MEMORANDUM

TO: Bill Swietlik, EPA

FROM: Patrick Goodwin, ERG

DATE: September 21, 2015

SUBJECT: Downstream Modeling Methodology and Supporting Documentation (DCN 

SE04455)

1.0 INTRODUCTION

ERG is providing technical support to the U.S. Environmental Protection Agency (EPA) 

for development of final effluent limitations guidelines and standards (ELGs) for the Steam 

Electric Power Generating Point Source Category (40 CFR 423). In support of the development 

of final effluent guidelines, ERG developed the following resources to evaluate pollutant 

concentrations in the receiving water reaches downstream from steam electric power plants and 

to assess the number of improved river miles associated with the regulatory options evaluated for 

the final rule (Options A, B, C, D, and E):

1

 Water Concentration Thresholds Spreadsheet.

 Total-to-Dissolved Water Concentration Conversion Factor Databases.

 Downstream Data Preparation Databases.

 Downstream Model Databases.

 Downstream Model Results Spreadsheets.

EPA’s economic contractor, Abt Associates, calculated downstream receiving water 

concentrations using EPA’s Risk-Screening Environmental Indicators (RSEI) model.

2 ERG then 

 

1 During development of the final rule, EPA decided not to base the final rule on Option F for existing sources due 

primarily to the high cost of that Option, particularly in light of the costs associated with other rulemakings expected 

to impact the steam electric power generating industry (see Section VIII.C.1 of the preamble). As a result, EPA 

chose not to conduct particular analyses for Option F to the same extent that it did for some of the other options 

considered.

2 EPA used pollutant loadings discharged to each receiving reach by steam electric power plants to estimate 

concentrations in downstream reaches. The RSEI model uses a simple dilution and first-order decay equation to 

calculate receiving water concentrations (metals are treated as conservative substances – i.e., incur no losses due to 

chemical reactions or biochemical degradation). The RSEI model assumes that the power plant’s annual discharge is

released at a constant rate throughout the year. In addition, EPA included pollutant loadings from EPA’s Toxics

Release Inventory (TRI) database for other industries to represent background pollutant concentrations in the 

downstream receiving waters. For further details on the RSEI model methodology and assumptions, see the Benefits 

and Cost Analysis for the Effluent Limitations Guidelines and Standards for the Steam Electric Power Generating 

Point Source Category (EPA-821-R-15-005).
Memorandum

September 21, 2015

Page 2

performed additional downstream modeling using these receiving water concentrations to assess 

the extent of water quality, wildlife, and human health benchmark exceedances downstream from 

discharges of steam electric power plant wastewater. The results of ERG’s downstream modeling 

are presented in Section 7.6 (Environmental and Human Health Improvements in Downstream 

Surface Water) of the Environmental Assessment for the Effluent Limitations Guidelines and 

Standards for the Steam Electric Power Generating Point Source Category (EA Report) (EPA821-R-15-006).

2.0 CHANGES IN DOWNSTREAM MODELING METHODOLOGY SINCE PROPOSAL

For the proposed rule, ERG compared the downstream receiving water pollutant 

concentrations (calculated using the RSEI model) to the same National Recommended Water 

Quality Criteria (NRWQC) and drinking water maximum contaminant level (MCL) water 

quality benchmarks used in the immediate receiving water (IRW) model national-scale analysis. 

Those water quality benchmarks are provided in Appendix C (Water Quality Module 

Methodology) of the EA Report. ERG also performed downstream wildlife and human health 

modeling using the downstream receiving water pollutant concentrations and the same wildlife 

and human health modules used in the IRW model national-scale analysis. These modules are 

described in Section 5.1.2 (Wildlife Module), Section 5.1.3 (Human Health Module), Appendix 

D (Wildlife Model Methodology), and Appendix E (Human Health Model Methodology) of the 

EA Report. 

Since proposal, ERG incorporated the following revisions to the downstream modeling 

methodology:

 ERG calculated dissolved pollutant concentrations, based on the total pollutant 

concentration outputs from RSEI, to allow for a more appropriate comparison 

with benchmarks that are based on dissolved concentrations; and

 ERG simplified the methodology by comparing the downstream modeled water 

concentrations to the thresholds that would trigger exceedances of wildlife and 

human health benchmarks, as opposed to performing full runs of the wildlife and 

human health modules (which would have generated prohibitively large output 

datasets due to the expanded number of river miles evaluated for the final rule).

The following sections describe these revisions in further detail.

2.1 Total-to-Dissolved Water Concentration Conversion Factors

The water quality module of the IRW model calculates total and dissolved pollutant 

concentrations in the immediate receiving waters of steam electric power plant discharges based 

on pollutant loadings and receiving water characteristics. The model then compares either the 

total pollutant or dissolved pollutant concentration to corresponding water quality benchmarks. 

Downstream water quality outputs from the RSEI model represent the total concentration of the 

pollutant in the water column. Exceedances of the following water quality, wildlife, and human 
Memorandum

September 21, 2015

Page 3

health benchmarks in the EA Report, however, are calculated using the dissolved concentration 

of the pollutant in the water column:

 Freshwater acute and chronic NRWQC for arsenic, cadmium, chromium VI, 

copper, lead, mercury, nickel, and zinc;

 No effect hazard concentrations (NEHC) of mercury for minks and eagles; and

 Reference dose of mercury for non-cancer human health effects.

To assess the extent of exceedances of these benchmarks in downstream waters, ERG 

calculated and applied conversion factors to translate the RSEI total concentration outputs into 

dissolved concentrations. ERG calculated these conversion factors by calculating ratios of 

dissolved pollutant concentration to total pollutant concentration using the outputs from the 

water quality module of the national-scale IRW model. The water quality module of the IRW 

model incorporates partition coefficients to estimate how much of the pollutants in the water 

column remain dissolved versus sorbed onto suspended solids. For each pollutant and immediate 

receiving water in the water quality module, ERG calculated the ratio of the dissolved 

concentration to the total concentration in the water column. ERG then averaged these ratios 

across all immediate receiving waters to arrive at one total-to-dissolved conversion factor for 

each of the ten modeled pollutants. ERG developed conversion factors under two separate 

industry baseline scopes: 1) the scope that accounts for the Coal Combustion Residuals (CCR) 

rulemaking (209 modeled immediate receiving waters, as discussed in the EA Report) and 2) the 

scope that accounts for both the CCR rulemaking and the Clean Power Plan [Clean Air Act 

Section 111(d)] (163 modeled immediate receiving waters, as discussed in Appendix I of the EA 

Report). Table 2-1 presents the resulting total-to-dissolved water concentration conversion 

factors.
Memorandum

September 21, 2015

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Table 2-1. Total-to-Dissolved Water Concentration Conversion Factors

Pollutant

Conversion Factor

(“With CCR” Scope) a

Conversion Factor

(“With CCR and CPP” Scope) b

Arsenic 0.730 0.729

Cadmium 0.259 0.256

Chromium VI 0.579 0.588

Copper 0.347 0.343

Lead 0.0572 0.0553

Mercury 0.127 0.125

Nickel 0.545 0.543

Selenium 0.496 0.494

Thallium 0.636 0.635

Zinc 0.219 0.216

Source: DCNs SE04582, SE04586.

Note: Numbers are rounded to three significant figures.

a – EPA incorporated these conversion factors in the database used to prepare water quality tables for the 

downstream model that accounts for the CCR rulemaking (DCN SE04583).

b – EPA incorporated these conversion factors in the database used to prepare water quality tables for the 

downstream model that accounts for the CCR and CPP rulemakings (DCN SE04587).

2.2 Water Concentration Thresholds for Wildlife and Human Health Exceedances

Since proposal, the RSEI output dataset increased in size from 24,439 downstream 

reaches (COMIDs) at proposal to 77,413 COMIDs for the final rule. Use of the same 

downstream modeling methodology used at proposal, which involved running the wildlife and 

human health modules on these water quality outputs, would have resulted in prohibitively large 

output databases of more than 10 million records. To reduce the model run times and amount of 

output data, ERG developed a simplified downstream modeling methodology that did not require 

execution of the wildlife and human health modules. This approach involved calculating the 

water pollutant concentrations that would result in exceedances if used as inputs to the wildlife 

and human health modules; ERG then compared the downstream receiving water pollutant 

concentrations to these “threshold” concentrations to identify the downstream reaches that would 

have at least one exceedance of a particular wildlife or human health benchmark. ERG used this 

approach to estimate the extent (in river miles) of wildlife and human health impacts in 

downstream surface waters under baseline conditions and the improvements under the modeled 

regulatory options (Options A, B, C, D, and E).

Table 2-2, Table 2-3, Table 2-4, and Table 2-5 present the calculated water concentration 

thresholds for the evaluated wildlife and human health benchmarks. See the spreadsheet

“Calculation of Water Concentration Thresholds for Wildlife and Human Health Exceedances”

(DCN SE04581) regarding the calculation of these thresholds.

ERG also used these water concentration thresholds in the analysis of water quality 

outputs from the case study models. See Section 8 of the EA Report and the memorandum 
Memorandum

September 21, 2015

Page 5

“Technical Approach for Case Study Water Quality Modeling of Aquatic Systems in Support of 

the Final Steam Electric Power Generating Industry Environmental Assessment” (DCN 

SE05570) for more information regarding the case study modeling methodology and results.

Table 2-2. Water Concentration Thresholds that Trigger NEHC Exceedances for Minks

Pollutant Total Concentration (mg/L) Dissolved Concentration (mg/L)

Arsenic 1.91E+00 --

Cadmium 2.10E-02 --

Chromium VI 2.95E+01 --

Copper 1.14E+00 --

Lead 7.52E-01 --

Mercury -- 1.54E-06

Nickel 1.56E+01 --

Selenium 2.31E-03 --

Thallium No NEHC Benchmark

Zinc 2.58E+00 --

Source: DCN SE04581.

Acronyms: mg/L (milligrams per liter).

Note: Numbers are rounded to three significant figures.

Table 2-3. Water Concentration Thresholds that Trigger NEHC Exceedances for Eagles

Pollutant Total Concentration (mg/L) Dissolved Concentration (mg/L)

Arsenic 5.60E+00 --

Cadmium 5.44E-02 --

Chromium VI 4.43E+01 --

Copper 1.13E+00 --

Lead 3.54E-01 --

Mercury -- 4.90E-07

Nickel 8.39E+01 --

Selenium 2.35E-03 --

Thallium No NEHC Benchmark

Zinc 4.14E-01 --

Source: DCN SE04581.

Acronyms: mg/L (milligrams per liter).

Note: Numbers are rounded to three significant figures.
Memorandum

September 21, 2015

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Table 2-4. Water Concentration Thresholds that Trigger Exceedances of Non-Cancer Human Health Benchmarks from Fish 

Consumption 

Cohort Note Arsenic (mg/L)

Cadmium 

(mg/L)

Chromium VI 

(mg/L) Copper (mg/L) Lead (mg/L)

Adult Subsistence (Cohort 

ID 14)

-- 9.15E-01 1.81E-03 2.44E+00 1.36E-01

No RfD Available

Adult Recreational (Cohort 

ID 7)

-- 2.82E+00 5.57E-03 7.52E+00 4.18E-01

Child 1 to <2 y.o.

Subsistence (Cohort ID 8)

Most sensitive child 

subsistence cohort

3.83E-01 7.56E-04 1.02E+00 5.67E-02

Child 1 to <2 y.o. 

Recreational (Cohort ID 1)

Most sensitive child 

recreational cohort

1.17E+00 2.31E-03 3.13E+00 1.74E-01

Child 16 to <21 y.o. 

Subsistence (Cohort ID 13)

Least sensitive child 

subsistence cohort

1.10E+00 2.18E-03 2.94E+00 1.63E-01

Child 16 to <21 y.o. 

Recreational (Cohort ID 6)

Least sensitive child 

recreational cohort

2.84E+00 5.61E-03 7.58E+00 4.21E-01

Cohort Note

Mercury a

(mg/L) Nickel (mg/L) Selenium (mg/L) Thallium (mg/L) Zinc (mg/L)

Adult Subsistence (Cohort 

ID 14)

-- 6.60E-08 1.22E+01 1.93E-03 5.11E-05 4.18E-01

Adult Recreational (Cohort 

ID 7)

-- 2.03E-07 3.76E+01 5.95E-03 1.58E-04 1.29E+00

Child 1 to <2 y.o.

Subsistence (Cohort ID 8)

Most sensitive child 

subsistence cohort

2.76E-08 5.10E+00 8.07E-04 2.14E-05 1.75E-01

Child 1 to <2 y.o. 

Recreational (Cohort ID 1)

Most sensitive child 

recreational cohort

8.46E-08 1.56E+01 2.47E-03 6.55E-05 5.36E-01

Child 16 to <21 y.o. 

Subsistence (Cohort ID 13)

Least sensitive child 

subsistence cohort

7.96E-08 1.47E+01 2.33E-03 6.16E-05 5.04E-01

Child 16 to <21 y.o. 

Recreational (Cohort ID 6)

Least sensitive child 

recreational cohort

2.05E-07 3.79E+01 5.99E-03 1.59E-04 1.30E+00

Source: DCN SE04581.

Acronyms: ID (identification number in Human Health module); mg/L (milligrams per liter); RfD (reference dose); y.o. (years old).

Note: Numbers are rounded to three significant figures.

a – Mercury benchmark is based on dissolved concentration in the water column. Benchmarks for all other pollutants are based on total concentration in the 

water column.
Memorandum

September 21, 2015

Page 7

Table 2-5. Water Concentration Thresholds that Trigger Excess Cancer Risk (Greater 

than 1-in-one-million) from Arsenic (Human Health Benchmark for Fish Consumption) 

Cohort Note Arsenic Concentration (mg/L)

Adult Subsistence (Cohort ID 14) -- 3.03E-03

Adult Recreational (Cohort ID 7) -- 9.33E-03

Child 1 to <2 y.o. Subsistence 

(Cohort ID 8)

Least sensitive child subsistence 

cohort 6.21E-02

Child 1 to <2 y.o. Recreational 

(Cohort ID 1)

Least sensitive child recreational 

cohort 1.90E-01

Child 6 to <11 y.o. Subsistence 

(Cohort ID 11)

Most sensitive child subsistence 

cohort 2.10E-02

Child 6 to <11 y.o. Recreational 

(Cohort ID 4)

Most sensitive child recreational 

cohort 5.53E-02

Source: DCN SE04581.

Acronyms: ID (identification number in Human Health module); mg/L (milligrams per liter); y.o. (years old).

Note: Numbers are rounded to three significant figures.

3.0 SUPPORTING DOCUMENTATION FOR DOWNSTREAM MODELING

A comprehensive listing of the downstream model supporting documentation is presented 

in Table 3-1 below. This table provides the document control number (DCN), title, and abstract 

of each record item, and identifies whether the record item contains Confidential Business 

Information (CBI).

Table 3-1. Downstream Modeling Record Items

DCN Title (Date) Abstract CBI a

SE04581 Final Steam EA -

Calculation of Water 

Concentration 

Thresholds for 

Wildlife and Human 

Health Exceedances 

(070615)

Spreadsheet used to calculate the water concentration thresholds 

that would result in exceedances if used as inputs to the wildlife 

and human health modules in the national-scale IRW model. 

These calculated thresholds are used in the Final Steam EA 

downstream models and case study models.

No

SE04582 Final Steam EA 

Downstream Model -

Calculation of Totalto-Dissolved Water 

Concentration 

Conversion Factors 

(CCR) (070615)

Database used to calculate the water concentration conversion 

factors (i.e., average ratio of dissolved water column concentration 

to total water column concentration) for each of the ten pollutants 

in the water quality module of the national-scale IRW model. 

These total-to-dissolved conversion factors are used in the Final 

Steam EA downstream models. This version is based on the IRW 

model that accounts for the CCR rulemaking.

Yes

SE04583 Final Steam EA 

Downstream Model -

Preparation of Water 

Quality Tables (CCR) 

(070915)

Database used to prepare the raw data tables of downstream 

receiving water concentrations, calculated using EPA’s RSEI 

model, for use in the EA downstream models. This includes 

application of the total-to-dissolved water concentration 

conversion factors. This version accounts for the CCR rulemaking.

Yes
Memorandum

September 21, 2015

Page 8

Table 3-1. Downstream Modeling Record Items

DCN Title (Date) Abstract CBI a

SE04584 Final Steam EA 

Downstream Model -

Baseline (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under baseline conditions. This version 

accounts for the CCR rulemaking.

Yes

SE04584A1 Final Steam EA 

Downstream Model -

Option A (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option A. This version accounts for 

the CCR rulemaking.

Yes

SE04584A2 Final Steam EA 

Downstream Model -

Option B (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option B. This version accounts for 

the CCR rulemaking.

Yes

SE04584A3 Final Steam EA 

Downstream Model -

Option C (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option C. This version accounts for 

the CCR rulemaking.

Yes

SE04584A4 Final Steam EA 

Downstream Model -

Option D (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option D. This version accounts for 

the CCR rulemaking.

Yes

SE04584A5 Final Steam EA 

Downstream Model -

Option E (CCR) 

(070915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option E. This version accounts for 

the CCR rulemaking.

Yes

SE04585 Final Steam EA 

Downstream Model -

Results Matrix (CCR) 

(071015)

Spreadsheet used to present the results of the downstream model 

in a reader-friendly format for incorporation into the EA Report. 

This version accounts for the CCR rulemaking.

No

SE04586 Final Steam EA 

Downstream Model -

Calculation of Totalto-Dissolved Water 

Concentration 

Conversion Factors 

(CPP) (072215)

Database used to calculate the water concentration conversion 

factors (i.e., average ratio of dissolved water column concentration 

to total water column concentration) for each of the ten pollutants 

in the water quality module of the national-scale IRW model. 

These total-to-dissolved conversion factors are used in the Final 

Steam EA downstream models. This version is based on the IRW 

model that accounts for the CCR and CPP rulemakings.

Yes

SE04587 Final Steam EA 

Downstream Model -

Preparation of Water 

Quality Tables (CPP) 

(081915)

Database used to prepare the raw data tables of downstream 

receiving water concentrations, calculated using EPA’s RSEI 

model, for use in the EA downstream models. This includes 

application of the total-to-dissolved water concentration 

conversion factors. This version accounts for the CCR and CPP 

rulemakings.

Yes
Memorandum

September 21, 2015

Page 9

Table 3-1. Downstream Modeling Record Items

DCN Title (Date) Abstract CBI a

SE04588 Final Steam EA 

Downstream Model -

Baseline (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under baseline conditions. This version 

accounts for the CCR and CPP rulemakings.

Yes

SE04588A1 Final Steam EA 

Downstream Model -

Option A (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option A. This version accounts for 

the CCR and CPP rulemakings.

Yes

SE04588A2 Final Steam EA 

Downstream Model -

Option B (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option B. This version accounts for 

the CCR and CPP rulemakings.

Yes

SE04588A3 Final Steam EA 

Downstream Model -

Option C (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option C. This version accounts for 

the CCR and CPP rulemakings.

Yes

SE04588A4 Final Steam EA 

Downstream Model -

Option D (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option D. This version accounts for 

the CCR and CPP rulemakings.

Yes

SE04588A5 Final Steam EA 

Downstream Model -

Option E (CPP) 

(081915)

Database used to identify those downstream reaches (and the total 

length of those reaches) whose modeled pollutant concentrations 

would trigger water quality, wildlife, and/or human health 

benchmark exceedances under Option E. This version accounts for 

the CCR and CPP rulemakings.

Yes

SE04589 Final Steam EA 

Downstream Model -

Results Matrix (CPP) 

(081915)

Spreadsheet used to present the results of the downstream model 

in a reader-friendly format for incorporation into the EA Report. 

This version accounts for the CCR and CPP rulemakings.

No

a – Record items containing CBI have been removed from the public record.