RCRA Delisting Technical Support Document 

Chapter 4: Risk and Hazard Assessment 

Revised October 2008 

Chapter 4 

Risk and Hazard Assessment 

What’s covered in Chapter 4: 

�	 Individual Risk and Hazard Estimation 

�	 Target Risk Levels 

- Target Risk 

- Target Hazard 

- Delisting Levels 

�	 Quantitative Estimation of Risk 

�	 Quantitative Estimation of Potential Noncancer Effects 

This chapter explains (1) back-calculation of pathway-specific delisting levels from target risk levels and 

subsequent selection of the final delisting level for each constituent in a petitioned waste and (2) aggregate 

risk analysis for one-time delisting petitions. Both determination of delisting levels and aggregate risk 

analysis involve risk assessment, but the calculations proceed in a different direction for the two cases. In 

determining delisting levels, the work begins by setting a target risk level, that reflects the maximum 

acceptable risk. Then, using waste volume and exposure parameters, the delisting level is back-calculated. 

This is done for each constituent and each potential exposure pathway (see Chapter 3), and the resulting 

constituent—and pathway-specific delisting levels are used to set the final delisting level for each constituent. 

It is important to note that even though the calculations proceed from the target risk to the delisting level 

(which is the reverse of the traditional risk assessment approach), the procedure is still based on standard risk 

assessment equations and methods — they are simply applied in reverse order. 

The back-calculation procedure contrasts with the method used to compute the aggregate risk for a one-time 

delisting petition. To determine aggregate risk, the calculations proceed in a forward direction. Beginning 

with the leachate and total waste concentrations for each constituent in the waste (source concentrations), the 

waste volume and exposure parameters are used to estimate the upper-bound excess lifetime cancer risks 

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(risk) and noncarcinogenic hazards (hazard). The risk is said to be aggregate because risks and hazards are 

summed separately for receptors (resident adults and children) across all applicable waste constituents and 

exposure pathways to obtain an estimate of the total individual risk and hazard for each receptor. 

Risk is the probability that a receptor will develop cancer. Risk is estimated based on a unique set of 

exposure, model, and toxicity assumptions. A risk of 1 x 10-6 means that one person out of 1 million persons 

assumed to be exposed under similar conditions could develop cancer as a result of lifetime exposure to one 

or more potential carcinogens. Hazard is defined as the potential for noncarcinogenic health effects as a 

result of exposure to constituents of concern, averaged over an exposure period of less than an entire lifetime. 

A hazard is not a probability but rather a measure (expressed as a ratio) of the magnitude of a receptor’s 

potential exposure relative to a standard exposure level. The standard exposure level is calculated over an 

exposure period such that there is no likelihood of adverse health effects to potential receptors, including 

sensitive populations (U.S. EPA 1989e). 

Risks and hazards typically are characterized for a single receptor and are referred to as individual risks and 

hazards (U.S. EPA 1989a; NC DEHNR 1997). “Risk Assessment Guidance for Superfund (RAGS), Volume 

I: Human Health Evaluation Manual (Part A)” ( U.S. EPA 1989a) states that human health risk assessments 

in the RCRA program have historically focused on high-end individual risk, and it has been recommended 

that risk assessors place their primary emphasis on characterizing high-end individual risks. All available 

guidance on target levels for risks and hazards is developed specifically for estimates of individual risk and 

hazard (see Sections 4.2 and 4.3). Therefore, the risk assessments performed for delisting using the DRAS 

will address only individual risks and hazards. 

4.1 INDIVIDUAL RISK AND HAZARD ESTIMATION 

Aggregate individual risk and hazard estimates are intended to convey information about the risks borne by 

individuals affected by releases from delisted wastes disposed of in Subtitle D nonhazardous waste 

management units. Aggregate risk assessments based on the equations described in this chapter provide 

quantitative risk and hazard estimates associated with potential exposure to constituents in wastes proposed 

for delisting. Both calculation of delisting levels and evaluation of aggregate risk involve use of risk 

equations described in the following sections. The cancer slope factors (CSF) for carcinogens and the oral 

and inhalation RfDs for noncarcinogens used in computing delisting levels and in the aggregate risk 

assessment are provided in Appendix A. Other chemical-specific variables (for example, bioaccumulation 

factors, Henry’s Law constants, and so on) are also provided in Appendix A. 

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4.2 TARGET RISK LEVELS 

Acceptable or target risk and hazard levels originated with the National Oil and Hazardous Substances 

Pollution Contingency Plan (NCP) (U.S. EPA 1990e). More recently, U.S. EPA has modified the target 

levels established in the NCP to reflect the contribution of background levels of contamination (U.S. EPA 

1994f). However, these modified target values are not discrete indicators of observed adverse effects. If a 

calculated risk falls within the target values, U.S. EPA may without further investigation conclude that a 

proposed action (such as delisting a petitioned waste) does not present an unacceptable risk. A calculated risk 

that exceeds these targets, however, would not in and of itself indicate that the proposed action is not safe or 

that it presents an unacceptable risk. Rather, a computed risk that exceeds a target value triggers further 

careful consideration of the underlying scientific basis for the calculation. Target risk and hazard levels are 

addressed in more detail in the following sections. Sections 4.2.1 through 4.2.5 describe target risk, target 

hazard, delisting levels, DRAS aggregate risk, and special limitations to the computation of each of these 

levels. 

4.2.1 Target Risk 

Risk estimates represent the incremental probability that an individual will develop cancer over a lifetime as 

a result of exposure to a carcinogenic chemical (U.S. EPA 1989a). As defined in the NCP, U.S. EPA has 

established the range of generally acceptable risks for known or suspected carcinogens as 1 x 10-6 to 1 x 10-4 

(U.S. EPA 1990e). Risks less than 1 x 10-6 (one additional cancer in 1,000,000 individuals) generally are not 

of concern, whereas risk levels exceeding 1 x 10-4 (one additional cancer in 10,000 individuals) are usually 

considered unacceptable. A risk level of 1 x 10-6 is used as a point of departure; that is, U.S. EPA generally 

must determine whether a waste qualifies for delisting if the petitioned waste’s risks are between 1 x 10-6 and 

1 x 10 -4. 

For calculation of delisting levels for multi year (multiple-batch) delistings, the default target risk in the 

DRAS is set to 1 x 10 -5. At this default target risk, delisting levels for waste constituents represent waste 

concentrations that pose a potential risk of one additional cancer in 100,000 individuals exposed. The 

delisting authority may change the target risk in DRAS to match their program requirements. For a one-time 

delisting, U.S. EPA performs a aggregate risk assessment for the petitioned waste. If the aggregate risk for 

the petitioned waste exceeds 1 x 10 -4, then the waste may not qualify for delisting. The delisting authority 

may also set the requirement for an aggregate risk more conservatively. 

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U. S. EPA Region 6 distinguished between three measures of risk: constituent—and exposure pathwayspecific risk, total exposure pathway risk, and aggregate risk in developing the DRAS. Constituent —and 

pathway-specific risk is calculated as shown in Equation 4-1. 

(4-1) 

where: 

Riskc,p = risk for waste constituent c for specific exposure pathway p 

LADD c,p = lifetime average daily dose for waste constituent c (mg/kg-day) via pathway p 

CSFc = cancer slope factor for waste constituent c (mg/kg-day)-1 

Equation 4-1 is re-arranged to compute LADDc,p from a given target risk level, when used in calculating 

delisting levels. 

Within a specific exposure pathway, an individual may be exposed to more than one chemical. The total risk 

associated with exposure to all chemicals through a single exposure pathway, or the total exposure pathway 

risk, is estimated as shown in Equation 4-2 (U.S. EPA 1989a). 

(4-2) 

where: 

Risk p = total risk from all constituents for a specific exposure pathway p 

Risk c,p = risk for waste constituent c for specific exposure pathway p 

At specific exposure points, individual receptors may be exposed via multiple exposure pathways (see Table 

2-2 in Chapter 2). Two steps are required to determine whether risks from multiple exposure pathways 

should be summed for a given individual receptor (U.S. EPA 1989a). The first step is to identify “reasonable 

exposure pathway combinations.” The second step is to determine whether it is likely that the “same 

individuals would consistently face the reasonable maximum exposure (RME) via more than one pathway.” 

For the purposes of one-time delisting risk assessments, U. S. EPA conservatively assumes that one individual 

would consistently encounter the RME via more than one pathway. Therefore, risks are summed across the 

receptor-exposure pathway combinations identified in Table 2-2. Hence, the aggregate risk from multiple 

exposure pathways is the sum of the total risks for the individual exposure pathways, expressed as shown in 

Equation 4-3. 

(4-3) 

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where: 

Riskcum = Aggregate risk from all constituents and all exposure pathways
 

Riskp = Total risk from all constituents for specific exposure pathway p
 

The equations used to calculate dose and risk levels are presented in Section 4.3. Appendix A presents the 

oral and inhalation CSFs used to compute risks for specific waste constituents. CSFs for constituents not 

identified in Appendix A-1 can be obtained from the following sources (listed in the preferred order): 

1. 	 U . S . E P A ’ s I R I S ( U . S . E P A 1 9 9 6 g ; a c c e s s i b l e o n l i n e a t 

“http://www.epa.gov/ngispgm3/iris/index.html”) 

2. 	 U.S. EPA (HEAST) (U.S. EPA 1997f) 

If relevant information is not available from these sources, the petitioner should, with the approval of the 

appropriate regulatory agency, contact the U.S. EPA National Center for Environmental Assessment (NCEA) 

office in Cincinnati, Ohio. NCEA personnel may be able to assist in developing the necessary toxicity values. 

4.2.2	 Target Hazard 

Standard risk assessment models assume that noncarcinogenic effects, unlike carcinogenic effects, exhibit 

a threshold; that is, they assume that there is a level of exposure below which no adverse effects will be 

observed (U.S. EPA 1989a). The potential for noncarcinogenic health effects resulting from exposure to a 

chemical is assessed by comparing daily exposure estimates to RfDs. An RfD is a daily intake rate that is 

estimated to pose no appreciable risk of adverse health effects, even to sensitive populations, over a specific 

exposure duration. Chronic RfDs are used to characterize noncarcinogenic hazards in order to maintain a 

conservative approach consistent with the purposes of the risk assessment. 

Appendix A-1 presents RfDs for a number of chemicals. RfDs for other chemicals can be obtained from the 

following sources (listed in the preferred order): 

1.	 U.S. EPA’s IRIS (U.S. EPA 1996g; accessible on-line at


 “http://www.epa.gov/ngispgm3/iris/index.html”) 


2. 	 U.S. EPA (HEAST) (U.S. EPA 1997f) 

If relevant information is not available from these sources, the applicant should, with the approval of the 

appropriate regulatory agency, contact the U.S. NCEA office in Cincinnati, Ohio. NCEA personnel may be 

able to assist in developing the necessary toxicity values. 

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The ratio between the daily exposure estimate for a constituent and that constituent’s RfD is known as a 

hazard quotient (HQ). When simultaneous exposure occurs to more than one constituent or through more 

than one pathway, a hazard index (HI), a summation of HQs, is used to express the potential for harm. The 

NCP target hazard for systemic toxicants is an HQ of 1.0 for a single chemical or an HI of 1.0. for multiple 

chemicals or pathways. U.S. EPA typically considers the need for reducing hazards if an HQ or HI exceeds 

1.0 for human receptors who may reasonably be expected to be exposed. U.S. EPA usually requires remedial 

action at locations where HQ or HI values significantly exceed 1.0. 

For a one-time delisting, U.S. EPA Region 6 performs a aggregate risk assessment for the petitioned waste. 

As in risk calculation, a distinction is made between three measures of hazard: constituent—and exposure 

pathway-specific hazard, total exposure pathway hazard, and aggregate hazard. The ultimate result of a 

aggregate risk assessment is an HI rather than an HQ because the assessment includes multiple pathways and 

may include multiple constituents. Consistent with the NCP and U.S. EPA guidance documents, if the HI 

for a waste exceeds 1.0, U.S. EPA considers whether to allow the waste to qualify for delisting. However, 

where HI values significantly exceed 1.0, U. S. EPA usually reviews the target organ hazard contribution, 

establishes specific delisting conditions, or denies the delisting petition. Constituent—and pathway-specific 

hazard is computed as shown in Equation 4-4. 

(4-4) 

where: 

HQc,p = hazard quotient for waste constituent c for exposure pathway p (unitless) 

ADDc,p = average daily dose for waste constituent c for pathway p (mg/kg-day) 

RfDc = reference dose for waste constituent c (mg/kg-day) 

As with carcinogenic constituents in a specific exposure pathway, a receptor may be exposed to multiple 

constituents associated with noncarcinogenic health effects. The total noncarcinogenic hazard for each 

exposure pathway is calculated using the procedures outlined by U.S. EPA (1989a). As in the procedure for 

calculating risks, the constituent—and pathway-specific hazards are summed for each receptor. The sum 

represents the total hazard for a given exposure pathway and is calculated as shown in Equation 4-5. 

(4-5) 

where: 

HIp = total hazard index for all waste constituents for specific exposure pathway p 

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HQc,p = hazard quotient of waste constituent c for specific exposure pathway p 

This summation methodology assumes that the health effects of the various chemicals to which a receptor 

is exposed are additive. 

As discussed in Section 4.2.1 for risks, a receptor may be exposed to chemicals associated with 

noncarcinogenic health effects through more than one exposure pathway. For the purposes of a risk 

assessment, U.S. EPA assumes that it is reasonable to estimate a receptor’s total hazard as the sum of the HIs 

for the exposure pathways identified in Table 2-2. Specifically, a receptor’s aggregate HI is the sum of the 

hazards for the individual exposure pathways, expressed as shown in Equation 4-6. 

(4-6) 

where: 

HIcum = aggregate hazard index for all constituents and all exposure pathways 

HIp = total hazard index for all constituents for specific exposure pathway p 

Generally, when the aggregate HI is assessed, an HIcum value less than or equal to 1.0 is considered to be 

protective of human health, whereas an HIcum value exceeding 1.0 indicates a potential for noncarcinogenic 

health effects (U.S. EPA 1989a). The more the HIcum value exceeds 1.0, the greater the level of concern. 

However, because RfDs do not have equal accuracy or precision, and because they are not based on the same 

severity of effects, the level of concern does not increase linearly as an Hicum value approaches and exceeds 

1.0 (U.S. EPA 1989a). 

The HIcum for a receptor can exceed 1.0 as a result of (1) the presence of one or more chemicals with an HQ 

exceeding 1.0 (HQc,p > 1.0 in Equation 4-5) or (2) summation of several chemical-specific HQs (as in 

Equation 4-5) or pathway-specific HIs (as in Equation 4-6) that are each less than 1.0. In the first case, the 

presence of at least one chemical-specific hazard greater than 1.0 is interpreted as indicating the potential for 

noncarcinogenic health effects. In the second case, a detailed analysis is required to determine whether the 

potential for noncarcinogenic health effects is accurately estimated by HIcum because the toxicological effects 

associated with exposure to multiple chemicals, often through different exposure pathways, may not be 

additive (U.S. EPA 1989a); therefore, HIcum may overestimate the potential for noncarcinogenic health effects. 

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To address this issue, chemical-specific hazards may be summed according to major health effects and target 

organs or systems (U.S. EPA 1989a). Table A-4 in Appendix A identifies target organs and systems that are 

affected by listed waste constituents. It is especially important to consider any differences related to the 

exposure route; this process is referred to as segregation of the HI. If the highest segregated HI exceeds 1.0, 

there is a potential for noncarcinogenic health effects. However, if the highest segregated HI is less than 1.0, 

HIcum (as calculated above) is overly conservative, and noncarcinogenic health effects are not likely to result 

from exposure to waste constituents, even if HI is greater than 1.0. Therefore, when HI for a specific cum cum 

volume of petitioned waste exceeds 1.0, it does not necessarily mean that the waste cannot be delisted; 

calculation of segregated HIs may show that HIcum is too conservative. Calculation of segregated HIs is 

beyond the scope of this DTSD and should be performed by a qualified toxicologist or risk assessor. 

Segregated HIs are not used in calculating delisting levels or aggregate hazards in subsequent sections. 

4.2.3 Delisting Levels 

For all standard multi year delisting petitions, the DRAS calculates delisting levels for each constituent of 

a petitioned waste at the default risk levels (a cancer risk of 1 x 10-5 and a noncancer hazard of 0.1) or the risk 

levels required by the delisting authority. The waste constituent concentrations of each batch of delisted 

waste should not exceed the established delisting levels. In determining the allowable concentrations for each 

constituent of a standard multi year petitioned waste, U.S. EPA Region 6 sets two delisting levels for each 

constituent: (1) a total concentration delisting level and (2) a leachate concentration delisting level. The total 

concentration delisting level is the maximum allowable concentration of a constituent in the whole waste. 

The total concentration of each constituent in the waste should not exceed the total concentration delisting 

level. The leachate concentration delisting level is the maximum allowable concentration of a constituent in 

leachate derived from the waste. The TCLP concentration of each waste constituent should not exceed the 

leachate concentration delisting level. Note that leachate concentrations from leach tests other than the TCLP 

may also be considered. 

The DRAS back-calculates a delisting level for each waste constituent and each exposure pathway that the 

program addresses. The DRAS generates leachate concentration delisting levels for all the groundwater 

exposure pathways (ingestion of groundwater, dermal absorption while bathing with groundwater, and 

inhalation of groundwater volatiles while showering) and for two of the air inhalation exposure pathways 

(inhalation of volatiles from a landfill and inhalation of volatiles from a surface impoundment). The DRAS 

generates total concentration delisting levels for the remaining air inhalation exposure pathway (inhalation 

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of particulates from a landfill), the surface water exposure pathways (ingestion of surface water and ingestion 

of fish), and the soil ingestion exposure pathway. 

To develop the final leachate concentration delisting level for a waste constituent, the DRAS calculates the 

constituent’s pathway-specific leachate concentration delisting levels for the groundwater pathways and the 

two relevant air exposure pathways and then selects the lowest of the pathway-specific values (including 

delisting levels derived from the SDWA MCL) as the final leachate concentration delisting level. Similarly, 

to develop the final total concentration delisting level for a waste constituent, the DRAS computes pathwayspecific values for the relevant pathways and then chooses the lowest as the final total concentration delisting 

level. 

The DRAS is a risk assessment tool that gives the user the flexibility to select alternative target risk levels. 

Delisting levels are determined according to the target risk level selected by the user. The selection of 

delisting levels is a risk management decision and, as such, should consider the selected risk level and other 

pertinent information. 

The delisting levels for each waste constituent are the maximum concentrations (total and TCLP) allowed for 

the constituent in any batch of the petitioned waste, based on the most sensitive pathway associated with 

exposure dependent on the TCLP waste constituent concentration and the waste constituent total 

concentration. Although the DRAS calculates a delisting level (at the target risks) for each of the 

groundwater pathways, only the most sensitive pathway for each constituent is selected as that constituent’s 

TCLP delisting level. The DRAS goes through the same procedure to determine the total delisting level for 

each waste constituent. These values are intended to be incorporated into the FR notice for a multi year 

delisting and are not to be exceeded in any batch sample of the delisted waste. Sections 4.3 and 4.4 present 

the algorithms used in the DRAS to calculate the delisting level concentrations for multi year delistings. If 

the concentration of each constituent in a petitioned waste is less than the total concentration delisting level, 

and if the waste TCLP concentration for each constituent is less than the leachate concentration delisting 

level, that waste may qualify to exit the hazardous waste management program. No single constituent 

concentration in the waste or TCLP concentration covered by a multi year delisting petition should exceed 

its delisting levels. 

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4.2.4 DRAS Aggregate Risk 

In addition to calculating the delisting levels for standard multi year delistings, the DRAS can perform a 

aggregate risk assessment for one-time disposal of waste in a landfill or surface impoundment waste 

management unit. The DRAS requires that the user input the total and TCLP concentrations for each 

constituent of a petitioned waste. The program then computes the aggregate risk and hazard (Riskcum and 

HIcum) for the waste. 

Computing the aggregate risk and hazard for a petitioned waste provides the user with a detailed analysis of 

the risks and hazards associated with the waste. The DRAS indicates which chemicals, pathways, and 

receptors are driving the risk and hazard for the waste. The DRAS computes the aggregate risk by summing 

the risks for all waste constituents for a given exposure pathway and then summing the risks for the pathways 

analyzed in the delisting risk assessment (see Equations 4-1 through 4-3). The DRAS computes the aggregate 

hazard by summing the HQs for all waste constituents for a given exposure pathway to obtain exposure 

pathway-specific HIs and then summing the HIs associated with the exposure pathways analyzed (see 

Equations 4-4 through 4-6). Sections 4.3 and 4.4 detail the equations and default parameters used in the 

DRAS to calculate the aggregate risk and hazard, respectively, for a petitioned waste. 

If a delisting petition is for a one-time exclusion, the results of the aggregate risk assessment may be used in 

lieu of the delisting levels. A one-time delisting does not require U.S. EPA to establish monitoring 

concentrations that must be met by each batch of waste to be managed under a promulgated exclusion. 

Therefore, the user may bypass the delisting levels, which are set at more conservative risk levels, in favor 

-4 -6 of the aggregate risk process that is conducted using U.S. EPA’s target risk range (1 x 10 to 1 x 10 for 

carcinogenic waste constituents and an HI of 1.0 to 0.1 for noncarcinogenic waste constituents). Use of the 

aggregate risk analysis allows the risk associated with an individual waste constituent to extend to a less 

conservative risk level as long as the aggregate risk for the entire petitioned waste lies below or within U.S. 

EPA’s target risk range. 

4.2.5 Special Limitations and Exceptions 

This section summarizes special limitations and exceptions to the methods outlined for calculating delisting 

levels, and risk and hazard. Earlier regulatory work performed using methods similar to those discussed 

herein has shown that a waste concentration ceiling may be needed to ensure that risk from direct exposure 

on site does not achieve unsafe levels, as is discussed in Section 4.2.5.1. Also, as described in Section 

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4.2.5.2, a special, constituent-specific waste concentration ceiling based on soil saturation levels is needed 

to prevent delisting of waste that may give rise to nonaqueous-phase liquids (NAPL) in groundwater. 

Because toxicity factors are not available for lead, its delisting level must be established outside the usual 

methods of the DRAS (See Section 4.2.5.3). Section 4.2.5.4 describes how risk and hazard levels are 

computed for PCDDs and PCDFs, and PCBs are addressed in Section 4.2.5.5. Finally other regulatory limits 

must be considered in the determination of delisting levels, including the Toxicity Characteristic regulatory 

levels (Section 4.2.5.6), maximum contaminant levels (MCLS) as described in Section 4.2.5.7. In addition, 

chemical detection limits are addressed in Section 4.2.5.8 and regulatory levels for aquatic biota are addressed 

in Section 4.2.5.9. 

4.2.5.1 Waste Constituent Concentration Ceiling 

Using standard chronic risk assessment algorithms, the DRAS generates upper-limit individual waste 

constituent source concentrations called delisting levels that are protective of the receptor for the defined 

exposure pathways. However, the delisting levels generated in this manner for certain constituents can be 

extremely high in their absolute concentrations (approaching those of pure compounds). Under certain 

circumstances, the generated delisting levels may not be the appropriate waste management concentration 

limits for the waste management unit. For example, a modeled chronic risk scenario (for example, volatile 

air emissions) cannot be relied upon if the source concentrations exceed certain limits (see Section 4.2.5.2 

soil saturation). Above certain concentrations, the compounds may exist in free phase and exhibit behavior 

different from that assumed by the model (for example., dissolution and transport of immobile hazardous 

constituents). Moreover, were wastes allowed to be disposed of at very high concentrations, acute exposures 

could occur at the waste management unit itself. Some of these high constituent concentrations could interact 

with and damage liner systems. Finally, the proposed HWIR Media Rule states that “EPA believes it is 

reasonable to classify media as highly contaminated if 1 percent of the volume of the media is contaminated 

with a particular constituent” (U.S. EPA 1996b). 

For these reasons, there is a need to assess the reasonableness of high concentration limits developed from 

the chronic risk models, particularly for liquid and volatile organics. The issue is a general policy concern 

for all chronic risk analyses, and its resolution is beyond the scope of the DRAS or this DTSD. However, 

the DRAS has been programmed to identify total waste constituents predicted at nominally high concentration 

levels (10,000 mg/kg or 1 percent by weight or greater). Delisting levels in excess of 10,000 mg/kg are 

highlighted in red on the Results Screen of the DRAS. When the DRAS computes a delisting level that 

exceeds 10,000 mg/kg, the petitioned waste containing constituents concentrations above this value will be 

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reviewed on a case-by-case basis. In the DRAS, the delisting levels (exit values) are not altered to reflect this 

ceiling review. The DRAS provides the user with the calculated delisting level based on the waste constituent 

concentrations, the waste volume, and the risk level selected by the user. 

The HWIR Waste Exemption Rule was proposed (in 60 FR 66344, December 21, 1995) with more than 80 

compounds in the non-wastewater leachate (NWT) category, which had criterion limits that exceeded 10,000 

mg/kg total waste concentration. As the DTSD methodologies are similar to those of the proposed HWIR, 

U.S. EPA Region 6 is aware that the chronic risk algorithms in the DRAS will generate delisting levels that 

exceed 10,000 mg/kg or 1 percent pure compound by weight when the constituent is not particularly toxic 

or does not readily migrate through the environment due to its physical and chemical properties. U.S. EPA 

Region 6 believes that delisting levels greater than 10,000 mg/kg, although they are predicted to be protective 

of chronically exposed resident receptors because they are back-calculated using acceptable risk assessment 

methodology, may pose a risk involving direct exposure on site. It is conceivable that some waste 

constituents for which high delisting levels are generated may exhibit another hazardous waste characteristic 

(such as ignitability) and that the petitioned waste may not be delisted as a result. However, not all waste 

constituents with elevated delisting levels will exhibit another hazardous waste characteristic and be excluded 

from delisting. Therefore, U.S. EPA will closely review any petition that includes a waste constituent 

concentration that exceeds 10,000 mg/kg or 1 percent pure chemical by weight. 

4.2.5.2 Soil Saturation Evaluation 

A soil saturation value represents the contaminant concentration in soil at which the adsorptive limits of the 

soil particles and the solubility limits of the available soil moisture have been reached. Above this 

concentration, pure or free-phase compound is expected in the soil. Three of the input parameters used to 

calculate soil saturation concentrations are constituent-specific: solubility (Sol), organic carbon partition 

coefficient (K oc) and Henry’s Law constant (H'). A chemical-specific soil saturation concentration is therefore 

dependent on the reference value used for each of these parameters and can vary significantly depending on 

the values selected for these three input parameters. Delisting decisions based on the soil saturation 

concentrations reported in Appendix A-3 should be made with caution. 

The derivation of the soil saturation concentration (C sat) is shown in Equation 4-7. 

(4-7) 

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where: 

Default 

Csat = soil saturation concentration (mg/kg) calculated 

Sol = solubility in water (mg/L-water) chem-specific (Appendix A-1) 

ñb = dry soil bulk density (kg/L) 1.5 (U.S. EPA 1996f) 

K = soil-water partition coefficient (L/kg) K @ f (chem-specific) d oc oc 

Koc = normalized distribution coefficient (L/kg) chem-specific (Appendix A-1) 

foc = fraction organic carbon content of soil (g/g) 0.006 or site-specific 

È = water-filled soil porosity (L /L ) 0.15 (U.S. EPA 1996f) w water soil 

È = air filled soil porosity (L /L ) 0.28 or n-È (U.S. EPA 1996f) a air soil w 

n = total soil porosity ( /L ) 0.43 or 1 - (ñ /ñ ) Lore soil b s 

ñs = soil particle density (kg/L) 2.65 3 H = Henry’s Law constant (atm-m /mol) chem-specific (Appendix A-1) 

H' = Henry’s Law constant (dimensionless) H x 41, where 41 is a unit 

conversion factor 

Equation 4-7 takes into account the amount of contaminant that is in the vapor phase in the pore spaces of 

the soil in addition to the amount dissolved in the soil’s pore water and sorbed to soil particles. If the soil 

saturation limit is exceeded, formation of a NAPL plume is possible for the organic constituents. If NAPLs 

are present, significant contaminant migration may occur within this NAPL phase, which is not accounted 

for in the EPACMTP fate and transport model. Additionally, compound solubility assumptions for water may 

be inappropriate if another free-phase liquid or mixture is present. Therefore, EPACMTP modeling results 

may underestimate potential groundwater concentrations at downgradient receptor wells when waste 

constituent concentrations exceed soil saturation values. 

The DRAS compares soil saturation values with calculated delisting levels and notifies the user when a waste 

constituent’s total delisting level exceeds its soil saturation value. When the DRAS calculates a delisting 

level greater than the calculated soil saturation, the delisting manager should consider setting the delisting 

level equal to the soil saturation value in accordance with soil screening guidance (U.S. EPA 1996f). 

4.2.5.3 Estimation of Potential Health Effects for Lead 

Toxicity factors (for example, RfDs or CSFs) are not available for lead. Therefore, consistent with U.S. EPA 

(1994f), the U.S. EPA OSW recommends that the potential for adverse health effects associated with 

exposure to lead be characterized through a direct comparison with medium-specific health-based levels. 

Specifically, the target level for lead in drinking water is 0.015 mg/L. The drinking water target level is based 

on the lead action level established under the U.S. EPA primary drinking water regulations. The U.S. EPA 

3 OSW recommends target levels at the POE for lead in soil and air of 100 mg/kg and 0.2 (μg)/m , respectively.

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The soil target level of 100 mg/kg is based on U.S. EPA guidance (1994g) that indicates that soil lead levels 

less than 400 mg/kg (based on lead concentrations in blood, as discussed below) are not of concern for 

remediation purposes. The U.S. EPA OSW has incorporated a margin of safety into the risk assessment 

process by allowing only 25 percent of the remediation threshold lead level to be attributable to releases from 

a waste management unit. Similarly, the protective air standard of 0.2 μg/m3

 is based on 25 percent of the 

National Ambient Air Quality Standard (NAAQS) quarterly average air concentration of 1.5 μg/m3

 (40 CFR 

3 50.12) converted to an annual basis value (0.9 μg/m ). 

Estimates of the potential for human health effects associated with potential exposure to lead are typically 

based on lead concentrations in blood. U.S. EPA guidance ( 1994g) recommends a maximum lead 

concentration in blood of 10 μg/deciliter (μg/dL), which is at the low end of the range of concern for adverse 

health effects in children. “Revised Draft Guidance for Performing Screening Level Risk Analyses at 

Combustion Facilities Burning Hazardous Wastes” (U.S. EPA 1994e) presents a mathematical model called 

the “Integrated Exposure U/BK [IEUBK] Model” for estimating lead levels in the blood of children on the 

basis of total lead uptake from exposures through diet, drinking water, air, and soil. When run with standard 

recommended default values (these generally represent national averages or “typical” values), U.S. EPA’s 

IEUBK model predicts that no more than 5 percent of children exposed to a lead concentration in soil of 400 

mg/kg will have lead concentrations in their blood exceeding 10 μg/dL (U.S. EPA 1994e). 

4.2.5.4 Estimation of Potential Health Effects for PCDDs and PCDFs 

PCDDs and PCDFs are toxic compounds that were first discovered as thermal decomposition products of 

polychlorinated compounds, including (1) the herbicide 2,4,5-T, (2) hexachlorophene, (3) PCBs, 

(4) pentachlorophenol, and (5) intermediate chemicals used to manufacture these compounds. In recent years, 

as chemical analytical methods have become more sensitive, additional sources of PCDDs and PCDFs have 

been identified, including (1) effluent from paper mills that use chlorine bleaches and (2) combustion sources, 

including forest fires, municipal and medical waste incinerators, and hazardous waste combustion units. 

“Health Assessment Document for 2,3,7,8-TCDD and Related Compounds” (U.S. EPA 1994b) concludes 

that there is adequate evidence that exposure to PCDDs and PCDFs results in a broad spectrum of cancer and 

noncancer effects in animals, some of which may occur in humans. The procedures for estimating risks 

associated with PCDDs and PCDFs are discussed below. 

There are 210 individual forms or “congeners” of PCDDs and PCDFs. U.S. EPA has developed procedures 

for assessing the cancer risks associated with exposure to the many PCDDs and PCDFs. These procedures 

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are used to assess risk on the basis of the relative toxicity of 2,3,7,8-TCDD, which is generally believed to 

be the most toxic form (U.S. EPA 1994c). Each congener is assigned a value referred to as a toxicity 

equivalency factor (TEF) that corresponds to its toxicity in relation to 2,3,7,8-TCDD. For example, 2,3,7,8­

TCDD has a TEF of 1.0, and other PCDDs and PCDFs have TEFs of 0.0 to 1.0. The U.S. EPA OSW and 

U.S. EPA guidance (1993d) recommend that all risk assessments include any PCDD or PCDF congener with 

chlorine molecules in the 2, 3, 7, and 8 positions. There are a total of 17 possible compounds that generally 

display dioxin-like toxicity. The TEF values for these 17 compounds are listed below. 

Dioxin Congener TEF 

(unitless) 

Furan Congener TEF 

(unitless) 

2,3,7,8-TCDD 1.000 2,3,7,8-Tetrachlorodibenzofuran 0.100 

1,2,3,7,8-Pentachlorodibenzo(p)dioxin 0.500 1,2,3,7,8-Pentachlorodibenzofuran 0.050 

1,2,3,4,7,8-Hexachlorodibenzo(p)dioxin 0.100 2,3,4,7,8-Pentachlorodibenzofuran 0.500 

1,2,3,6,7,8-Hexachlorodibenzo(p)dioxin 0.100 1,2,3,4,7,8-Hexachlorodibenzofuran 0.100 

1,2,3,7,8,9-Hexachlorodibenzo(p)dioxin 0.100 1,2,3,6,7,8-Hexachlorodibenzofuran 0.100 

1,2,3,4,6,7,8-Heptachlorodibenzo(p)dioxin 0.010 1,2,3,7,8,9-Hexachlorodibenzofuran 0.100 

1,2,3,4,5,7,8,9-Octachlorodibenzo(p)dioxin 0.001 2,3,4,6,7,8-Hexachlorodibenzofuran 0.100 

1,2,3,4,6,7,8-Heptachlorodibenzofuran 0.010 

1,2,3,4,7,8,9-Heptachlorodibenzofuran 0.010 

1,2,3,4,6,7,8,9-Octachlorodibenzofuran 0.001 

Source: U.S. EPA 1994c 

Of the 2,3,7,8-substituted PCDD and PCDF congeners, a CSF has been developed for only 2,3,7,8-TCDD. 

The carcinogenic potential of the other sixteen 2,3,7,8-substituted congeners is derived by assigning TEF 

values that compare the toxicity of the 16 other toxic congeners to that of 2,3,7,8-TCDD; the CSF for 2,3,7,8­

TCDD is based on evidence from experiments with rodents. The TEF values based on receptor binding 

studies or a sensitive measure of receptor binding,—specifically, induction of aryl hydrocarbon hydroxylase 

enzyme activity. The TEFs are based on the activity of the compounds in short-term toxicity assays that are 

considered predictive of the compounds’ ability to cause cancer in animals during long-term experimental 

carcinogenicity studies. Consequently, “CSFs” derived from the TEF values for these 16 toxic congeners are 

less certain than the CSF for 2,3,7,8-TCDD. Consistent with previous guidance (U.S. EPA 1994c), U.S. EPA 

Region 6 recommends that a risk assessment for PCDDs and PCDFs be completed by calculating the (TEQ) 

for 2,3,7,8-TCDD. The total TEQ is calculated by converting the dioxin congener concentrations to a 

2,3,7,8-TCDD concentration using the TEFs for all 17 dioxin congeners. 

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To assess the risk association with a mixture of PCDDs and PCDFs reported in a delisting petition using the 

TEF procedure, the following steps should be performed: 

1. Review the delisting petition for all PCDD and PCDF analytical determinations for the delisting 

sample. 

2. Multiply the congener concentrations in the sample by the TEFs listed above to express the 

concentrations as 2,3,7,8-TCDD equivalents (TEQs). 

3. Sum the products calculated in Step 2 to obtain the total TEQ for the delisting sample. 

4. Use the delisting sample’s total TEQ as the sample concentration of 2,3,7,8-TCDD to be entered 

in the DRAS under the chemical name “TCDDioxin 2,3,7,8" or( CAS) number 1746016. 

All of the 17 PCDD or PCDF congeners with chlorine molecules in the 2, 3, 7, and 8 positions are highly 

bioaccummulative compounds. For highly bioaccummulative chemicals, the accumulation of these chemicals 

up the food chain is an exposure pathway of potential concern. The DRAS does not consider potential 

impacts from ingestion of production terrestrial food products (fruit/vegetables) or animal food products 

(beef, pork, chicken). Ingestion of contaminated food products may be a potential exposure pathway for 

bioaccumulating compounds encountered under continuous exposure conditions. Bioaccummulative 

compounds may cause adverse impacts via the production terrestrial food products pathway if these 

compounds continuous long term releases from a landfill are expected to result in concentrations approaching 

some equilibrium at the point of exposure over a duration of years. Region 6 recommends analysis of the 

terrestrial food products pathway, in addition to the DRAS analysis, should the user have reason to expect 

that this compound’s above ground releases from a landfill will occur continuously over a period of years to 

decades. Consult a risk assessor or toxicologist with questions regarding risk via the terrestrial food 

(fruit/vegetable and animal) products pathways. 

4.2.5.5 Estimation of Potential Health Effects for PCBs 

U.S. EPA guidance (1988c, 1994d, 1994e) recommends that all PCB congeners (209 different chemicals) be 

addressed as a mixture having a single carcinogenic potency in a risk assessment. This recommendation was 

based on the U.S. EPA drinking water criteria for PCBs (U.S. EPA 1988c), which in turn were based on 

available toxicological information with the following limitations: 

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C The only PCB for which a CSF had been developed was Aroclor 1260; there was no 

agreed— upon procedure for applying this CSF to similar mixtures with less chlorine 

content. 

C Available physical, chemical, fate-and-transport, and toxicological information on individual 

congeners was limited, primarily because separation and synthesis of pure congeners can be 

technically difficult. 

C The number of tests conducted with various PCB mixtures and specific PCB congeners to 

demonstrate similar toxicological effects was very limited. 

4.2.5.5.1 Coplanar PCBs 

Since the compilation of “Drinking Water Criteria Document for Polychlorinated Biphenyls (PCBs)” (U.S. 

EPA 1988c), additional research on PCBs has been reported. The most important result of this research is 

the finding that some of the moderately chlorinated PCB congeners can have dioxin-like effects (U.S. EPA 

1992c; 1994d; ATSDR 1995). These dioxin-like congeners include are listed below. 

PCB Congener 

TEF 

(unitless) PCB Congener 

TEF 

(unitless) 

3,3',4,4'-Tetrachlorobiphenyl 0.0005 2,3,3',4,4',5-Hexachlorobiphenyl 0.0005 

2,3,3',4,4'-Pentachlorobiphenyl 0.0001 2,3,3',4,4',5'-Hexachlorobiphenyl 0.0005 

2,3,4,4',5-Pentachlorobiphenyl 0.0005 2,3',4,4',5,5'-Hexachlorobiphenyl 0.00001 

2,3',4,4',5-Pentachlorobiphenyl 0.0001 3,3',4,4',5,5'-Hexachlorobiphenyl 0.01 

2',3,4,4',5-Pentachlorobiphenyl 0.0001 2,2',3,3',4,4',5-Hheptachlorobiphenyl 0.0001 

3,3',4,4',5-Pentachlorobiphenyl 0.1 2,2',3,4,4',5,5'-Heptachlorobiphenyl 0.00001 

2,3,3',4,4',5,5'-Heptachlorobiphenyl 0.0001 

Source: U.S. EPA 1996h 

The listed congeners have four or more chlorine atoms with few substitutions in the ortho positions (positions 

designated as 2, 2', 6, and 6'). They are sometimes referred to as coplanar PCBs because the rings can rotate 

into the same plane if they are not blocked from rotation by ortho-substituted chlorine atoms; in this 

configuration, the shape of the PCB molecule is very similar to that of a PCDF molecule. Studies have shown 

that these dioxin-like congeners can then react with the aryl hydrocarbon receptor; this reaction is believed 

to initiate the adverse effects of PCDDs and PCDFs. As reported in “PCBs: Cancer Dose-Response 

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Assessment and Application to Environmental Mixtures” (U.S. EPA 1996h), the World Health Organization 

has used various test results to derive interim TEFs ranging from 0.1 to 0.00001 for the PCB congeners listed 

above. Additional congeners are suspected of producing similar reactions, but not enough data are available 

to derive TEF values for them. U.S. EPA Region 6 recommends that permitting authorities estimate risks 

associated with coplanar PCBs by computing a TEQ for PCBs and then applying a slope factor for dioxin on 

the chemical input screen of the DRAS. 

4.2.5.5.2 Other PCBs 

In addition to the coplanar (dioxin-like) PCB congeners, the remaining PCBs should be evaluated in a risk 

assessment. Based on consideration of the accumulated research on PCBs, (especially a recent carcinogenesis 

study of Aroclors 1016, 1242, 1254, and 1260 and a number of studies of the transport and bioaccumulation 

of various congeners) U.S. EPA derived three new CSFs to replace the former single CSF for PCBs (U. S. 

EPA 1996h). These new CSFs became effective in IRIS (U.S. EPA 1996g) on October 1, 1996. Additional 

studies are still being performed on PCBs. Therefore, the three CSFs are subject to revision as additional 

information becomes available. The three CSFs and the criteria for their use are listed below. 

CSF 

(mg/kg-day)-1 Criteria for Use 

2 Food chain exposure; sediment or soil exposure 

Early-life (infant and child) exposure via all routes to all PCB mixtures 

0.4 

(less used) 

Ingestion of water-soluble (less chlorinated) congeners 

Inhalation of evaporated (less chlorinated) congeners 

0.07 Congeners with more than four chloride atoms per molecule less than 0.5 percent of the total PCBs 

Source: U.S EPA 1996h 

A CSF of 2 mg/kg-day-1 will be used for a risk assessment. The DRAS will perform the PCB risk analysis 

using the PCB CSF of 2 mg/kg-day-1 to generate a conservative estimate of the risk associated with PCB 

exposure. However, the CSF of 0.07 mg/kg-day-1 can be used in risk assessments where the PCB mixture 

contains less than 0.5 percent PCB congeners with more than four chlorine atoms. This risk calculation 

would have to be made by the delisting manager. 

All of the coplanar (dioxin-like) PCB congeners are highly bioaccummulative compounds. For highly 

bioaccummulative chemicals, the accumulation of these chemicals up the food chain is an exposure pathway 

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of potential concern. The DRAS does not consider potential impacts from ingestion of production terrestrial 

food products (fruit/vegetables) or animal food products (beef, pork, chicken). Ingestion of contaminated 

food products may be a potential exposure pathway for bioaccumulating compounds encountered under 

continuous exposure conditions. Bioaccummulative compounds may cause adverse impacts via the 

production terrestrial food products pathway if these compounds continuous long term releases from a landfill 

are expected to result in concentrations approaching some equilibrium at the point of exposure over a duration 

of years. Region 6 recommends analysis of the terrestrial food products pathway, in addition to the DRAS 

analysis, should the user have reason to expect that this compound’s above ground releases from a landfill 

will occur continuously over a period of years to decades. Consult a risk assessor or toxicologist with 

questions regarding risk via the terrestrial food (fruit/vegetable and animal) products pathways. 

4.2.5.6 Toxicity Characteristic Rule Evaluation 

The TC Rule was published on March 29, 1990, in 55 FR 11798. The TC Rule established regulatory levels 

for 40 nonhydrolyzing or minimally hydrolyzing constituents (see Table 4-1). A solid waste exhibits the 

characteristic of toxicity if, the (TCLP), extract from a representative sample of the waste contains a 

concentration equal to or greater than the concentration listed in 40 CFR 261.24(a) for any of the waste 

constituents listed. The regulatory levels were generated based on a health-based chronic toxicity limit and 

a DAF of 100. The health-based chronic toxicity reference levels for the toxicity characteristic constituents 

were generated using chronic RfDs, carcinogenic risk-specific doses, or MCLs. 

The TC rule regulatory levels are not to be exceeded. Because the DRAS generates levels using different 

modeling and exposure scenarios, delisting levels for the 40 constituents may be below the regulatory levels. 

Therefore, the DRAS compares a waste constituent’s leachate concentration delisting level concentration, 

called the maximum allowable TCLP concentration, with the waste constituent’s TC Rule regulatory level. 

If the maximum allowable (see p. 4-23) TCLP concentration exceeds the TC Rule regulatory level, the DRAS 

notifies the user of the exceedance. 

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TABLE 4-1 


TC RULE REGULATORY LEVELS
 

 Regulatory Level 

Chemical (mg/L) 

Arsenic 5.0 

Barium 100 

Benzene 0.5 

Cadmium 1.0 

Carbon Tetrachloride 0.5 

Chlordane 0.03 

Chlorobenzene 100 

Chloroform 6.0 

Chromium 5.0 

Cresol 200 

m-Cresol 200 

o-Cresol 200 

p-Cresol 200 

2,4-D 10.0 

1,4-Dichlorobenzene 7.5 

1,2-Dichloroethane 0.5 

1,1-Dichloroethylene 0.7 

2,4-Dinitrotoluene 0.13 

Endrin 0.02 

Heptachlor 0.008 

Hexachlorobenzene 0.13 

Hexachloro-1,3-butadiene 0.5 

Hexachloroethane 3.0 

Lead 5.0 

Lindane 0.4 

Mercury 0.2 

Methoxychlor 10.0 

Methyl ethyl ketone 200.0 

Nitorbenzene 2.0 

Pentachlorophenol 100.0 

Pyridine 5.0 

Selenium 1.0 

Silver 5.0 

Tetrachloroethylene 0.7 

Toxaphene 0.5 

Trichloroethylene 0.5 

2,4,5-Trichlorophenol 400 

2,4,6-Trichlorophenol 2.0 

2,4,5-TP Acid (Silvex) 1.0 

Vinyl chloride 0.2 

Source: U.S. EPA 1990d 

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4.2.5.7 MCL Evaluation 

In 1974, Congress passed the Safe Drinking Water Act. This law requires U. S. EPA to determine safe levels 

of chemicals in drinking water that do or may cause health problems. These non-enforceable levels, which 

are based solely on possible health risks and exposures, are called MCL goals (MCLGs). Based on MCLGs, 

U. S. EPA sets enforceable standards called MCLs (see Appendix A-2). MCLs are set as close to the MCLGs 

as possible, considering the ability of public water systems to detect contaminants and remove them using 

suitable treatment technologies. 

In addition to calculating a waste constituent’s delisting level the ingestion of groundwater based on the 

preselected target risk level, the DRAS calculates the waste constituent’s delisting level for ingestion of 

groundwater where the groundwater concentration is set at the existing MCL. In some cases, the waste 

constituent’s groundwater ingestion delisting level based on the MCL will be less conservative than that 

calculated based on the selected risk level. 

When the DRAS indicates that groundwater ingestion is the limiting groundwater pathway, the user has the 

option of considering either of the groundwater ingestion pathway delisting levels: the risk-based maximum 

acceptable TCLP concentration or the MCL-based maximum allowable TCLP concentration. By default, 

however, the DRAS will select the more conservative of the two in summarizing delisting levels. 

4.2.5.8 Detection Limit Evaluation 

Often a delisting petition contains waste sample analytical results that for a number of the constituents 

analyzed for but not detected. A nondetect may be reported at the method detection limit (MDL), an 

estimated quantitation limit (EQL), or a reporting level (RL). A number of variables influence a detection 

limit including the instrumentation used, the waste matrix, and the analytical test method. In some instances, 

the reported detection limit for a given chemical may exceed an allowable delisting level at the target risk 

level. If the user suspects the analyte to be a potential waste constituent, the user can employ DRAS to 

determine whether the reported detection limit for a waste constituent exceeds a delisting level at the target 

risk level. 

The user enters a detection level concentration as the concentration in DRAS and selects a check box 

identifying the concentration as a detection limit. The DRAS determines the risk and hazard associated with 

a fraction of the waste constituent value. The default fraction used in the risk assessment is 0.5 (50 percent 

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of the detection limit). The program tracks waste constituent risks associated with actual measured 

concentrations and risks associated with 50 percent of the entered detection limit values. The risk assessment 

results for a value entered as a waste constituent’s detection limit are presented in italics to distinguish them 

from risk assessment results for measured waste constituent concentrations. 

4.2.5.9 Estimation of Ecological Effects 

A simple framework was developed for ecological assessment of wastes petitioned for delisting based on 

EPA's Framework for Ecological Risk Assessment (U.S. EPA, 1992d). During the problem formulation 

phase, a suite of ecological exposure scenarios were reviewed to determine potential ecological exposure 

pathways. At best, one can only infer that an ecosystem is protected from chemical stressors. The 

toxicological data support the evaluation of individuals, populations, and occasionally communities, but are 

inadequate to address the complexities of an ecosystem in most cases. Thus, the approach taken in the 

assessment was to estimate protective levels for the populations and communities expected to be exposed to 

wastes disposed to a landfill or surface impoundment, the WMUs evaluated in the DRAS program. 

Ecological Exposure 

The ecological risk assessment is based upon an approach that evaluates the movement of waste constituents 

from their WMUs, through different pathways, to the points where ecological receptors are expected to be 

exposed to these constituents. In selecting the ecological exposure pathways, U.S. EPA Region 6 considered 

the evaluated WMUs, the landfill and surface impoundment, and the potential for ecological exposure at these 

managed non-Subtitle C units. Potential exposure to two generic ecosystems was reviewed: A 

freshwater-based ecosystem and a terrestrial-based ecosystem. Terrestrial-based exposure from wastes 

disposed to landfills is minimized by regulations. Under 40 CFR §258, minimum criteria are established for 

all municipal solid waste landfills. These criteria, including wetland and floodplain controls, disease vector 

control and cover material requirements minimize terrestrial exposure. In Region 6, state regulations impose 

these criteria on industrial solid waste landfills. Given the measures employed to control terrestrial exposure, 

Region 6 believes terrestrial exposures to be minimal. Other regions should consult their respective states 

for regulations imposing criteria on industrial landfills. Nevertheless, Region 6 considered the potential for 

exposure to the freshwater ecosystem from wastewater runoff from a landfill receiving delisted wastes. 

Although most RCRA delistings involve solid materials, liquid wastes may on occasion be delisted to surface 

impoundments. There are no federal regulations establishing criteria for municipal or industrial surface 

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impoundments. The DRAS does not consider the potential for exposure to the freshwater ecosystem from 

wastewater runoff from a surface impoundment receiving delisted wastes. In the case of a delisting to a 

surface impoundment, Region 6 encourages the delisting manager to consider conducting a waste specific 

ecological risk assessment. The risk assessment should consider potential ecological pathways such as acute 

exposure to avian species. 

Ecological Assessment Endpoint 

The “Framework for Ecological Risk Assessment” defines assessment endpoints as explicit expressions of 

the environmental value that is to be protected (U.S. EPA 1992d). The Framework document goes on to 

define measurement endpoints as the measurable attributes related to the environmental value chosen as the 

assessment endpoint (U.S. EPA 1992d). Although it is conceptually useful to distinguish between the two 

types of endpoints, the practical implications in a predictive regulatory assessment require that both types be 

evaluated concurrently. In other words, assessment endpoints are not useful if they do not have measurable 

attributes, and measurement endpoints are not useful for assessment endpoints lacking ecological significance. 

For ecological receptors, an environmental value for the protection of populations or communities was 

selected as an assessment endpoint over a value to protect a given species. 

Ecological Benchmarks 

Ecological benchmarks were developed for the protection of the aquatic community. These ecological 

benchmarks are referred to as toxicity reference values (TRVs) and were developed from a variety of 

ecological receptors based on the availability of data for a given waste constituent. The TRV is developed 

to protect the entire community, not one particular species. In general, TRVs (the measurement endpoints) 

were selected for consistency with the Agency's “Framework for Ecological Risk Assessment” (U.S. EPA 

1992d). Region 6 believes the ecological analysis is conservative with respect to the overall assessment 

endpoint (e.g., sustainability of the reproducing populations) because of the way the source, fate and transport 

parameters are set and how the TRVs are developed. However, the degree to which this conservativeness 

transfers to ecosystems is not known. See Appendix A-1 for the list of aquatic TRVs currently used in the 

DRAS analyses. 

The ecological assessment focused on inferring the sustainability of populations and communities within the 

aquatic ecosystem. Therefore, TRVs were derived from measurement endpoints (i.e., reproductive, 

developmental, growth, survival, and mortality) from which such inferences could be made. Reproductive 

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studies (e.g., number of viable young per female) were preferred over other endpoints. The aquatic TRVs 

defaulted to a more conservative no effects level (or concentration) approach for ecological receptors. For 

populations of fish and aquatic invertebrates (represented by daphnids), a hierarchical approach was taken 

for use of data sources in deriving aquatic TRVs. The first choice was the final chronic values (FCVs) from 

the Ambient Water Quality Criteria (AWQC) effort by the EPA Office of Water (U.S. EPA 1998d). If these 

benchmarks were not available, then a freshwater aquatic TRV was selected from the draft Protocol for 

Screening Level Ecological risk Assessment at Hazardous Waste Combustion Facilities (U.S. EPA. 1998e). 

Finally, TRVs were selected from the Toxicological Benchmarks for Screening Potential contaminants of 

Concern for Effects on Aquatic Biota (Suter and Tsao, 1996). 

Ecological Example Calculation 

The following outlines an example calculation for determining whether the waste constituent concentration 

is predicted to exceed the protective concentration of the constituent in the aquatic community at the point 

of exposure. The aquatic toxicity reference value (TRV) is the constituent concentration in the ecosystem 

that should not be exceeded to protect the aquatic community. The allowable ecological delisting level 

concentration is the delisting waste constituent concentration that will not exceed the aquatic TRV and is 

specific to the waste volume/chemical/waste management unit/receptor. 

Receptor: Aquatic ecosystem 

Pathway: Waste Constituent Concentration � WMU � Runoff � Surface water � Aquatic 

TRV 

WMU: Landfill 

Equations 2-48 through 2-54 are used to calculate the predicted surface water concentration for a given waste 

constituent delisted to a landfill, based on the waste constituent’s concentration and volume reported in the 

petition. The overland to surface water model used in this pathway encompasses both the WMU release and 

the fate and transport portions of the calculation. The DRAS compares the waste constituent’s predicted 

surface water concentration to the constituent’s aquatic TRV. If the predicted surface water concentration 

exceeds the aquatic TRV, the petitioned waste has exceeded the ecological assessment. The DRAS notifies 

the user of any waste constituent whose total concentration is predicted to exceed the aquatic TRV in the 

modeled aquatic ecosystem. 

The DRAS compares the predicted ambient concentration with the TRV for protection of the ambient water 

community. For highly bioaccummulative waste constituents (log Kow > 4.0 or BCF > 100), such as PCBs 

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or dioxin, the user should consider that bioaccumulation of the waste constituent through the food web may 

occur. Under these circumstances, the ambient TRV value may not be protective. The current version of the 

DRAS does not account for bioaccumulation of waste constituents in the aquatic community. When assessing 

ecological impacts from the runoff of waste constituents having a log Kow > 4.0 or a BCF > 100, the user 

should consider the potential for bioaccummulative impacts within the food web or benthic community. 

4.3 QUANTITATIVE ESTIMATION OF RISK 

This section presents the equations used to calculate pathway delisting levels and risks associated with 

exposures to carcinogenic chemicals constituents via groundwater, surface water, air, and soil pathways. 

Calculation of delisting levels and hazards associated with exposures to noncarcinogenic chemicals is 

addressed in Section 4.4. The equations used are adapted from the Technical Support Document HWIR (U.S. 

EPA 1995b) and U.S. EPA OSW delisting dockets (U.S. EPA 1993a, and 1994a). The general risk equations 

presented in Section 4.2.1 are implemented for specific constituents and exposure pathways. Riskc,p is first 

computed for each constituent using an expanded form of Equation 4-1, and is summed across all constituents 

to calculate Riskp for each exposure pathway as shown in Equation 4-2. Risks for all groundwater exposure 

pathways and surface exposure pathways are summed separately by the DRAS to assist the user in 

understanding which pathways contribute most to risk. The DRAS finally sums groundwater exposure 

pathway risk and surface exposure pathway risk as shown in Equation 4-3 to provide the user with the 

aggregate risk, Risk cum, for all exposure pathways. Constituent- and pathway-specific subscripts are omitted 

in subsequent equations for simplicity of presentation; the context indicates whether the value being 

calculated is constituent- or pathway-specific. 

For groundwater exposures, the DRAS computes constituent-specific delisting levels and aggregate risks for 

the following groundwater pathways: ingestion of groundwater, dermal exposure to groundwater via bathing 

(adult and child), and inhalation of volatile constituents released from groundwater during showering. The 

equations used to compute the risks associated with groundwater exposures are discussed in Section 4.3.1. 

For surface water, the DRAS computes delisting levels and aggregate risks for the following exposure 

pathways: ingestion of drinking water and ingestion of fish, both from surface water bodies receiving runoff 

from landfills. The equations used to calculate risks from exposure to surface water are discussed in Section 

4.3.2. For air, the DRAS computes delisting levels and aggregate risks associated with exposure via 

inhalation of particulates and volatiles from solid wastes disposed of in landfills and inhalation of volatiles 

from liquid wastes disposed of in surface impoundments. Finally, the DRAS computes delisting levels and 

aggregate risks associated with exposure via (child) ingestion of soil contaminated by particles eroded from 

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a landfill by wind and deposited on the soil from the air. The equations used to calculate risks associated with 

the inhalation exposure pathways and with soil ingestion are discussed in Sections 4.3.3 and 4.3.4, 

respectively. 

4.3.1 Groundwater Exposure Pathways 

Three groundwater exposure pathways are evaluated in the DRAS: ingestion of groundwater, dermal 

exposure to groundwater via bathing, and inhalation of volatile constituents released from groundwater during 

showering. The receptor is assumed to be an adult for the ingestion and showering pathways; both adult and 

child receptors are addressed for the bathing pathway. Each pathway is discussed below. 

4.3.1.1 Ingestion of Groundwater 

This section discusses computation of leachate concentration delisting levels and of risks associated with 

exposure of adults to carcinogenic waste constituents via the groundwater ingestion pathway. Section 

4.3.1.1.1 presents the equations used to compute pathway delisting levels for standard multi year delistings. 

Section 4.3.1.1.2 presents the equations and methods used to compute constituent-specific risk and pathwayspecific risk. 

4.3.1.1.1 Delisting Level Analysis 

A pathway leachate concentration delisting level for groundwater ingestion (Cdl-ingest ) is calculated for each 

carcinogenic constituent using the maximum allowable constituent concentration in groundwater for ingestion 

exposure (Cgw-ingest ). Cgw-ingest is computed at the target risk level using Equation 4-8. 

(4-8) 

where: 

Default 

Cgw-ingest = 

TR = 

EF = 

maximum allowable constituent concentration 

 in groundwater for ingestion (mg/L) 

individual target risk level (unitless) 

exposure frequency (days/year) 

calculated

User specified (default 1 x 10-5

 See Section 4.2.3) 

350 (U.S. EPA 1997b) 

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AT = averaging time (years) 75 (U.S. EPA 1997b) 

IFWadj = water ingestion factor, age-adjusted 1.07 (Equation 3-4)

 [LCyear]/[kgCday] 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

The waste constituent’s Cgw-ingest is multiplied by its volume-adjusted DAF to generate the constituent’s 

Cdl-ingest, as shown in Equation 4-9. Note that since some inorganic constituents now have DAFs that vary 

based on the landfill leachate concentration, DRAS now uses an iterative procedure to determine the leachate 

concentration-volume-adjusted DAF pairing that corresponds to the maximum allowable receptor well 

concentration. The DRAS may in some cases extrapolate between known leachate-DAF pairings in order 

to determine the appropriate delisting level. 

(4-9) 

where: 

Default 

Cdl-ingest = pathway leachate concentration delisting calculated

 level for groundwater ingestion (mg/L) 

DAF va = waste volume-adjusted dilution-attenuation Equation 2-5

 factor (unitless) 

Cgw-ingest = maximum allowable constituent concentration Equation 4-8

 in groundwater for ingestion (mg/L) 

If the TCLP concentration for a given waste constituent exceeds its pathway leachate concentration delisting 

level, Cdl-ingest , the waste may not qualify for a standard multi year delisting. The waste may qualify for a onetime delisting, depending on the results of risk analysis calculations presented Section 4.3.1.1. 2. 

4.3.1.1.2 Risk Analysis 

The basic risk equation used to develop Equations 4-8 and 4-9, in which the target risk is specified and the 

leachate concentration delisting level is back-calculated, is solved in the forward direction to compute the risk 

from groundwater ingestion for each carcinogenic constituent, given a concentration of that constituent in 

groundwater, as shown in Equation 4-10. 

(4-10) 

where: 

Default 

Risk = cancer risk for carcinogens (unitless) calculated 

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C = gw constituent concentration in groundwater (mg/L) Equation 2-6 

IFWadj = water ingestion factor, age-adjusted 1.07 (Equation 3-4)

 [LCyear]/[kgCday] 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

To compute the individual constituent risks for groundwater ingestion, the user enters each waste constituent 

TCLP concentration into the DRAS at the specified prompt. The DRAS computes a waste constituent’s Cgw 

by dividing the waste constituent TCLP concentration by the constituent’s DAFva (see Equation 2-6). Using 

Equation 4-10, the DRAS calculates the groundwater ingestion- specific risk for each constituent. Then the 

DRAS uses Equation 4-2 and sums the individual constituent risks to determine the total risk for the 

groundwater ingestion pathway. 

4.3.1.2 Adult and Child Dermal Contact with Groundwater During Bathing 

This section describes computation of leachate concentration delisting levels and risks associated with 

exposure of adults and children to carcinogenic waste constituents via dermal contact with groundwater 

during bathing. Section 4.3.1.2.1 presents the equations used to compute pathway delisting levels for 

standard multi year delisting. Section 4.3.1.2.2 presents the equations used to compute constituent-specific 

risk and pathway-specific risk for one-time delistings. 

4.3.1.2.1 Delisting Level Analysis 

Pathway leachate concentration delisting levels for dermal contact with groundwater used for bathing 

(C ) are calculated using the target risk level. The DRAS computes C for the adult and child dl-dermal dl-dermal 

receptors using appropriate exposure parameters for each and then sets the final C using the results for dl-dermal 

the more sensitive of the two receptors. C is calculated for each carcinogenic constituent and is a dl-dermal 

function of the maximum allowable constituent concentration in groundwater for dermal exposure, Cgw-dermal, 

which is computed so as not to exceed the acceptable risk level for dermal exposure during bathing. Cgw-dermal 

is multiplied by the constituent’s DAFva to generate Cdl-dermal , as shown in Equation 4-11. 

(4-11) 

where: Default 

Cdl-dermal = pathway leachate concentration delisting level for calculated 

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 groundwater dermal contact (mg/L) 

DAFva = waste volume-adjusted dilution attenuation Equation 2-5

 factor (unitless) 

Cgw-dermal = maximum allowable constituent concentration Equations 4-12 through 4-14

 in groundwater for dermal exposure (mg/L) 

Cgw-dermal is computed differently for organic and inorganic constituents. For organic constituents, Cgw-dermal 

is calculated using one of two methods, depending on whether the elapsed time from the beginning of bathing 

(called the event duration, t ) is greater or less than the time required for the flux of chemical through the event 

skin to reach steady state (a chemical-specific constant called t

*

). For inorganic constituents, the method used 

is independent of t . event 

Equation 4-12 is used to compute Cgw-dermal for an organic constituent when tevent is less than t

* 

. 

(4- 12) 

where: 

Default 

Cgw-dermal = maximum allowable constituent concentration calculated

 in groundwater for dermal exposure (mg/L) 

DAevent = dose absorbed per unit area per event Equation 4-15

 (mg/cm -event) 2 

Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1) 

ô = lag time (hr) chem-specific (Appendix A-1) 

event t = duration of event (hr/event) 0.25( adult) or 

0.33 (child) (U.S. EPA 1997b) 

C is compute * d for organic constituents using Equation 4-13 when t is greater than or equal to t . gw-dermal event 

(4-13)

where: 

Default 

Cgw-dermal = maximum allowable constituent concentration calculated

 in groundwater for dermal exposure (mg/L) 

DAevent = dose absorbed per unit area per event Equation 4-15 

(mg/cm -event) 2 

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RCRA Delisting Technical Support Document 

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Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1) 

t = duration of event (hr/event) 0.25 (adult) or event 

0.33 (child) (U.S. EPA 1997b) 

ô = lag time (hr) chem-specific (Appendix A-1) 

B = Bunge constant (unitless) chem-specific (Appendix A-1) 

Cgw-dermal is computed for inorganic constituents using Equation 4-14.

(4-14) 

 where: 

Default 

Cgw-dermal = maximum allowable constituent concentration calculated 

in groundwater for dermal exposure (mg/L) 

DA event = dose absorbed per unit area per event Equation 4-15 

(mg/cm -event) 2 

Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1) 

event t = duration of event (hr/event) 0.25( adult) or 

0.33 (child) (U.S. EPA 1997b) 

To generate C , one must first generate the dose absorbed per unit area per event (DA .). DA is gw-dermal event event 

calculated separately for adults and children using Equation 4-15. 

(4-15) 

where: 

Default 

DA event

AT 

DAD 

BW 

EV 

EF 

ED 

= 

= 

= 

= 

= 

= 

= 

dose absorbed per unit area per event 

 (mg/cm -event) 2 

averaging time (years) 

dermally absorbed dose (mg/kg-day) 

body weight (kg) 

- adult 

- child 

event frequency (events/day) 

exposure frequency (days/year) 

exposure duration (years) 

- adult 

- child 

Calculated

75 (U.S. EPA 1997b) 

Equation 4-16 

72 (U.S. EPA 1997b) 

15 (U.S. EPA 1997b) 

1 (U.S. EPA 1997b) 

350 (U.S. EPA 1997b) 

30 (U.S. EPA 1997b) 

6 (U.S. EPA 1997b) 

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2 Askin = exposed skin surface area (cm )

- adult 20,000 (U.S. EPA 1997b) 

- child 7,900 (U.S. EPA 1997b) 

The DAD, or dermally absorbed dose for contact with water during bathing, is computed for carcinogens 

using Equation 4-16. 

(4-16) 

where: 

Default 

DAD = dermally absorbed dose (mg/kg-day) calculated 

TR = individual target risk level (unitless) user specified (1 x 10-5 default) 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

If the TCLP concentration for a given waste constituent exceeds its pathway leachate concentration delisting 

level, C , the waste may not qualify for a standard multi year delisting. However, the waste may qualify dl-dermal 

for a one-time delisting, depending on the results of risk steps calculations presented Section 4.3.1.2.2.. 

4.3.1.2.2 Risk Analysis 

In the risk analysis, the DRAS uses the equations used in computation of the delisting levels, but instead of 

back-calculating from the target risk level to a maximum allowable constituent concentration in leachate, the 

equations are used to calculate the risk given some constituent concentration expected in the groundwater.

 The risk for a given carcinogenic constituent associated with dermal contact during bathing is computed for 

the adult and child receptors using Equation 4-17. 

(4-17) 

where: 

Default 

R = risk from dermal contact during bathing for calculated 

constituent (unitless) 

DAD = dermally absorbed dose (mg/kg-day) Equation 4-18 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

The dermally absorbed dose (DAD), needed in Equation 4-17, is calculated as shown in Equation 4-18 (U.S. 

EPA 1992b). 

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(4-18) 

where: 

Default 

DAD = dermally absorbed dose (mg/kg-day) calculated 

DA event = dose absorbed per unit area per event 

(mg/cm -event ) 2 Equations 4-19 through 4-21 

EV = event frequency (events/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 

- adult 30 (U.S. EPA 1997b) 

- child 6 (U.S. EPA 1997b) 

Askin = exposed skin surface area (cm )2 

- adult 20,000 (U.S. EPA 1997b) 

- child 7,900 (U.S. EPA 1997b) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

BW = body weight (kg) 

- adult 72 (U.S. EPA 1997b) 

- child 15 (U.S. EPA 1997b) 

The dose absorbed per unit area per event (DA ) must be calculated using Equation 4-19, 4-20, or 4-21, event 

depending on the type of waste constituent in question and the relationship between the duration of the bath 

(t ) and the time required for the flux of chemical through the skin to reach steady state (t

*

, a chemical- event 

specific constant). For organic constituents, when t is less than t

* 

, DA is computed using Equation 4-19 event event 

(U.S. EPA 1992b). 

(4-19) 

where: 

Default 

DA event = dose absorbed per unit area per event calculated 2 (mg/cm -event)

C = gw constituent concentration in groundwater (mg/L) Equation 2-6
 

Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1)
 

ô = lag time (hr) chem-specific (Appendix A-1)
 

t = duration of event (hr/event) 0.25(adult) or
 event 

0.33 (child) (U.S. EPA 1997b) 

For organic constituents, when t is greater than or equal to t

*

, Equation 4-20 is used (U.S. EPA 1992b). event 

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RCRA Delisting Technical Support Document 

Chapter 4: Risk and Hazard Assessment 

Revised October 2008 

(4-20) 

where: 

Default 

DA event = dose absorbed per unit area per event calculated 2 (mg/cm -event)


C =
 gw constituent concentration in groundwater (mg/L) Equation 2-6 

Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1) 

t = duration of event (hr/event) 0.25 (adult) event

0.33 (child) (U.S. EPA 1997b) 

B = Bunge constant (unitless) chem-specific (Appendix A-1) 

ô = lag time (hr) chem-specific (Appendix A-1) 

For inorganic constituents, DA is calculated using Equation 4-21. event 

(4-21) 

where: 

Default 

DA event = dose absorbed per unit area per event calculated 2 (mg/cm -event)

C = gw waste constituent conc. in groundwater (mg/L) Equation 2-6 

Kp

w = skin permeability constant in water (cm/hr) chem-specific (Appendix A-1) 

t = duration of event (hr/event) 0.25 (adult) event 

0.33 (child) (U.S. EPA 1997b) 

Using Equation 4-17, the DRAS calculates the risk associated with groundwater exposure via dermal 

absorption during bathing for each waste constituent. Subsequently, the DRAS uses Equation 4-2 and sums 

the constituent-specific risks to determine the total risk for the groundwater dermal absorption pathway. 

4.3.1.3 Adult Inhalation of Volatiles from Groundwater During Showering 

This section describes computation of pathway leachate concentration delisting levels and risks associated 

with exposure to carcinogenic constituents in groundwater via inhalation of volatiles by adults while 

showering. Section 4.3.1.3.1 presents the equations used to calculate the delisting levels for standard multi 

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year delistings. Section 4.3.1.3.2 presents the equations used to calculate constituent-specific and pathwayspecific risk for one-time delistings. 

The method used to determine waste constituent-specific exposures resulting from inhalation of constituents 

in tap water is based on information used to support the “Petroleum Refinery Listing Determination” (U.S. 

EPA 1997a). This project used a method developed by McKone (1987) to estimate a time-varying constituent 

concentration in three compartments: the shower, the bathroom, and the house. Therefore, U.S. EPA Region 6 

uses simplified versions of the equations that compute a conservative constant concentration for each of the 

three compartments. The simplified equations are based on the steady-state model developed by McKone 

and Bogen (1992), which calculates a chemical concentration in air for each of the three compartments. 

4.3.1.3.1 Delisting Level Analysis 

The pathway total waste constituent concentration delisting levels for carcinogenic constituents inhaled when 

groundwater is used for showering (C ) are calculated for each constituent using the maximum allowable dl-inhale 

concentration in groundwater for inhalation (Cgw-inhale ) at the specified target risk level. To generate Cdl-inhale , 

Cgw-inhale is multiplied by the constituent’s DAFva as shown in Equation 4-22. 

(4-22) 

where: 

Default 

Cdl-inhale = pathway leachate concentration delisting calculated 

level for shower inhalation (mg/L) 

DAF va = waste volume-adjusted dilution attenuation Equation 2-5 

factor (unitless) 

Cgw-inhale = maximum allowable constituent concentration Equation 4-24 

in groundwater used for showering (mg/L) 

To generate Cgw-inhale , one must first generate the sum of products of the maximum allowable air concentration 

and corresponding exposure time in three compartments during showering (CETmax) using Equation 4-23. 

(4-23) 

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where: 

Default 

CETmax = sum of products of maximum allowable calculated (Equation 4-23) 

constituent air concentration in specific compartment 3 and corresponding exposure time (mg-days/m )

TR = individual target risk level (unitless) 1 x 10-5 (U.S. EPA 1997b) 

or user-specified 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

EV = event frequency (events/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

IFAadj = inhalation factor, age-adjusted 10.7 (Equation 3-3) 

([m3

-year]/[kg-day]) 

ETcomp = exposure time in each compartment (days/shower) (McKone and Bogen 1992) 

- shower 0.00792 

- bathroom 0.0338 

- house 0.625 

CSF = inhal constituent inhalation cancer slope factor chem-specific (Appendix A-1) 

 (mg/kg-day)-1 

Once CETmax for a given waste constituent is known, Cgw-inhale is back-calculated based on the linear 

relationship of the transient shower concentration equations as described in Equations 4-31 through 4-33. 

In other words, the linearity of these equations allows us to simply calculate the fractions of the average 

compartment concentrations to the groundwater concentration in the forward calculations and then divide 

CETmax by the sum of products of concentration fraction and corresponding exposure time to get Cgw-inhale . 

This requires that one forward calculation be performed at an arbitrary TCLP concentration before the 

backward calculations begin. The allowable groundwater concentration can then be computed using Equation 

4-24. 

(4-24) 

where: 

Default 

Cgw-inhale = maximum allowable constituent concentration calculated

 in groundwater used for showering (mg/L) 

CET max = sum of products of maximum allowable calculated (Eq. 4-23) 

constituent air concentration in specific compartment 

and corresponding exposure time (mg-days/m )3 

FGW, comp = fraction of average waste constituent concentration Equation 4-25

 in the compartment over the groundwater waste 

 constituent concentration based on a forward calculation 

ET comp = exposure time in each compartment (days/shower) (McKone and Bogen 1992) 

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- shower 0.00792 

- bathroom 0.0338 

- house 0.625 

The relationship between the average waste constituent concentration in a given compartment (determined 

using Equations 4-28 to 4-33) divided by an established waste concentration in the groundwater is calculated 

in Equation 4-25. This relationship is unique to each chemical since it is based on the volatilization of the 

chemical in the compartment. However, the relationship between this calculated fraction and the allowable 

delisting concentration is linear. 

(4-25) 

where: 

FGW, comp = fraction of average waste constituent concentration Equation 4-25 

in the compartment over the groundwater waste 

constituent concentration based on a forward calculation 

CF-avg, comp =	 average compartment concentration Equations 4-28 - 4-30 

from a forward calculation (mg/m3) 

C = F-gw-inhale	 groundwater concentration that CFavg is based
 

on (mg/L)
 

Once Cgw-inhale is determined from Equation 4-24, the DRAS performs the back-calculation described in 

Equation 4-22 to generate the pathway delisting level (C ). If the TCLP concentration of a given waste dl-inhale 

constituent exceeds its pathway leachate concentration delisting level, C , the waste may not qualify for dl-inhale 

a standard multi year delisting. However, the waste may qualify for a one-time delisting, depending on the 

results of risk analysis calculations presented Section 4.3.1.3.2. 

4.3.1.3.2 Risk Analysis 

In the risk analysis, the DRAS computes the risk for an adult receptor exposed to carcinogenic waste 

constituents through inhalation of air concentrations contaminated during showering with the groundwater 

and subsequent exposure. The shower model used in this analysis is based on the equations presented in 

McKone (1987). The shower model estimates the change in the shower air concentration based on the mass 

of waste constituent lost by the contaminated groundwater and the air exchange rate between the various 

model compartments (shower, bathroom, and house). The constituent-specific risk is calculated for each 

inhalation compartment using Equation 4-26. 

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(4-26) 

where: 

Default 

R = risk from inhalation during showering calculated

 for constituent (unitless) 

CET = sum of products of maximum allowable calculated (Equation 4-27) 

constituent air concentration in specific compartment 3 and corresponding exposure time (mg-days/m )

1000L/m 3= conversion from cubic meters to liters 

CSFinhal = constituent inhalation cancer slope factor chem-specific (Appendix A-1)

 (mg/kg-day)-1 

IFAadj = inhalation factor, age-adjusted ([m 3-year]/[kg-day]) 10.67 (Equation 3-3) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

EV = event frequency (events/day) 1 (U.S. EPA 1997b) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

Cair,i is the sum of the exposure concentrations in the shower, bathroom, and house compartments computed 

using Equation 4-27. 

(4-27) 

where: 

Default 

CET = sum of products of maximum allowable calculated (Equation 4-27) 

constituent air concentration in specific compartment 3 and corresponding exposure time (mg-days/m )

C = average constituent air conc. in shower (mg/L) Equation 4-28 avg,s 

Cavg,b = average constituent air conc. in bathroom (mg/L) Equation 4-29 

Cavg,h = average constituent air conc. in house (mg/L) Equation 4-30 

ETcomp = exposure time in each compartment (days/shower) U.S EPA 1997b 

- shower 0.00792 

- bathroom 0.0338 

- house 0.625 

To calculate the total constituent air concentration inhaled, the average constituent-specific air concentration 

(Cavg,i ) that the individual is exposed to in each inhalation compartment (shower, bathroom, and house) is 

calculated using Equations 4-28 through 4-30, respectively. 

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(4-28) 

where: 

Default 

C avg,s 

t 

N 

= 

= 

= 

average air concentration in shower (mg/L) 

calculational time step index 

total number of time steps for the time in the 

 shower 

calculated 

57 (n/ Ät)

and: 

Ät 

n 

ys,t 

= 

= 

= 

time step (min) 

time in shower (min) 

air concentration in shower at time t (mg/L) 

0.2 (McKone 1987) 

11.4 (U.S. EPA 1997b) 

Equation 4-31 

(4-29) 

where: 

Default 

Cavg,b = average air concentration in bathroom (mg/L) calculated 

t 

N 

= 

= 

calculational time step index 

total number of time steps for the time in the 243 (n/Ät)

Ät = 

 bathroom excluding time in shower 

time step (min) 0.2 (McKone 1987) 

n = time in bathroom excluding time in shower (min) 48.6 (U.S. EPA 1997b) 

yb,t = air concentration in bathroom at time t (mg/L) Equation 4-32 

and: 

(4-30) 

where: 

Default 

Cavg,h 

t 

N 

n 

= 

= 

= 

= 

average air concentration in house (mg/L) 

calculational time step index 

total number of time steps for the time in the 

 house excluding time in the bathroom and shower 

time in house excluding shower and bathroom 

 time (min) 

calculated 

4500 (n/Ät)

900 (U.S. EPA 1997b)

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Ät = time step (min) 0.2 (McKone 1987) 

yh,t = air concentration in house at time t (mg/L) Equation 4-33 

For each of the compartments, the average concentration of the waste constituent in the air is dependent on 

calculation of the gas phase of the waste constituent in that compartment. For example, calculation of the 

average air concentration in the shower, C , is dependent on calculation of the gas phase constituent avg,s 

concentration in the shower at time t (ys,t+1 ) as derived using Equation 4-31. The air exchange rate between 

the shower and the bathroom is included in the estimation of the gas phase concentration of the constituents 

in the shower (McKone 1987). 

where: 

Default 

ys,t+1 = gas phase constituent concentration in the shower calculated

 at end of time step (mg/L) 

ys,t = gas phase constituent concentration in the shower calculated (0.0 for 1st time step)

 at beginning of time step (mg/L) 

Qgs = volumetric gas exchange rate between shower 100 (McKone 1987)

 and bathroom (L/min) 

yb,t = gas phase constituent concentration in bathroom calculated (0.0 for 1st time step)

 at beginning of time step (mg/L) 

(4-31) 

(t - t ) = t=1 t calculational time step (min) 0.2 (McKone 1987) 

s I = shower water use (L/min) 5.5 (McKone 1987) 

C gw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

em,s f = fraction of constituent emitted from shower (Equation 4-34) 

water use (unitless) 

V s = volume of shower stall (L) 2,300 (U.S. EPA 1997b) 

To calculate the average concentration of the waste constituent in the air of the bathroom, Cavg,b , the gas phase 

constituent concentration in the bathroom at time t (yb,t+1 ) is calculated using Equation 4-32. Equation 4-32 

is calculated based on total bathroom water use of 125 L/day and exposure duration in bathroom of 1 hr/day 

(U.S. EPA 1997b). 

(4-32) 

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where: 

Default 

yb,t+1 = gas phase constituent concentration in the calculated

 bathroom at end of time step (mg/L) 

yb,t = gas phase constituent concentration in the calculated (0.0 for 1st time step)

 bathroom at for previous time step (mg/L) 

Qgs = volumetric gas exchange rate between shower 100 (McKone 1987)

 and bathroom (L/min) 

ys,t = gas phase constituent concentration in the shower calculated (0.0 for 1st time step)

 for previous time step (mg/L) 

Qgb = volumetric gas exchange rate between bathroom 300 (McKone 1987) 

and house (L/min) 

yh,t = gas phase constituent concentration in bathroom calculated (0.0 for 1st time step)

 at for previous time step (mg/L) 

(t - t ) = calculational time step (min) 0.2 (McKone 1987) t+1 t 

Ib = bathroom water use (L/min) 2.08 (McKone 1987) 

Cgw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

fem,b = fraction of constituent emitted from bathroom (Equation 4-34)

 water use (unitless) 

Vb = volume of bathroom (L) 13,600 (U.S. EPA 1997b) 

To calculate the average air concentration of a waste constituent in the house, Cavg,h , the gas phase constituent 

concentration in the house at time t (yh,t+1 ) is calculated using Equation 4-33. Equation 4-33 is calculated 

based on total bathroom water use of 201 L/day and 16 hours of household exposure (U.S. EPA 1997b). 

(4-33) 

where: 

Default 

yh,t+1 = gas phase constituent concentration in the house calculated

 at end of time step (mg/L) 

yh,t = gas phase constituent concentration in the house calculated (0.0 for1st time step)

 for previous time step (mg/L) 

Qgb = volumetric gas exchange rate between the 300 (McKone 1987)

 bathroom and house (L/min) 

yb,t = gas phase constituent concentration in the calculated (0.0 for1st time step)

 bathroom for previous time step (mg/L) 

Qgh = volumetric gas exchange rate between house 2,325 (McKone 1987)

 and atmosphere (L/min) 

ya,t = gas phase constituent concentration in assumed 0.00

 the atmosphere (mg/L) 

t+1 t (t - t ) = calculational time step (min) 0.2 (McKone 1987) 

Ih = water use in house - other than bathroom (L/min) 0.21 (McKone 1987) 

C gw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

fem,h = fraction of constituent emitted from household (Equation 4-34) 

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 water use - other than bathroom (unitless) 

Vh = volume of house (L) 310,000 (U.S. EPA 1997b) 

In order to calculate the gas phase constituent concentration in the three compartments (shower, bathroom 

and house) at time t, the fraction of the waste constituent emitted from the contaminated groundwater (fem) 

must be calculated for each compartment. The fraction emitted for the shower compartment (f ) is em, s 

calculated using the formula in Equation 4-34. The fraction emitted for the bathroom compartment and the 

house compartment are calculated using the general formula in Equation 4-38. The fraction of the gas phase 

saturation (f ) is unique to each compartment, thereby determining a unique fraction emitted (f ) for each sat em 

compartment. 

(4-34) 

where: 

Default 

fem, i = fraction of constituent emitted from contaminated calculated

 groundwater for each compartment i (unitless) 

N = Dimensionless mass transfer coefficient (unitless) Equation 4-36 

sat f = Fraction of gas phase saturation for each Equation 4-35

 compartment i (unitless) 

The fraction of the gas phase saturation (fsat) for each compartment is calculated using Equation 4-35. 

em 

(4-35)

where: 

Default 

fsat = fraction of gas phase saturation for each calculated

 compartment i (unitless) 

yi,t = gas phase constituent concentration in each 

 compartment i (mg/L) 

ys,t = for shower compartment Equation 4-31 

yb,t = for bathroom compartment Equation 4-32 

yh,t = for house compartment Equation 4-33 

H’ = dimensionless Henry’s law constant (H × 41) chem-specific (Appendix A) 

Cgw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

To calculate the fraction of waste constituent emitted from the contaminated groundwater (f ), the 

dimensionless overall mass transfer coefficient (N) is determined using Equation 4-36. 

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(4-36) 

where: 

Default 

N = dimensionless mass transfer coefficient (unitless) calculated 

Kol = overall mass transfer coefficient (cm/sec) Equation 4-37 

dp = droplet diameter (cm) 0.098 (McKone 1987) 

h = nozzle height (cm) 180 (McKone 1987) 

vt = terminal velocity (cm/sec) 400 (McKone 1987) 

To calculate the dimensionless overall mass transfer coefficient (N), the overall mass transfer coefficient (Kol) 

is computed using Equation 4-37. 

(4-37) 

where: 

Default 

Kol = Overall mass transfer coefficient (cm/sec) Calculated 

â = Proportionality constant (cm/sec) -1/3 216 (McKone 1987) 2 Dw = Diffusion coefficient in water (cm /sec) Chem-specific (Appendix A) 2 Da = Diffusion coefficient in air (cm /sec) Chem-specific (Appendix A) 

Hì = Dimensionless Henry’s law constant (H × 41) Chem-specific (Appendix A) 

The fraction of the gas phase saturation (fsat) is unique to each compartment, thereby determining a unique 

fraction emitted (fem) for each compartment. The fraction emitted from the bathroom or household water use 

is a function of the input transfer efficiency (or maximum fraction emitted) and the driving force for mass 

transfer. For the bathroom, the fraction emitted is calculated as follows: 

(4-38) 

where 

Default 

fem,b = fraction of constituent emitted from Calculated 

bathroom water use (unitless) 

yb = gas phase constituent concentration in Equation 4-32 

the bathroom (mg/L) 

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HN = Dimensionless Henry’s law constant Chem-specific (Appendix A) 

C gw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

åtransfer = transfer efficiency for non-shower water use Equation 4-40 

(unitless) 

fmax,b = maximum transfer efficiency for bathroom 0.5 (McKone, 1987) 

water use (unitless) 

The fraction emitted from the household water use is calculated as follows: 

(4-39) 

where 

Default 

fem, h = fraction of constituent emitted from Calculated

 bathroom water use (unitless) 

yh = gas phase constituent concentration in the Equation 4-33

 house (mg/L) 

HN = Dimensionless Henry’s law constant 41 × Henry’s Law Constant 

C gw = waste constituent concentration in the receptor Equation 2-6

 well (mg/L) 

åtransfer = transfer efficiency for non-shower water use Equation 4-40 

fmax, h = maximum transfer efficiency for household 0.66 (McKone, 1987)

 water use (unitless) 

The transfer efficiency is calculated using the following equation based on Little (1992). 

(4-40) 

where 

Default 

åtransfer = transfer efficiency for non-shower water use Calculated 2 Dl = diffusion coefficient in water (cm /s) chem-specific (Appendix A-1) 2 Da = diffusion coefficient in air (cm /s) chem-specific (Appendix A-1) 

HN = Dimensionless Henry’s law constant chem-specific (Appendix A-1)

 (HN = 41 × Henry’s Law Constant) 

There is a theoretical maximum air concentration that can occur in a compartment. The DRAS calculates that 

theoretical maximum concentration, C , for each compartment using the Ideal Gas Law. The DRAS air-max 

calculates C for all three compartments and compares the compartment concentration to the theoretical air-i 

maximum. If C for a compartment is larger than C , C for the compartment is set equal to C . air-i air-max air-i air-max 

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The C or C for each compartment are totaled in Equation 4-27 and the sum is used in Equation 4-26 air-i air-max 

to calculate the inhalation risk for each constituent. Then the risk for each waste constituent is summed using 

Equation 4-2 to obtain the total pathway-specific risk associated with shower inhalation of volatiles from 

groundwater during showering. 

4.3.2 Surface Water Exposure Pathways 

The surface water exposure pathways addressed by the DRAS include ingestion of surface water and 

ingestion of fish. An adult receptor is assured for both pathways. The pathway delisting levels and risks for 

these pathways are calculated using a method described in EPA Headquarters delisting docket materials (U.S. 

EPA 1993a, 1993b). The method uses the USLE to calculate the rate of erosion of a petitioned waste from 

a landfill to a surface water body. The receiving water body is assumed to be a stream. The amount of eroded 

waste delivered in runoff to the stream is calculated using a sediment delivery ratio, and the volume of surface 

water into which runoff occurs is determined by estimating the size of the stream into which the eroded waste 

is likely to be transported. 

For the purposes of delisting, the stream is assumed to be a second-order or fifth-order stream (U.S. EPA 

1993a). A second-order stream is assumed to be the smallest stream capable of supporting fish, whereas a 

fifth-order stream is assumed to be the smallest stream capable of serving as a community water supply. For 

the surface water ingestion pathway, drinking of unfiltered, untreated water is assumed. Clean Water Act 

regulations restrict the potential for such exposure, making this a very conservative exposure pathway. 

Delisting decisions based on this exposure pathway should reviewed carefully. The surface water ingestion 

analyses are based on the assumption that a chemical is transported to a fifth-order stream; the fish ingestion 

analyses are based on the assumption that the chemical is transported to a second-order stream. 

Sections 4.3.2.1 and 4.3.2.2 describe the equations and default parameters used to calculate the pathway 

delisting levels and risks associated with the ingestion of surface water and ingestion of fish exposure 

pathways. 

4.3.2.1 Ingestion of Surface Water 

This section describes the computation of the total concentration delisting levels and risks associated with 

exposure of adults to carcinogenic waste constituents via ingestion of surface water. Section 4.3.2.1.1 

presents the equations used to compute pathway delisting levels for carcinogenic waste constituents for 

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standard multi year delisting petitions. Section 4.3.2.1.2 presents the equations and methods used to compute 

constituent-specific and pathway-specific risks for one-time delistings. 

4.3.2.1.1 Delisting Level Analysis 

Pathway total concentration delisting levels for carcinogens associated with ingestion of surface water 

-5 (C ) are calculated using the user specified target risk level (default 1 x 10 ). To calculate C , the dl-water dl-water 

maximum allowable concentration of a waste constituent in surface water (Csw) must first be calculated. The 

DRAS then back-calculates C using C . Equation 4-41 is used to calculate C . dl-water sw sw 

(4-41) 

where: 

Default 

Csw = maximum allowable concentration of constituent calculated

 in surface water (mg/L) -5 TR = individual target risk level (unitless) User specified (default 1 x 10 )


EF = exposure frequency (days/year) 350 (U.S. EPA 1998b)
 

AT = averaging time (years) 75 (U.S. EPA 1997b)
 

IFWadj = water ingestion factor, age-adjusted 1.07 (Equation 3-4)


 ([L-yr]/[kg-day]) 

CSForal = oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

After is calculated, C , the waste constituent’s pathway total concentration delisting level, C , for the sw dl-water 

surface water ingestion pathway is computed as shown in Equation 4-42. 

(4-42) 

where: 

Default 

Cdl-water = pathway total concentration delisting level for calculated 

ingestion of surface water (mg/kg) 

Csw = maximum allowable concentration of constituent Equation 4-41

 in surface water (mg/L) 

C = 5th-stream concentration of eroded waste and soil in Equation 4-43
 

 fifth-order stream (kg/L)
 

C5th-stream is calculated as shown in Equation 4-43. 

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(4-43) 

where: 

Default 

C5th-stream = concentration of eroded waste and soil in calculated

 fifth-order stream (kg/L) 

Aw = rate of soil and waste erosion from landfill Equation 2-51 

(kg/[acre-year]) 

Q5th = volumetric flow in fifth-order stream (L/year) 3.4 x 1011 (U.S. EPA, 1993a) 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

If the total concentration of a given constituent in the waste exceeds its pathway total concentration delisting 

level, C , the waste may not qualify for a standard multi year delisting. However, the waste may qualify dl-water 

for a one-time delisting, depending on the results of risk analysis calculations presented in Section 4.3.2.1.2. 

4.3.2.1.2 Risk Analysis 

This section describes how the DRAS computes risks associated with the surface water ingestion exposure 

pathway. The DRAS computes each carcinogenic waste constituent’s risk for surface water ingestion 

pathway as shown in Equation 4-44. 

(4-44) 

where: 

Default 

Risk = risk for surface water ingestion for calculated

 waste constituent (unitless) 

C = concentration of constituent in surface water (mg/L) Equation 4-45 sw 

IFWadj = water ingestion factor, age-adjusted 1.07 (Equation 3-4) 

([L-year]/[kg-day]) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

The concentration of a constituent in surface water, Csw, is computed using Equation 4-45. 

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(4-45) 

where: 

Default 

Csw = concentration of waste constituent in surface calculated 

water (mg/L) 

Ctotal waste = total concentration of constituent in waste (mg/kg) waste-specific 

Aw = rate of waste erosion from landfill (kg/[acre-year]) Equation 2-49 

Q = volumetric flow of stream (L/year) (U.S. EPA 1994a) stream 

- for fifth-order stream 3.4 x 1011 

- for second-order stream 3.3 x 109 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

Using Equation 4-44, the DRAS calculates the risks from ingestion of surface water for each constituent and 

then, using Equation 4-2, sums the individual constituent risks to determine the total risk for the surface water 

ingestion pathway. 

4.3.2.2 Ingestion of Fish 

This section describes computation of total concentration delisting levels and of risks associated with adult 

ingestion of fish that are exposed to petitioned carcinogenic waste constituents that erode from a waste 

disposal unit and are transported to surface water. The bioaccumulation factor (BAF) values used are for a 

trophic level 4 (carnivorous) fish (U.S. EPA 1995e). 

Section 4.3.2.2.1 presents the equations used to compute carcinogenic waste constituent pathway delisting 

levels for standard multi year delistings. Section 4.3.2.2.2 presents the equations and methods used to 

compute constituent-and pathway-specific risks for one-time delistings. 

4.3.2.2.1 Delisting Level Analysis 

This section describes the equations used to calculate the pathway total concentration delisting levels for fish 

ingestion (C ) for carcinogens. To perform these calculations, the DRAS must first calculate the maximum dl-fish 

allowable fish tissue concentration such that the target risk level is not exceeded (Cfish ). Knowing the 

allowable fish tissue concentration (Cfish ), we can backcalculate to determine the maximum allowable total 

surface water concentration (Csw). Using Csw, the maximum allowable concentration of the dissolved phase 

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of the constituent in the waste (C ) is then calculated. Finally, C is used to calculate C . The first step, dw dw dl-fish 

then, is to calculate Cfish for the carcinogenic constituent using Equation 4-46. 

(4-46) 

where: 

Default 

Cfish = maximum allowable concentration of calculated

 constituent in fish tissue (mg/kg) 

TR = target risk level (unitless) user specified (1 x 10-5 default) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

CRfish = fish consumption rate (kg/day) 0.02 (U.S. EPA 1997b) 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

Now, the maximum allowable concentration of the constituent in the total waste is determined, Csw, using 

Equation 4-47 and the appropriate factor, either the BCF or the BAF, as discussed in Section 2.3.4.2, 

Concentration of Waste Constituent in Fish (Cfish ). 

(4-47) 

where: 

Default 

C = dw maximum allowable dissolved phase water calculated
 

concentration (mg/L)
 

Cfish = maximum allowable concentration of Equation 4-43


 constituent in fish tissue (mg/kg) 

BAF = fish bioaccumulation factor (L/kg) or chem-specific (Appendix A-1) 

BCF = fish concentration factor (L/kg) chem-specific (Appendix A-1) 

Using Equation 4-48, the maximum allowable concentration of the waste concentration in water column in 

the second order stream (Csw) is calculated: 

(4-48) 

4-48 
 

 

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where: 

Default 

Cdw = Dissolved phase water concentration (mg/L) Calculated 

Csw = Waste concentration in water column (mg/L) Equation 4-47 

Kdsw = 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) 

Finally, C is used in Equation 4-49 to back calculate the delisting level (C ) for the fish ingestion sw dl-fish 

pathway. 

(4-49) 

where: 

Default 

Cdl-fish = pathway total concentration delisting level calculated

 for fish ingestion (mg/kg) 

Csw = Waste concentration in water column (mg/L) Equation 4-48 

Aw = rate of waste erosion from landfill (kg/[acre-year]) Equation 2-51 

Q2ndstream = volumetric flow of second-order stream (L/year) 3.3x109 

(U.S.EPA 1994a) A = area of waste management unit (acres) Equation 2-12 or 2-13 

If the total concentration of a given waste constituent exceeds its total concentration delisting level, C , dl-fish 

the waste may not qualify for a standard multi year delisting. However, the waste may qualify for a one-time 

delisting, depending on the results of risk analysis calculations presented in Section 4.3.2.2.2. 

4.3.2.2.2 Risk Analysis 

In the risk analysis, the DRAS program computes the risk associated with the fish ingestion exposure 

pathway. Each carcinogenic waste constituent’s risk for the fish is computed using Equation 4-50. 

(4-50) 

where: 

Default 

4-49 
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Risk = risk from fish ingestion for constituent (unitless) calculated
 

Cfish = concentration of constituent in fish tissue (mg/kg) Equation 4-51
 

Fc = fraction contaminated (unitless) 1.0 (U.S. EPA 1997b)
 

CRfish = fish consumption rate (kg/day) 0.02 (U.S. EPA 1997b)
 

ED = exposure duration (years) 30 (U.S. EPA 1997b)
 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b)
 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1)
 

BW = body weight (kg) 72 (U.S. EPA 1997b)
 

AT = averaging time (years) 75 (U.S. EPA 1997b)
 

Cfish is calculated as shown in Equation 4-48. 

(4-51) 

where: 

Default 

Cfish = concentration of constituent in fish tissue (mg/kg) calculated 

C = dw concentration of dissolved phase of constituent Equation 4-48 

 in waste water (mg/L) 

BAF = fish bioaccumulation factor (L/kg) or chem-specific (Appendix A-1) 

BCF = fish bioconcentration factor (L/kg) chem-specific (Appendix A-1) 

First the DRAS calculates the constituent-specific risk for fish ingestion using the total waste constituent 

concentration. Then the program sums all the constituent-specific risks for the fish ingestion pathway using 

Equation 4-2 to determine the total risk for the fish ingestion pathway. 

4.3.3 Air Exposure Pathways 

The DRAS includes an analysis of the human health risks associated with exposure of adults to carcinogens 

via three air exposure pathways: (1) inhalation of particulates from solid wastes disposed of in landfills, (2) 

inhalation of volatiles from solid wastes disposed of in landfills, and (3) inhalation of volatiles from liquid 

wastes disposed of in surface impoundments. The DRAS also includes analysis of the human health risks 

associated with exposure of children to carcinogens via air exposure by ingestion of soil contaminated by 

deposition of particulate matter from solid wastes disposed of in landfills. The delisting level for the air 

exposure pathways is expressed in terms of the total constituent concentration in the waste. 

The risks associated with inhalation exposure to particulate and volatile matter can typically be determined 

only when inhalation HBNs are available. Inhalation cancer slopes derived from oral potency values are only 

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used when specifically recommended by IRIS or when target organ endpoints are far from the respiratory 

system (U.S. EPA 2006). 

Releases and transport of contaminated particulates from landfills and releases of volatiles from landfills and 

surface impoundments are evaluated using methods described in U.S. EPA Headquarters delisting dockets 

(U.S. EPA 1993b,1994a). Particulate emissions are determined by estimating respirable contaminated 

particulate emissions from wind erosion of landfill surfaces (U.S. EPA 1985a). The DRAS uses the AP-42 

methodologies for calculating dust emissions resulting from on-site vehicular traffic and waste loading and 

unloading operations (U.S. EPA 1985b). 

Emission of volatiles from landfills is evaluated with Shen’s modification of Farmer’s equation, which was 

developed by U.S. EPA’s OAQPS (U.S. EPA 1984). The modified Farmer’s equation (Equation 2-33), which 

was developed to estimate atmospheric emissions from covered landfills. Estimates of emissions of volatile 

organics from liquid wastes in surface impoundments use a methodology developed by the U.S. EPA OAQPS 

and known as SIMS (U.S. EPA 1990a, 1990b). The approach used by SIMS estimates (1) individual liquidand gas-phase mass transfer coefficients for each pollutant, (2) equilibrium constants for each pollutant based 

on Henry’s Law constants, (3) overall mass transfer coefficients for each pollutant, and (4) emissions based 

on the pollutant mass balance in the vicinity of the surface impoundment. 

The DRAS calculates emission rates for particulates and volatiles and uses a Gaussian dispersion model to 

predict worst-case average chemical concentrations 1,000 feet downwind of a hypothetical land disposal 

facility, as documented in U.S. EPA’s “Ambient Air Dispersion Model” (U.S. EPA 1985c). The equations 

and default parameters used to calculate the concentrations of respirable particulates and volatiles at the POE 

are presented in Sections 2.3.1 and 2.3.2, respectively. 

The following sections describe the equations and default parameters used to calculate pathway delisting 

levels and assess risks associated with exposure to waste constituents via the air exposure pathways. Section 

4.3.3.1 presents the equations used to compute pathway delisting levels for standard multi year delistings. 

Section 4.3.3.2 presents the equations used to compute constituent-and pathway-specific risks for one-time 

delistings. 

4.3.3.1 Delisting Level Analysis 

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Before pathway delisting levels for carcinogens associated with the air inhalation exposure pathways can be 

computed, the constituent’s maximum allowable respirable air concentration at the POE (Cair) must be 

calculated. Cair is calculated at the target risk level using Equation 4-52. 

(4-52) 

where: 

Default 

Cair = constituent’s maximum allowable respirable calculated 3 air concentration at POE (mg/m ) -5 TR = target risk (unitless) user specified (default 1 x 10 )


AT = averaging time (years) 75 (U.S. EPA 1997b)
 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b)
 

IFAadj = inhalation factor, age-adjusted ([m 3

-year]/[kg-day]) 10.67 (Equation 3-3)
 

CSF = inhalation cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1)
 

Cair is used with the air transport equation (Equation 2-25) and the air emission equations (Equation 2-33 for 

volatiles and Equation 2-24 for particulates) to back-calculate the pathway total concentration delisting levels. 

The following sections describe the methods used to calculate delisting levels for the landfill particulates 

inhalation pathway (Cdl-air-p), the landfill volatiles inhalation pathway (Cdl-air-v), and the surface impoundment 

volatiles inhalation pathway (Co). If the total concentration of a given carcinogenic waste constituent exceeds 

Cdl-air-p, or if the TCLP concentration of a constituent exceeds either Cdl-air-v or Co, the waste may not qualify 

for a standard multi year delisting. However, the waste may qualify for a one-time delisting, depending on 

the results of risk analysis calculations presented in Section 4.3.3.2. 

Landfill Particulate Delisting Levels 

First the maximum allowable emission rate of the constituent, Qp, is back-calculated from Cair (see equation 

4-52) using Equation 4-53. 

(4-53) 

where: 

Default 

Q p = maximum allowable emission rate of waste calculated

 constituent particulates (mg/s) 

Cair = constituent’s maximum allowable air Equation 4-52

 concentration at POE (mg/m )3 

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3z = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

Lv = distance from the virtual point to the compliance Equation 2-27

 point located 1,000 feet (304.8 m) downwind (m) 

F = frequency that wind blows across waste unit 0.15 (U.S. EPA 1994a)

 toward the receptor (unitless) 

Then the pathway waste concentration delisting level for respirable landfill particulates in air (Cdl-air-p ) is 

calculated by setting the maximum allowable total emission rate for respirable particulates, Qp10, as equal to 

Qp and using Equation 4-54. 

(4-54) 

where: Default 

Cdl-air-p = pathway total concentration delisting level calculated

 for respirable landfill air particulates (mg/kg) 

Qp10 = emission rate of waste constituent particulates up set equal to Qp (Equation 4-53)

 to 10 ìm (mg/s) 

E = w10 wind erosion emissions rate of particulates up Equation 2-9 

 to 10 ìm (g/hr) 

Ev10 = vehicle travel emissions rate of particulates Equation 2-19

 up to 10 ìm (g/hr) 

El10 = waste loading and unloading emissions rate of Equations 2-21 and 2-22

 particulates up to 10 ìm (g/hr) 

Landfill Volatile Delisting Levels 

First the maximum allowable volatile emission flux of the constituent, Ei, from the landfill is calculated by 

setting the maximum allowable total emission rate of volatiles, Qv, as equal to Qp (calculated in 

Equation 4-53) and using Equation 4-55. 

(4-55) 

where: 

Default 

Ei = maximum allowable volatile emission flux of calculated 

4-53 
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 constituent (g/s) 

Qv = maximum allowable constituent volatile emission equal to Qp (Equation 4-53)

 rate (mg/s) 

1000 mg/1g conversion factor 

Then Ei is used to calculate the pathway waste constituent concentration delisting level for volatiles from a 

landfill (C ) using Shen’s modification of Farmer’s Landfill Volatilization Equation as presented in dl-air-v 

Equation 4-56 (adopted from EPA-450/3-84-020; Evaluation and Selection of Models for Estimating Air 

Emissions from Hazardous Waste Treatment, Storage, and Disposal Facilities). 

(4-56) 

where: 

Default 

Cdl-air-v = pathway waste constituent concentration calculated

 delisting level for volatiles from landfill (mg/kg) 

Ei = maximum allowable volatile emission Equation 4-55

 flux of constituent (g/s) 

d = depth of soil cover (m)	 0.1524 (U.S. EPA 1994a) 

PT	 = total sand porosity (unitless) 0.4 (U.S. EPA 1994a) 

= 2 Da diffusivity of constituent in air (cm /s) chem-specific (Appendix A-1) 

Pa = air-filled sand porosity (unitless) 0.4 (U.S. EPA 1994a) 2 Aexposed = surface area (m ) Equation 2-15
 

Csi = saturation vapor concentration of i in landfill Equation 4-57
 

1x10 6 = conversion from kilograms to milligrams
 4	 2 2 1x10	 = conversion from m to cm 

Finally, the saturation vapor concentration (C ) of waste constituent i in the landfill is calculated using si

Equation 4-57. 

(4-57)

where: 

Default 

3 Csi = saturation vapor concentration of i in landfill (g/m ) Calculated


MW = molecular weight of waste constituent i chem-specific (Appendix A-1)
 3	 -5 R = universal gas constant (atm-m /mole- EK) 8.21 x 10 

T = standard temperature (EK) 298 

Vp = Vapor Pressure (atm) chem-specific (Appendix A-1) 

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Surface Impoundment Volatile Delisting Levels 

The pathway leachate concentration delisting level for volatiles from waste disposed of in a surface 

impoundment is calculated by setting the maximum allowable total emission rate of volatiles, Q , as equal v 

to Qp (calculated in Equation 4-53) and using Equation 4-58. 

(4-58) 

where: Default 

Cdi-air-si = pathway leachate concentration delisting level for calculated

 volatiles from surface impoundment (mg/L) 

Qv = maximum allowable constituent volatile emission set equal to Qp (Equation 4-53)

 rate (mg/s) 

tf = time for constituent concentration to reach 1 % Equation 4-59 

of C (days) dl-air-si 

3 Vsi = volume of liquid in surface impoundment (m ) unit-specific (user-provided) 

K = overall mass transfer coefficient (m/s) Equation 4-60 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

tr = retention time of liquid in surface impoundment Equation 2-40


 (days)
 

The time for the constituent concentration to reach one percent of C , t , is computed using Equation 4-59. di-air-si f 

(4-59) 

where: 

Default 

f t = time for constituent concentration to reach calculated 

dl-air-si 1 percent of C (days) 

Vsi = volume of liquid in surface impoundment (m ) 3 unit-specific (user provided) 

K = overall mass transfer coefficient (m/s) Equation 4-60 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

The overall mass transfer coefficient, K, is calculated using Equation 4-60. 

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(4-60) 

where: 

Default 

K = overall water to air mass transfer coefficient (m/s) calculated 

Kl = liquid-phase mass transfer coefficient (m/s) Equation 4-61 

Kg = gas-phase mass transfer coefficient (m/s) Equation 4-62 

Keq = equilibrium constant (unitless) Equation 4-63 

Kl

 is calculated using Equation(4-61). 

(4-61) 

where: 

Default 

Kl = liquid-phase mass transfer coefficient (m/s) calculated 

U10 = wind speed (m/s) 5.73 (U.S. EPA 1994a) 2 Dw = diffusion coefficient in water (cm /s) chem-specific (Appendix A-1) 2 -6 Dether = diffusion coefficient of ether (cm /s) 8.56 x 10 (U.S. EPA 1994a) 

Kg is calculated using Equation (4-62). 

(4-62) 

where: 

Default 

Kg = gas phase mass transfer coefficient (m/s) calculated 

U10 = wind speed (m/s) 5.73 (U.S. EPA 1994a) 

Scg = Schmidt number on gas side (unitless) Equation 4-64 

de = effective diameter of surface impoundment (m) unit-specific (user-provided) 

Keq is calculated using Equation 4-63. 

(4-63) 

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where: 

Default 
 

Keq = equilibrium constant (unitless) calculated
 3 H = Henry’s Law constant (atm-m /mol) chem-specific (Appendix A-1) 3 -5 R = universal gas law constant (atm-m /mol-K) 8.21 x 10
 

T = standard temperature (K) 298
 

Scg is calculated using Equation 4-64. 

(4-64) 

where: 

Default 

Scg = Schmidt number on gas side (unitless) calculated 

ìa = viscosity of air (g/[cm-sec]) 0.000181 (U.S. EPA 1990a) 3 -3 ña = density of air (g/cm ) 1.2 x 10 (U.S. EPA 1990a) 2 Da = diffusivity of constituent in air (cm /s) chem-specific (Appendix A-1) 

4.3.3.2 Risk Analysis 

Risk as a function of the effective air concentration of a particulate or volatile carcinogenic constituent (Cair) 

is calculated for each of the three air exposure pathways as shown in Equation 4-65. 

(4-65) 

where: 

Default 

Risk = risk for waste constituent (unitless) calculated 3 Cavg = constituent’s air concentration at POE (mg/m )


Particulates Equation 4-66
 

Landfill volatiles Equation 4-67
 

Surface Impoundment volatiles Equation 4-71
 

IFAadj = inhalation factor, age-adjusted 10.67 (Equation 3-3) 3 ([m -year]/[kg-day]) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

CSF = constituent inhalation cancer slope factor chem-specific (Appendix A-1) 

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 (mg/kg-day)-1 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

Cavg at the POE is determined separately for each of the three air exposure pathways using the air emission 

and transport equations derived in Section 2.3 and presented below for each pathway. Using Equation 4-65, 

the DRAS calculates the risks for receptors at the POE for the three air exposure pathways. The program then 

sums the individual constituent risks using Equation 4-2 to determine the total risk for each of the air 

inhalation pathways. 

Landfill Particulates 

The average air concentration of a waste constituent at the POE (Cavg) derived from the particulate released 

from the landfill is calculated using Equation 4-66. 

(4-66) 

where: 

Default 

C avg = downwind concentration of waste constituent 

 at POE (mg/m )3 

calculated

Qp10 = emission rate of waste constituent particulates Equation 2-24

 up to 10 ìm (mg/s) 

3z = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

L v = distance from virtual point to compliance Equation 2-27

 point located 1,000 ft (304.8 m) downwind (m) 

F = frequency that wind blows across waste unit 0.15 (U.S. EPA 1994a)

 toward receptor (unitless) 

Landfill Volatiles 

For the to landfill volatiles inhalation pathway, Cavg for landfill volatile emissions is calculated using 

Equation 4-67. 

(4-67)

where: 

Default 

Cavg = downwind concentration of waste constituent at calculated 

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3 POE (mg/m )

Qv = constituent volatile emission rate (mg/s) Equation 4-68 

3z = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

Lv = distance from virtual point to compliance Equation 2-27

 point located 1,000 feet (304.8 m) downwind (m) 

F = frequency that wind blows across waste unit 0.15 (U.S. EPA 1994a)

 toward receptor (unitless) 

For volatile emissions from a covered landfill, the emission rate, Q , is calculated using Equation 4-68. v

(4-68) 

where: Default 

Qv = constituent volatile emission rate (mg/s) calculated 

Ei = emission flux of constituent (g/s) Equation 4-69 

1000mg/1g conversion factor to mg 

The emission flux of a volatile waste constituent released from the landfill, Ei

, is calculated using 

Equation 4-69. Equation 4-69 is essentially Shen’s modification of Farmer’s landfill volatilization equation 

(U.S. EPA 1984). Farmer’s equation was developed by U.S. EPA’s Office of Air Quality Planning and 

Standards (OAQPS) to estimate the rate of emission of volatiles from a covered landfill (U.S. EPA 1984). 

(4-69)

where: 

Default 

Ei = maximum allowable volatile emission calculated

 flux of constituent (g/s) 

C = waste concentration of constituent (mg/kg) waste-specific (user-provided) waste 

d = depth of soil cover (m)	 0.1524 (U.S. EPA 1994a) 

PT	 = total sand porosity (unitless) 0.4 (U.S. EPA 1994a) 

= 2 Da diffusivity of constituent in air (cm /s) chem-specific (Appendix A-1) 

Pa = air-filled sand porosity (unitless) 0.4 (U.S. EPA 1994a) 2 Aexposed	 = surface area (m ) Equation 2-15 

Csi	 = saturation vapor concentration of i in landfill Equation 4-70 

1x10 6	 = conversion from kilograms to milligrams 4	 2 2 1x10	 = conversion from m to cm 

And the saturation vapor concentration (Csi) of waste constituent i in the landfill is calculated using 

Equation 4-70. 

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(4-70) 

where: 

Default 3 Csi = saturation vapor concentration of i in landfill (g/m ) Calculated


pi = vapor pressure of waste constituent i (atm) chem-specific (Appendix A-1)
 

MWi = molecular weight of waste constituent i chem-specific (Appendix A-1)
 3 -5 R = universal gas constant (atm-m /mol- K) 8.21 x 10
 

T = standard temperature (K) 298
 

Surface Impoundment Volatiles 

For the surface impoundment volatiles inhalation pathway, Cavg for use in Equation 4-65 is calculated using 

Equation 4-71. 

(4-71) 

where: 

Default 

Cavg = downwind concentration of waste constituent at calculated 3 POE (mg/m )

Qv = constituent volatile emission rate (mg/s) Equation 4-72 

F = frequency wind blows from sector of interest 0.15 (U.S. EPA 1994a) 

3z = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

Lv = distance from virtual point to compliance Equation 2-27


 point located 1,000 feet (304.8 m) downwind (m) 


Qv for volatile emissions from a surface impoundment is computed using Equation 4-72. 

(4-72) 

where: 

Default 

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Qv = volatile emission rate of constituent i from calculated

 the surface impoundment (mg/s) 

C = liquid waste concentration of constituent (mg/L) waste-specific (user-provided) waste 

3 Vsi = volume of liquid in surface impoundment (m ) unit-specific (user-provided) 

K = overall mass transfer coefficient (m/s) Equation 4-60 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

tr = retention time for liquid in surface impoundment Equation 2-40


 (days) 
 

tf = time for constituent concentration to reach 1 Equation 4-59
 

percent of Co (days)
 

4.3.4 Soil Exposure Pathway 

This section presents equations used to calculate pathway total concentration delisting levels and risks 

associated with exposure of children to soils contaminated with carcinogenic waste constituents at a POE 

1,000 feet from a waste disposal unit. The soil contamination is assumed to be the result of deposition of 

waste particulates from the air. The equations used to compute the delisting levels and risks are based on a 

method described in U.S. EPA Headquarters delisting docket materials (U.S. EPA 1993a, 1993b). 

Section 4.3.4.1 presents the equations used to compute pathway total concentration delisting levels for 

standard multi year delistings. Section 4.3.4.2 presents the equations and methods used to compute 

constituent-and pathway-specific risks for one-time delistings. 

4.3.4.1 Delisting Level Analysis 

To calculate the pathway total concentration delisting levels for exposure to carcinogenic waste constituents 

via ingestion of soil ,C , the maximum allowable concentration of each constituent in soil at the POE, C , dl-soil soil 

must first be computed. Csoil is the maximum concentration of the waste constituent in soil that does not 

cause the risk from soil ingestion to exceed the target level. Csoil is calculated using Equation 4-73. 

(4-73) 

where: 

Default 
 

Csoil = maximum allowable concentration of calculated


 constituent in soil at POE (mg/kg)
 -5 TR = target risk level (unitless) user-specified (default 1 x 10 )

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AT = averaging time (years) 75 (U.S. EPA 1997b)
 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b)
 

IFSadj = soil ingestion factor ([mg/year]/[kg/day]) 113 (Equation 3-2)
 

CSForal = constituent oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1)
 

Using Csoil, the DRAS back-calculates the maximum allowable constituent concentration in air at the POE, 

Cavg. Assumptions include a constituent accumulation period in the soil of 1 year and a depth of mixing of 

1 cm Cavg is calculated as shown in Equation 4-74. 

(4-74) 

where: 

Default 

Cavg = maximum allowable concentration of calculated 3 constituent in air at POE (mg/m )


Csoil = maximum allowable concentration of Equation 4-73


 constituent in soil at POE (mg/kg)
 3 ñb = soil bulk density (g/cm ) 1.45 (Brady 1984) 

t = soil thickness from which particles can be 0.01 (U.S. EPA 1994a)

 ingested (m) 

vd = deposition velocity (m/s) 0.03 (U.S. DOE, 1984) 

Before the pathway delisting level can be calculated, it is necessary to compute the annual average emission 

rate of particles up to 30 ìm in size from the disposal unit as shown in Equation 4-75. 

(4-75) 

where: 

Default 

ET30 = annual average emission rate of particles up to calculated

 30 ìm (g/hr) 

Ew30 = wind erosion emission rate of particulates up to Equation 2-16

 30 ìm (g/hr) 

Ev30 = vehicle travel emissions rate of particulates up to Equation 2-20

 30 ìm (g/hr) 

El30 = loading and unloading emission rate of particles Equation 2-23

 up to30 ìm (g/hr) 

Finally, once Cair and ET30 are known, the total concentration delisting level is computed using Equation 4-76. 

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(4-76) 

where: 

Default 
 

Cdl-soil = pathway total concentration delisting level calculated


 for soil ingestion (mg/kg)
 

Cavg = maximum allowable concentration of Equation 4-74
 3 constituent in air at POE (mg/m )

Gz = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1986a) 

Lv = distance virtual point to compliance Equation 2-27

 point located 1,000 feet (0.3048 km) downwind (m) 

ET30 = annual average emission rate of particles up to Equation 4-75 ( also, see

 30 ìm (g/hr) Section 2.3.1.3) 

F = frequency that wind blows across waste unit 0.15 (U.S. EPA 1994a)

 toward receptor (unitless) 

If the total concentration for a given carcinogenic waste constituent exceeds its pathway total concentration 

delisting level, C , the waste may not qualify for a standard multi year delisting. However, the waste may dl-soil 

qualify for a one-time delisting, depending on the results of risk analysis calculations presented Section 

4.3.4.2. 

4.3.4.2 Risk Analysis 

In the risk analysis, the DRAS computes the risk associated with the soil ingestion exposure pathway. The 

risk for each carcinogenic waste constituent associated with soil ingestion at the POE is calculated. Children 

have higher daily soil intake rates when comparing their contact rates and lower body weights to those of 

adults (U. S. EPA 1997b). However, the child’s exposure duration is considerably shorter than the adult’s. 

For carcinogens, exposure is averaged over a lifetime (75 years; U. S. EPA 1997b). Exposure to 

contaminants via soil ingestion is calculated as a weighted average of exposures for 6 years as a child and as 

an adult. The total exposure is still averaged over a lifetime. For the soil ingestion exposure scenario, the 

constituent-specific cancer risk from ingestion of soil is calculated using Equation 4-77. 

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(4-77) 

where: 

Default 

Risk = risk for waste constituent (unitless) Calculated 

Csoil = conc. of constituent in soil at POE (mg/kg) Equation 4-78 

IFSadj = soil ingestion factor ([mg/year]/[kg/day]) 113 (Equation 3-2) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

CSForal = oral cancer slope factor (mg/kg-day) -1 chem-specific (Appendix A-1) 

AT = averaging time (years) 75 (U.S. EPA 1997b) 

Csoil is calculated based on the constituent’s rate of deposition onto the soil. Assuming that at any given time 

the soil is burdened with 1 year’s accumulation of constituent from air deposition and that the mixing depth 

of the constituent into the soil is 1 cm, Equation 4-78 is used to calculate Csoil. 

(4-78) 

where: 

Default 
 

Csoil = conc. of constituent in soil at the POE (mg/kg) calculated
 

qd = constituent deposition rate (mg/m 2

-s) Equation 4-79
 3 ñb = soil bulk density (g/cm ) 1.45 (Brady 1984) 

t = soil thickness of particles that can be ingested (m) 0.01 (U.S. EPA 1994a) 

The constituent deposition rate is determined using the deposition velocity and the constituent concentration 

in air, as shown in Equation 4-79. 

(4-79) 

where: 

Default 

qd = constituent deposition rate (mg/m -s) 2 calculated 

vd = deposition velocity (m/s) 0.03 (U.S. DOE, 1984) 

C avg = downwind concentration of waste constituent at Equation 4-80 

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3 POE (mg/m )

Cavg is calculated using Equation 4-80. 

(4-80)

where: 

Default 

C avg = downwind concentration of waste constituent at 

 POE (mg/m )3 

calculated

Qp30 = emission rate of waste constituent particulates Equation 2-30

 up to 30 ìm (mg/s) 

3z = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

L v = distance from virtual point to compliance Equation 2-27

 point located 1,000 feet (304.8 m) downwind (m) 

F = frequency that wind blows across waste unit 0.15 (U.S. EPA 1994a)

 toward receptor (unitless) 

Using Equation 4-77, the DRAS calculates the risk associated with ingestion of soil for each constituent. Then 

the program sums the constituent-specific risks using Equation 4-2 to determine the total risk for the soil 

ingestion pathway. 

4.4 QUANTITATIVE ESTIMATION OF POTENTIAL NONCANCER EFFECTS 

This section presents the equations used to calculate pathway delisting levels and aggregate hazards 

associated with exposures to noncarcinogenic chemicals via the groundwater, surface water, air, and soil 

exposure pathways. Calculation of delisting levels and risks associated with exposures to carcinogenic 

chemicals is described in Section 4.3. The equations used below to compute pathway delisting levels and 

aggregate hazards are adapted from the “HWIR Technical Support Document” (U.S. EPA 1995b) and U.S. 

EPA OSW delisting dockets (U.S. EPA 1993a, 1993c, 1994a). The general hazard equations presented in 

Section 4.2.2 are implemented for specific constituents-and exposure pathways. The constituent and 

pathway-specific HQc,p, is computed for each constituent using an expanded form of Equation 4-4. Then the 

HQ is summed across all constituents to calculate the HI , for each exposure pathway, as shown in c,p p 

Equation 4-5. The DRAS sums all the HIp values for the groundwater exposure pathways and the surface 

exposure pathways separately to assist the user in identifying the most important exposure pathways. Finally, 

the aggregate HIcum is calculated by summing the HIp across all the groundwater and surface exposure 

pathways, as shown in Equation 4-6. Constituent-and pathway-specific subscripts are omitted in subsequent 

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equations for simplicity of presentation; the context indicates whether the value being calculated is 

constituent-or pathway-specific. 

For exposures to noncarcinogens, the DRAS evaluates the following groundwater pathways: ingestion of 

groundwater, dermal exposure to groundwater during bathing (adult and child), and inhalation exposure to 

volatile groundwater components during showering. The equations used to compute the associated with 

groundwater exposures are discussed in Section 4.4.1. For surface water, the DRAS evaluates hazards 

associated with the following exposure pathways: ingestion of drinking water and ingestion of fish from 

surface water bodies contaminated by runoff from landfills; the relevant equations are discussed in Section 

4.4.2. For air exposure pathways, the DRAS evaluates hazards associated with inhalation of particulates and 

volatiles from solid wastes disposed of in landfills and inhalation of volatiles from liquid wastes disposed of 

in surface impoundments; the relevant equations are discussed in Section 4.4.3. Finally, the DRAS evaluates 

hazards associated with a child receptor’s ingestion of soil contaminated by air deposition of particulates 

eroded from a landfill. The relevant equations are discussed in Section 4.4.4. 

4.4.1 Groundwater Exposure Pathways 

For noncarcinogens, the DRAS evaluates exposure to groundwater using the same pathways that it uses for 

carcinogens: ingestion of groundwater, dermal exposure to groundwater during bathing, and inhalation of 

volatiles during showering with groundwater. The receptor is assumed to be an adult for the ingestion and 

showering pathways; both adult and child receptors are considered for bathing pathway. The following 

sections address calculation of constituent-and pathway-specific HQs for each constituent and groundwater 

pathway. 

4.4.1.1 Adult Ingestion of Groundwater 

The DRAS computes leachate concentration delisting levels and hazards associated with exposure of adults 

to noncarcinogenic waste constituents via the groundwater ingestion pathway. Section 4.4.1.1.1 presents the 

equations used to compute pathway delisting levels for standard multi year delistings. Section 4.4.1.1.2 

presents the equations and methods used to compute the constituent-specific HQ and the pathway-specific 

HI for one-time delistings. 

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4.4.1.1.1 Delisting Level Analysis 

Pathway leachate concentration delisting levels for noncarcinogens associated with ingestion of groundwater, 

Cdl-ingest, are determined based on a default target hazard quotient (THQ) of 0.1 in order to account for HQ from 

other pathways and/or chemicals. To calculate Cdl-ingest , the DRAS first computes the maximum allowable 

groundwater concentration for the ingestion pathway for the waste constituent Cgw-ingest using Equation 4-81. 

(4-81) 

where: 

Default 

Cgw-ingest = maximum allowable constituent concentration calculated

 in groundwater for ingestion (mg/L) 

THQ = target hazard quotient (unitless) user specified (0.1 default) 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

CR = water consumption rate (L/day) 2 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

Once Cgw-ingest is calculated, Cdl-ingest can be computed. Cgw-ingest is multiplied by the waste constituent’s DAFva 

to generate Cdl-ingest as shown in Equation 4-82. 

(4-82) 

where: 

Default 
 

Cdl-ingest = pathway leachate concentration delisting level calculated


 for groundwater ingestion (mg/L)
 

DAFva = waste volume-adjusted dilution attenuation factor equation 2-5


 (unitless)
 

Cgw-ingest = maximum allowable constituent concentration in equation 4-81


 groundwater for ingestion (mg/L)
 

If the TCLP concentration for a given waste constituent exceeds its Cdl-ingest , the waste may not qualify for a 

standard multi year delisting. However, the waste may qualify for a one-time delisting, depending on the 

results of hazard analysis calculations presented in Section 4.4.1.1.2. 

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4.4.1.1.2 Hazard Analysis 

In the hazard analysis, the DRAS computes the hazard associated with the groundwater ingestion exposure 

pathway. The DRAS calculates the noncarcinogenic waste constituent-specific HQ for ingestion of 

groundwater using Equation 4-83. 

(4-83) 

where: 

Default 

HQ = constituent-specific hazard quotient for calculated

 ingestion of groundwater (unitless) 

CR = water consumption rate (L/day) 2 (U.S. EPA 1997b) 

Cgw = waste constituent concentration in groundwater Equation 2-6

 (mg/L) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

The HQs for all noncarcinogenic constituents in the petitioned waste are summed using Equation 4-5 to 

determine the HIp for the groundwater ingestion pathway. 

4.4.1.2 Adult and Child Dermal Contact with Groundwater During Bathing 

The DRAS computes of leachate concentration delisting levels and hazards associated with adult and child 

exposures during bathing through dermal absorption of noncarcinogenic constituents in groundwater. Section 

4.4.1.2.1 presents the equations used to compute pathway delisting levels for standard multi year delistings. 

Section 4.4.1.2.2 presents the equations and methods used to compute constituent-specific HQs and pathwayspecific HIs for one-time delistings. 

4.4.1.2.1 Delisting Level Analysis 

Pathway leachate concentration delisting levels for noncarcinogens associated with adult and child dermal 

exposures to groundwater during bathing, Cdl-dermal, are based on the target hazard quotient. The DRAS 

computes delisting levels for the adult and child receptors after separately determining their respective 

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exposures. The DRAS then selects the delisting level that corresponds to the more sensitive receptor. Cdl-dermal 

is calculated as the product of the waste constituent’s maximum allowable groundwater concentration for 

dermal exposure during bathing, Cgw-dermal, and the waste constituent DAFva generated by the EPACMTP 

model (see Section 2.2). Equation 4-84 shows this calculation. 

(4-84) 

where: 

Default 

Cdl-dermal = pathway leachate concentration delisting calculated

 level for groundwater dermal contact (mg/L) 

DAF va = waste volume-adjusted dilution Equation 2-6

 attenuation factor (unitless) 

Cgw-dermal = maximum allowable constituent concentration Equation 4-12, 4-13, or 4-14

 in groundwater for dermal exposure (mg/L) 

For organic waste constituents, Cgw-dermal is computed using one of two methods, depending on whether the 

duration of the bath t is greater or less than the time required for the flux of the chemical through the skin event 

to reach steady-state (t

*

, a chemical-specific constant). For inorganic waste constituents, only one method 

is used. The maximum allowable constituent concentration in groundwater is computed using Equation 4-12 

for organic constituents when t is less than t

*

, Equation 4-13 for organic constituents when t is greater event event 

than or equal to t

*

, and Equation 4-14 for inorganic constituents. 

The dose of the waste constituent absorbed during bathing per unit area per event (DA ) is used in each event 

equation to calculate Cgw-dermal. DAevent is calculated separately for adults and children using Equation 4-85; 

the appropriate default parameters are used for the adult and child receptors. 

(4-85) 

where: 

Default 2 DA = dose absorbed per event (mg/cm -event) calculated event

AT = averaging time (years) 

-adult 30 (U.S. EPA 1997b) 

-child 6 (U.S. EPA 1997b) 

DAD = dermally absorbed dose (mg/kg-day) Equation 4-86 

BW = body weight (kg) 

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- adult 72 (U.S. EPA 1997b) 

- child 15 (U.S. EPA 1997b) 

EV = event frequency (events/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 

-adult 30 (U.S. EPA 1997b) 

-child 6 (U.S. EPA 1997b) 

Askin = exposed skin surface area (cm )2 

- adult 20,000 (U.S. EPA 1997b) 

- child 7,900 (U.S. EPA 1997b) 

The DAD for dermal contact with groundwater needed in Equation 4-85 is calculated using Equation 4-86. 

DAD is the product of the THQ and the RfD for a given noncarcinogenic waste constituent. 

(4-86) 

where: 

Default 

DAD = dermally absorbed dose (mg/kg-day) calculated 

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

Cdl-dermal is calculated using Equation 4-84 for each noncarcinogenic waste constituent in the petitioned waste. 

If the TCLP concentration of a given waste constituent exceeds C , the waste may not qualify for a dl-dermal 

standard multi year delisting. However, the waste may qualify for a one-time delisting, depending on the 

results of hazard analysis calculations presented in Section 4.4.1.2.2. 

4.4.1.2.2 Hazard Analysis 

In the hazard analysis, the DRAS computes the hazard associated with the dermal contact with groundwater 

during bathing exposure pathway. The DAD for exposure to a noncarcinogenic waste constituent during 

bathing is computed using Equation 4-87. 

(4-87) 

where: 

Default 

DAD = dermally absorbed dose (mg/kg-day) calculated 2 DA = dose absorbed per event (mg/cm -event) Equation 4-19, 4-20, or 4-21 event 

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EV = event frequency (events/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 

- adult 30 (U.S. EPA 1997b) 

- child 6 (U.S. EPA 1997b) 

Askin = exposed skin surface area (cm )2 

- adult 20,000 (U.S. EPA 1997b) 

- child 7,900 (U.S. EPA 1997b) 

AT = averaging time (years) 

- adult 30 (U.S. EPA 1997b) 

- child 6 (U.S. EPA 1997b) 

BW = body weight (kg) 

- adult 72 (U.S. EPA 1997b) 

- child 15 (U.S. EPA 1997b) 

DA is computed using Equation 4-19 for organic constituents when t is less than t

*

, Equation 4-20 for event event 

organic constituents when t is greater than or equal to t

*

, and Equation 4-21 for inorganic constituents. event 

All three equations use the groundwater well concentration, Cgw, which is equal to the TCLP concentration 

of the constituent divided by the DAFva (see Equation 2-6). 

The constituent-specific HQ for the dermal contact with groundwater during bathing exposure pathway is 

computed as shown in Equation 4-88 (U.S. EPA 1992b). 

(4-88) 

where: 

Default 

HQ = constituent-specific hazard quotient for dermal calculated

 contact with groundwater during bathing (unitless) 

DAD = dermally absorbed dose (mg/kg-day) Equation 4-87 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

Using Equation 4-88, the DRAS calculates the constituent-specific HQs for groundwater exposure via dermal 

absorption during bathing and, using Equation 4-5, sums the HQs to determine the total HIp for the 

groundwater dermal absorption pathway. 

4.4.1.3 Adult Inhalation of Volatiles from Groundwater During Showering 

The DRAS computes leachate concentration delisting levels and hazards associated with exposure of adults 

to noncarcinogenic volatile constituents groundwater during showering. Exposure of children is not evaluated 

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for this pathway because the adult resident receptor scenario has been found to be protective of children (U.S. 

EPA 1997a). 

The method used to determine waste constituent-specific exposures resulting from inhalation of constituents 

in tap water is based on information used to support the “ Petroleum Refinery Listing Determination” (U.S. 

EPA 1997a). This project used a method developed by McKone (1987) to estimate a time-varying constituent 

concentration in three compartments: the shower, the bathroom, and the house. 

The constraints associated with implementing the risk algorithms in a QuattroPro Version 6.0 spreadsheet 

precluded incorporating and solving time-varying shower equations because a complex system of macros and 

linked analyses would have been required. Therefore, U.S. EPA Region 6 uses simplified versions of the 

equations that compute a conservative constant concentration for each of the three compartments. The 

simplified equations are based on the steady-state model developed by McKone and Bogen (1992), which 

calculates a chemical concentration in air for each of the three compartments. 

Section 4.4.1.3.1 presents the equations used to compute delisting levels for standard multi year delistings.

 Section 4.4.1.3.2 presents the equations used to compute the constituent-specific HQ and pathway-specific 

HI for one-time delistings. 

4.4.1.3.1 Delisting Level Analysis 

The pathway leachate concentration delisting levels for noncarcinogens associated with adult inhalation 

exposure to groundwater during showering, C , are back-calculated from the maximum allowable dl-inhale 

groundwater concentration for the showering exposure pathway, Cgw-inhale . Specifically, Cdl-inhale is calculated 

as the product of Cgw-inhale and the constituent’s DAFva using Equation 4-89. 

(4-89) 

where: 

Default 

Cdl-inhale = pathway leachate concentration delisting calculated

 level for shower inhalation (mg/L) 

DAF va = waste volume-adjusted dilution attenuation Equation 2-5

 factor (unitless) 

Cgw-inhale = maximum allowable constituent concentration Equation 4-91

 in groundwater for inhalation (mg/L) 

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To calculate Cgw-inhale , one must first calculate the maximum allowable concentration volatilized into air, 

CET , during showering. For noncarcinogens, CET , is calculated by DRAS using the target hazard max max 

quotient and the constituent’s inhalation RfD. Equation 4-90 shows how CETmax , is calculated. 

(4-90) 

where: 

Default 

CET = sum of products of maximum allowable calculated (Equation 4-27) 

constituent air concentration in specific compartment 3 and corresponding exposure time (mg-days/m )

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 

RfD = constituent inhalation reference dose (mg/kg-day) chem-specific (Appendix A-1) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 3 IR = inhalation rate (m /day) 20 (U.S. EPA 1997b) 

EV = event frequency (showers/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

CETmax , is the maximum allowable constituent concentration in air that will not allow the acceptable hazard 

level to be exceeded. Once CETmax , is calculated for a given waste constituent, the DRAS calculates Cgw-inhale 

based on the inhalation exposure in the shower, bathroom, and house compartments combined. The DRAS 

calculates Cgw-inhale using Equation 4-91. 

(4-91) 

where: 

Default 

Cgw-inhale = maximum allowable constituent concentration calculated

 in groundwater used for showering (mg/L) 

CET max , = sum of products of maximum allowable calculated (Equation 4-90) 

constituent air concentration in specific compartment 

and corresponding exposure time (mg-days/m )3 

FGW, comp = fraction of average waste constituent concentration Equation 4-25

 in the compartment over the groundwater waste 

 constituent concentration based on a forward calculation 

ET comp = exposure time in compartment (days/shower) (U.S. EPA 1997b) 

- shower 0.00792 

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- bathroom 0.0338 

- house 0.625 

If the TCLP concentration for a given waste constituent exceeds C , the waste may not qualify for a dl-inhale 

standard multi year delisting. However, the waste may qualify for a one-time delisting, depending on the 

results of hazard analysis calculations presented in Section 4.4.1.2.3. 

4.4.1.3.2 Hazard Analysis 

In the hazard analysis, the DRAS computes the hazard associated with adult inhalation exposure to a 

noncarcinogenic waste constituent during showering with groundwater. To determine total inhalation 

exposure to volatilized waste constituents in the house during and after showering, the DRAS computes the 

air concentration of waste constituents in three compartments of the house; the shower, the bathroom and the 

house. The DRAS computes each constituent’s HQ for this exposure pathway using Equation 4-92. 

(4-92) 

where: 

Default 

HQ = hazard quotient for inhalation during showering calculated

 for constituent (unitless) 

CET = sum of products of maximum allowable calculated (Equation 4-27) 

constituent air concentration in specific compartment 3 and corresponding exposure time (mg-days/m )

EV = event frequency (showers/day) 1 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ET comp = exposure time in compartment (days/shower) (U.S. EPA 1997b) 

- shower 0.00792 

- bathroom 0.0338 

- house 0.625 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 3 IR = inhalation rate (m /day) 20 (U.S. EPA 1997b) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

RfD = constituent’s inhalation reference dose chem-specific (Appendix A-1)

 (mg/kg-day) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

1000L/m 3 conversion factor for cubic meters to liters 

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CET is calculated by summing the waste constituent concentration for each of the three compartments in the 

house using Equation 4-27 and the appropriate default parameters. There is a theoretical maximum air 

concentration that can occur in a compartment. The DRAS calculates that theoretical maximum 

concentration, C , for each compartment using the Ideal Gas Law. The DRAS calculates C for all three air-max air-i 

compartments and compares the compartment concentration to the theoretical maximum. If C exceeds air-i 

C for a given waste constituent, C is reset to the value of C . Using Equation 4-92 the user air-max air-i air-max 

calculates the constituent-specific HQs for groundwater exposure via inhalation during showering. The 

DRAS then sums these HQs using Equation 4-5 to determine the HIp for the pathway. 

4.4.2 Surface Water Exposure Pathways 

The surface water exposure pathways addressed by the DRAS include ingestion of surface water and 

ingestion of fish. An adult receptor is assumed for both pathways. The pathway delisting levels and hazards 

for these pathways are calculated using a method described in EPA Headquarters delisting docket materials 

(U.S. EPA 1993a, 1993b). The method uses the USLE to calculate the rate of erosion of a petitioned waste 

from a landfill to a surface water body. The receiving water body is assumed to be a stream. The amount 

of eroded waste delivered in runoff to the stream is calculated using a sediment delivery ratio, and the volume 

of surface water into which runoff occurs is determined by estimating the size of the stream into which the 

eroded waste is likely to be transported. 

For the purposes of delisting, the stream is assumed to be a second-order or fifth-order stream (U.S. EPA 

1993a). A second-order stream is assumed to be the smallest stream capable of supporting fish, whereas a 

fifth-order stream is assumed to be the smallest stream capable of serving as a community water supply. The 

surface water ingestion analyses are based on the assumption that a noncarcinogenic waste constituent is 

transported to a fifth-order stream; the fish ingestion analyses are based on the assumption that the chemical 

is transported to a second-order stream. 

Sections 4.4.2.1 and 4.4.2.2 describe the equations and default parameters used to calculate the pathway 

delisting levels and hazard associated with the ingestion of surface water and ingestion of fish exposure 

pathways. 

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4.4.2.1 Ingestion of Surface Water 

The DRAS computes total concentration delisting levels and hazards associated with exposure of adults to 

noncarcinogenic waste constituents via the surface water ingestion pathway. Section 4.4.2.1.1 presents the 

equations used to compute pathway total concentration delisting levels for standard multi year delistings. 

Section 4.4.2.1.2 presents the equations and methods used to compute constituent- and pathway-specific 

hazards for one-time delistings. 

4.4.2.1.1 Delisting Level Analysis

 Pathway total concentration delisting levels for noncarcinogens associated with ingestion of surface water, 

C , are computed for the target hazard quotient.. To calculate C , it is necessary to first calculate the dl-water dl-water 

maximum allowable concentration of the waste constituent in surface water, Csw, using Equation 4-93. The 

DRAS then calculates C using C . dl-water sw 

(4-93) 

where: 

Default 

Csw = maximum allowable concentration of calculated

 constituent in surface water (mg/L) 

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 

RfD = reference dose (mg/kg-day) chem-specific (Appendix A-1) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

CR = water consumption rate (L/day) 2.0 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

Once C is known, C is calculated using Equation 4-94. sw dl-water 

(4-94) 

where: 

Default 

Cdl-water = pathway total waste concentration delisting calculated 

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 level for ingestion of surface water (mg/kg) 

Csw = maximum allowable concentration of Equation 4-93


 constituent in surface water (mg/L)
 

C5th-stream = concentration of constituents in fifth-order stream Equation 4-43


 (kg/L)
 

If the total concentration of a constituent in the waste exceeds its pathway total concentration delisting level, 

Cdl-water, the waste may not qualify for a standard multi year delisting. However, the waste may qualify for 

a one-time delisting, depending on the results of hazard analysis calculations presented in Section 4.4.2.1.2. 

4.4.2.1.2 Hazard Analysis 

In the hazard analysis, the DRAS computes hazards associated with the surface water ingestion pathway. The 

DRAS calculates each noncarcinogenic waste constituent’s HQ for this pathway using Equation 4-95. 

(4-95)

where: 

Default 
 

HQ = constituent-specific hazard quotient calculated


 for ingestion of surface water (unitless) 

CR = water consumption rate (L/day) 2.0 (U.S. EPA 1997b) 

Csw = concentration of constituent in surface Equation 4-45

 water (mg/L) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

RfD = reference dose (mg/kg-day) chem-specific (Appendix A-1) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

Equation 4-45, which is used to compute Csw, is based on the USLE and the sediment delivery ratio for 

overland transport of eroded waste to surface water as described in the delisting docket (U.S. EPA 1993a). 

Using Equation 4-95, the DRAS calculates the HQ associated with ingestion of surface water for each waste 

constituent. The program then sums the HQs using Equation 4-5 to determine the total HIp for the surface 

water ingestion pathway. 

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4.4.2.2 Ingestion of Fish 

This section describes the computation of total pathway concentration pathway delisting levels and hazards 

associated with exposure of adults to noncarcinogenic waste constituents via ingestion of fish. Section 

4.4.2.2.1 presents the equations used to compute constituent-specific total concentration delisting levels for 

standard multi year delistings. Section 4.4.2.2.2 presents the equations used to compute constituent-and 

pathway-specific hazards for one-time delistings. The BAF values used are for a trophic level 4 (carnivorous) 

fish (U.S. EPA 1995e). 

4.4.2.2.1 Delisting Level Analysis 

This section describes the equations used to calculate the pathway total concentration delisting levels for 

exposure to noncarcinogenic constituents through fish ingestion, C . The DRAS first calculates the dl-fish 

maximum allowable constituent concentration in surface water that will not result in a constituent 

concentration in fish tissue in excess of the maximum edible concentration. The maximum allowable fish 

tissue concentration for a waste constituent, Cfish , is calculated at the target hazard quotient using 

Equation 4-96. 

(4-96) 

where: 

Default 

Cfish = maximum allowable concentration of calculated

 constituent in fish tissue (mg/kg) 

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 

RfD = reference dose (mg/kg-day) chem-specific (Appendix A-1) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

CRfish = fish consumption rate (kg/day) 0.02 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

Once Cfish is calculated, the corresponding maximum allowable concentration of the of the constituent freely 

dissolved in surface water is calculated using Equation 4-97. 

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(4-97)

where: 

Default 

Cdw = maximum allowable dissolved phase water calculated

 concentration in the second order stream (mg/kg) 

Cfish = maximum allowable concentration of constituent Equation 4-96

 in fish tissue (mg/kg) 

BAF = bioaccumulation factor of waste constituent (L/kg) chem-specific (Appendix A-1)

 or 

BCF = bioconcentration factor of waste constituent (L/kg) chem-specific (Appendix A-1) 

Using Equation 4-98, the maximum allowable dissolved phase water in the second order stream is converted 

to the total concentration of the waste constituent in water column (Csw): 

 ( 4-98) 

where: 

Default 

Cdw = Dissolved phase water concentration (mg/L) Equation 4-97 

Csw = Waste concentration in water column (mg/L) calculated 

Kdsw = Suspended sediments/surface water partition chem-specific (Appendix A-1)

 coefficient (L water/kg suspended sediment) 

TSS = Total suspended solids concentration (mg/L) Equation 2-52 

1 x 10-6 Units conversion factor (kg/mg) 

Finally, C is used in Equation 4-99 to back calculate the delisting level (C ) for the fish ingestion sw dl-fish 

pathway. 

(4-99) 

where: 

Cdl-fish = pathway total concentration delisting 

 level for fish ingestion (mg/kg) 

Default 

calculated

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Csw = maximum allowable concentration in water Equation 4-98 

column of constituent in 2nd order stream (mg/L) 

Aw = rate of waste ans soil erosion from landfill Equation 2-51 

(kg/[acre-year]) 

Q2ndstream = volumetric flow of second-order stream (L/year) 3.3 x 109 

(U.S. EPA 1994a) 

A = area of waste management unit (acres) Equation 2-12 or -13 

If the total concentration of a given waste constituent exceeds its pathway total concentration delisting level, 

Cdl-fish, the waste may not qualify for a standard multi year delisting. However, the waste may qualify for a 

one-time delisting, depending on the results of hazard analysis calculations presented in Section 4.4.2.2.2. 

4.4.2.2.2 Hazard Analysis 

In the hazard analysis, the DRAS computes the hazards associated with the fish ingestion exposure pathway. 

The DRAS computes each noncarcinogenic waste constituent’s HQ for this pathway using Equation 4-100. 

(4-100) 

where: 

Default 

HQ = hazard quotient (unitless) calculated 

Cfish = concentration of constituent in fish tissue (mg/kg) Equation 4-51 

CRfish = fish consumption rate (kg/day) 0.02 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

ED = exposure duration (years) 30 (U.S. EPA 1997b) 

BW = body weight (kg) 72 (U.S. EPA 1997b) 

RfD = reference dose (mg/kg-day) chem-specific (Appendix A-1) 

AT = averaging time (years) 30 (U.S. EPA 1997b) 

The concentration of the waste constituent in fish tissue, Cfish , is computed using Equation 4-51. This 

equation is based on an estimate of the dissolved fraction (Cdw) of the waste constituent concentration present 

in a second-order stream. The DRAS sums the HQs for the noncarcinogenic waste constituents using 

Equation 4-5 to determine the total HIp for the fish ingestion pathway. 

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4.4.3 Air Exposure Pathways 

The DRAS includes an analysis of the human health hazards associated with exposures to noncarcinogens 

via three air exposure pathways: (1) inhalation of particulates from solid wastes disposed of in landfills, 

(2) inhalation of volatiles from solid wastes disposed of in landfills, and (3) inhalation of volatiles from liquid 

wastes disposed of in surface impoundments. An adult receptor is assumed for all three pathways. The 

delisting level for the inhalation of particulates pathway is expressed in terms of the total constituent 

concentration in the waste. Leachate concentration delisting levels are generated for the inhalation of 

volatiles pathways. 

The risks associated with inhalation exposure to particulate and volatile matter can typically be determined 

only when inhalation HBNs are available. Inhalation reference concentrations derived from oral reference 

doses are only used when specifically recommended by IRIS or when target organ endpoints are far from the 

respiratory system (U.S. EPA 2006). 

The methods used to evaluate the release and transport of noncarcinogenic particulates and volatiles from 

waste disposal units are the same as those used for carcinogens; only the assessment calculations differ. A 

discussion of the source of the methods is presented in Section 4.3.3. 

The following sections describe the equations and default parameters used to calculate pathway delisting 

levels and assess hazards associated with exposure to waste constituents via the air exposure pathways. 

Section 4.4.3.1 presents the equations used to compute pathway delisting levels for standard multi year 

delistings. Section 4.4.3.2 presents the equations used to compute constituent-specific and pathway-specific 

hazards for one-time delistings. 

4.4.3.1 Delisting Level Analysis 

This section describes the equations used to calculate air exposure pathway delisting levels for 

noncarcinogenic waste constituents. Before the delisting levels can be calculated, each constituent’s 

maximum allowable respirable air concentration at the POE, Cair, must be calculated. Cair is calculated at the 

target hazard quotient, as shown in Equation 4-101. 

(4-101) 

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where: 

Default 

Cair = constituent’s maximum allowable respirable air calculated 3 concentration at POE (mg/m )

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 3 RfC = constituent reference concentration (mg/m ) chem-specific (Appendix A-1)
 

AT = averaging time (years) 30 (U.S. EPA 1997b)
 

ED = exposure duration (years) 30 (U.S. EPA 1997b)
 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b)
 

Cair is used with the air transport equation (Equation 2-25) and the particulate and volatile air emission 

equations (Equation 2-24 for particulates, and Equation 2-33 for volatiles) to back-calculate the pathway total 

concentration and leachate concentration delisting levels, respectively. The following sections describe the 

methods used to calculate delisting levels for landfill particulates (Cdl-air-p), landfill volatiles (Cdl-air-v), and 

surface impoundment volatiles (Co). If the total concentration of a given noncarcinogenic waste constituent 

exceeds Cdl-air-p, or the TCLP concentration of a constituent exceeds either Cdl-air-v or Co, the waste may not 

qualify for a standard multi year delisting. However, the waste may qualify for a one-time delisting, 

depending on the results of hazard analysis calculations presented in Section 4.3.3.2. 

Landfill Particulate Delisting Levels 

First the maximum allowable emission rate of the constituent, Qp, is calculated using in Cair using 

Equation 4-102. 

(4-102) 

where: 

Default 

Q p

Cair 

3z

U 

L v

F 

Finhal 

= 

= 

= 

= 

= 

= 

= 

maximum allowable total constituent 

 emission rate (mg/s) 

constituent’s maximum allowable air 

 concentration at POE (mg/m )3 

vertical dispersion coefficient (m) 

mean annual wind speed (m/s) 

distance from virtual point to compliance 

 point located 1,000 feet (304.8 m) downwind (m) 

frequency that wind blows across waste unit 

 toward receptor (unitless) 

fraction of particles inhaled 

calculated

Equation 4-101

Equation 2-26 

4 (U.S. EPA 1994a) 

Equation 2-27

0.15 (U.S. EPA 1994a)

0.125 (U.S. EPA 1994a) 

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Then the pathway total concentration delisting level for respirable landfill in air particulates, Cdl-air-p, is 

calculated by setting the maximum allowable total emission rate for respirable particulates, Qp10, as equal to 

Qp and using Equation 4-103. 

(4-103) 

where: 

Default 

C = dl-air-p pathway total concentration delisting level for calculated
 

 respirable landfill particulates in air (mg/kg)
 

Q = p10 emission rate of waste constituent particulates set equal to Qp (Equation 4-102) 

 up to 10 ìm (mg/s) 

Ew10 = wind erosion emission rate of particulates up Equation 2-9

 to 10 ìm (g/hr) 

Ev10 = vehicle travel emission rate of particulates Equation 2-19

 up to 10 ìm (g/hr) 

El10 = waste loading and unloading emission rate of Equations 2-21 and 2-22

 particulates up to 10 ìm (g/hr) 

Landfill Volatile Delisting Levels 

First the maximum allowable volatile emission flux of the constituent, Ei

, from the landfill is calculated by 

setting the maximum allowable total emission rate of volatiles, Qv, as equal to Qp (calculated in 

Equation 4-102) and using Equation 4-104. 

(4-104) 

where: 

Default 

Ei = maximum allowable volatile emission flux of calculated 

 constituent (g/s) 

Q v = maximum allowable constituent volatile p set equal to Q (Equation 4-102)

 emission rate (mg/s) 

1000mg /g conversion factor 

Then, Ei is used to calculate the pathway total delisting level concentration for volatiles from a landfill, 

Cdl-air-v, using Equation 4-105. 

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(4-105) 

where: 

Default 

Cdl-air-v = pathway total waste concentration delisting level calculated

 for volatiles from landfill (mg/kg) 

Ei = maximum allowable volatile emission Equation 4-55

 flux of constituent (g/s) 

d = depth of soil cover (m)	 0.1524 (U.S. EPA 1994a) 

PT	 = total sand porosity (unitless) 0.4 (U.S. EPA 1994a) 

= 2 Da diffusivity of constituent in air (cm /s) chem-specific (Appendix A-1) 

Pa = air-filled sand porosity (unitless) 0.4 (U.S. EPA 1994a) 2 Aexposed = surface area (m )	 Equation 2-15 

Csi	 = saturation vapor concentration of i in landfill Equation 4-106 3 (g/m )


1x10 6 = conversion from kilograms to milligrams
 4	 2 2 1x10	 = conversion from m to cm 

Finally, the saturation vapor concentration (C ) of waste constituent i for volatiles from a landfill (C ) si	 dl-air-v 

is calculated using Equation 4-106. 

(4-106) 

where: 

Default 3 Csi = saturation vapor concentration of i in landfill (g/m ) Equation 4-56

pi = vapor pressure of waste constituent i (at) chem-specific (Appendix A-1) 

MW = molecular weight of waste constituent i chem-specific (Appendix A-1) 3	 -5 R	 = universal gas constant (atm-m /mol- K) 8.21 x 10 

T	 = standard temperature (K) 298 

Surface Impoundment Volatile Delisting Levels 

The pathway leachate concentration delisting level for volatiles from waste disposed of in a surface 

impoundment is calculated by setting the maximum allowable total emission rate of volatiles, Qv, as equal 

to Qp (calculated in Equation 4-102) and using Equation 4-107. 

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(4-107) 

where: 

Default 

Cdl-air-si = pathway leachate concentration delisting level calculated

 for volatiles from surface impoundment (mg/L) 

Qv = maximum allowable constituent volatile emission set equal to Qp (Equation 4-102)

 rate (mg/s) 

tf = time for constituent concentration to reach 1 Equation 4-59 

 percent of Co (days) 3 Vsi = volume of liquid in surface impoundment (m ) unit-specific (user- provided) 

K = overall mass transfer coefficient (m/s) Equation 4-60 

A = area of waste management unit (acres) Equation 2-12 or 2-13 

tr = retention time for liquid in surface Equation 2-40


 impoundment (days)
 

4.3.3.2 Hazard Analysis 

Hazard as a function of the effective air concentration of a particulate or volatile noncarcinogenic 

constituents, Cair, is calculated for each of the three air exposure pathways as shown in Equation 4-108. 

(4-108) 

where: 

Default 

HQ 

Cair 

ED 

EF 

RfC 

= 

= 

= 

= 

= 

hazard quotient for waste constituent (unitless) 

effective concentration of constituent in air

 at POE (mg/m )3 

Particulates 

Landfill volatiles 

Surface impoundment volatiles 

exposure duration (years) 

exposure frequency (days/year) 

constituent reference concentration (mg/m ) 3 

calculated 

Equation 4-66 

Equation 4-68 

Equation 4-73 

30 (U.S. EPA 1997b) 

350 (U.S. EPA 1997b) 

Chem-specific (Appendix A-1) 

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AT = averaging time (years) 30 (U.S. EPA 1997b) 

Cair at the POE is determined separately for each of the three air exposure pathways using the air emission 

and transport equations derived in Section 2.3 and presented in Section 4.3.3.2. 

Using Equation 4-108, the DRAS calculates the waste constituent HQs for receptors at the POE for the three 

air exposure pathways. The program then sums the individual constituent HQs using Equation 4-5 to 

determine the HIp for each of the air inhalation pathways. 

4.4.4 Soil Exposure Pathway 

This section presents equations used to calculate pathway total concentration delisting levels and hazards 

constituents associated with exposures of children to soils contaminated with noncarcinogenic waste 

constituents at a POE 1,000 feet from a waste disposal unit. The soil contamination is assumed to be the 

result deposition of waste particulates from the air. The equations used to compute the delisting levels and 

hazards are based on a method described in U.S. EPA Headquarters delisting docket materials (U.S. EPA 

1993a, 1993b). 

Section 4.4.4.1 presents the equations used to compute pathway total concentration delisting levels for 

standard multi year delistings. Section 4.4.4.2 presents the equations and methods used to compute 

constituent-specific and pathway- hazards for one-time delistings. 

4.4.4.1 Delisting Level Analysis 

This section describes computation of the pathway total concentration delisting level for exposure to 

noncarcinogens via ingestion of soil, C . First, the maximum allowable concentration of each constituent dl-soil 

in soil at the POE, C , is calculated. C is the maximum concentration of the waste constituent in soil that soil soil 

does not cause the risk from soil ingestion to exceed the target hazard quotient. Csoil is calculated using 

Equation 4-109. 

(4-109) 

where: 

Default 

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Csoil = maximum allowable concentration of calculated

 constituent in soil at the POE (mg/kg) 

THQ = target hazard quotient (unitless) user specified (0.1 default 

U.S. EPA 1997b) 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

BWc = body weight of child (kg) 15 (U.S. EPA 1997b) 

AT = averaging time (years) 6 (U.S. EPA 1997b) 

CRsoil = soil consumption rate (mg/day) 200 (U.S. EPA 1997b) 

ED = exposure duration (years) 6 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

Using Csoil, the DRAS back-calculates the maximum allowable constituent concentration in air at the POE, 

C , assuming a constituent accumulation period in the soil of 1 year. C is calculated, at a depth of mixing air air 

of 1 cm as shown in Equation 4-110. 

(4-110) 

where: 

Default 

Cair = maximum allowable concentration of calculated 3 constituent in air at POE (mg/m ) 3 ñb = soil bulk density (g/cm ) 1.45 (Brady 1984) 

t = soil thickness from which particles can be 0.01 (U.S. EPA 1994a)

 ingested (m) 

C = soil maximum allowable concentration of 

 constituent in soil at the POE (mg/kg) Equation 4-109 

vd = deposition velocity (m/s) 0.03 (U.S. DOE, 1984) 

Finally, once Cair is known, the total concentration delisting level is computed using Equation 4-111. 

(4-111) 

where: 

Default 

Cdl-soil = pathway total concentration delisting level 

 for soil ingestion (mg/kg) 

calculated

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Cair = maximum allowable concentration of constituent Equation 4-110 3 in air at POE (mg/m )

Gz = vertical dispersion coefficient (m) Equation 2-26 

U = mean annual wind speed (m/s) 4 (U.S. EPA 1994a) 

Lv = distance from virtual point to compliance Equation 2-27

 point located 1,000 feet (304.8 m) downwind (m) 

ET30 = annual average emission rate of particles up to Equation 4-75 (also see

 30 ìm (g/hr) Section 2.3.1.3) 

F = frequency that wind blows across 0.15 (U.S. EPA 1994a)

 waste unit toward receptor (unitless) 

If the total concentration for a given noncarcinogenic waste constituent exceeds its pathway total 

concentration delisting level, C , the waste may not qualify for a standard multi year delisting. However, dl-soil 

the waste may qualify for a one-time delisting, depending on the results of hazard analysis calculations 

presented in Section 4.4.4.2. 

4.4.4.2 Hazard Analysis 

In the hazard analysis, the DRAS computes the hazard associated with the soil ingestion exposure pathway. 

The hazard for each noncarcinogenic waste constituent associated with soil ingestion at the POE is calculated. 

Only the child receptor is considered. The focus on children reflects the higher daily soil intake rates for 

children and their lower body weights. The constituent-specific hazard from ingestion of soil is calculated 

using Equation 4-112. 

(4-112) 

where: 

Default 

HQ = constituent hazard quotient (unitless) calculated 

Csoil = concentration of constituent in soil POE (mg/kg) Equation 4-78 

CRsoil = soil consumption rate (mg/day) 200 (U.S. EPA 1997b) 

ED = exposure duration (years) 6 (U.S. EPA 1997b) 

EF = exposure frequency (days/year) 350 (U.S. EPA 1997b) 

BWc = body weight of child (kg) 15 (U.S. EPA 1997b) 

RfD = constituent reference dose (mg/kg-day) chem-specific (Appendix A-1) 

AT = averaging time (years) 6 (U.S. EPA 1997b) 

Csoil is calculated based on the constituent’s rate of deposition onto the soil. Assuming that at any given time 

the soil is burdened with 1 year’s accumulation of constituent from air deposition and that the mixing depth 

of the constituent into the soil is 1 cm, Equation 4-78 is used to calculate Csoil. Then, using Equation 4-112, 

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the DRAS calculates the HQ associated with ingestion of soil for each constituent. Finally this program sums 

the constituent-specific HQs using Equation 4-5 to determine the total HIp for the soil ingestion pathway. 

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