# Toxic Weighting Factors Methodology

U.S. Environmental Protection Agency Office of Water Engineering and Analysis Division

1200 Pennsylvania Avenue, NW Washington, D.C. 20460

March 2012

EPA - 820 -R- 12

- 005i CONTENTS
Page

1. I NTRODUCTION

................................ ................................ .............................................................. 1-1 1.1 Introduction References ................................................................................................... 1-3

2. C LARIFICATION OF K EY TWF C ONCEPTS AND T ERMS ................................................................. 2-1

3. T HE TWF E QUATION .................................................................................................................... 3-1 3.1 The TWF Equation References ........................................................................................ 3-3

4. T HE A QUATIC L IFE C OMPONENT OF THE TWF E QUATION .......................................................... 4-1 4.1 The Aquatic Life Component of the TWF Equation References ..................................... 4-2

5. T HE H UMAN H EALTH C OMPONENT OF THE TWF E QUATION ....................................................... 5-1 5.1 Equation for Calculating the TWF Human Health Value for Noncarcinogens ............... 5-1 5.2 Equation for Calculating the TWF Human Health Value for Carcinogens ..................... 5-2 5.3 Data Sources for Calculating the Human Health Compon ent of the TWF Equation ........................................................................................................................... 5-3 5.4 Calculating the TWF Human Health Value for Dioxins .................................................. 5-4 5.5 Variable Factors Used in the Human Health Component of the TWF Equation ............. 5-6 5.5.1 Relative Source Contribution Factors ................................................................. 5-6 5.5.2 Bioaccumulation and Bioconcentration Factors ................................................. 5-6 5.6 The Human Health Component of the TWF Equation References .................................. 5-7

6. EPA’ S U SE OF TWF S TO S UPPORT E FFLUENT G UIDELINES R ULEMAKINGS AND P ROGRAM P LANNING ..................................................................................................................................... 6-1 6.1 Use of TWFs in Cost

- Effectiveness Analysis .................................................................. 6-1 6.2 Use of TWFs in Effluent Guidelines Program Planning .................................................. 6-3 6.3 EPA’s Use of TWFs to Support Effluent Guidelines Rulemakings and Program Planning References ........................................................................................................ 6-6

7. L IMITATIONS AND A PPROPRIATE U SES OF T OXIC W EIGHTING F ACTORS .................................... 7-1 7.1 Limitations of Source Data Used to Derive TWFs .......................................................... 7-1 7.2 Limitations of Discharge Data to Which TWFs Are Applied ......................................... 7-1 7.3 Appropriate Uses of TWFs .............................................................................................. 7-3 7.4 Limitations and Appropriate Uses of Toxic Weighting Factors References ................... 7-4ii LIST OF TABLES
Page

Table 3 -1. TWF for Copper ....................................................................................................................... 3-2

Table 5

- 1. Toxic Equivalency Factors (TEF), Bioaccumulation Factors (BAF), and TWF Values for Dioxin Compounds ................................................................................................................. 5-5

Table 6 -1. Examples of Effluent Guidelines Cost- Effectiveness Analysis Ranked by Dollars Per TWPE Removed

........................................................................................................................... 6-2

Table 6 -2. TWPE Ranking of Industrial Categories Based on 2008 DMR Data ....................................... 6-4

Table 6 -3. TWPE Ranking of Industrial Categories Based on 2008 TRI Data ......................................... 6-5iii LIST OF FIGURES
Page
Figure 3 -1. TWF Equation ......................................................................................................................... 3-1
Figure 5 -1. Equation for Calculating the TWF Human Health Value for Noncarcinogens ....................... 5-1
Figure 5 -2. Equation for Calculating the Human Health Component of TWFs for Carcinogens .............. 5-2
Figur e 5 -3. Hierarchy of Data Sources for Calculating the Human Health Component of TWFs ............ 5-31

- 1 1. I NTRODUCTION The purpose of this report is to explain the concept, calculation, use, and limitations of Toxic Weighting Factors (TWFs). The U.S. Environmental Protection Agency’s Engineering and Analysis Division of the Office of Water (EPA) began developing TWFs in the early 1980s to compare the relative cost effectiveness of treatment technology opti ons for national industrial discharge regulations known as effluent guidelines. EPA e xpanded its application of TWFs in 2003 when it began using them in effluent guidelines program planning to rank industries by their total annual toxic- weighted discharges to identify
those that may warrant additional research (U.S. EPA, 2004).

EPA has calculated TWFs for 1,064 chemicals based on information about the concen trations in

water at which they become harmful to aquatic life, along with information about the levels in fish tissue at which they become harmful to humans. Information on harmful effects is used to weight the toxicity of
chemicals relative to copper which is a common toxic pollutant in industrial waste streams . By assigning TWFs to toxic chemicals, EPA can add together varying amounts of different chemicals, each with different toxicities, to calculate the total toxic -weighted pounds in a waste stream, referred to as Toxic Weighted Pound Equivalents (TWPE). For example, nickel, which is less toxic than copper, ha s a TWF of 0.11; whereas acrylonitrile , which is more tox ic than copper , has a TWF of 2.3 . A facility that annually discharges 50 pounds of nickel and 10 pounds of acrylonitrile (50 x 0.11 + 10 x 2.3 = 28.5 TWPE ) has a waste stream that is almost twice as toxic as a facility that discharges 100 pound s of nickel and 2 pounds of acrylonitrile (100 x 0.11 + 2 x 2.3 = 15.6 TWPE ). TWPE

- based estimates of the toxicity of waste streams are solely used by EPA to better understand how treatment technologies, individual facility discharges, and industry discharges compare to one another, not to as sess impacts t o aquatic life or human
health.

The equations for calculating TWFs are contained in a set of Excel Worksheets known as the
TWF Database. The TWF D atabase uses aquatic life values that have already been calculated for various purposes such as for use in water quality criteria . Hu man health values are derived in the TWF D atabase using information from disparate sources. EPA combines aquatic life and human health effects information whenever possible when calculating TWFs to help compensate for t he incompleteness of data sources and to simplify TWF calculation and waste stream evaluation. The majority of TWFs, 58 percent (6 16 ), are derived solely from aquatic life values. Five percent of TWFs (54) are derived solely from human health values. Thirty- seven percent of T WFs (3 94 ) are derived from both aquatic life and human health effects information. If EPA were to separate the aquatic life and human health values during TWF calculation, there would be 1,010 chemicals with aquatic life- based TWFs (61 6 + 3 94 = 1,010) and 448
chemicals with human health- based TWFs (54 + 3 94 = 4 48). The parsing of industry rankings by separate aquatic life and human health categories would defeat the purpose of TWFs, which is to help simplify the1

- 2 interpretation of dispara te discharge data. Moreover, there is no clear metric for decision makers to use in choosing between the results of separate aquatic life and human health

- based industry rankings.

EPA recognizes that the approach of combining aquatic life and human health values is not suitable for rigorous applications, such as analyses performed for risk assessments or water quality

- based permitting. EPA acknowledges that the TWF approach of combining aquatic life and human health values mixes different types of effects data, amplifies uncertainties in the underlying data, and obscures the relative contribution of aquatic life and human health values in the calculation of TWFs. EPA also recognizes that the toxic effects of many chemicals are frequently different in humans than they are in aquatic life. Nevertheless, EPA considers the combination of aquatic life and human health effects information in the calculation of TWFs to be a reasonable approach for integrating disparate toxic effects information from different sources for the purpose of informing regulatory decisions about the relative cost effectiveness of treatment technologies, and for indentifying industries that may warrant additional analysis.

It should be noted that although the TWF Database contains TWFs for 1,064 chemicals, only a small subset of chemicals with TWFs are typically assessed by the effluent guidelines program. The number of chemicals with TWFs regulated by effluent guidelines for any particular industrial category,
and thus assessed in a cost eff ectiveness analysis, typically range anywhere from one to 20. Effluent guidelines program planning assessment only examines the approximately 200 chemicals with TWFs that

are annually reported by facilities to be discharged to surface water nationwide. EPA data show that the
total amount and toxic- weighted amount of chemicals discharged to surface water is declining over time as industries adopt increasingly effective treatment technologies.

This report incorporates comments from an independent peer review panel EPA convened to examine the TWF methodology ( ERG , 2006). The majority of the panel’s suggestions concerned improving the clarity of the explanation of how TWFs are developed. It was also apparent from the panel’s comments that the TWF Methodology did not adequately discuss the limitations of TWFs and their specific uses by EPA. EPA prepared a detailed response report to address the peer review panel’s comments ( U.S. EPA, 2011). Moreover, EPA has endeavored to respond to the panel’s suggestions in this report by more clearly explaining how TWFs are derived from toxic effects data and more precisely describing EPA’s specific uses of TWFs in effluent guidelines regulatory evaluation and planning. EPA
has also revised this report to more thoroughly describe the limitations of the data used to calculate TWFs, and the limitations of EPA’s facility discharge data to which TWFs are applied. EPA will continue to include a section on TWFs in the Technical Support Document accompanying effluent guidelines program plans summarizing any revisions to exis ting T WFs or development of new TWFs.1

- 3 EPA has continually developed and refined TWFs during the past three decades as new chemical toxicity data became available. However, given the current extensive set of chemicals for which TWFs have been calculated, EPA does not anticipate developing new TWFs in the foreseeable future

. Nevertheless, EPA may decide to revise certain TWFs as new chemical toxicity data become available. EPA may also decide to revise this report as needed to further clarify the TWF methodology in response to public comments. EPA w ill use the peer r eviewers ’ comments to infor m any future refinement of TWFs or revisions to the TWF methodology.

The body of this report is organized into six chapters. The first four chapters discuss TWF terminology and t he equations and data used to calculate TWFs. The chapter e xplaining the aquatic life component of TWFs is much shorter than the chapter explaining the human health component . This is because the TWF Database uses aquatic life values that have already been calculated by other sources,
whereas human health values a re calculated in the TWF Database and thus require more detailed explanation. The last two chapters summarize EPA’s use of TWFs and explain the ir limitations

.

1.1 Introduction References

1. ERG. 2006. Eastern Research Group, Inc. Toxic Weighting Factor Development in Support of CWA 304(m) Planning Process . ( June ). EPA

- HQ

- OW -2004-0032-1634.

2. U.S. EPA. 2004. Technical Support Document for the 2004 Effluent Guidelines Program Plan

. EPA 821R-04-014. Washington, DC. (August). EPA

- HQ

- OW -2003-0074-1346 through 1352.

3. U.S. EPA. 2011. U.S. Environmental Protection Agency. Response to Peer Review Comments on EPA Report: Toxic Weighting Factor Development in Support of CWA 304(m) Planning Process

. Washington, DC. EPA

- HQ

- OW

- EPA

- HQ

- OW -2010-0824, D CN 07500.2

- 1 2. C LARIFICATION OF K EY TWF C ONCEPTS AND T ERMS The effluent guidelines program only uses TWFs as an indicator of the relative potential of chemicals to cause harm prior to discharge, and never to assess harmful effects of chemicals in the
envir onment. This is a critical point to understand about TWFs and, consequently it is emphasized throughout this report. The toxic effects of chemicals discharged to aquatic environments are highly variable and site specific. Adequate assessment of the toxic effects of chemicals in aquatic environments requires site specific information beyond the scope of TWFs.

T oxicity refers to the degree to which a substance can harm an organism. A central concept of toxicology is that toxic effects are variable depending on the chemical structure of a substance, the dose an organism receives, and an organism’s susceptibility to increasing doses. All chemicals can cause harm
at a certain level. A chemical is considered to be toxic if a small amount of it can cause harm and relatively non

- toxic if only large amounts of it are harmful.

The toxic properties of a waste stream begin to change as soon as it is discharged to surface

water . T he potenti al toxic effects of chemicals vary depending on their concentration in a waste stream, the ir dilution once they are discharged , the ir inherent potential for per sistence or degradation in the
environment , and the physical and chemical characteristics of the waterbody to which they are discharged, such as temperature and pH. After mixing with surface water, toxic chemicals may degrade through photochemical and microbial processes, or compartmentalize through particulate adsorption, sediment deposition, or vol atilization. Biological alteration in the aquatic environment may decrease or increase the toxicity of a chemical, such as when certain anaerobic bacteria metabolize inorganic mercury into methyl mercury, making it biologically available

. Aquatic life effects from toxic chemicals are generally considered to be either acute or chronic. Acute effects are well defined and occur relatively quickly, such as death within a couple of days. Chronic effects are more subtle, developing and persisting for long periods over the course of an organism’s life time. Chronic effects are typically measured in terms of physiological decline, such as reduced growth a nd reproduction, but they may be also measured or in terms of the lethality of a
chemical

.

Human h ealth effects f rom toxic chemicals are divided into two categories: non
- threshold effects, such as carcinogenicity, and threshold effects, such as acute or chronic toxicity. EPA assumes that there is no safe level of exposure to carcinogens, and thus there is no effects threshold. The risk of developing cancer from a certain dose of a chemical for a given duration of exposure is therefore not an ef fect threshold, but rather represents an indicator of potential level of harm. Noncarcinogens are generally
assumed to have an identifiable exposure threshold, both for individuals and populations, below which2

- 2 effects are not observable. For example, EPA water quality criteria for noncarcinogens represent a t ype of
toxic effects threshold because they define maximum ambient water chemical concentrations that can
exi st before harmful effects begin to occur.

The human health comp onent of TWFs is derived almost entirely from information concerning fish consumption, which is the most significant pathway of exposure t o water borne toxic chemicals. Certain highly toxic chemicals, such as mercury, may become concentrated at harmful levels in the types of fish commonly eaten by humans through bioaccumulation, bioconcentration, and biomagnification.
Bioaccumulation refers to the buildup of toxic chemicals in organic tissues from all combined routes of exposure, inclu ding food consumption, water ingestion, and contact with water and sediments. Bioaccumulation occurs when an organism absorbs a toxic chemical faster than it metabolizes or excretes it. The longer a toxic chemical persists in organic tissue, the greater the potential for adverse effects, even if environmental levels of the chemical are not very high. Bioconcentration is a related but more specific term, referring to uptake and accumulation of a toxic chemical from contact with water alone.
B ioaccumulation typically accounts for significantly more chemical build up in tissues than
bioconcentration. Biomagnification occurs when chemicals become increasingly concentr ated up the levels of a food chain through predation. Chemi cal bioavailability, biota fat content, an d rate of transfer among food chain l evels can affect biomagnifications potential. Chemicals such as dioxins are particularly prone to biomagnification bec ause they are highly persistent in the environment and accumulate in fatty tissues.3

- 1 3. T HE TWF E QUATION EPA convened a work group in 1981 to develop TWFs for use in evaluating the cost

effectiveness of treatment technology options for effluent guidelines rulemakings (U.S. EPA, 1988). The work group decided to simplify TWF calculation by combining information about toxic effects to aquatic
life and human health. The equation is illustrated in Figure 3-1 . The 1981 work group also d ecided to normalize the aquatic life and human health values to a standard benchmark so that different amounts of different chemicals could be added toge ther to estimate the total toxicity of a waste stream. The work group chose the freshwater chronic aquatic life criterion for copper as the benchmark scaling factor
because it is a common and well studied toxic chemical in industrial waste streams. 

# ( ) HH AQ TWF 6.56.5 +      

=

Where:

5.6 = scaling fac tor (original copper freshwater chronic aquatic

- life criterion of 5.6 μg/L); AQ = Aquatic Life Value (μg/L); and HH = Human Health Value (μg/L).


### Figure 3-1. TWF Equation
The aquatic life value is ta ken directly from one of several toxicity databases, depending on the
best source of information availab le for each chemical. The human health value is calculated for each
chemical using data from multiple sources of best available information depending on whe ther the chemical is a carcinogen or noncarcinogen. Data sources for TWFs are discussed in Sections 4 and 5

.
The chronic freshwater aquatic life criterion for copper, however, has been revised three times since it was first p ublished in 1980 due to advances in the scientific understanding of its toxic effects. The original co pper freshwater chronic aquatic life criterion of 5.6 micrograms per liter (μg/L ) was revised to 12.0 μg/L in 1984, then changed to 9.0 μg/L in 1998, and revised again in 2007 to a criterion based on
site

- specific water quality fac tors ( U.S. EPA, 2007a ). A s shown in Figure 3 -1 , EPA has continued to use the original 5.6 μg/L value for the TWF benchmark in order to provide consistency in interpretation of
TWF -bas ed analys es from year to year.

The nature of the copper criterion helps to illustrate the complexity of determining the potential
toxic effects of a chemical. C opper frequently occurs naturally in surface water because it is an abundant

tra ce element in the earth’s crust . At low concentrations it is an essential micronutrient for virtually all plants and animals, but at higher concentrations copper is toxic to aquatic life. To better account for the waterbody-specific nature of copper toxicity, EPA published revised water quality criteria for copper in 2007 (U.S. EPA , 2007a ) based on a chemical speciation model, called the Biotic Ligand Model (BLM)3

- 2 (U.S. EPA, 2007b). The BLM calculates the toxicity of copper to aquatic organisms as a function of
certain chemical and phy sical characteristics of a receiving waterbody. It also s imultaneous ly calculates
ionic c oncentrations of other chemicals that can either bind with copper and render it biologically unavailable, or compete with copper for binding sites on aquatic organisms, such as fish gills. Although the BLM can potential ly improve the accuracy of predictions of the ecotoxicity of copper and other metals, its use requires significant data input, including water temperature along with concentrations of nine separate che mical constituents: pH, alkalinity, calcium, magnesium, sodium, sulfate, potassium, chloride, and dissolved organic carbon.

The BLM

- based copper criterion provides no national value that can be us ed in TWF calculations because it is site specific. Therefore, when calculating the TWF for copper, EPA normalizes the 1998 copper criterion of 9.0 μg/L to the original 1980 copper criterion of 5.6 μg/L . The 1998 copper criterion is
used because it is the most recent copper criterion prior to the BLM

- based copper criterion

. The copper human health value of 444 μg/L, derived from the reference d ose contained in the 1998 EPA Region 3
Risk -based Concentration Table and noted in Section 5, is also normalized to the original 1980 copper
criterion of 5.6 μg/L.

Con sequently, the TWF for copper is 0.635 rather than 1.0, as illustrated in Table 3-1

.

Table 3-1. TWF for Copper

Aquatic Life Value (μg/L ) Human Health Value (μg/L ) Application of TWF Scaling Factor Toxic Weighting Factor Copper 9 (1998 Criterion) 4 4 4 5.6/9.0 + 5.6/ 444 0.63 5
Although the 1981 cost effectiveness workgroup selected copper as the benchmark, any chemical
could have been used for weighting relative chemical toxicities . Chemicals with toxicities close to the mean toxicity of a group of chemicals, however, would serve as the most balanced benchmark for scaling purposes. In the most recent version of the TWF D atabase, compiled in 2007, there are 258 chemicals
with TWF s gr eater than that of copper and 805 chemicals with TWFs less than the TWF for copper.

The c omplete list of TWFs , the values used in the aquatic life and human health components of TWFs, the values of the constant and variable factors used to calculate TWFs, and all data sources are
contained in the TWF Database. The TWF Database is available in the public docket for the effluent guidelines planning process and can be accessed through docket number “EPA- HQ

- OW -2008-0517” at www.regulations.gov (U.S. EPA, 2007c ). The 2007 TWF Database in the docket is the most current version as of the date of this report

. This report is also available in “EPA

- HQ

- OW -2008-0517.” Any3

- 3 future revised versions of the TWF Database and reports wil l be posted to the effluent guidelines program planning docket, which is updated annually and can be found by searching www.regulations.gov.

3.1 The TWF Equation References

1. U.S. EPA. 1988. U.S. Environmental Protection Agency. Office of Water . Ehrensberger , K., and R. Rico. Cost

- effectiveness Analysis for Effluent Guidelines . Washington, DC. ( May ).

2. U.S. EPA . 2007a. U.S. Environmental Protection Agency. Office of Water. Aquatic Life Ambient Freshwater Quality Criteria: Copper . EPA -822-R-07-001. Washington, DC. ( February ). EPA

HQ

- OW -2006-0771-1214 . Available online at: http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/pollutants/copper/upload/2009_0 4_27_criteria_copper_2007_criteria-full.pdf

.
3. U.S. EPA . 2007b. U.S. Environmental Protection Agency. Office of Water. Aquatic Life Ambient Freshwater Quality Criteria – Copper, 2007 Revision

. EPA -822-R-07-001. Washington, DC. (February). Available online at: http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/pollutants/copp er/2007_index.cf

m

.
4. U.S. EPA . 2007c. U.S. Environmental Protection Agency. Office of Water. Toxic W eighting F actor Database. Washington, DC. (September). EPA

- HQ

- OW -2008-0517 DCN 04661.4

- 1 4. T HE A QUATIC L IFE C OMPONENT OF THE TWF E QUATION

EPA selects aquatic life effects data to calculate TWFs using a three tiered hierarchy in orde r of decreasing data quality :

1. F reshwater chronic aquatic life criteria published by EPA as National Recommended Water Quality Criteria (NRWQC) ( U.S. EPA, 2 009 ); 2. E ffects data based on te sting of aquatic organisms; and 3. E stimated toxicity effects data derived from various nationally

- recognized sources.
Based on this hierarchy, aquatic life values are available for all chemicals for which TWFs have been developed, with the notable exception of certain dioxin compounds as ill ustrated in Table 5-1 . If aquatic life effects values are not available for a particular d ioxin compound, then the copper -normalized human
health value is used as the TWF for that chemical

. Conversely, TWFs for 6 16 chemicals are based solely on aquatic life values because human health values are not available.

Aquatic life effects values from freshwater enviro nments, only, is used in TWF calculations.
S al twater aquatic effects values are not used because they have been calculated for only a small set of
chemicals due to the complexities of determining toxicity effects in mari ne environments.

Chronic long

- term e ffects values rather than acute short

- term effects values are used in most
TWFs because they are more available

. For a few TWFs , however, acute values are used because chronic
values are not available. Acute effects are typically expressed as t oxicity endp oints . A n acute toxicity endpoint of LD50, for example, is the lethal dose of a chemical that kills 50 percent of test organisms in specified period, usually within 96 hours or less. Chronic effects are more subtle and develop and persist
for a relat ively long period of time, typically one- tenth or more of the life sp an of an organism. Chronic
effects are measured in terms of physiological decline, such as reduced growth and reproduction, or in
terms of the lethality of a chemical

.

F reshwater chronic aquati c life criteria published as NRWQC, available for 72 chemicals, are the highest quality source data for the aquatic life component of TWFs because they have been meticulously developed, extensively reviewed, and constitute national guidance for protection of the vast majority of aquatic freshwater communities in the United States. T he Criterion Continuo us Concentration (CCC)
values from freshwater c hronic aquatic life criteria are used in the aquatic life component of the TWF equation. The CCC is a chronic value representing the highest in

- stream concentrati on of a toxicant to which organisms can be exposed indefinitely without causing unacceptable effects. Thorough discussions of the CCC and other elements of NRWQC are contained in the met hodology for devel oping aquatic life criteria, titled Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses (U.S. EPA, 1985).4

- 2 Aquatic organism test data, comp osing the second tier of source data for the aquatic life component of TWFs, are available for only 46 chemicals. The results of tests on three types of indicator organisms are used in TWFs: fish, macroinvertebrates, and algae. Chronic effects values from organism
tests indicate concentrations above which long- term effects are likely to occur. Acute toxicity test results,
such as those for short -term lethal concentrations, are used when chronic toxicity information is not
available for a chemical. Organism test data were obtained from two complementary EPA databases: the Ecotoxicity Database (ECOTOX) ( U.S. EPA, 2011a ) and the Assessment Tools for Evaluation of Risk Database (ASTER) ( U.S. EPA, 2011b ).

E stimates of chronic toxicity , taken from several sources, are used as TWF aquatic life values
when NRWQC or organism test data are not available. The primary source of chronic toxicity estimates is EPA’s Quantitative Structure Activity Relationship (QSAR) model, which derives statistical relationships between chemical properties and biological effects. QSAR toxicity estimates are maintained in ASTER. The TWF Database contains a complete list of other less frequently cited sources of toxicity estimates.

4.1 The Aquatic Life Component of the TWF Equation References

1. U.S. EPA. 1985. U.S. Environmental Protection Agency. Office of Research and Development. Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aq uatic Organisms and Their Uses . Washington, DC. Available online at: http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/upload/85guidelines.pdf

.
2. U.S. EPA . 2009. U.S. Environmental Protection Agency. Office of Water. National Recommended Water Quality Criter ia . Washington, DC. Available online at: http://water.epa.gov/scitech/swguidance/standards/current/upload/nrwqc-2009.pdf

.
3. U.S. EPA. 2011a. U.S. Environmental Protection Agency. Office of Research and Development. ECOTOX Database Website. Washington, DC. Available online at: http://cfpub.epa.gov/ecotox

.
4. U.S, EPA . 2011b. U.S. Environmental Protection Agency. Office of Research and Development. ASsessment Tools for the Evaluation of Risk (ASTER) Website. Washington, DC. Available online at: http://www.epa.gov/med/Prods_Pubs/aster.htm

.5

- 1 5. T HE H UMAN H EALTH C OMPONENT OF THE TWF E QUATION

The derivation of the human health component of the TWF equation is more complex than that of the aquatic life component , which is simply an effects value taken from a nationally -recognized source.
The human health values are derived from separate equations to estimate effects thresholds for noncarcinogens and non-threshold cancer risk- based factors for carcinogens. These e quations are based
on EPA’s Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health: Technical Support Document Volume 1: Risk Assessment (U.S. EPA, 2000). The noncarcinogen and carcinogen human health values are based on assumptions about fish consumption, which is the most common pathwa y for human exposure to toxic chemicals. Both values represent maximum possible toxic chemical concentrations in fish, based on the best available toxicological science, that are considered to minimize the long-term exposure risk to the general population.

5.1 Equation for Calculating the TWF Human Health Value for Noncarcinogens

If a chemical is not consi dered to have the potential to cause ca ncer in humans, based on EPA’s Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005), the human health effects val ue is derived using a threshold concentration for noncancer adverse effects, which is referred to as the oral reference dose (RfD). More specifically, the RfD is the amount of a noncarcinogen to which a person may be exposed each day for a lifetime of 70 y ears without suff ering harmful effects. The RfD includes sensitive subgroups. Usually doses less than the RfD are unlikely to be associated with adverse health effects. However, RfD uncertainty may span up to an order of magnitude. The equation for calculating the TWF human health value for noncarcinogens , shown in Figure 5-1 , uses the same adult body weight (70 kg), and fish consumptio n rate (17.5 grams per day), that are use d in the calculation of water quality criteria (U.S. EPA, 2000). It should be noted that, in accordance with the NRWQC methodology, a different fish consumption rate of 18.7 grams per day is used for calculating the TWF human health value for mercury.

TWF Human Health Noncarcinogenic Value ( μ g L ) =

Reference Dose � mg kg day � × Relative Soure Contribution (%) × Bodyweight ( kg ) × μ g to mg Conversion
Daily Fish Consumption Estimate � kg day � × Bioaccumulation or Bioconcentration Factor � L kg �

= RfD � mg kg day � × RSC (%) × 70 kg × 1000 μ g mg
0. 0175 � kkkk dddddd � × BAF or BCF � L kg �

a In accordance with the NRWQC methodology, a different fish consumption rate of 18.7 grams per day is used for calculating the TWF human health value for mercury

.



### Figure 5-1. Equation for Calculating the TWF Human Health Value for Noncarcinogens5

- 2 5.2 Equation for Calculating the TWF Human Health Value for Carcinogens
EPA does not calculate human health effects thresholds for carcinogens because the Agency
assumes that there is no safe le vel of exposure

. Instead, EPA e stimates the lifetime risk of developing cancer using a cancer potency factor based on a linear relationship between dose and response. The cancer potency factor represents an upper -bound estimate of the lifetime probability that an individual will develop cancer from a specific life time dose of a carcinogen. Lifetime cancer risk is expressed in terms of the likelihood that one person in an equally exposed population of a specific size would develop cancer if exposed continuously 24 hours per day to a spec ific concentration of a carcinogen throughout a lifetime of 70 years.

TWF Human Health Carcinogenic Value ( μ g L ) =

Cancer Risk Level
Cancer Potency Factor � kkkk ∙dddddd

mm � × Relative Soure Contribution (%) × Bodyweight ( kg ) × μ g to mg Conversion

Daily Fish Consumption Estimate � kg day � × Bioaccumulation or Bioconcentration Factor � L kg �

= 10 − 5

Cancer Potency Factor � kkkk∙dddddd

mm � × RSC (%) × 70 kg × 1000 μ g mg 0. 0175 � kkkk dddddd � × BAF or BCF � L kg �

a In accordance with the NRWQC methodology, a different fish consumption rate of 18.7 grams per day is used for calculating the TWF human health value for mercury

. 

### Figure 5-2 . Equation for Calculating the Human H ealth Component of TWFs for Carcinogens
The level of cancer risk that is of concern is a matter of personal, community, and regulatory judgment. In general, EPA considers a lifetime risk of developing cancer below one in a million (10 -6 ) to
be so small as to be negligible, and cancer risk of more than one in ten thousand (10 -4 ) to be sufficiently large to warrant action, such as the remediation of a Superfund site. The need for action to address a
lifetime cancer risk between10 -6 and 10 -4 is eva luated on a case -by- case basis.

When TWFs were first developed in 1981, EPA dec ided to use a cancer level of 10 -5 for
calcul ating the human health value of TWFs for carcinogens to be consistent with the 10 -5 cancer risk level used in the calculation of NRWQC . Howeve r, a more protective risk level of 10 -6 began to be used in the calculation of water quality criteria when the NRWQC methodology was revised in 2000 (U.S. EPA, 2000). In order to maintain data consistency for comparing TWF

- based analytical results from yea r to year , EPA decided to continue using 10 -5 in TWF calculations as shown in Figure 5-2

.5

- 3 T he equation for calculating the TWF human health value for carcinogens uses the same adult body weight (70 kg), and fish consumption rate (17.5 grams per day), that are used in the TWF human
health value for non carcinogens

.

5.3 Data Sources for Calculating the Human Health Component of the TWF Equation
Since there is no single comprehensive source of toxicity information, EPA draws data from multiple sources to calculate TWF human health values. As illustrated in Figure 5-3 , RfD and cancer potency factors are taken from a hierarchy of sources based on criteria outlined in Section 2.4.6 of the Technical Suppor t Document for Water Quality

- Based Toxics Control (U.S. EPA, 1991).

Tier I

• EPA National Recommended Water Quality Criteria, published after 2000 ( EPA, 2007 )

• EPA Integrated Risk Information System (IRIS) Oral RfDs or Oral Cancer Potency Factors ( EPA, 2011a )

Tier II

• EPA’s Health Effects Assessment Summary Tables (HEAST)

• EPA’s Region III Risk

- Based Concentration Table (RCB)

• California Environmental Protection Agency health assessments and toxicity criteria (CalEPA)

Tier III

• Criteria from EPA National Recommended Water Quality Criteria Documents , prior to 2000

• Agency for Toxic Substances and Disease Registry (ATSDR) profiles



### Figure 5-3. Hierarchy of Data Sources for Calculating the Human Health Component of TWFs
EPA’s Integrated Risk Information System (IRIS) and NRWQC published after 2000 are preferred sources of RfDs and cancer potency factors for calculating TWF human health values. IRIS contains qu antitative and qualitative information on eff ects that may result from exposure t o environmental contaminants (U.S. EPA, 2011a). IRIS information is used to support risk assessments, policy development, and regulatory activities. IRIS has varying levels of data for most of the chemicals typically ass essed by the effluent guidelines program. NRWQC are developed through rulemaking

process.

If RfDs or cancer potency factors are not available in IRIS or NRWQC developed after 2000, then other sources are checked beginning with the Health Effects Assessment Summary Tables (HEAST).
The HEAST tables are compiled from toxicity studies by EPA’s National Center for Environmental Assessment (NCEA ) for analysis of Superfund and hazardous waste sites. If RfDs or cancer potency factors are not available in HEAST, the n EPA Region 3’s Risk -based Concentration Table ( RBC ) is checked. The RBC is similar to HEAST and also used to support analysis of Superfund and hazardous5

- 4 waste sites EPA, ( U.S. EPA, 2011b) . If RfDs or cancer potency factors are not available in HEAST or RBC, California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (CalEPA) health assessments and toxicity criteria are checked . If none of these data sources have RfD s or cancer potency factors for a chemical, then NRWQC developed prior to 2000 are check ed . If still no data are available, then EPA searches the Agency for Toxic Substances and Disease Registry (ATSDR) toxicological profiles (ASTDR, 2011).

If a chemical has both noncarcinogenic and carcinogen ic effects, and if both the RfD and cancer potency factor are from the same tier in the data hierarchy, then the TWF is calculated using the cancer potency factor. For example, if IRIS contains both an RfD and cancer slope factor for a chemical, then the
TWF is calculated using the cancer slope factor. If, however, the RfD and cancer potency factor for a chemical come from sources on different tiers in the data hierarchy, then the value from the higher tier data source is used. For example, if an RfD for a chemical is available from IRIS but the cancer potency factor is only available from the EPA Region 3 RBC table, then the TWF is calculated using the RfD.

5.4 Calculating the TWF Human Health Value for Dioxins

The calculation of the TWF human health value for dioxin compoun ds differs slightly from that of other carcinogens. The term dioxin refers to a set of chlorinated compounds that share similar chemical

structure s and toxic properties. Dioxin compounds, which are among the chemicals in the TWF Database with the highest T WF values, have widely rangi ng toxicities and potential to increase the risk of cancer

. Dioxin compounds includ e seven polychlorinated dibenzodioxins (PCDDs) and 10 polychlorinated dibenzofurans (PCDFs). There are TWF values for all of these dioxin compoun ds in the TWF Database.

PCDDs and PCDFs are unintentional byproducts of most forms of combustion and several industrial chemical processes. Although release of dioxins to the environment has dropped sharply during the past two decades – by 90 perc ent or m ore – they are still a concern because of their long persistence in the environment and high toxicity. Reservoirs of 2,3,7,8-tetrachlorodibenzo-p-dioxin, for instance, which
has the second highest TWF after radium

- 228, can persist for decades in soils and sediments before eventually entering the human food chain. Dioxin compounds, moreover, especially PCDDs, have high octanol to water partition coefficient values, which means that they easily bioaccumulate in the fatty tissues of fish and biomagnify up the food chain.

Because dioxin compounds have widely ranging toxicities and typically occur together in mixtures, the World Health Organization has developed Toxic Equivalency Factors (TEF) to assess their toxic potency relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin. As shown in Table 5-1 , the cancer potency factor of each dioxin compound is adjusted by a TEF relative to t he cancer potency factor of 2,3,7,8- tetrachlorodibenzo-p-dioxin (U.S. EPA, 1985).5

- 5 Table 5-1 . Toxic Equivalency Factors (TEF), Bioaccumulation Factors (BAF), and TWF Values for Dioxin Compounds

Dioxin TEF TEF

Adjusted

Cancer Potency Factor BAF TWF Aquatic Life Value

(μg/L ) a TWF Human Health Value (μg/L ) a TWF Tetrachlorodibenzo

- p
- dioxin, 2,3,7,8

- 1 160,000 31,160 5,600,000 697,984,000 703,584,000 Pentachlorodibenzo

- p
- dioxin, 1,2,3,7,8

- 1 160,000 30,934 NA 692,928,000 692,928,000 Pentachlorodibenzofuran, 2,3,4,7,8

- 0.5 80,000 49,760 NA 557,312,000 557,312,000 Hexachlorodibenzofuran, 2,3,4,6,7,8

- 0.1 16,000 22,859 NA 51,204,160 51,204,160 Hexachlorodibenzofuran, 1,2,3,7,8,9

- 0.1 16,000 21,120 NA 47,308,800 47,308,800 Tetrachlorodibenzofuran, 2,3,7,8

- 0.1 16,000 19,549 29,474 43,790,080 43,819,554 Hexachlorodibenzo

- p
- dioxin, 1,2,3,4,7,8

- 0.1 16,000 10,490 NA 23,498,240 23,498,240 Hexachlorodibenzofuran, 1,2,3,6,7,8

- 0.1 16,000 6,299 NA 14,109,440 14,109,440 Hexachlorodibenzo

- p
- dioxin, 1,2,3,7,8,9

- 0.1 16,000 4,730 NA 10,595,840 10,595,840 Hexachlorodibenzo

- p
- dioxin, 1,2,3,6,7,8

- 0.1 16,000 4,266 NA 9,556,480 9,556,480 Hexachlorodibenzofuran, 1,2,3,4,7,8

- 0.1 16,000 2,571 NA 7,632,640 5,760,000 Pentachlorodibenzofuran, 1,2,3,7,8

- 0.05 8,000 6,815 NA 5,760,000 7,632,640 Heptachlorodibenzofuran, 1,2,3,4,7,8,9

- 0.01 1,600 13,545 NA 3,033,984 3,033,984 Heptachlorodibenzo

- p
- dioxin, 1,2,3,4,6,7,8

- 0.01 1,600 1,835 NA 411,136 411,136 Heptachlorodibenzofuran, 1,2,3,4,6,7,8

- 0.01 1,600 383 NA 85,760 85,760 Octachlorodibenzofuran 0.0001 16 590 5,600 986 2,021 Octachlorodibenzo

- p
- dioxin 0.0001 16 440 700 1,321 6,586

Methylmercury 1,225,029 NA 23,494 23,494 Source: Toxic E quivalency F actors (TEFs) for PCBs, PCDDs, PCDFs for H umans and W ildlife ( Van den Berg, 1998 ).

a - The aquatic life and human health component values in this table hav e been normalized to copper. There are no TWF aquatic life values fo r most dioxins

. b - Methymercury data are shown for comparison. Currently there is no cancer potency factor for methylmercury, nor is there an aquatic life value.
Given the high toxicity of dioxins and their high potential for biomagnification in aquatic food chains, it is worth noting that only a small number of facilities discharge small amounts of dioxins to
surface water. In 2008, the most recent year for which discharge monitoring data were available for this report, only six facilities submitted DMRs showing the discharge of dioxin and dioxin- like compounds. Four facilities in the pulp and paper point source category reported a total annual discharge of 0.000208 pounds of dioxins (145,000 TWPE). Two facilities in the chl orine and chlorinated hydrocarbons point source category reported a total annual discharge of 0.000619 pounds of dioxins (436,000 TWPE) ( U.S. EPA, 20 10 ).5

- 6 5.5 Variable Factors Used in the Human Health Component of the TWF Equation
Two variable factors representing sources of exp osure and aqu atic food chain bioaccumulation

are u sed in the human health component of the TWF equation to adjust the otherwise constant factors representing RfD, cancer potency factor , body weight, and fish consumption rate: 1) relative source contribution factors and, 2) bioaccumulation and bioconcentration factors.

5.5.1 Relative Source Contribution Factors
The relative source contribution (RSC) factor is used in the calculation of the human health component of TWFs to apportion exposure pathways of a chemical among drinking water, fish consumption, skin contact, or inhalation. The RSC is applied to both the RfD for noncarcinogens and the cancer potency factor for carcinogens. The RSC approach is documented in the Methodology for Deriving Ambient Water Quality Criteria for the Protectio n of Human Health (U.S. EPA, 2000). EPA, however,
has published RSCs for only 28 chemicals . The RSCs were calculated as part of the development of NRWQC. For TWF human health calculations for all other chemicals, fish consumption is assumed to be the only exposure pathway, and thus an RSC of 100 percent is applied. Consequently, the drinking water exposure pathway is assumed to be zero percent for most TWFs. EPA considers this assumption to be
reasonab le for the purpose of screeni ng- level analysis, considering that most Americans drink treated water, or tested well water, and thus generally are not exposed to unsafe levels of toxic chemicals with
TWFs through drinking water.

5.5.2 Bioaccumulation and Bioconcentration Factors

Toxic chemicals have a range of physical characteristics which determine how readily they partition from an aqueous phase into an organic phase in the surface water environment. In fish, chemicals typically partition into the organic phase in the gills. An aquatic bio accumulation factor (BAF) is the ratio of a toxic chemical in an organism to its concentration in the ambient water, considering exposure from all potential pathways. An aquatic bioconcentration factor (BCF) is the ratio of a toxic chemical in an organism to its concentration in the ambient water, considering direct contact with the water as the only exposure pathway. Bioaccumulation typically accounts for significantly more chemical build up in tissues than bioconcentration, especially for highly toxic and highly persistent chemicals like dioxin compounds. National- level BAFs are not available, however, because they are complex to develop.
BCFs , on the other hand, are readily available , and EPA uses them to calculate most TWFs with exception of the 17 dioxin compounds and methylmercury as shown in Table 5-1

.

EPA esti mates BAFs for dioxins because of their extremely high potential to bioaccumulate. EPA
derives BAFs for d ioxins by combining estimates of the percentage of fish tissue that is fat with estimates of the tendency of a dioxin to be absorbed in fat. These estimates were developed by the Great Lakes5

- 7 Water Quality Initiative (GLWQI) ( U.S. EPA, 1995). EPA applied th e GLWQI estimates to equations for
BAF calculation published in EPA’s Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health (U.S. EPA, 2000).

EPA estimates the BAF for methylmercury differently than it does for dioxins. Unlike other NRWQC, the water quality criterion for methymercury is based on fish tissue concentration rather than on ambient water concentration. For the purpose of calculating methymercury TWFs, EPA converts the fish tissue criterion of 0.3 milligrams m ethylmercury per kilogram in fish tissue to a national BAF estimate of 1,225,029 using EPA published methodology ( EPA , 2001 ).

The TWF Database contains the complete list of sources for BCFs used in TWF calculations

. Most BCFs come from the following sources:

• “Estimating Bioconcentration Potential from Octanol/Water Partition Coefficients” ( Veith and Kosian , 1983 );

• Assessment Tools for Evaluation of Risk (ASTER) Database (U.S. EPA, 1999);

• “Evaluation of Waterborne Exposure Pathways to Paragraph 4(c) Po llutants” (Little, 1983); and

• Handbook of Chemical Property Estimation Methods – Environmental Behavior of Organic Compounds (Lyman et al. , 1981).

5.6 The Human Health Component of the TWF Equation References

1. ASTDR. 2011. Agency for Toxic Substances and Disease Registry. ASTDR Website. Available online at: http://www.atsdr.cdc.gov/

.
2. Little, Arthur D. 1983. “Evaluation of W aterborne Exposure P athways to P aragraph 4(c) Pollutants.” Draft Report. (April 28).

3. Lyman WJ, WF Reehl, and DH Rosenblatt. 1981. Handbook of C hemical P roperty Estimation Methods - Environmental Behavior of Organic Compounds . New York, NY: McGraw

- Hill Book Company.

4. U.S. EPA. 1985. U.S. Environmental Protection Agency. Office of Health and Environme ntal Assessment. Health Assessment Document for Polychlorinated Dibenzo-p-dioxins . Washington, DC. EPA -600-8-84-814F.

5. U.S. EPA. 1991. U.S. Environmental Protection Agency. Office of Water. Technical Support Document for Water Quality-Based Toxics Control . Washington, DC. (March). EPA

- HQ

- OW

2004-0032-0935

6. U.S. EPA. 1995. U.S. Environmental Protection Agency. Great Lakes Water Quality Initiative Tech nical Support Document for the P rocedure to D etermine Bioaccumulation Factors. EPA

820-B-95-005. (March).

7. U.S. EPA . 1999. U.S. Environmental Protection Agency. Assessment Tools for Evaluation of Risk (ASTER) Database . Duluth, MN: U.S. EPA, Environmental Research Laboratory.5

- 8 8. U.S. EPA . 2000. U.S. Environmental Protection Agency. Office of Water. Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health: Technical Support Document Volume 1: Risk Assessment . EPA -822-B-00-005. Washington, DC. ( October ).

9. U.S. EPA . 2001. U.S. Environmental Protection Agency. 66 FR 1344. Water Quality Criteria: Notice of Availability of Water Quality Criterion for the Protection of Human Health: Methylmercury , (January 8).

9. U.S. EPA . 2005. U.S. Environmental Protection Agency. Guidelines for Carcinogen Risk Assessment . EPA/630/P -03/001F. Washington, DC. ( March ). Available online at: http://www.epa.gov/raf/publications/pdfs/CANCER_GUIDELINES_FINAL_3-25-05.pdf

. 10. U.S. EPA. 2007. U.S. Environmental Protection Agency. Water Quality Standards Handbook: Second Edition. EPA -823-B-94-005. Washington, DC. (June). Available online at: http://water.epa.gov/scitech/swguidance/standards/handbook/
11. U.S. EPA. 2010. U.S. Environmental Protection Agency. Technical Support Document for the 2010 Effluent Guidelines Program Plan. EPA 820R-10-021. Washington, DC. (December). EPAHQ

- OW -2008-0517 DCN 07320.

12. U.S. EPA . 2011a. U.S. Environmental Protection Agency. Integrated Risk Information System (IRIS) Website. Available online at: http://www.epa.gov/IRIS/

.
13. U.S. EPA . 2011b. U.S. Environmental Protection Agency. Mid- Atlantic Risk Assessment – Human Health Risk Assessment Website. Available online at: http://www.epa.gov/reg3hwmd/risk/human/index.htm

. 14. Van den Berg, et al., 1998. Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for Humans and Wildlife. Environ. Health Perspect. 106:775-792.
15. Veith, GD and P. Kosian. 1983. “Estimating Bioconcentration Potential from Octanol/Water Partition Coefficients.” In D. Mackay et al. (Eds.) “Physical behavior of PCBs in the Great Lakes”. Ann Arbor Scie. Publ. Ann Ar bor, MI:269 -282.6

- 1 6. EPA’ S U SE OF TWF S TO S UPPORT E FFLUENT G UIDELINES R ULEMAKINGS AND P ROGRAM P LANNING EPA’s effluent guidelines program uses TWFs for two specific purposes. Since the early 1980s,
EPA has used TWFs t o help inform regulatory decision making abo ut the relative cost effectiveness of technology options to treat effluent containing toxic chemicals. In 2003, EPA began using TWFs in effluent guidelines program planning to rank indus tries by their total annual toxic

- weighted discharges to identify majo r sources nationwide tha t may warrant further analysis.

EPA has issued effluent guidelines for 56 industrial categories encompassing 35,000 to 45,000 facilities that discharge directly to waters of the United States . The effluent guidelines program has als o issued pretreatment standards that apply to approximately 12,000 facilities that discharge to publically owned treatment works (POTWs). Each year, effluent guidelines and pretreatment standards prevent the discharge of more than 1.2 billion pounds of tox ic pollutants.

EPA does not use TWFs in water quality- based permitting or risk assessment because they do not provide the information on site

- specific water chemistry, environmental alteration of chemicals, exposure pathways, and organism susceptibility re quired to adequately set permit limits or assess risk. Moreover, EPA only uses TWFs to help inform decision making, and never as a sole source of information on which to base policy.

6.1 Use of TWFs in Cost

- Effectiveness Analysis
As explained in the introducti on of this report, TWFs are used to calculate Toxic Weighted Pound Equivalents (TWPEs), which are employed in cost

- effectiveness analysis. Cost effectiveness is defined as the incremental annual cost, expr essed in 1981 constant dollars, per incremental tox ic weighted pound of pollutant removed by a treatment technology (U.S. EPA, 1988). As shown in Table 6-1 , the use of
constant 1981 dollar values allows effluent guidelines developed and revised at different times t o be compared with one another.

Cost

- effectiveness analysis is used to help evalua te a set of treatment technologies to identify the

one that has the lowest cost per pound of toxic pollut ants from a waste stream. It should be noted that there is no absolute scale for determining whether or not a treatment technology option is cost effec tive. The cost effectiveness of treatment technologies are considered to be high or low relative to one another.

Although not required by the Clean Water Act, cost effectiveness analysis is a useful tool for effluent guidelines regulatory analysis. It is, however, just one of many factors considered during evaluation of treatment technology option s. Moreover, cost

- effectiveness analysis is not always used in effluent guidelines regulatory development. It is only used to help assess regulatory options for in dustries6

- 2 that discharge toxic chemicals classified by the Clean Water Act (CWA) as “Priority Pollutants,” which are 126 chemicals specified in the Act, and for substances the CWA classifies as “Non -conventional
P ollutants , ” such as chlorides, iron, and amm onia. Substances the CWA classifies as “C onventional Pollutants,” such as oil and grease, biochemical oxygen demand, and total suspended solids are not
included in cost

- effectiveness analysis because they are addressed in a different analogous analysis tha t pertains to all pollut ants, not just toxic chemicals.

Table 6-1 . Examples of Effluent Guidelines Cost

- Effectiveness Analysis Ranked by Dollars Per TWPE Removed

Industry Estimated TWPE Discharged Prior to Rul e (thousands) Estimated TWPE Discharged after Implementation of Rule (thousands) Estimated TWPE Removed by Rule (thousands) Cost

- Effectiveness of Rule (1981 Dollars per TWPE removed) Inorganic Chemicals I 32,503 1,290 31,213 <1 Battery Manufacturing 4,126 5 4,121 2 Nonferrous Metals Manufacturing I 6,653 313 6,340 4

Organic Chemicals 54,225 9,735 44,490 5 Inorganic Chemicals II 605 27 578 6 Nonferrous Metals Manufacturing II 1,004 12 992 6

Porcelain Enameling 1,086 63 1,023 6 Centralized Waste Treatment 3,372 1,267 2,105 5

- 7

Canmaking 12 0 12 10 Metal Finishing 3,305 3,268 37 12 Pesticides 2,461 371 2,090 14 Copper Forming 70 8 62 27 Iron & Steel 1,053 853 200 27 Pulp & Paper 61,713 2,628 59,085 39 Coil Coating 2,289 9 2,280 49 Metal Products & Machinery 140 70 70 50

Nonferrous Metals Forming 34 2 32 69 Foundries 2,308 39 2,269 84 Pharmaceuticals 89,790 471 89,320 47

- 96 Aluminum Forming 1,340 90 1,250 121 Electronics I 9 3 6 404 S ource: Adapted from Cost

- Effectiveness Analysis of Effluent Limitation Guidelines and Standards for the Centralized Waste Treatment Industry (U.S. EPA, 2000 ).
Data used in conducting cost

- effectiveness analyses may come from a variety of sources. Most typically, however, data are compiled through industry surveys and effluent sampling and analysis specifically designed for individual effluent guidelines rulemakings. The number of chemicals assessed in6

- 3 a cost effectiveness -analysis may range from only a few to more than 100 depending on the nature of an industry’s waste streams. For the centralized waste treatment industry, for example, cost effectiveness was assessed for technologies to treat a set of 146 chemicals.

6.2 Use of TWFs in Effluent Guidelines Program Planning

CWA Sectio n 304(b) requires EPA to annually review industrial categories for which effluent guidelines have been promulgated to determine if any revisions are warranted, and to identify possible new industrial categories for regulation. Section 304(m) requires EPA to publish a biennial plan announcing the schedule for the annual reviews, as well as the schedule for completing any effluent guidelines revisions or new r egulations identified during the annual review s. EPA has published a draft program plan eve ry o dd-numbered year since 1989 and a final program plan every even -nu mbered year
since 1990.

EPA began using TWPEs in 2003 to rank industrial categories by their total annual discharge amounts to supp ort development of the 2004 effluent guidelines program plan ( U.S. EPA, 2004 ). The use of TWPEs as an indicator of the relative potential for industrial waste streams to cause harm prior to discharge assists EPA in setting priorities for effluent guidelines rulemakings so as to achieve the greatest environmental and health benefits from toxic discharge reduction. It is important to note that TWPE

based industry ranking is just one of four factors considered in effluent guidelines program planning, in addition to consideration of public comments. EPA uses the fol lowing three additional factors in its annual review of industrial categories:

1. The performance and cost of applicable and demons trated wastewater treatment technologies, process changes, or pollution prevention alternatives that could effectively reduce th e pollutants in the industrial category’s wastewater and, consequently, reduce the potential harm to human health or the environment associated with pollutant discharges;

2. The affordability or economic achievability of the identified wastewater treatment technologies , process change s, or pollution p revention measures ; and

3. Opportunities to eliminate inefficiencies or impediments to pollution prevention or technological innovation or opportunities t o promote innovative approaches . This factor might also prom pt EPA during an annual review to decide against identifying an existing set of effl uent guidelines or pretreatment standards for revision where the pollutant source is already efficiently and effectively controlled by other regulatory or nonregulatory pro grams.

EPA calculates two TWPE estimates for each industrial category based on data from individual facility discharge monitoring reports (DMRs ) and EPA’s Toxics Release Inventory (TRI). Facilities are required to submit DMRs to EPA or state permitting ag encies as a condition of their National Pollutant6

- 4 Discharge Elimination System (NPDES) water discharge permits. DMRs are stored in the Permit Compliance System (PCS) and the Integrated Compliance Information System for the National Pollutant Discharge Elimination System (ICIS

- NPDES). EPA began replacing PCS with ICIS -NPDES in 2006. Until the transition is complete, EPA retrieves ce rtain data from both databases.

TRI is the common name for Section 313 of the Emergency Planning and Community Right- to

Know Act (EPCRA) (U.S. EPA, 2009a) . F acilities above certain reporting thresholds, such as size and amount of toxic chemicals discharged, must annually report estimates of discharges of certain toxic
chemicals listed under EPCRA. Unlike DMRs, TRI reports are primarily based on estimates rather than on sampling and analysis. TRI reports, however, include estimates of toxic chemical waste streams sent to
POTWs , which DMRs do not. TRI also includes estimates of some pollutant discharges that are not currently regulated (i.e., pollutants not in DMRs).

When facilities discharge indirectly, POTWs remove varying amounts of toxic chemicals from industrial waste streams, depending to the type of treatment process they use, before discharging the
treated effluent to surface waters. This type of discharge is referred to as an indirect discharge. For the purpose of effluent guidelines program planning, EPA primarily uses TRI data to help estimate the amount of indirect discharges from industrial sources. EPA only uses T RI data to support effluent guidelines program planning, not for cost- effectiveness analysis.

Table 6-2. TWPE Ranking of Industrial Categories Based on 2008 DMR Data a

Rank Point Source Category Number of Disch argers Total Annual Pounds Total TWPE

1 Nonferrous Metals Manufacturing 68 1,760,000,000 955,000

2 Fertilizer Manufacturing 54 160,000,000 818,000 3 Petroleum Refining 727 7,740,000,000 618,000

4 Organic Chemicals, Plastics And Synthetic Fibers b 545 4,740,000,000 512,000

5 Pulp, Paper And Paperboard 267 2,350,000,000 510,000 6 Ore Mining And Dressing 117 558,000,000 339,000 7 Textile Mills 106 39,500,000 247,000 8 Waste Combustors 306 3,970,000,000 245,000

9 Inorganic Chemicals Manufacturing b 206 1,790,000,000 228,000

10 Miscellaneous Foods And Beverages 152 110,000,000 193,000

Source: Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan ( U.S. EPA, 2009b ).

a Excludes categories currently or recently under review for a potential rulemaking or rulemaking revision. b Excludes facilities manufacturing chlorine or chlorinated hydrocarbons, as these discharges are currently undergoing review for possible rulemaking revision

.6

- 5 Table 6-2 shows the ten industrial categories with the highest current TWPE discharges accordin g the 2008 DMR data analyzed for the 2010 effluent guidelines program plan. Although DMR and TRI data show that EPA regulatory programs are reducing discharges of toxic chemicals over time, discharge amounts of individual toxic chemicals vary from year to year, depending largely on economic conditions. The heaviest industries, such as metal manufacturing, petroleum refining, and ore mining , produce the highest toxic discharges, and thus tend to be among the highest ranked industries from year to year.

Table 6-3 shows the ten industrial categories with the highest TWPE discharges accordin g the 2008 TRI data analyzed for the 2010 effluent guidelines program plan. T he discrepancies between Table
6-2 and Table 6-3 are the result of differing DMR and TRI reporting requirements. TRI contains discharge data for fewer facilities due to reporting thresholds that only require la rger facilities to submit
data. TRI also reflects a lower estimate of the pounds of pollutant discharged, for the same reason.

Table 6-3. TWPE Ranking of Industrial Categories Based on 2008 TRI Data a

Rank Point Source Category Number of Dischargers Total Annual Pounds Total TWPE 1 Pulp, Paper And Paperboard 250 19,300,000 523,000 2 Petroleum Refining 298 25,700,000 410,000 3 Organic Chemicals, Plastics And Synthetic Fibers b 679 27,600,000 137,000 4 Ore Mining And Dressing 34 491,000 109,000 5 Plastics Molding And Forming 123 2,200,000 74,700 6 Inorganic Chemicals Manufacturing b 162 8,510,000 71,300 7 Nonferrous Metals Manufacturing 122 4,190,000 38,700 8 Pesticide Chemicals 73 2,090,000 35,500 9 Carbon Black Manufacturing 7 279 27,600 10 Timber Products Processing 122 48,400 27,300 Source: Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan ( U.S. EPA, 2009b ).

a Excludes categories currently or recently under review for a potential rulemaking or rulemaking revision. b Excludes facilities manufacturing chlorine or chlorinated hydrocarbons, as these discharges are currently undergoing review for possible rulemaking revision

. For the purpose of effluent guidelines program planning analysis , EPA adds the TWPEs
calculated fr om TRI and DMR data to produce a total TWPE value for each industrial category. EPA takes this approach because the combination of DMR and TRI data helps to overcome certain limitations
in each data source and provides a more comprehensive portrait of total toxic discharges . A thorough discussion of the effluent guidelines program, th e effluent g uidelines planning process, use of TWFs and
TWPEs in industry ranking, and the complete list of industry ranking s is contained in the Technical Support Document for the Preliminary 2010 E ffluent Guidelines Program Plan (U.S. EPA, 2009b).6

- 6 6.3 EPA’s Use of TWFs to Support Effluent Guidelines Rulemakings and Program Planning References

1. U.S. EPA . 1988. U.S. Environmental Protection Agency. Office of Water. Ehrensberger, K., and R. Rico. Cost

- effectiveness Analysis for Effluent Guidelines . Office of Wate r. Washington, DC. ( May ).

2. U.S. EPA. 2000. U.S. Environmental Protection Agency. Office of Water. Cost

- Effectiveness Analysis of Effluent Limitation Guidelines and Standards for the Centralized Waste Treatment Industry

. Washington, DC. ( December ). Availab le online at: http://water.epa.gov/scitech/wastetech/guide/treatment/upload/2006_12_28_guide_cwt_final_effe ctive.pdf

.
3. U.S. EPA. 2004. Tec hnical Support Document for the 2004 Effluent Guidelines Program Plan

. EPA 821R-04-014. Washington, DC. (August). EPA

- HQ

- OW -2003-0074-1346 through 1352.

4. U.S. EPA . 2009a. U.S. Environmental Protection Agency. 2009 Toxics Release Inventory National Analysis Overview . Washington, DC. (Unknown). Available online at: http://www.epa.gov/tri/tridata/tri09/nationalanalysis/overview/2009TRINAOverviewfinal.pdf

.
5. U.S. EPA. 2009b. Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan . EPA 821R-09-006. Washington, DC. (October). EPA

- HQ

- OW -2008-0517-0515.7

- 1 7. L IMITATIONS AND A PPROPRIATE U SES OF T OXIC W EIGHTING F ACTORS Appropriate uses of TWFs are constrained by the limitations of the data used to derive them, and by limitations in the discharge data to which TWFs a re applied. This section discusses the appropriate
uses of TWFs given the data limitations.

7.1 Limitations of Sour ce Data Used to Derive TWFs

Due to the lack of a single comprehensive source of to xicity information for the range of chemicals assessed by the effluent guidelines prog ram, EPA must combine disparate sources o f effects
information in the calculation of TWF s. The data used to derive TWFs were compiled for different purposes at different times. Effects information used in the aquatic life component of TWFs is compiled primarily from three different sources, each based on different underlying analytical method ologies. Similarly, RfDs and cancer potency factors used to derive the human health component of TWFs are compiled from a hierarchy of multiple sources based on data quality and availability . I nconsistent source data is the primary reason why TWFs are not suitable for more rigorous applications, such as analyses to support water-quality based permitting or risk assessment.

In addition, complete data are lacking for the aquatic life and human health components of the majority of chemicals for which TWFs have been calculated. Only 37 percent of TWFs (349 of 1,064 ) are based on both aquatic life and hum an health effects informa tion. The majority of TWFs, 62 percent (661), are based solely on aquatic life values. The remaining five percent of TWFs (54) are derived solely from
human health values.

7.2 Limitations of Discharge Data to Which TWFs Are Applied
There are usually no data limitations issues when conducting cost

- effectiveness analyses because the data are typically compiled through industry surveys and effluent sampling specifically designed for
individual effluent guidelines rulemaking s. For the screening- level analysis, ho wever, TWF

- based analysis of PCS/ICIS

- NPDES data, must be caveated to note the limitations of these two databases, which are the only sources of national level facility discharge data available to EPA for effluent guidelines program planning analysis.

One of the biggest limitations of PCS/ICIS -NPDES is that for many industries, annual discharge amounts can only be calculated for facilities classified as major dischargers , which are typically the largest facilities with the most toxic discharges . Facilities with NPDES permits are not required to report
the actual quantitie s of chemicals they discharge. Instead, they must only report the discharge flow rate, such as gallons per day, and the concentration of chemicals in the waste stream. EPA combines the flow and concentration information to produce annual discharge load estimates expressed as pounds discharged7

- 2 per year. Facilities classified as minor discharge r s, however, are not required to report their dischar ge flow. Some states report both flow and concentration information for minor discharger s, others do not. In addition, there is considerable inconsistency in how state permitting authorities classify facilities as either major or minor. Moreover, t he majority of facilities are classified as minor dischar ger s in some industries with significant toxic discharges , such as most coal and ore mining facilities. State permitting authorities are required to consider the following six factors when classifying facilities as major or minor (U.S. EPA, 2010):

1. Toxic po llutant potential ; 2. Discharge flow to stream flow ratio ; 3. Conventional pollutant loading ; 4. Public health impact; 5. Water quality factors ; and 6. Proximity to coastal waters

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In addition to incompleteness of data reported to PCS/ICIS

- NPDES, there are several other important limitations to the database (U.S. EPA, 2009).

• PCS/ICIS

- NPDES contain data only for the pollutants facilitie s are required to monitor as stipulated in their NPDES permits. Facilities are not required to monitor or report all of the pollutants the y actually discharge.

• PCS/ICIS

- NPDES include very limited data characterizing indirect discharges from facilities to POTWs.

• Many of the pollutant parameters reported to PCS/ICIS- NPDES are not individual chemical compounds. For example, “oil and grease” is a common industrial parameter reported to PCS/ICIS- NPDES that can contain a wide variety of industrial chemicals

. Moreover, ma ny elemental chemicals have multiple ionic species with differing toxic properties . Levels of these multiple ionic species are frequently reported as a single total value. For example, the various ionic forms of mercury may be reported together as “total mercury” levels in a waste stream.

• Most reported flow rates do not indicate the type of wastewater. Thus total flow rates of industrial process wastewater may be diluted by stormwater or noncontact cooling water.

• Facilities are identified by SIC code, not by effluent guidelines point source category. For some SIC codes, it may be difficult or impossible to identify the actu al industry that is the source of the reported wastewater discharge.

A primary advantage of TRI is that , unlike PCS/ICIS

- NPDES, facilities are required to report

annua l discharge amounts of chemicals rather than separate flow and concentration values. How ever, a disadvantage of TRI is that facilities are only required to report estimated discharge amounts rather than7

- 3 actual sampled discharge amounts

. There are several additional notable limitations of the data reported to TRI ( U.S. EPA, 2009).

• Information in TRI does not represent total nationa l discharges because not all facilities are required to report to TRI. Small business with less than ten employees and facilities that do not meet discharge amount reporting thresholds are not required to report to TR I. Thus the completeness of discharge information v aries from industry to industry.

• The list of chemicals that facilities are required to report to TRI is not all - inclusive and has varied over time.

• TRI requirements encourage facilities to report some compounds as present at one-half the detection level if a facility suspects that the compound has the potential to b e present, even if analysis of discharge samples shows the compound is below its detection level. Many companies adopt this conservative reporting approach. F or facilities with high discharge flows, this can result in large estimates of pounds discharged with no sampling data to determine that the compound was actually present.

• As with PCS/ICIS

- NPDES , certain chemicals are reported as a class, such as polycyclic aromatic compounds, dioxin compounds, and metal compounds. Because the individual compounds in the class may h ave widely varying toxicities, actual toxicities of waste streams can be di fficult to estimate accurately.

• Facilities are identified by NAICS code, not by effluent guidelines point s ource category. For some NAICS codes, it may be difficult or impossible to identify the point source category that is the actual source of the toxic wastew ater discharge.

• Only facilities in certain NAICS codes are required to report, so some sources of toxic chemical discharges are not included in TRI

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7.3 Appropriate Uses of TWFs
In spite of the limitations of the TWF source data and the discharge data to whi ch they are applied, EPA considers T WFs appropriate for use in effluent guidelines planning analysis to identify
industries or facilities that may warrant additional research. Moreover, TWPE -based ranking of industries
is just one of four factors that EPA considers when setting priorities for effluent guidelines program

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EPA also considers TWFs appropriate for use in the calculation of cost

- effectiveness values because such values only serve as indicators of the relative cost effectiveness of treatment tech nology options and not as absolute metrics. Moreover, as with TWPE -based industry ranking in effluent guidelines program planning, cost- effectiveness analysis is not used as the sole basis for decision making about technology options , but rather as one of m any factors that EPA considers.

EPA’s effluent guidelines program cautions other organizations that may consider using TWFs for applications other than the specific ones for which they were designed. Other potential users of TWFs should carefully evaluate TWF source data limitations, the limitations of EPA national discharge7

- 4 databases, and the uncertainties created by the TWF approach of combining aquatic life and human health effects values before deciding to u se TWFs.

7.4 Limitations and Appropriate Uses of Toxic Weighting Factors References

1. U.S. EPA. 2009. U.S. Environmental Protection Agency. Office of Water. Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan . EPA 821-R-09-006. Washington, DC. ( October ). EPA

- HQ

- OW -2008-0517-0515.

2. U.S. EPA. 2010. U.S . Environmental Protection Agency. Office of Water. NPDES Permit Writers Manual. Washington, DC. EPA -833-K-10-001. (September). Available online at: http://www.epa.gov/npdes/pubs/pwm_2010.pdf

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