the waste management unit exposed (m ) 2 fraction of the area exposed (unitless) area of the waste management unit (acres) Default calculated Equation 2-14 Equation 2-12 (one-time) or 2­ 13 (multi-year) Although particles greater than 10 ì m in size generally are not considered respirable, U.S. EPA Region 6 also calculated the emission rate for particle sizes up to 30 ì m in order to assess the potential impact of deposition and ingestion of such particles (see Section 2.3.1.3). U.S. EPA Region 6 used the distributions of wind-eroded particles presented in AP-42 (U.S. EPA 1985b) to estimate an

2-23RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 emission rate for particles up to 30 ì m in size . c Specifically, these distributions indicate that the release rate for particles up to 30 ì m in size should be approximately twice the release rate calculated for particles 10 ì m in size. Equation 2-16 shows this relationship. (2-16) where: E w30 E w10 =

= wind erosion emission rate for particles up to 30 ì m (g/hr) wind erosion emission rate for particles up to 10 ì m (g/hr) Default calculated Equation 2-9

Vehicle Emissions
U.S. EPA Region 6 used Equation 2-17 as described in AP-42 (U.S. EPA 1985b) to calculate vehicle emissions:

(2-17)

where: Default E v = vehicle emissions (g/hr) calculated k = constant; 0.36 for particles up to 10 ì m and (U.S. EPA 1986b) 0.8 for particles up to 30 ì m s = percent silt content of waste (unitless) 8 (U.S. EPA 1986b) S = mean vehicle speed (kilometers per hours [km/hr]) 24 (U.S. EPA 1986b) W = mean vehicle weight (tons) 15 (U.S. EPA 1986b) w = mean number of wheels per vehicle 6 for W=15 tons (U.S. EPA 1986b) N

p = number of days per year with at least 0.01 inch of 90 (U.S. EPA 1986b) precipitation (days/year) VKT = vehicle kilometers traveled = kilometers/trip x the Equation 2-18 calculated number of round trips made per day (km-trips/day) Conservatively assuming that a minimum of 7.4 vehicles traverse the waste daily, based on the 95 th percentile of vehicles per day reported in Table 3A-1 of the 1987 U.S. EPA survey of industrial Subtitle

c Particles less than or equal to 30 ì m in diameter can be transported for considerable distances downwind; those larger than 30 ì m are likely to settle within a few hundred feet. See AP-42 (U.S. EPA. 1985b and the “Draft Superfund Exposure Assessment Manual” (U.S. EPA 1986b). 2-24RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 D waste facilities in the United States (Westat 1987 and U.S. EPA 1997h), and adding the number of daily trips based on the petitioned waste volume, VKT can be calculated using Equation 2-18.
(2-18)

minimum round trips per day where: Default VKT = vehicle kilometers traveled (km-trips/day) calculated Site Width = (m) Equation 2-15 TPD min = 7.4 (95 th percentile from Subtitle D Survey) ñ w = waste density (tons per cubic yard) 1.42 (U.S. EPA 1993a) 3 V = annual volume of waste (yd /year) delisting petition-specific Vehicle emissions, E and E , can then be calculated using Equation 2-17 as shown in Equations 2-19 v10 v30 and 2-20.

(2-19)

and

(2-20)

U.S. EPA assumes a maximum total annual generation rate for the petitioned waste to estimate a daily disposal amount in tons per day. Therefore, assuming that 15-ton vehicles are used, the number of trips can be determined. U.S. EPA Region 6 realized that particulate emissions from vehicles are related to the size and weight of the vehicles used for waste transport as well as the number of vehicle trips made. The assumption of small vehicle use (with more trips) is conservative.

2-25RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Waste Loading and Unloading Emissions
U.S. EPA Region 6 used Equation 2-21, which is based on an AP-42 (U.S. EPA 1985b) methodology, to calculate emissions from waste loading and unloading operations, E l . In Equation 2-21, E lis given in units of kilograms of waste released per metric ton of waste disposed of.

(2-21)

where: Default E l = emissions from waste loading and unloading calculated operations (kg/metric ton) K

p = batch drop particle size multiplier (dimensionless): U.S. EPA 1986b

S 0.36 for particles up to 10 ì m
S 0.73 for particles up to 30 ì m s = silt content of waste (percent) 8 (U.S. EPA 1986b) U = mean annual wind speed (m/s) 4 (U.S. EPA 1986b) DH = drop height of material from truck (m) 2 (U.S. EPA 1986b) M = moisture content of waste (percent) 1 (U.S. EPA 1986b) 3 Yd = dumping device capacity (m ) 10 (U.S. EPA 1986b) Based on the petitioned waste volume, emissions from waste loading and unloading operations are calculated for particles up to 10 and 30 ì m in size as shown in Equations 2-22 and 2-23, respectively.

(2-22)

where: Default = waste loading and unloading emission rate of calculated particles up to 10 ì m (kg/metric ton)

2-26RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

(2-23)

where: Default = waste loading and unloading emission rate of calculated particles up to 30 ì m (kg/metric ton) To convert the emission rate from kilogram per metric ton to the required grams per hour, U.S. EPA Region 6 assumes that the waste has an average density of 1.686 g/cm 3 (U.S. EPA 1993a), which is equal to 1.42 tons per cubic yard (or 1.289 metric tons per cubic yard). Using the petitioned waste volume (V) in cubic yards per year and the waste density, the Agency determines the emission rate in grams per hour as follows:

Total Respirable Particulate Emissions
U.S. EPA Region 6 calculated the total annual average emissions of respirable particles ( E T10 ) by summing E for wind erosion, E for vehicle travel, and E for waste loading and unloading w10 v10 l10 operations. Equations 2-7 and 2-8 were then combined to produce Equation 2-24.
( 2-24)
where: Default Q p10 = emission rate of waste constituent particles calculated up to 10 ì m (mg/s) á = mass fraction of waste constituent (unitless) waste-specific (equal to total concentration in waste in mg/mg) E w10 = wind erosion emission rate of particles up Equation 2-9 to 10 ì m (g/hr) E v10 = vehicle travel emission rate of particles Equation 2-19 up to 10 ì m (g/hr) 2-27RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 E l10 = waste loading and unloading emission rate Equation 2-21 and 2-22 of particles up to 10 ì m (g/hr)
2.3.1.2 Calculation of Particulate Constituent Concentration in Air at the POE
U.S. EPA Region 6 used the AADM, modified for a landfill source (U.S. EPA 1985c), to calculate the downwind concentrations of constituents released from a land disposal facility. The model assumes that (1) the emission rate is constant over time, (2) the emissions arise from an upwind virtual point source with emissions occurring at ground level, and (3) that no atmospheric destruction or decay occurs. U.S. EPA Region 6 calculated the downwind concentration of a constituent at the POE ( C avg ) using Equation 2-25.
(2-25)
where: Default C avg = downwind concentration of waste constituent at POE calculated 3 (mg/m ) Q p = emission rate of waste constituent particles (mg/s) Equation 2-24 Ó

z = vertical dispersion coefficient (m) Equation 2-26 U = mean annual wind speed (m/s) 4 (U.S. EPA 1986b) L v = distance from the virtual point to the compliance point Equation 2-27 located 1,000 feet (304.8 m) downwind (m) F = frequency that wind blows from the sector of 0.15 interest (unitless) The Pasquill-Gifford vertical dispersion coefficient, Ó z , can be calculated using Equation 2-26 as described in the “Industrial Source Complex (ISC) Dispersion Model User’s Guide” (U.S. EPA 1986a). (2-26) where: Default
= Ó

z vertical dispersion coefficient (m) calculated a = coefficient for stability class D 32.093 (from Appendix B, Table B-5) L = distance from center of the uncovered waste (site width)/2 + 0.3048 km area to compliance point 1,000 feet downwind (km) (U.S. EPA 1986b) b = coefficient for stability class D 0.81066 (from Appendix B, Table B-5)
U.S. EPA Region 6 uses Equation 2-27 to calculate the distance from the virtual point to the compliance point 1,000 feet (304.8) downwind.
2-28RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008
(2-27) where: Default

v L = distance from the virtual point to the compliance calculated point located 1,000 feet downwind (km) L = distance from center of the uncovered waste area to (U.S. EPA 1986b) the (site width)/2 + 0.3048 km compliance point 1,000 feet (304.8 m) downwind (km) L' = virtual distance (the distance necessary to convert Equation 2-28 from an ideal point source to a volume source) (km) The virtual distance is calculated as the distance required for the transverse standard deviation of the Gaussian plume, ó y , to grow to half the width of the site. This distance will be different for each stability class and is calculated for Pasquill Stability Category D using Equation 2-28. (2-28)
where: Default L' = virtual distance (km) calculated ) 0.5 x = 1⁄2 the width of the area exposed (m) 0.5*(A exposed p = Pasquill Stability Category D coefficient (unitless) 68.26 (from Appendix B, Table B-6) q = Pasquill Stability Category D coefficient (unitless) 0.919 (from Appendix B, Table B-6) As stated previously, U.S. EPA Region 6 used a value of 4 m/s for wind speed ( U ). Based on information in the Climatic Atlas of the United States (Visher 1954), U.S. EPA Region 6 assumed that the average frequency ( F ) that the wind blows from various directions for many U.S. cities is 0.15. Using these values and the calculated values for Ó zand L vas inputs, the average downwind concentration of emissions can be calculated using Equation 2-25. The concentration of each hazardous constituent actually inhaled can be calculated using Equation 2-29. 3 U.S. EPA Region 6 assumed that a moderately active person breathes 20 m of air per day based on information in U.S. EPA’s “Exposure Factors Handbook” (U.S. EPA 1997b). (2-29)

2-29RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

where: Default C inh = concentration of waste constituents inhaled (mg/day) calculated 3 C avg = downwind conc. of waste constituent at POE (mg/m )	 Equation 2-25 3 20 = Adult inhalation rate (m /day) (US EPA, 1997b)
2.3.1.3 Calculation of Air Deposition Rates and Resulting Soil Concentrations at the POE
U.S. EPA Region 6 used the steps described below to investigate air deposition of the annual total emissions of particles less than or equal to 30 ì m in size (that is, total suspended particles [ E T30 ]) to soil 1,000 feet from the edge of a disposal unit. First, U.S. EPA Region 6 summed E for wind erosion, E w30 v30 for vehicle travel, and E l30 for waste loading and unloading operations (calculated in Equations 2-16, 2­ 20, and 2-23, respectively) to arrive at the total emission rate of particles up to 30 ì m in size ( Q p30 ) as shown in Equation 2-30.
(2-30)
where: Default Q p30 = emission rate of waste constituent particles calculated up to 30 ì m (mg/s) á = mass fraction of waste constituent (unitless)	 waste-specific (equal to total concentration in waste in mg/mg) E w30 = wind erosion emission rate of particles up to Equation 2-16 30 ì m (g/hr) E v30 = vehicle travel emission rate of particles up to Equation 2-20 30 ì m (g/hr) E l30 = waste loading and unloading emission rate of Equation 2-23 particles up to 30 ì m (g/hr)
U.S. EPA Region 6 calculated the average downwind particulate concentration in the air 1,000 feet from the disposal unit boundary using the AADM, as described previously (see Equation 2-25). U.S. EPA Region 6 then calculated the flux of particles hitting the ground at the downwind point using Equation 2­ 31. (2-31)

where:
2-30RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Default 2 q d = rate of deposition (mg/m /s) calculated v d = deposition velocity (m/s) 0.03 (U.S. Department of Energy [U.S. DOE] 1984) C avg = downwind concentration of waste constituent at POE Equation 2-25 3 (mg/m ) The effective deposition velocity is a function of friction velocity, surface roughness height, particle density, and particle size (U.S. DOE 1984). U.S. EPA Region 6 estimated a value of 0.03 m/s for the effective deposition velocity for all particles less than or equal to 30 ì m in size as described below. The ranges of values for friction velocity and surface roughness height can be obtained from Figure B-4 in Appendix B of this document. To be conservative, U.S. EPA Region 6 assumed that the terrain on which particles are deposited is a field of grass with blades up to about 5 cm high (a surface not susceptible to erosion but with a capacity for trapping particles) and assumed a wind speed of 4 m/s. These assumptions led to a roughness height of 2.0 cm and a friction velocity of 50 cm/s (see Figure B-3 in Appendix B). A particle density of 2.61 g/cm 3 (an average particle density for clay particles) was assumed for the petitioned waste (U.S. EPA 1994a). Figure B-6 in Appendix B presents curves of deposition velocity as a function of particle size for several particle densities and roughness heights. U.S. EPA Region 6 used these curves to obtain deposition velocities for particle sizes of 1, 10, and 30 ì m (0.001, 0.018, and 0.07 m/s, respectively). To obtain an effective deposition velocity for particles less than or equal to 30 ì m in size, U.S. EPA Region 6 calculated the mean velocities for the two particle size ranges, 1 to 10 ì m (0.0095 m/s) and 10 to 30 ì m (0.044 m/s), and then used the average of these two mean velocities to derive an effective deposition velocity of 0.03 m/s for use in Equation 2-31. Finally, U.S. EPA Region 6 calculated the resulting soil concentration ( C soil ) after 1 year of accumulation. U.S. EPA Region 6 conservatively assumed no constituent removal for 1 yr (that is, no leaching, volatilization, soil erosion, or degradation). To calculate C soil , U.S. EPA Region 6 used Equation 2-32.
(2-32)

where: Default C soil = concentration of constituent in soil at calculated the POE (mg/kg) 2-31RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

2 q d = rate of deposition in mg/m /s Equation 2-31 3 ñ b = soil bulk density (kg/m ) 1450 (Brady 1984) t = soil thickness from which particles can be ingested 0.01 (U.S. EPA 1994a) (m)
U.S. EPA Region 6 selected a value of 1450 kg/m 3 for soil bulk density; this value is the midpoint of the 3 range for soil bulk density (1200 to 1700 kg/m ) cited in The Nature and Properties of Soils (Brady 1984). U.S. EPA Region 6 also selected a value of 1 cm as the thickness of the soil surface that will be available for mixing and ingestion. U.S. EPA Region 6 believes that a value of 1 cm is reasonable, given the assumption that no constituent removal would occur for 1 yr (U.S. EPA 1994a).
2.3.2 Calculation of Chemical Concentration in Air at the POE-Volatiles Petitioned wastes potentially contain volatile organic compounds (VOC). Therefore, U.S. EPA Region 6 evaluates the potential threat to human health resulting from atmospheric transport and inhalation of volatile constituents from a petitioned waste. U.S. EPA Region 6 evaluates the potential influence of volatiles in petitioned waste on air quality for two source terms: landfills and surface impoundments. U.S. EPA Region 6 derives an annual waste generation rate and estimated emissions from landfills using Farmer’s equation (Farmer and others 1978). Estimates of emissions of VOCs from disposal of wastewaters in surface impoundments are computed with U.S. EPA’s Surface Impoundment Modeling System (U.S. EPA 1990a, 1990b).The emission rates derived for the two disposal scenarios are entered in U.S. EPA’s AADM, a steadystate, Gaussian plume dispersion model, to predict the concentrations of constituents 1,000 feet downwind of a hypothetical disposal facility. For a complete description and discussion of the AADM, refer to (U.S. EPA 1985c).
2.3.2.1 Calculation of Volatile Emissions from a Landfill Using Modified Farmer's Equation Shen’s modification of Farmer’s equation, which was developed by U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS), is used to estimate the rate of emission of volatiles from a covered landfill (U.S. EPA 1984). This equation provides the rate of volatile emission instead of the flux rate by multiplying by the landfill surface area. U.S. EPA Region 6 determined that Farmer’s equation would provide a reliable estimate of volatile emissions from a landfill. The rate of emission from the landfill is calculated using Equation 2-33.

2-32RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

(2-33)

where: Default E i = emission rate of chemical I (g/sec) calculated 2 D a = diffusion coefficient of constituent in air (cm /sec) chem-specific (Appendix A-1) 4 2 2 1/1x10 = conversion factor for diffusivity from cm to m 2 A exposed = surface area (m ) Equation 2-15 3 C si = saturation vapor concentration of I in landfill (g/m ) Equation 2-35 L = depth of soil cover (m) 0.1524 (U.S. EPA 1994a) P a = air-filled sand porosity (dimensionless) 0.40 (U.S. EPA 1994a) P T = total sand porosity (dimensionless) 0.40 (U.S. EPA 1994a) W /W = weight fraction of waste constituent I (mg/Kg) waste-specific

i 1kg/1000g= weight fraction conversion factor for (g/g) to (mg/Kg) 1g/1000mg=weight fraction conversion factor for (g/g) to (mg/Kg) It may be assumed that (1) the total porosity of dry sand is about 40 percent ( P T = 0.40) (U.S. EPA 1994a), (2) the percentage of air-filled pore space in the sand above the landfill is 40 percent ( P a= 0.40) (U.S. EPA 1994a), and the concentration of the constituent at the surface is negligible. Therefore, Equation 2-33 can be simplified to:

(2-34)
A conservative assumption is made that the waste constituent I is in a state of pure component at saturation vapor concentration. The saturation vapor concentration of a waste constituent ( C si ) in the landfill can be calculated as shown in Equation 2-35.
(2-35) where: Default 3 C si = saturation vapor concentration of I in landfill (g/m ) calculated p i = vapor pressure of waste constituent I (atm) chem-specific (Appendix A-1) MW i = mole weight of waste constituentI (g/mole) chem-specific (Appendix A-1) 3

- 5 R = molar gas constant (atm-m /mole- E K) 8.21 x 10 T = standard temperature ( E K) 298 K (U.S. EPA 1994a)

2-33RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.3.2.2 Calculation of Downwind Volatile Waste Constituent Concentration in Air at the POE—Landfill A landfill system may include a cover above the waste to limit volatile emissions. Therefore, U.S. EPA Region 6 conservatively assumed that a minimum of a 6-inch daily cover was present. U.S. EPA Region 6 used the area of the landfill to calculate atmospheric emissions and transport because the entire surface area will form a source of volatile waste constituents.
U.S. EPA Region 6 used the AADM, modified to estimate emissions from a landfill (U.S. EPA 1994a), to calculate the downwind concentrations of volatile constituents released from the landfill. The model assumes that (1) the emission rate is constant over time, (2) the emissions arise from an upwind virtual point source with emissions occurring at ground level and (3) no atmospheric destruction or decay of the constituent occurs. U.S. EPA Region 6 calculated the average downwind concentration of a constituent as shown in Equation 2-25. The volatile emission rate, Q , is calculated using Equation 2-36. v

(2-36)

where: Default

v Q = volatile emission rate (mg/sec) calculated E i = chemical flux (g/sec) Equation 2-34 The emission rate is then used to compute the downwind concentration ( C avg ) by employing the AADM (Equation 2-25). To calculate the POE and the mass inhaled ( C inh ), U.S. EPA Region 6 assumes that a 3 moderately active person breathes 20 m of air per day (U.S. EPA 1997b). The mass inhaled ( C inh ) is calculated as shown in Equation 2-37.
(2-37)
where: Default C inh = mass of waste constituent inhaled (mg/day) calculated 3 C avg = downwind conc. of waste constituent at POE (mg/m ) Equation 2-25 3 20 = Adult inhalation rate (m /day) (US EPA, 1997b)
2-34RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

C inh is computed for each contaminant in the petitioned waste and is then compared to the inhalation levels of concern.
2.3.2.3 Calculation of Volatile Emissions from a Hypothetical Surface Impoundment
U.S. EPA Region 6 uses the Surface Impoundment Modeling System (SIMS) (U.S. EPA 1990a, 1990b) to calculate emissions from a hypothetical disposal impoundment. SIMS was developed by U.S. EPA’s OAQPS and is available for downloading along with its documentation from the OAQPS web site at “www.epa.gov.oar.oaqps”. The equations contained in SIMS to estimate emissions are presented in this section. Surface impoundments can be divided into two general categories: treatment and disposal impoundments. In treatment impoundments, wastewater containing particles, biochemical oxygen demand (BOD), or photodegradable constituents is introduced into a lagoon. The particles settle to the bottom of the unit, and a combination of biological, photochemical, and volatilization mechanisms causes destruction or removal of dissolved contaminants. An impoundment may be artificially aerated to speed up these processes. The partially treated and clarified wastewater is then drawn off for further treatment and is discharged via a permitted outfall (such as an [NPDES] outfall). The settled sludge may be dredged and landfilled separately. In disposal impoundments, the wastewater does not flow through the unit; instead, the impoundment is sized such that all the water evaporates or infiltrates to groundwater. Disposal impoundments are defined as units that receive wastewater for ultimate disposal rather than for storage or treatment. Generally, wastewater is not continuously fed to or discharged from these types of impoundments. Therefore, the assumption of an equilibrium bulk concentration, which is applicable for flow-through impoundments, is not applicable for disposal impoundments; the concentrations of VOCs in a disposal impoundment decrease with time. The emission estimating procedure accounts for the decreasing liquid-phase concentrations the driving force for air emissions. For a disposal impoundment that contains no biomass, the biomass concentration equals zero, and no biodegradation of pollutants occurs in the impoundment. Further information can be found in the “Background Document for the Surface Impoundment Modeling System (SIMS) Version 2.0" (U.S. EPA 1990b). U.S. EPA Region 6 runs the SIMS model for each chemical until the fraction emitted to air reaches 1.0 percent, indicating that a significant fraction of the
2-35RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 waste constituent has left the unit. For the lighter nonpolar compounds, this generally occurs after approximately 11 days. U.S. EPA Region 6 computes the average rate of emission from the impoundment by dividing the total mass of each contaminant by the number of days required for all of the contaminant to volatilize. The total rate of volatile emissions from the surface impoundment Q v is computed using Equation 2-38.

(2-38)

where: Default Q v = total emission rate of volatiles (mg/sec) calculated 3 C o = initial surface impoundment concentration (gm/m ) waste-specific t f = time for constituent concentration to reach 1 Equation 2-39 percent of C o (days) K = overall mass transfer coefficient (m/s) Equation 2-41 A si = area of the surface impoundment (acres) Equation 2-12 or 2-13 t r =	 retention time for liquid in a surface impoundment Equation 2-40 (days) 3 V

si = volume of liquid in the surface impoundment (m ) delisting petition-specific 4046.8 = conversion of acres to square meters 86,400 = conversion of days to seconds 1000 = conversion of grams to milligrams The time for the constituent concentration to reach 1 percent of C ois computed with Equation 2-39.
(2-39 )

where: Default t = calculated f time for constituent concentration to reach 1 percent of C o (days) 3 V

si = volume of liquid in the surface impoundment (m ) waste-specific K = overall mass transfer coefficient (m/s) Equation 2-41 2 A si = area of the surface impoundment (m ) waste-specific The retention time for liquid in a surface impoundment ( t r ) is computed with Equation 2-40. 2-36RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

(2-40)

where: Default

r t = retention time for liquid in surface impoundment calculated (days) V

si = volume of liquid in the surface impoundment (m ) 3 waste-specific (user-provided) Qf = flow rate of liquid into surface impoundment (m /yr) waste-specific (user-provided) 3 The overall mass transfer coefficient is calculated using Equation 2-41.
(2-41 )

where: Default K = overall mass transfer coefficient (m/s) calculated K l = liquid side mass transfer coefficient (m/s) Equation 2-42 K eq = equilibrium constant (unitless) Equation 2-43 K g = gas mass phase transfer coefficient (m/s) Equation 2-44

K lis calculated using Equation 2-42.
(2-42)

where: Default K l = liquid side mass transfer coefficient (m/s) calculated U 10 = wind speed at 10 m (m/s) 5.73 (U.S. EPA 1994a) 2 D w = diffusion coefficient in water (cm /s) chemical-specific 2

- 6 D ether = diffusion coefficient of ether (cm /s) 8.56 x 10 (U.S. EPA 1994a)

K eg , also called the dimensionless Henry’s Law Constant (H’), is calculated using Equation 2-43. (2-43 )

2-37RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 where: Default K eq = equilibrium constant (unitless) calculated 3 H = Henry’s Law constant (atm-m /g-mol) chemical-specific 3

- 5 R = universal gas law constant (atm-m /g-mol K) 8.21 x 10 T = standard temperature (K) 298 K is calculated using Equation 2-44.

g
(2-44 )

where: Default K g = gas mass phase transfer coefficient (m/s) calculated U 10 = wind speed at 10 m (m/s) 5.73 (U.S. EPA 1994a) d e = effective diameter of surface impoundment (m) Equation 2-45 S cg = Schmidt number on gas side (unitless) Equation 2-46 The effective diameter of the surface impoundment is a function of the size of the surface impoundment, and therefore, is directly related to the to the volume of liquid waste petitioned for the delisting ( V si ). The effective diameter is calculated in Equation 2-45.
(2-45)

where: Default d e = effective diameter of surface impoundment (m) calculated A si = area of the surface impoundment (acres) Equation 2-12 or 2-13 4,046.8 = conversion from acres to square meters

S cg is calculated using Equation 2-46.
(2-46 )

where:
2-38RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 Default S cg = Schmidt number on gas side (unitless) calculated μ a = viscosity of air (gm/cm-s) 0.000181 (U.S. EPA 1990b) 3

- 3 ñ a = density of air (g/cm ) 1.2 x 10 (U.S. EPA 1990b) 2 D a = diffusivity of constituent in air (cm /s) chem-specific (Appendix A-1) Average emission rates (in gm/hr) are computed using this methodology and are then used as input for the atmospheric dispersion modeling analysis described below.
2.3.2.4 Calculation of Downwind Waste Constituent Concentration in Air at the POE Surface Impoundment Calculation of the downwind waste constituent concentration at the POE depends on the assumed size of the disposal unit. The assumptions necessary to determine the size of the unit depend on whether the unit is considered to be a covered landfill or a surface impoundment. U.S. EPA Region 6 calculates downwind concentrations separately for each configuration of the waste management unit. U.S. EPA Region 6 uses Equation 2-25 in Section 2.3.1.2 to calculate downwind concentrations for a landfill. Section 2.3.2.4 presents U.S. EPA Region 6’s calculation of downwind waste constituent concentrations at the POE for a hypothetical surface impoundment.
U.S. EPA Region 6 uses the AADM (U.S. EPA 1985c) to calculate the downwind concentrations of constituents released from a surface impoundment. The model assumes that (1) the emission rate is constant over time, (2) the emissions arise from an upwind virtual point source with emissions occurring at ground level, and (3) no atmospheric destruction or decay of waste constituents occurs. U.S. EPA Region 6 calculates the average downwind concentration of a constituent as shown in Equation 2-25. The downwind concentrations of emissions at the POE for a hypothetical surface impoundment (C avg ) can be calculated as shown in Equation 2-47.

(2-47) where:

avg C

v Q

z 3 = = = downwind conc. of waste constituent at POE (mg/m ) 3 total emission rate of volatiles (mg/s) vertical dispersion coefficient (m) Default calculated Equation 2-38 Equation 2-26
2-39RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 U = mean annual wind speed (m/s) 4 (U.S. EPA 1986b) L v = distance from the virtual point to the compliance point Equation 2-27 located 1,000 feet (304.8 m) downwind (m) F = frequency that wind blows from the sector of interest 0.15 (U.S. EPA 1986b) (unitless) The mass of a constituent inhaled (C inh ) downwind from a hypothetical surface impoundment is computed with the equation that is used to compute inhaled concentrations of volatiles emitted from landfills (Equation 2-37).
2.3.3 Calculation of Waste Constituent Concentration in Surface Water Exposure through the surface water pathway results from erosion of hazardous materials from the surface of a solid waste landfill and transport of these constituents to nearby surface water bodies. U.S. EPARegion 6 uses the universal soil loss equation (USLE) (Wischmeier and Smith 1978) to compute longterm soil and waste erosion from a landfill in which delisted waste has been disposed. The USLE is used to calculate the amount of waste that will be eroded from the landfill. In addition, the size of the landfill is computed using the waste volume estimate provided by the petitioner and the preprocessing calculation for the EPACML (U.S. EPA 1991b). The volume of surface water into which runoff occurs is determined by estimating the expected size of the stream into which the soil is likely to erode (Keup 1985). The amount of soil delivered to surface water is calculated using a sediment delivery ratio (Mills and others 1982). Finally, a portion of the solid phase that is transported to a surface water body is assumed to be dissolved in surface water column. The dissolved fraction of the waste constituent in the water column is determined by a partitioning equation (U.S. EPA 1998b). U.S. EPA Region 6 uses conservative values for all variables likely to influence the potential for soil erosion and subsequent discharge to surface water. By using conservative values, U.S. EPA Region 6 is providing reasonably conservative estimates of the concentrations of waste constituents in surface water.
U.S. EPA Region 6 calculates erosion and discharge of contaminants to surface water using the USLE (Wischmeier and Smith 1978). The USLE is used to calculate the annual amount of soil and waste eroded, as shown in Equation 2-48. (2-48) where: Default A eroded = soil and waste eroded (tons/acre/yr) calculated
2-40RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 RF = rainfall erosion factor (1/yr) 300 (Wischmeier and Smith 1978) K ef = soil erodibility (tons/acre) 0.3 (Wischmeier and Smith 1978) LS = slope length, or topographic factor petition-specific; see Appendix B, Table (dimensionless) B-2 (Wischmeier and Smith 1978) CM = cover & management factor (dimensionless) 1 (Wischmeier and Smith 1978) P = support practice factor (dimensionless) 1 (Wischmeier and Smith 1978) Rainfall erosion factor (RF) values range from 20 to 550 per year. A value of 300 was chosen as a conservative estimate (Wischmeier and Smith 1978). The distribution of rainfall erosion factor values for the United States is given in Figure B-1 Appendix B (Wischmeier and Smith 1978). Values greater than 300 occur in only a small portion of the southeastern United States. By selecting a value of 300 for its analysis, U.S. EPA Region 6 ensures that a reasonable worst-case scenario is provided for most possible landfill locations within the United States. Soil erodibility (K ) factors range from 0.1 to 0.4 ton per acre (see Table B-1 in Appendix B). A value of ef 0.3 was selected for the analysis; it corresponds to the clay loam, clay, and silty clay loam soil types (Wischmeier and Smith 1978). U.S. EPA Region 6 believes that these soil types represent a reasonable worst-case approximation of the types of soil material present at a landfill. Topographic factor (LS) values range from 0.06 to 12.9 (see Table B-2 in Appendix B) and account for the influence of slope length and steepness on erosion potential (Wischmeier and Smith 1978). The value is a function of the slope and slope length (see the equation at the bottom of Table B-2 in Appendix B). This parameter is partly petition-specific in that the slope length is calculated from the area of the landfill. The slope, however, is not petition-specific and is conservatively assumed to be 5%. Cover and management factors (CM) range from 0.4 to 1.0 (see Table B-3 in Appendix B). A value of 1.0 reflects dedicated disposal practices at a facility. Support practice factor (P) values range from 0.25 to 1.0 (see Table B-4 in Appendix B). This factor reflects the influence of conservation practices on erosion potential. If conservation practices are used (for example, contouring or terracing), the potential for erosion is lower. A support practice factor value of 1.0 means that no support practice is used. This value was therefore chosen as the most conservative value for erosion potential.

2-41RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.3.3.1 Computing the Amount of Soil Delivered to Surface Water
U.S. EPA Region 6 computes the percentage of eroded material that is delivered to surface water (the sediment delivery ratio, S d ) based on the assumption that some eroded material will be redeposited between the landfill and the surface water body. U.S. EPA Region 6 assumes a distance (D) of 100 m to the nearest surface water body d and uses the sediment delivery ratio equation developed by Mills and others (1982) as shown in Equation 2-49.

(2-49)

where: Default S d = sediment delivery ratio (unitless) calculated 0.77 = constant (unitless) assumed (U.S. EPA 1994a) D = 100, distance to stream or river (m) assumed (U.S. EPA 1994a)
U.S. EPA Region 6 multiplies the total annual mass of eroded material by the sediment delivery ratio to

s Using a sediment delivery ratio the determine the mass of soil and waste delivered to surface water (A )
. of 0.28, A s can be calculated as shown in Equation 2-50. (2-50)
where: Default A s = soil and waste mass delivered to surface water (kg/acre/yr) calculated A eroded = amount of soil and waste eroded (tons/acre/yr) Equation 2-48 S d = sediment delivery ratio (unitless) 0.28 (Equation 2-49) The total annual amount of soil and waste eroded from the landfill is then calculated using Equation 2-51.
(2-51)

dRefer to the draft “National Survey of Solid Waste (Municipal) Landfill Facilities” (U.S. EPA 1988b). This report shows that 3.6 percent of the surveyed landfill facilities are located within 1 mile (1,609 m) of a river or stream and that the average distance from these facilities to the closest river or stream is 1,936 feet (586 m). Therefore, the assumption of D = 100 m is conservative. 2-42RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 where: Default A w = rate of soil and waste erosion from landfill calculated (kg/acre/yr) A s = soil and waste mass delivered to surface water Equation 2-50 calculated (kg/acre/yr) F exposed = fraction of area exposed to erosion (unitless) 0.0041 (U.S. EPA 1994a) The fraction of waste exposed to erosion, F exposed , is based on the assumption that 1 month’s worth of waste is uncovered at any one time that is, 30 days/(365 days/yr x 20 yr) = 0.0041. U.S. EPA Region 6’s assumption that 1 month’s worth of waste would be left uncovered at any one time and thus would be readily transportable by surface water runoff is conservative. The minimum criteria for municipal solid waste landfills (MSWLF) set forth in 40 CFR Part 258 require that disposed waste be covered with 6 inches of earthen material at the end of each operating day or at more frequent intervals (40 CFR 258.21). Although a facility might request a temporary waiver of this cover requirement because of extreme seasonal weather conditions, it is highly unlikely that 1 month’s worth of waste would be exposed all the time during the active life of the facility (that is, 20 years).
2.3.3.2 Determining the Volume of Surface Water
U.S. EPA Region 6 selected a representative volume or flux rate of surface water based on stream order, which is a system of taxonomy for streams and rivers. A stream that has no other streams flowing into it is referred to as a first-order stream. Where two first-order streams converge, a second-order stream is created. Where two second-order streams converge, a third-order stream is created. Stream order has proven to be a good predictor of flow parameters, including average length, drainage area, mean flow, width, depth, and velocity (Keup 1985). Data indicate that second-order streams have a flow rate of 9 about 3.7 cubic feet per second (3.3 x 10 L/yr). The second-order stream was selected for analysis as the smallest stream capable of supporting recreational fishing. Fifth-order streams were also chosen for analysis as the smallest streams capable of serving as community water supplies. Fifth-order stream flow is on the order of 380 cubic feet per second (3.4 x 10 11 L/yr) (U.S. EPA 1994a).
2.3.3.3 Computing the Waste Constituent Concentration in Surface Water The waste constituent concentration in a surface water body near a land disposal facility is calculated using Equation 2-52. (2-52)

2-43RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008

where: Default

sw C = concentration of waste constituent in surface water calculated (mg/L) A = area of the waste management unit (acres) Equation 2-12 or 2-13

w A = rate of waste erosion from landfill (kg/acre/yr) Equation 2-51

stream Q = volume of stream (L/yr) (U.S. EPA 1994a) - for 5 order stream th 3.4 x 10 11 - for 2 order stream nd 3.3 x 10 9 C total waste = waste constituent concentration in delisted waste (mg/kg) waste-specific
U.S. EPA Region 6’s assumption that surface water runoff from a land disposal facility will be uncontrolled and will enter an adjacent surface water body also is conservative because 40 CFR 258.26 requires adequate run-on and run-off controls at MSWLFs. In addition, 40 CFR 257.33 and 258.27 prohibit a point source or nonpoint source discharge of pollutants that violates any requirements of theClean Water Act, including NPDES requirements and any requirement of an approved area-wide or statewide water quality management plan.
2.3.4. Calculation of Dissolved Phase Waste Concentration in Surface Water ( C dw )
U.S. EPA OSW (U.S. EPA 1998b) recommends the use of Equation 2-53 to calculate the fraction of the surface water concentration (C ) of a waste constituent that is dissolved in the water column ( C ). sw dw
(2-53)

w here: Default C dw = Dissolved phase water concentration (mg/L) Calculated C sw = Waste concentration in water column (mg/L) Equation 2-52 Kd sw = Suspended sediments/surface water partition chem-specific (Appendix A-1) coefficient (L water/kg suspended sediment) TSS = Total suspended solids concentration (mg/L) Equation 2-54 1 x 10 -6 Units conversion factor (kg/mg) The use of Equation 2-53 to calculate the concentration of the waste constituent dissolved in the water column is consistent with U.S. EPA (1994e) and U.S. EPA (1998b). The total suspended solids
2-44RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 concentration ( TSS ) is derived as a function of the soil and waste mass delivered to the surface water from the landfill and the background suspended solids concentration. The TSS is calculated as follows:
(2-54)

where: Default TSS = total suspended solids concentration (mg/L) calculated A w = rate of soil and waste from the landfill (kg/acre/yr) Equation 2-51 nd 9 Q = volume of stream (L/yr) - for 2 order stream 3.3 x 10 (U.S. EPA 1994)

stream A = area of the waste management unit (acres) Equation 2-12 or 2-13 SS b = background suspended solids concentration (mg/L) 10 ppm (U.S. EPA 1994a) The total waste constituent concentration in the water column ( C sw ) in a second order stream is calculated by using the Equation 2-52. The surface water partition coefficient ( Kd sw ) is discussed below.
2.3.4.1 Partitioning Coefficients for Suspended Sediment-Surface Water ( Kdsw )
Partition coefficients ( Kd ) describe the partitioning of a compound between sorbing material, such as soil, soil pore-water, surface water, suspended solids, and bed sediments. For organic compounds, Kdhas been estimated to be a function of the organic-carbon partition coefficient and the fraction of organic carbon in the partitioning media. For metals, Kdis assumed to be independent of the organic carbon in the partitioning media and, therefore, partitioning is similar in all sorbing media. The soil-water partition coefficient ( Kd s ) describes the partitioning of a compound between soil pore-water and soil particles, and strongly influences the release and movement of a compound into the subsurface soils and underlying aquifer. The suspended sediment-surface water partition coefficient ( Kd sw ) coefficient describes the partitioning of a compound between surface water and suspended solids or sediments. Organics For organics (including PCDDs and PCDFs), soil organic carbon is assumed to be the dominant sorbing component in soils and sediments. Therefore, Kdvalues were calculated using the following fraction organic carbon ( f

OC ) correlation equations from Review Draft Addendum to the Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions (U.S. EPA 1993d):

2-45RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008
(2-55)

U.S. EPA (1993d), from literature searches, states that f OCcould range as follows:

C 0.05 to 0.1 in suspended sediments - for which a mid-range value of f = 0.075 oc,sw generally can be used. Consistent with the Region 6 Combustion Risk Assessment Protocol (U.S. EPA 1998b) guidance document, this DTSD uses mid-range f oc

values recommended by U.S. EPA (1993d). Kd swvalues were calculated using K oc values recommended for each compound in this DTSD. The variables ( Kd sw , TSS) in the Equation 2-53 are site-specific. Therefore, the use of the default values will introduce an under- or overestimation of C dw . The degree of uncertainty associated with TSS is dependent on the suspended solids of the nearest second order stream and how this value compares to the default background suspended solids value (20 mg/L). Uncertainty associated with the variable Kd sw is associated with estimates of the fraction organic content ( f

OC ). Because f OC

values can vary widely for different locations in the same medium, using default f OC

values may result in significant uncertainty in specific cases.
2.3.4.2 Concentration of Waste Constituent in Fish (C fish ) The waste constituent concentration in fish is calculated using either a waste-specific bioconcentration factor ( BCF ) or a waste-specific bioaccumulation factor ( BAF ). For compounds with a log K owless than 4.0, BCF s are used. Compounds with a log K owgreater than 4.0 are assumed to have a high tendency to bioaccumulate, therefore, BAF s are used. Appendix A-1 provides a detailed discussion on the sources of the waste constituent-specific BCF and BAFvalues, and the methodology used to derive them. BCF and BAF values are generally based on dissolved water concentrations. Therefore, when BCF or BAFvalues are used, the waste constituent concentration in fish is calculated using dissolved water concentrations. The equations used to calculate fish concentrations are described in the subsequent subsections.

2-46RCRA Delisting Technical Support Document Chapter 2: Estimation of Chemical Releases and Media Concentrations Revised October 2008 2.3.4.3 Calculation of Fish Concentration ( C fish) from Bioconcentration Factors Using Dissolved Phase Water Concentration
U.S. EPA OSW recommends the use of Equation 2-56 to calculate fish concentration from BCF s using dissolved phase water concentration. The use of this equation is further described in Appendix A-1. (2-56)

where: Default C

fish = Concentration of waste constituent in fish calculated (mg/kg FW tissue) C dw = Dissolved phase water concentration of waste Equation 2-53 constituent (mg/L)
= BCF fish Fish bioconcentration factor of waste constituent (L/kg) chem-specific (Appendix A-1) The dissolved phase water concentration ( C dw ) is calculated by using Equation 2-53 above. Chemical-specific BCF fishvalues are presented in Appendix A-1. The use of Equation 2-56 to calculate fish concentration is consistent with U.S. EPA (1994e) and U.S. EPA (1998b) .
2.3.4.4 Calculation of Fish Concentration (C fish ) from Bioaccumulation Factors Using Dissolved Phase Water Concentration
U.S. EPA OSW recommends the use of Equation 2-57 to calculate fish concentration from BAF s using dissolved phase water concentration.
(2-57)
where: Default C

fish = Concentration of waste constituent in fish calculated (mg/kg FW tissue) C dw = Dissolved phase water conc. of waste constituent (mg/L) Equation 2-53 BAF fish = Fish bioconcentration factor of waste constituent (L/kg) chem-specific (Appendix A-1)
The dissolved phase water concentration ( C dw ) is calculated by using Equation 2-53. Chemical-specific bioaccumulation factor (BAF fish) values are presented in Appendix A-1.

2-47