## MEMORANDUM

TO: Bill Swietlik, EPA

FROM: Patrick Goodwin , ERG

DATE: September 21 , 2015

SUBJECT: Downstream Modeling Methodology and Supporting Documentation (DCN

SE0 44 5 5 )

1.0 I NTRODUCTION


### 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 mil es 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 Database s

.

 Downstream Model Database s

.

 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 o f the other options considered.

2 EPA used pollutant loading s discharged to each receiving reach by steam electric power plant s to estimate concentrations in downstream reaches. The RSEI model uses a simple dilution and first

- order decay equation to calcul ate 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 t hroughout the year. In addition, EPA included pollutant loadings from EPA’s Toxic s Release Inventory (TRI) database for other industries to represent background pollutant concentrations in the downstream receiving waters. For further details on the RSEI mo del 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 the se 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 down stream 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 Sourc e Category (EA Report) ( EPA

821

- R

- 15

- 006 )

.
2.0 C HANGES IN 

### D OWNSTREAM 

### M ODELING 

### M ETHODOLOGY 

### S INCE 

### P ROPOSAL


### For the proposed rule, ERG compared the downstream receiving water pollutant concentrations (calculated using the RSEI model) to the same National Recommen ded 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 Quali ty 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 c alculated dissolved pol lutant 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 s implified the methodology by comparing the downstream mode led 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 d ue 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 co ncentration 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 bench marks in the EA R eport, 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;

 N o effect hazard concentr ations (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 natio nal

- 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 w ater 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 fa ctor 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 i n the EA Report) and 2) the

s cope 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 Page 4

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. 0 572 0. 0 553 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 the se 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 R SEI 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 develop ed 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 wildlif e 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 be nchmark. 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 Exceeda nces” (DCN SE04581) regarding the calculation of these thresholds

.
ERG also used these water concentration threshold s 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 Threshold s that Trigger NEHC Exceedance s for Mink s

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 Threshold s that Trigger NEHC Exceedance s for Eagle s

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 Page 6

Table 2

- 4 . Water Concentration Threshold s 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 subs istence 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 subs istence 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 subs istence 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. B enchmarks for all other pollutants are based on total concentration in the water column.

### Memorandum

September 21 , 2015 Page 7

Table 2

- 5 . Water Concentration Threshold s 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 S UPPORTING 

### D OCUMENTATION FOR 

### D OWNSTREAM 

### M ODELING



### 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 threshold s

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 d ownstream m odel s and case study models. No

SE04582 Final Steam EA

Downstream Model

- Calculation of Total

to

- 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 model s

. This version is based on the IRW model that accounts for the CCR rule making. 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 downst ream m odel s

. 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

accou nts 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, wildlif e, 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, wildlif e, 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 R eport. This version accounts for the CC R rulemaking. No

SE04586 Final Steam EA

Downstream Model

- Calculation of Total

to

- 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 model s

. This version is based on the IRW model that accounts for the CCR and CPP rulemaking s

. 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 wat er concentrations , calculated using EPA’s RSEI model , for use in the EA d ownstream m odel s

. This includes application of the total

- to

- dissolved water concentration conversion factors. This version accounts for the CCR and CPP rulemaking s

. 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 rulemaking s

. Yes

SE04588A1 Final Steam EA

Downstream Model

- Option A (CPP) (081915) Database used to identify those downstream reaches (and the total

length of those reache s) 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 rulemaking s

. 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 a ccounts for the CCR and CPP rulemaking s

. 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 trigge r water quality, wildlife, and/or human health benchmark exceedances under Option C. This version accounts for the CCR and CPP rulemaking s

. 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 rulemaking s

. 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 concentration s would trigger water quality, wildlife, and/or human health benchmark exceedances under Option E. This version accounts for the CCR and CPP rulemaking s

. 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 R eport. This version accounts for the CCR and CPP rulemaking s

. No

a

– Record items containing CBI have b een removed from the public r e cord.