MEMORANDUM

TO: Bill Swietlik, EPA

FROM: Jill Lucy, ERG

DATE: September 21, 2015

SUBJECT: Review of Toxic Weighting Factors in Support of the Final Steam Electric 

Effluent Limitations Guidelines and Standards (DCN SE04479)

To support its revision of the effluent limitations guidelines and standards (ELGs) for the 

Steam Electric Power Generating Point Source Category (40 CFR Part 423), EPA calculated

annual pollutant loadings from steam electric power plants that discharge to receiving waters, 

either directly or via pass through at publicly owned treatment works (POTWs). For the 

evaluated wastestreams, EPA calculated the loadings for baseline (existing) conditions and the 

pollutant removals for regulatory options. EPA used the pollutant removals (baseline loadings 

minus option loadings) to evaluate the effectiveness of the regulatory option treatment 

technologies, to estimate benefits gained from removing pollutants discharged from plants, and 

to evaluate the cost-effectiveness of the regulatory option in reducing the pollutant loadings. In 

addition to expressing pollutant loadings in pounds of pollutant discharged per year, EPA used

toxic weighting factors (TWFs) to account for differences in toxicity across pollutants. EPA 

calculated a toxic-weighted pound equivalent (TWPE) value for each pollutant discharged to 

compare pollutant mass loadings based on normalized toxicity. To perform this comparison, 

EPA multiplied the mass loadings of pollutant in pounds per year (lbs/yr) by the pollutantspecific TWF to derive a “toxic-equivalent” loading (lb-equivalent/yr), or TWPE.

EPA explained the concept, calculation, use, and limitations of TWFs in the March 2012 

document, Toxic Weighting Factors Methodology (EPA-820-R-12-005). The document 

described how EPA first developed the TWF equation that combines information about toxic 

effects to aquatic life and human health. The document also explained how EPA determined the 

aquatic life component and human health component of the TWF calculation. Finally, the 

document discussed that ELG rulemakings and program planning are the appropriate uses for 

TWFs and discussed the limitations of the TWFs. EPA used TWFs developed using this 

methodology to support its proposed revised ELGs for the steam electric power generating 

industry (see 78 FR 34432, June 7, 2013). EPA received comments on the proposed ELGs that 

EPA should review toxicity benchmarks used to calculate TWFs for copper and other metals1

and identify any more recent data.

 

1 See EPA-HQ-OW-2009-0819-4655.
Memorandum

September 21, 2015

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The purpose of this memorandum is to provide background on how EPA used TWFs for 

the steam electric power generating industry ELG development and to document the review of 

toxicity benchmarks, including identifying new data and determining whether the new data are 

more appropriate for TWF development. ERG reviewed the benchmarks (and underlying input 

data) included in the calculation of 16 pollutant TWFs. EPA chose these 16 pollutants due to 

their high modeled pollutant loadings for the evaluated wastestreams discharged by steam 

electric power plants. Based on this analysis, EPA revised TWFs for seven pollutants. This 

memorandum includes the following sections:

 Section 1: Overview of TWF Methodology– general methodology that EPA used to 

develop TWFs.

 Section 2: Review of TWFs for Selected Steam Electric Power Generating Industry 

Pollutants of Concern – evaluation of toxicity benchmarks and other input data that 

EPA used to calculate TWFs. 

 Section 3: Summary of Revised TWFs.

 Section 4: References – a list of references used in support of this memorandum.

 Attachment A – details on TWF calculations and equation parameters.

 Attachment B – summary of how EPA used the TWFs.

1. OVERVIEW OF TWF METHODOLOGY 

Toxicity is the degree to which a substance can harm or damage an organism, such as 

aquatic species, wildlife, or humans. As described in EPA’s Toxic Weighting Factors 

Methodology, EPA Office of Water (OW) Engineering and Analysis Division (EAD) uses TWFs 

to compare the relative cost-effectiveness of treatment technology options for ELGs and to 

relatively rank industries by their TWPEs as part of the effluent guidelines program planning. 

EPA has developed TWFs for 1,064 chemicals [U.S. EPA, 2012a]. EPA calculated the TWFs 

using the MS ExcelTM TWF Database for all pollutants except for boron; EPA used a separate 

analysis to update the boron TWF in 2008 [U.S. EPA, 2012b; Abt, 2008].

EPA calculated TWFs for pollutants in water environments using two toxicity 

components:

1. Aquatic Life Value (AQ): the concentration in water at which the pollutant 

becomes harmful to aquatic life.

2. Human Health Value (HH): the concentration in water that corresponds to a 

human health effect threshold for noncarcinogenic pollutants and a non-threshold 

cancer risk-based factor for carcinogenic pollutants. 
Memorandum

September 21, 2015

Page 3

AQ is a pollutant concentration in the water (μg/L) that is protective of aquatic life. EPA 

selected AQ using the following benchmark values, in order of descending preference:

 Freshwater chronic benchmark value (e.g., National Recommended Water Quality 

Criteria (NRWQC) developed by EPA).

 Freshwater acute benchmark value.

 Salt water chronic benchmark value.

 Salt water acute benchmark value.

Section 2.1 discusses the review of aquatic life benchmark values used in calculating 

TWFs for the selected steam electric power generating industry pollutants of concern. 

HH is a pollutant concentration in the water (μg/L) that is protective for humans that eat 

fish and shellfish (i.e., aquatic organisms). As outlined in the Toxic Weighting Factors 

Methodology, EPA assumed the primary exposure pathway is fish consumption and assumed the 

drinking water exposure pathway is negligible. This assumption is reasonable for screening-level 

analyses, considering that most Americans drink treated water and generally are not exposed to 

unsafe levels of toxic chemicals via the drinking water exposure route. 

EPA uses different equations or input values based on the following two factors: 1) 

whether the pollutant is noncarcinogenic or carcinogenic and 2) whether EPA OW Health and 

Ecological Criteria Division (HECD) used the 1980 or 2000 NRWQC methodology for its 

National Recommended Water Quality Criteria: 2002. Human Health Criteria Calculation 

Matrix (EPA-822-R-02-012) [U.S. EPA, 2002a].2 Table 1-1 presents the four different equations 

that EPA used to calculate HH in the TWF Database.

Table 1-1. Equations Used to Calculate the Human Health (HH) Component of Pollutant 

Toxic Weighting Factors (TWFs)

Methodology and 

Pollutant Type Equation

1980 methodology, 

noncarcinogen, 

HH =

RfD × BW × 1000

FI × BCF Equation 1

 

2 EPA’s National Recommended Water Quality Criteria: 2002 Human Health Criteria Calculation Matrix (EPA822-R-02-047) provided information regarding the calculation of the human health criteria contained in the 

document entitled, National Recommended Water Quality Criteria: 2002 (EPA-822-R-02-047). The National 

Recommended Water Quality Criteria: 2002 Human Health Criteria Calculation Matrix document provides cancer 

potency factors; reference doses; relative source contributions; fish intake values; and equations used to derive the 

human health criteria for EPA’s 2002 NRWQC.
Memorandum

September 21, 2015

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Methodology and 

Pollutant Type Equation

organism 

consumption only

Where: HH = Human health value (μg/L)

RfD = Reference dose, as milligrams per kilogram-day (mg/kg/day)

BW = Body weight for an adult, 70 kg [U.S. EPA, 2000]

1000 = μg/mg conversion

FI = Fish consumption rate, 0.0065 kg/day [U.S. EPA, 2000]

BCF = Bioconcentration factor (L/kg)a

1980 methodology, 

carcinogen, 

organism 

consumption only

HH =

(

Crisk

CPF ) × BW × 1000

FI × BCF

Equation 2

Where: HH = Human health value (μg/L)

Crisk = Cancer risk level, typically between 1 ×10-4 and 1 ×10-6

CPF = Cancer potency factor (kg-day/mg)

BW = Body weight for an adult, 70 kg [U.S. EPA, 2000]

1000 = μg/mg conversion

FI = Fish consumption rate, 0.0065 kg/day [U.S. EPA, 2000]

BCF = Bioconcentration factor (L/kg) a

2000 methodology, 

noncarcinogen, 

organism 

consumption only

HH =

RfD × RSC × BW × 1000

FI × BCF

Equation 3

Where: HH = Human health value (μg/L)

RfD = Reference dose, as milligrams per kilogram-day (mg/kg/day)

RSC = Relative source contribution (%)

BW = Body weight for an adult, 70 kg [U.S. EPA, 2000]

1000 = μg/mg conversion

FI = Fish consumption rate, 0.0175 kg/day [U.S. EPA, 2000]b

BCF = Bioconcentration factor (L/kg) a

2000 methodology, 

carcinogen, 

organism 

consumption only

HH =

(

Crisk

CPF ) × BW × 1000

FI × BCF

Equation 4

Where: HH = Human health value (μg/L)

Crisk = Cancer risk level, typically between 1 ×10-4 and 1 ×10-6

CPF = Cancer potency factor (kg-day/mg)

BW = Body weight for an adult, 70 kg [U.S. EPA, 2000]

1000 = μg/mg conversion

FI = Fish consumption rate, 0.0175 kg/day [U.S. EPA, 2000] b

BCF = Bioconcentration factor (L/kg) a

a – Bioaccumulation factors (BAFs) may also be used in place of bioconcentration factors (BCFs) [U.S. EPA, 2000].

b – The default fish consumption rate for general adult population and sport anglers is 0.0175 kg/day [U.S. EPA, 

2000]. For mercury TWF calculations (noncarcinogenic pollutant), EPA OW EAD used a fish consumption rate of 

0.0187 kg/day, the fish intake rate used as a basis for the 1998 NRWQC. For the 2002 NRWQC, EPA OW HECD 

used the fish consumption rate of 0.0175 kg/day for mercury [U.S. EPA, 2002a].
Memorandum

September 21, 2015

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Attachment A provides more details on the calculation of TWFs using AQ and HH and 

descriptions of the input values used to calculate HH. Section 2.2 discusses ERG’s review of 

human health benchmark values used in calculating HH for the selected steam electric power 

generating industry pollutants of concern.

2. REVIEW OF TWFS FOR SELECTED STEAM ELECTRIC POWER GENERATING INDUSTRY 

POLLUTANTS OF CONCERN 

ERG focused the TWF review described in this memorandum on 16 pollutants found in 

steam electric power plant wastestreams, listed in Table 2-1. ERG selected these pollutants for 

review because of the high modeled pollutant loadings from steam electric power plants. Table 

2-1 also indicates whether the current TWF for that pollutant is based on AQ, HH, or both values

and references the current equations being used to calculate HH.

For the 16 reviewed pollutants, EPA set AQ equal to freshwater chronic criteria values. 

When developing the TWF methodology, EPA selected the freshwater chronic criteria as the 

most preferred value of AQ due to their availability [U.S. EPA, 2012a]. 

EPA derived the HH values from separate equations to estimate effects thresholds for 

noncarcinogens and non-threshold cancer risk-based factors for carcinogens (see Table 1-1). Of 

the evaluated pollutants, only arsenic is considered carcinogenic for purposes of the TWF 

calculation. Although cadmium, chromium VI, and nickel are potential carcinogens, EPA has not 

developed final cancer potency factors for these three pollutants. Therefore, EPA used the 

equation for noncarcinogenic pollutants to calculate HH for cadmium, chromium VI, and nickel.

Table 2-1. Proposal TWFs: Values and Calculation for 16 Pollutants 

Pollutant Proposal TWF AQ Calculation HH Calculation

Aluminum 0.0647 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Arsenic 4.041 Freshwater chronic criteria Equation 2: 1980 methodology; carcinogens

Boron 0.0083 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Cadmium 23.1 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Chloride 0.00002435 Freshwater chronic criteria No Value

Chromium VI 0.517 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Copper 0.635 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Iron 0.0056 Freshwater chronic criteria No Value

Lead 2.24 Freshwater chronic criteria No Value

Manganese 0.07043 Freshwater chronic criteria HH = 100a

Mercury 117 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Nickel 0.1089 Freshwater chronic criteria Equation 1: 1980 methodology; noncarcinogens

Selenium 1.121 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Thallium 1.027 Freshwater chronic criteria Equation 1: 1980 methodology; noncarcinogens

Vanadium 0.035 Freshwater chronic criteria No Value

Zinc 0.0469 Freshwater chronic criteria Equation 3: 2000 methodology; noncarcinogens

Source: U.S. EPA, 2012b; Abt, 2008.
Memorandum

September 21, 2015

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a EPA set HH for manganese at 100 μg/L for human health protection from consumption of aquatic organisms only

[U.S. EPA, 2002b].

The remainder of this section describes ERG’s review of the data used to calculate TWFs 

for the 16 pollutants listed in Table 2-1. In addition to reviewing the toxicity benchmarks (i.e., 

freshwater chronic aquatic life criteria, reference doses, and cancer potency factors) for the 16 

pollutants, ERG also reviewed data for the BCFs used in the calculation of the HH value. Based 

on this review, EPA updated TWF calculations for seven pollutants (see Section 3).

2.1 Review Benchmark Values for AQ

As described in Section 1, EPA determined AQ for each pollutant as a concentration in 

the water (μg/L) that is protective of aquatic life. EPA’s 2012 document, Toxic Weighting 

Factors Methodology, describes how EPA selected AQ based on a three-tiered hierarchy in order 

of decreasing data quality [U.S. EPA, 2012a]. The three data sources are:

 Tier 1: Aquatic life criteria published by EPA OW HECD as NRWQC.

 Tier 2: Effects data based on testing of aquatic organisms.

 Tier 3: Estimated toxicity effects data derived from various nationally-recognized 

sources.

EPA used aquatic life benchmarks from Tier 1 data sources where available. EPA used

Tier 2 data sources if the Tier 1 data sources did not include an aquatic life benchmark for the 

pollutant. EPA used Tier 3 data sources if the Tier 1 and Tier 2 data sources did not include an 

aquatic life benchmark for the pollutant.

Table 2-2 summarizes the AQ review for the 16 pollutants, the current AQ used to 

calculate the TWF, and the data source of the AQ. ERG performed the following steps in 

reviewing the current AQ:

 For pollutants using an NRWQC (Tier 1) data source, ERG determined if the 

aquatic life benchmark is the most recent. See the EPA NRWQC web site, 

http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm.

 For pollutants using Tier 2 or Tier 3 data sources, ERG confirmed there are no 

aquatic life benchmarks from the Tier 1 data source.

 For pollutants using Tier 2 or Tier 3 data sources, ERG determined if there are 

other data sources and aquatic life benchmarks that can potentially be used to 

update AQ.
Memorandum

September 21, 2015

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Table 2-2. Summary of AQ Reviews for Selected Steam Electric Pollutants of Concern

Pollutant

AQ Used in TWF 

Calculation

(μg/L)

Data Source (Hierarchy 

of Data Source) ERG Review

Aluminum 87 NRWQC web site (Tier 1) No update (most recent value from 1988).

Arsenic 150 NRWQC web site (Tier 1) No update (most recent value from 1995).

Boron 750 EPA’s Quality Criteria for 

Water 1986 (Tier 2)

No update. NRWQC web site refers to EPA’s 

Quality Criteria for Water 1986. See discussion 

in Section 2.1.2.

Cadmium 0.25 NRWQC web site (Tier 1) No update (most recent value from 2001).

Chloride 230,000 NRWQC web site (Tier 1) No update (most recent value from 1986).

Chromium VI 11 NRWQC web site (Tier 1) No update (most recent value from 1995).

Copper 9 2002 NRWQC [U.S. EPA, 

2012b] (Tier 1)

No update. See discussion in Section 2.1.1.

Iron 1,000 NRWQC web site (Tier 1) No update (most recent value from 1986).

Lead 2.5 NRWQC web site (Tier 1) No update (most recent value from 1980).

Manganese 388 Birge, 1979 (Tier 2 or 3) Update. No NRWQC; ERG identified a 1996 

reference that discusses aquatic life criteria for 

manganese. See discussion in Section 2.1.2.

Mercury 0.77 NRWQC web site (Tier 1) No update (most recent value from 1995).

Nickel 52 NRWQC web site (Tier 1) No update (most recent value from 1995).

Selenium 5 NRWQC web site (Tier 1) No update (most recent value from 1999).

Thallium 40 EPA Ambient Water Quality 

Criteria [U.S. EPA, 1980a] 

(Tier 2)

Update. No NRWQC; ERG identified a 1996

reference that discusses aquatic life criteria for 

thallium. See discussion in Section 2.1.2.

Vanadium 160 Birge, et. al, 1977 (Tier 2) Update. No NRWQC; ERG identified a 1996 

reference that discusses aquatic life criteria for 

vanadium. See discussion in Section 2.1.2.

Zinc 120 NRWQC web site (Tier 1) No update (most recent value from 1995).

Source: U.S. EPA, 2012b; U.S. EPA, 2014.

Acronyms: AQ (aquatic life value); NRWQC (National Recommended Water Quality Criteria).

2.1.1 Review of Tier 1 Data Sources for the Aquatic Life Benchmark

Of the 16 pollutants included in this review, EPA has set NRWQC (Tier 1 data source) 

for 12 pollutants. EPA used the NRWQC as AQ when calculating TWFs. The only pollutant 

with a potential updated aquatic life benchmark based on the revised NRWQC is copper.

The current AQ for copper is 9 μg/L (based on the NRWQC set in 1998). In 2007, EPA 

published a revised methodology for calculating freshwater chronic aquatic life criteria for 

copper; however, the recommendation is to develop criteria specific to the water body 

characteristics (e.g., temperature, pH, dissolved organic compounds, suspended particles, and 

presence of various cations and anions including those that comprise hardness and alkalinity). 

EPA recommends that states use a Biotic Ligand Model (or bioavailability model) to set aquatic 
Memorandum

September 21, 2015

Page 8

life criteria for copper. Rather than developing a single national value, the document 

recommends calculating the final chronic value (FCV) for a specific water body chemistry [U.S. 

EPA, 2007]. For a national-scale, screening level analysis, EPA decided to continue using the 

2002 NRWQC value of 9 μg/L in TWF calculations.

2.1.2 Review of Tier 2 and Tier 3 Data Sources for Pollutants without Tier 1 Data Sources

Of the 16 pollutants included in this review, four did not have NRWQC benchmarks and 

instead relied on Tier 2 or Tier 3 data sources for their AQ value. To develop TWFs, EPA used

Tier 2 data sources for the boron, thallium, and vanadium aquatic life benchmarks. EPA 

developed the aquatic life benchmarks for boron and thallium as part of ambient water quality 

criteria analyses in 1986 and 1980, respectively. EPA used a 1979 EPA document as the data 

source for the manganese aquatic life benchmark. ERG was unable to locate the 1979 reference 

to determine if it was a Tier 2 (effects data based on testing of aquatic organisms) or Tier 3 

(estimated toxicity effects data derived from various nationally-recognized sources) data source.

For the current review, ERG identified a Tier 2 data source that included aquatic life 

benchmarks for these pollutants, which could potentially be used to update the AQs for boron, 

manganese, thallium, and/or vanadium. Toxicological Benchmarks for Screening Potential 

Contaminants of Concern for Effects on Aquatic Biota: 1996 (Suter & Tsao, 1996) is a 

Department of Energy document which includes U.S. chronic National Ambient Water Quality 

Criteria (NAWQC) and Secondary Chronic Values (SCV). SCVs, used for pollutants that do not 

have NAWQC, are calculated using the “Tier II method” described in the EPA's Proposed Water 

Quality Guidance for the Great Lakes System. SCVs are aquatic benchmarks established with 

fewer data than are required for the NAWQC. SCVs for boron, manganese, thallium, and 

vanadium are based on multiple studies (included in the reference document’s Appendix A) that 

measured impacts to aquatic organisms. Suter & Tsao, 1996 included the following SCVs:

 Boron: 1.6 μg/L. 

 Manganese: 120 μg/L.

 Thallium: 12 μg/L.

 Vanadium: 20 μg/L.

EPA decided to not update the AQ value for boron in the TWF calculations. The current 

AQ value for boron is based on a 1986 aquatic life benchmark that EPA OW HECD includes on 

its NRWQC web site. EPA completed a review of the boron TWF calculations in 2008, more 

recent than the Suter & Tsao reference [Abt, 2008].

Based on this review, EPA decided to update the AQ values for manganese, thallium, and 

vanadium. The SCVs for these three pollutants are from a Tier 2 reference (Suter & Tsao, 1996) 

based on a more recent analysis than the current AQs. EPA decided to update the AQs for these

three pollutants in the TWF calculations to match the SCVs identified.
Memorandum

September 21, 2015

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2.2 Review Benchmark Values for HH

The equations and human health benchmarks EPA used to calculate HH depend on 

whether the pollutant is noncarcinogenic or carcinogenic. For noncarcinogenic pollutants, EPA 

used a reference dose. For carcinogenic pollutants, EPA calculated HH using a cancer potency 

factor and cancer risk level. EPA classified the data sources for the reference doses and cancer 

potency factor into the following three tiers, as outlined in EPA’s Toxic Weighting Factors 

Methodology document:

Tier 1

 EPA NRWQC, published after 2000.

 EPA Integrated Risk Information System (IRIS) Oral reference doses or Oral 

cancer potency factors [Most recently available online].

Tier 2

 U.S. EPA’s Health Effects Assessment Summary Tables (HEAST) [Most recent 

table is from 1997].

 U.S. EPA’s Region III Risk-Based Concentration (RBC) Table [Most recent 

version is dated November 2013; see Summary Table available at 

http://www.epa.gov/reg3hwmd/risk/human/rbconcentration_table/Generic_Tables/index.htm].

 California Environmental Protection Agency health assessments and toxicity 

criteria (CalEPA).

Tier 3

 U.S. EPA NRWQC documents, published before 2000.

 Agency for Toxic Substances and Disease Registry (ATSDR) profiles.

EPA used reference doses and cancer potency factors from Tier 1 data sources where 

available. EPA used Tier 2 data sources if the Tier 1 data sources did not include a reference 

dose or cancer potency factor for the pollutant. EPA used Tier 3 data sources if the Tier 1 and 

Tier 2 data sources did not include a reference dose or cancer potency factor for the pollutant. In

cases where a pollutant has both a reference dose and cancer potency factor (e.g., arsenic), EPA 

calculated HH using the cancer potency factor and cancer risk level, unless the reference dose 

was from a higher tier data source.

Table 2-3 summarizes reviews for the 16 pollutants included in this analysis, the current 

reference dose or cancer potency factor used to calculate HH, the data source of the reference 

dose or cancer potency factor, and the results of ERG’s review. As part of the review, ERG 

performed the following steps:

 For pollutants using the Tier 1 data sources, ERG determined if the reference dose

or cancer potency factor is the most recent available. See the EPA NRWQC web 

site, http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm

and EPA IRIS web site, http://www.epa.gov/IRIS/.
Memorandum

September 21, 2015

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 For pollutants using Tier 2 or Tier 3 data sources, ERG confirmed that there is no 

reference dose (or cancer potency factor) from the Tier 1 data sources.

 For pollutants using Tier 2 or Tier 3 data sources, ERG determined if the

reference dose (or cancer potency factor) is the most recent available. 

 For Tier 3 data sources, ERG confirmed that there is no reference dose (or cancer 

potency factor) available from Tier 2 data sources.

 For pollutants with no reference dose (or cancer potency factor), ERG determined

if any data sources are available.
Memorandum

September 21, 2015

Page 11

Table 2-3. Summary of Reference Dose (RfD) and Cancer Potency Factor (CPF) Reviews 

for Selected Steam Electric Generating Industry Pollutants of Concern

Pollutant

RfD (or CPF) Used in 

TWF Development 

Data Source (Hierarchy 

of Data Source) ERG Review Notes

Aluminum RfD: 1.0 mg/kg/day EPA Region III RBC

Table (Tier 2)

No update (most recent value in RBC table). 

No Tier 1 data.

Arsenic RfD: 0.0003 mg/kg/day

CPF: 1.75 kg-day/mg

NRWQC (Tier 1) Update. The CPF value in IRIS is 1.5 kgday/mg. See discussion in Section 2.2.1.

Boron RfD: 0.2 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

Cadmium RfD: 0.0005 mg/kg/day IRIS web site (Tier 1) Update. Value used in current TWF 

calculations is for intake of water. IRIS also 

includes a diet-based RfD of 0.001 mg/kg/day. 

See discussion in Section 2.2.1.

Chloride No value No value No update. No data for chloride RfD.

Chromium VI RfD: 0.003 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

Copper RfD: 0.04 mg/kg/day EPA Region III RBC 

Table (Tier 2)

No update (most recent value in RBC table). 

No Tier 1 data.

Iron RfD: 0.3 mg/kg/day EPA Region III RBC 

Table (Tier 2)

Update. Most recent EPA Region III RBC 

Table lists the RfD as 0.7 mg/kg/day. See 

discussion in Section 2.2.2. No Tier 1 data.

Lead No value No value No data for lead RfD.

Manganese RfD: 0.14 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

a

Mercury RfD: 0.0001 mg/kg/day NRWQC [U.S. EPA, 

2002a] (Tier 1)

No update (most recent RfD developed by EPA

in 2001).

b

Nickel RfD: 0.02 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

Selenium RfD: 0.005 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

Thallium RfD: 0.000068

mg/kg/day

NRWQC [U.S. EPA, 

2002a] (Tier 1)

No update (most recent RfD developed by 

EPA).

Vanadium RfD: 0.001 mg/kg/day EPA_NCEA Provisional 

Value from Region III 

data table, dated 

10/8/2004c

Update. EPA Region III RBC Table lists an 

RfD for vanadium and vanadium compounds as 

0.005 mg/kg/day. See discussion in Section 

2.2.2. No Tier 1 data.

Zinc RfD: 0.3 mg/kg/day IRIS web site (Tier 1) No update (most recent value in IRIS).

Acronyms: CPF (cancer potency factor); IRIS (Integrated Risk Information System); NCEA (National Center for 

Environmental Assessment); NRWQC (National Recommended Water Quality Criteria); RBC (Risk-Based 

Concentration); RfD (reference dose); TWF (toxic weighting factor).

a The current TWF calculation for manganese uses HH equal to 100 μg/L [U.S. EPA, 2012b]. Although there is a 

reference dose for manganese, there is no BCF to calculate the HH using the methodology equations. Therefore, 

EPA did not revise the current HH used to calculate the TWF.

b See http://www.epa.gov/mercury/exposure.htm.

c Reference as listed in TWF Database.

2.2.1 Review of Tier 1 Data Sources for Reference Doses and Cancer Potency Factors

EPA used Tier 1 data sources for reference doses and cancer potency factors for 10 of the 

16 pollutants included in ERG’s review. Based on a review of these Tier 1 data sources, ERG 
Memorandum

September 21, 2015

Page 12

identified the following updates to reference doses and cancer potency factors used to calculate 

HH:

 For arsenic, IRIS lists the cancer potency factor as 1.5 kilogram-day per milligram 

(kg-day/mg). The 2002 National Recommended Water Quality Criteria 2002: 

Human Health Criteria Calculation Matrix lists the cancer potency factor for 

arsenic as 1.75 kg-day/mg. This latter value was “derived from the risk per unit 

concentration in drinking water from IRIS: 5 x 10-5risk per μg/L in drinking 

water” [U.S. EPA, 2002a]. EPA decided to update the HH calculation with the

cancer potency factor currently recommended on the IRIS web site.

 For cadmium, IRIS lists the reference dose for diet-intake as 0.001 milligram per 

kilogram per day (mg/kg/day) and for water-intake as 0.0005 mg/kg/day. Current 

TWF calculations use the water-intake reference dose. EPA used the ambient 

water quality criteria equation to calculate HH based on consumption of 

organisms only. EPA decided the use of the diet-intake reference dose is more 

consistent with the methodologies used for other pollutants than the use of the 

water-intake reference dose for the TWF calculation and updated this value.

2.2.2 Review of Tier 2 and Tier 3 Data Sources for Reference Doses and Cancer Potency 

Factors

EPA did not have Tier 1 data sources for reference doses and cancer potency factors for 

six of the 16 pollutants included in ERG’s review. ERG confirmed that no Tier 1 data were 

available for those pollutants. Next, ERG determined if the reference dose (or cancer potency 

factor) based on Tier 2 or Tier 3 data sources was the most recently available. 

U.S. EPA’s Region III Risk-Based Concentration (RBC) Table, a Tier 2 data source, 

included reference doses for four of the six pollutants without a Tier 1 data source. Aluminum 

and copper reference doses used to calculate HH matched the U.S. EPA Region III RBC Table; 

therefore, EPA did not make any changes to HH values for those two pollutants. U.S. EPA 

Region III’s RBC Table included updated reference doses for iron and vanadium. EPA decided 

to update the following reference doses used to calculate HH:

 For iron, use the reference dose of 0.7 mg/kg/day as listed in the latest version of 

the U.S. EPA Region III RBC Table (replace 0.3 mg/kg/day in TWF 

calculations).

 For vanadium, use the reference dose of 0.005 mg/kg/day as listed in the latest 

version of the U.S. EPA Region III RBC Table (replace 0.001 mg/kg/day in TWF 

calculations).

For two of the six pollutants (chloride and lead), ERG did not identify any Tier 2 or Tier 

3 data sources. Therefore, EPA made no changes to the TWF calculations and this results in an 

HH value of zero (i.e., there is no human health component in the TWF for these two pollutants).
Memorandum

September 21, 2015

Page 13

2.2.3 Bioconcentration Factors (BCFs) and Bioaccumulation Factors (BAFs)

A BCF is the ratio of the pollutant concentration in the tissue of an aquatic organism to 

the pollutant concentration in the ambient water. For all 16 pollutants included in this review, 

EPA used BCFs (not BAFs) to calculate the HH value for TWFs, because national-level BAFs 

are not available for these pollutants. BAFs are developed to account for pollutants entering fish 

tissue from both the ambient water and from food sources; this makes them more complex to 

develop. 

ERG performed a cursory review of the BCFs used in the TWF calculations and 

determined that updates were needed only if the BCF (or BAF) used in the TWF calculation did 

not correspond to the BCF (or BAF) used in development of EPA’s NRWQC. ERG identified 

one update: revising the BCF for copper to be 36 rather than 360. EPA’s 1980 Ambient WQC 

document lists the weighted-average BCF for the edible portion of freshwater and estuarine fish 

as 36 (not 360). The overall mean BCF, including scallops, mussels, clams, and oysters, is 290 

[U.S. EPA, 1980b]. The 2002 National Recommended Water Quality Criteria 2002: Human 

Health Criteria Calculation Matrix lists the BCF for copper as 36 [U.S. EPA, 2002a].
Memorandum

September 21, 2015

Page 14

3. SUMMARY OF REVISED TOXIC WEIGHTING FACTORS

Based on a review of the TWF general methodology and documentation available, EPA 

decided to update the TWF calculations as outlined in Table 3-1. Attachment B provides details 

on how EPA used TWFs for the steam electric ELGs.

Table 3-1. Revised TWFs

Pollutant TWF Calculation Update Previous TWF Revised TWF

Arsenic Revised CPF. 4.04 3.47

Cadmium Revised RfD (diet-based). 23.1 22.8

Copper Revised BCF. 0.635 0.623

Iron Revised RfD – EPA does not have a BCF to use 

in the HH calculation; therefore, RfD update 

does not change the TWF.

0.00560 0.00560

Manganese Revised AQ benchmark. 0.0704 0.103

Mercury Revise the fish consumption rate used to 

calculate HH for mercury to 0.0175 kg/day

(same as other pollutants using the 2000 

Methodology).

117 110

Thallium Revised AQ benchmark; use the 2000 

methodology. NRWQC updated in 2003 [U.S. 

EPA, 2003].

1.03 2.85

Vanadium Revised AQ benchmark.

Revised RfD – EPA does not have a BCF to use 

in the HH calculation; therefore, RfD update 

does not affect the TWF.

0.035 0.280

Acronyms: AQ (aquatic life value); BCF (bioconcentration factor); CPF (cancer potency factor); HH (human health 

value); NRWQC (National Recommended Water Quality Criteria); RfD (reference dose); TWF (toxic weighting 

factor).

4. REFERENCES

1. Abt. 2008. Abt Associates, Inc. “Revised Draft – Updating the Boron TWF.” 

(December).

2. Birge, WJ. 1979. Aquatic Toxicity Tests on Inorganic Elements Occurring in Oil Shale 

(EPA-600/9-80-022). Oil Shale Symposium - Sampling, Analysis and Quality Assurance 

(March).

3. Birge, et.al. 1977. WJ Birge, JA Black, and AG Westerman. Evaluation of Aquatic 

Pollutants Using Fish and Amphibian Eggs as Bioassay Organisms. In SW Nielsen, G. 

Migaki, and DG Scarpelli (Eds.) Symp. Animals Monitors Environ. Pollut. 1977, Storrs, 

CT 12:108-118.

4. Suter & Tsao. 1996. G.W. Suter II and Tsao, C.L. Toxicological Benchmarks for 

Screening Potential Contaminants of Concern for Effects on Aquatic Biota: 1996 

Revision. ES/ER/TM-96/R2. Prepared for the U.S. Department of Energy. (June).
Memorandum

September 21, 2015

Page 15

5. U.S. EPA. 1980a. Ambient Water Quality Criteria for Thallium (EPA-440-5-80-074). 

Washington, DC. (October).

6. U.S. EPA. 1980b. Ambient Water Quality Criteria for Copper (EPA-440-5-80-036). 

Washington, DC. (October).

7. U.S. EPA. 1986. Quality Criteria for Water 1986 (EPA-440-5-86-001). Washington, DC. 

(May).

8. U.S. EPA. 2000. Methodology for Deriving Ambient Water Quality Criteria for the 

Protection of Human Health (2000) (EPA-822-B-00-004). (October).

9. U.S. EPA. 2002a. National Recommended Water Quality Criteria: 2002. Human Health 

Criteria Calculation Matrix (EPA-822-R-02-012). (November).

10. U.S. EPA. 2002b. National Recommended Water Quality Criteria: 2002 (EPA-822-R-02-

047). (November).

11. U.S. EPA. 2003. Revised National Recommended Water Quality Criteria for the 

Protection of Human Health. (December). Available online at:

http://water.epa.gov/scitech/swguidance/standards/criteria/health/15table-fs.cfm. 

12. U.S. EPA. 2005. Guidelines for Carcinogen Risk Assessment (EPA-630-P-03-001F). Risk 

Assessment Forum. Washington, DC. (March).

13. U.S. EPA. 2007. Aquatic Life Ambient Freshwater Quality Criteria—Copper (EPA-822-

R-07-001). Washington, DC. (February).

14. U.S. EPA. 2012a. Toxic Weighting Factors Methodology (EPA-820-R-12-005). 

Washington, DC. (March).

15. U.S. EPA. 2012b. TWF Database (EPA-HQ-OW-2008-0517-0713). MS ExcelTM. 

Washington, DC.

16. U.S. EPA. 2014. National Recommended Water Quality Criteria (web site). 

http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm. Accessed

March 2014.

17. U.S. EPA Region III. 2013. Ecological Risk Assessment (web site). 

http://www.epa.gov/reg3hwmd/risk/eco/btag/sbv/fw/screenbench.htm. Accessed March 

2014.
Attachment A

September 21, 2015

Page A-1

Attachment A. TWF Calculations

This attachment provides more detailed information on how EPA calculated TWFs and 

the parameters used to determine the human health component (HH).

Equation to Calculate TWF

EPA calculated TWFs for a given pollutant by comparing that pollutant’s AQ and HH to 

a scaling factor value for AQ and HH. EPA combined aquatic life and human health effects 

information to help compensate for the incompleteness of data sources, to simplify wastestream 

evaluations, and to simplify the TWF calculation. In some cases, a pollutant TWF may be 

derived solely from an AQ, and less commonly, a pollutant TWF may be derived solely from a 

HH. In its methodology, EPA noted that the approach of combining the AQ and HH is not 

appropriate for more rigorous analysis, such as risk assessments [U.S. EPA, 2012a]. However,

the combining of these effects is a reasonable approach for the purposes used in ELGs, 

specifically “informing regulatory decisions about the relative cost effectiveness of treatment 

technologies, and for identifying industries that may warrant additional analysis.” Equation A-1 

shows the calculation of TWFs:

TWF =

5.6

AQ +

5.6

HH

Where:

5.6 = scaling factor (original copper freshwater chronic aquatic-life criterion of 

5.6 micrograms per Liter [μg/L])

AQ = Aquatic Life Value (μg/L)

HH = Human Health Value (μg/L)

Equations to Calculate HH

As discussed in Section 1 of the memorandum, EPA used different equations or input 

values based on the following two factors: 1) whether the pollutant is noncarcinogenic or 

carcinogenic and 2) whether EPA OW HECD used the 1980 or 2000 NRWQC methodology for 

its 2002 NRWQC analyses [U.S. EPA, 2002a]. Noncarcinogenic pollutants are those pollutants 

that EPA does not consider to have the potential to cause cancer in humans based on EPA’s 

Guidelines for Carcinogen Risk Assessment (2005). 

The 1980 methodology, compared to the 2000 methodology, used a different equation for 

noncarcinogenic pollutants and a different fish consumption rate for all pollutants. EPA OW 

HECD presented both methodologies in the 2002 EPA document, National Recommended Water 

Quality Criteria 2002: Human Health Criteria Calculation Matrix [U.S. EPA, 2002a]. In 2002, 

EPA OW HECD began using the 2000 methodology to calculate the NRWQC. At that time, 

EPA OW HECD chose to continue using the 1980 methodology for 17 pollutants. EPA OW 

Equation A-1
Attachment A

September 21, 2015

Page A-2

HECD planned to update the NRWQC for the subset of pollutants at a later date.3 For purposes 

of calculating TWFs, EPA OW EAD used these same equations to calculate HH.

The TWF Database included calculations using the following methodologies (equations 

and inputs) for HH, based on the consumption of aquatic organisms:

 1980 methodology to calculate the NRWQC for noncarcinogenic pollutants based 

on the consumption of organisms only. See Equation 1 in Table 1-1. 

 1980 methodology to calculate NRWQC for carcinogenic pollutants based on the 

consumption of organisms only. See Equation 2 in Table 1-1. 

 2000 methodology to calculate the NRWQC for noncarcinogenic pollutants based 

on the consumption of organisms only. See Equation 3 in Table 1-1. 

 2000 methodology to calculate NRWQC for carcinogenic pollutants based on the 

consumption of organisms only. See Equation 4 in Table 1-1. 

For noncarcinogenic pollutants, EPA calculated HH using a reference dose, or other 

benchmark protective of human health from the consumption of aquatic organisms (i.e., fish and 

shellfish). If a benchmark for the consumption of aquatic organisms only is not available, EPA 

uses a reference dose or benchmark based on the consumption of water and aquatic organisms.

The calculation of HH for noncarcinogenic pollutants in the 1980 NRWQC methodology differs 

from the 2000 methodology in two ways: 1) no use of the Relative Source Contribution (RSC)

factor and 2) different fish consumption intake value.

Description of Input Values to the HH Equations

For noncarcinogenic pollutants, EPA calculated HH using the reference dose. A pollutant 

reference dose is an estimate of a daily oral exposure to the human population that is likely to be 

without an appreciable risk of deleterious effects during a lifetime. Reference doses may be 

derived from a no observed adverse effect level (NOAEL), lowest observed adverse effect level 

(LOAEL), or benchmark dose, with uncertainty factors generally applied to reflect limitations of 

the data. See U.S. EPA Risk Assessment Glossary at 

http://www.epa.gov/risk_assessment/glossary.htm for additional details. 

The relative source contribution (RSC) factor is the percentage of pollutant exposure 

apportioned to the fish consumption (or fish and water consumption) pathway. Other exposure 

 

3

In 2003, EPA HECD proposed new criteria for 14 of the 17 pollutants (plus 1,1-dichloroethylene). For these 

proposed new criteria, EPA HECD used the 2000 methodology [U.S. EPA, 2003]. EPA updated the thallium 

methodology (only updated pollutant that is also included in EPA OW’s list of 16 pollutant TWFs evaluated in this

memorandum). Note that EPA HECD continues to recommend using the 1980 methodology for arsenic, nickel, and 

chloroform. 
Attachment A

September 21, 2015

Page A-3

pathways include air (inhalation), dermal, and other dietary intake. From EPA’s 2012 Toxic 

Weighting Factors Methodology:

“The RSC approach is documented in the Methodology for Deriving Ambient Water 

Quality Criteria for the Protection 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 reasonable for the 

purpose of screening-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.”

A bioconcentration factor (BCF) is the ratio of the pollutant concentration in the tissue 

of an aquatic organism to the pollutant concentration in the ambient water. For TWF 

development, EPA relied on BCF values, rather than bioaccumulation factors (BAFs). The 

rationale for this approach is described in EPA’s 2012 Toxic Weighting Factors Methodology:

“An aquatic bioaccumulation 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.”

For carcinogenic pollutants, EPA calculated HH using a cancer potency factor, or cancer 

slope factor. EPA used cancer potency factors to estimate the risk of cancer associated with 

exposure to a carcinogenic pollutant. EPA’s Integrated Risk Information System (IRIS)4defines 

a cancer slope factor as “an upper bound, approximating a 95% confidence limit, on the 

increased cancer risk from a lifetime exposure to an agent (pollutant) by ingestion.” EPA then 

weighted this estimated cancer risk against a selected cancer risk level. For NRWQC 

development, EPA uses a cancer risk level of 10-6(one in 1,000,000 people), which reflects an 

appropriate risk level for the general population and is consistent with Agency-wide practices. 

EPA noted that both the 10-5(one in 100,000 people) and 10-6cancer risk levels are appropriate 

for determining risks to the general population. EPA recommends using the cancer risk level of 

10-4(one in 10,000 people) when evaluating risks for highly exposed populations [U.S. EPA, 

2000]. The 2000 NRWQC methodology and the 1980 NRWQC methodology recommend a 

cancer risk level of 10-6[U.S. EPA, 2002a]. For the HH calculations, EPA OW EAD used a 

cancer risk factor of 10-5(i.e., one in 100,000 people).

 

4 http://www.epa.gov/iris/help_ques.htm.
Attachment B

September 21, 2015

Page B-1

Attachment B. How EPA Uses TWFs

EPA received comments on the use of TWFs for the proposed Steam Electric Power 

Generating Point Source Category ELGs. One comment noted that TWFs assume a uniform 

characterization of toxicity of pollutants from all discharges and relate the toxicity to pollutant 

loadings rather than to pollutant concentrations in the receiving water. This attachment

summarizes the use of the TWFs in the ELG program. For a more detailed summary, see the 

Toxic Weighting Factors Methodology [U.S. EPA, 2012a].

EPA used TWFs as a way to better understand how treatment technologies, individual 

plant discharges, and industry discharges compare to one another [U.S. EPA, 2012a]. For 

development of ELGs, EPA used available data such as survey responses, sampling episode data, 

and other wastewater monitoring data to characterize discharges from a specific plant. EPA 

calculated plant-specific (and for the Steam Electric ELG, wastestream-specific) pollutant 

loadings as mass per year (e.g., lb/yr or grams/yr) based on the pollutant concentration in the 

discharge and the discharge flow rate. EPA calculated pollutant loadings to establish baseline 

(current) discharge quantities of pollutants and calculated regulatory option pollutant loadings to 

determine pollutant removals. To determine the toxicity of a specific wastestream, EPA used

TWFs to allow the addition of discharges across multiple pollutants. The result is the TWPE.

EPA used the pollutant removals to evaluate the effectiveness of the regulatory option 

treatment technologies, estimate benefits gained from pollutant removals, and evaluate the costeffectiveness of the regulatory options.

EPA used TWFs and the estimated TWPEs as an indicator of the relative potential of a 

pollutant to cause harm and not to represent actual aquatic or human health impacts that may 

occur at specific locations due to these pollutant loadings. To assess impacts to aquatic life or 

human health, EPA used the pollutant loadings discharged to the receiving water, as lbs/year, as 

input to the national-scale immediate receiving water (IRW) model used for the environmental 

assessment. EPA used the model to predict pollutant concentrations in the specific receiving 

water body, sediment, and fish tissue. Using the concentration in the fish tissue, EPA calculated

an average daily dose (ADD) for humans consuming fish. EPA did not use TWFs or TWPEs in 

the national-scale IRW model. See the Environmental Assessment for the Effluent Limitations 

Guidelines and Standards for the Steam Electric Power Generating Point Source Category

(EPA-821-R-15-006) for further details on the environmental assessment and IRW model.