TASK ORDER 2014-19 UNDER

CONTRACT EP-C-13-009

PEER REVIEW OF THE DRAFT AQUATIC LIFE AND AQUATICDEPENDENT WILDLIFE SELENIUM WATER QUALITY CRITERIA 

FOR THE SAN FRANCISCO BAY AND DELTA (2014)

PEER REVIEW SUMMARY REPORT

July 28, 2015

Submitted to:

U.S. Environmental Protection Agency

Office of Water, Office of Science and Technology

Health and Ecological Criteria Division

1200 Pennsylvania Avenue, NW

Washington, DC 20460

Attn: Joe Beaman

Beaman.Joe@epa.gov

Submitted by:

Eastern Research Group, Inc.

110 Hartwell Avenue

Lexington, MA 02421

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CONTENTS

1.0 INTRODUCTION.............................................................................................................................. 1

1.1 BACKGROUND ............................................................................................................................... 1

1.2 PEER REVIEWERS............................................................................................................................ 1

2.0 SUMMARIES ORGANIZED BY CHARGE QUESTION............................................................................ 2

Overall Document Organization and Structure......................................................................................... 2

2.1 Please comment on the overall construction and clarity of the document, i.e., is the 

layout of the document logical and is the document clearly written? Are there better 

ways to arrange and/or present the information? If so, please explain and describe. ............... 2

Questions on Use of Ecosystem-Scale Selenium Model ........................................................................... 4

2.2 Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco 

Bay and Delta (SFBD) varies from fresh to brackish to marine, and many of the species 

(or close relatives of the species) evaluated for the draft national freshwater criterion 

(USEPA, 2014 pp 57-62; 78-79 provided as supporting material), live in the estuary. 

Given these considerations, discuss whether it is reasonable to use the draft national 

freshwater selenium criterion (tissue elements) for aquatic life in the SFBD. Are there 

better ways to estimate toxicity benchmark values for key aquatic life species, many 

of which are anadromous, in the estuary? Please explain........................................................... 4

2.3 Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth 

is most likely the most sensitive endpoint. A recent study, De Riu et al. (2014) 

(provided), using green sturgeon observed selenium concentrations of 6 – 7 mg/kg dw 

(whole body) in the control treatment, which appears to reflect essentially a ‘noeffect’ concentration for growth or other effects, and is similar to the draft national 

whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this 

study (and any other relevant data on green sturgeon), is the draft national 

benchmark value of 8.1 mg/kg dw protective of the juvenile growth endpoint for 

green sturgeon? Please explain your answer............................................................................... 7

2.4 Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values 

from environmental data. FWS has suggested that a biphasic model (Beckon et al., 

2008 and Brian and Cousens, 1989, cited in review document) may be a better model 

to represent effects of selenium, since a biphasic model shows a more representative 

progression of effect to living species (selenium is a micronutrient, but is toxic in both 

deficit concentrations and excessive concentrations). Studies on selenium effects in 

the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in 

review document) do not appear to consider this issue, and study designs do not 

appear to take the issue into account........................................................................................ 10

a. Given that many of the studies in the literature often use a small (or zero) control 

concentration of selenium, and a low level of effect is observed (reflective of a 

deficient concentration), is the response to increasing levels of selenium (into the 

excessive toxic amounts) better represented using a biphasic model or log-logistic 

model? Would the slope of the regression be skewed in either model, due to 

incomplete modeling of deficit effects? .............................................................................. 10
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b. Discuss whether it would be reasonable and appropriate to use a biphasic model 

to determine EC10 values for studies in the literature on representative or key 

species for the SFBD (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 

1990, cited in review document), or whether the results of using a standard loglogistic model would produce more representative results................................................ 13

2.5 Species and Diet: Discuss whether the appropriate key species and food chains (webs) 

have been identified and appropriately applied for the SFBD................................................... 14

a. Discuss whether the diets of the key species chosen in each of the identified food 

webs are reasonable based on the literature (sources cited in review document) 

and whether the calculations of the Trophic Transfer Factors (TTFs) are 

scientifically supportable based on those diets (see specific questions below). Are 

there better ways to calculate TTFs for mixed diets for the key species? Please 

explain.................................................................................................................................. 14

Specifically for Clam-eating Fish:................................................................................................ 15

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile 

white and green sturgeon, and splittail scientifically supportable based on the 

available literature (Zeug et al., 2014, provided; USFWS, 2008 pp. 51-59, provided 

as supporting material; and Feyrer et al., 2003, cited in review document)? Is it 

scientifically supportable to postulate that splittail do not consume as much 

Corbula, and are therefore not as much at-risk as sturgeon?............................................. 15

c. Recent literature indicates that green sturgeon are more sensitive to selenium 

than white sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited 

in review document). Studies have also shown that adult green sturgeon primarily 

use the SFBD as a migratory corridor to freshwaters for spawning 

(http://exxexpartnership.com/wp-content/uploads/2010/10/GreenSturgeon-

_1_.pdf, cited in review document). If it is known that egg maturation in adult 

green sturgeon is completed prior to entering the estuary, discuss whether it is 

scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from 

elevated levels of selenium in Corbula in the SFBD (please provide references). 

Please comment on and discuss the potential for selenium exposure of adult 

green sturgeon during migration to play a role in reproductive toxicity, given the 

current state of knowledge on the reproductive biology and life history of green 

sturgeon............................................................................................................................... 17

Specifically for Clam-eating Birds:.............................................................................................. 19

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup 

are scientifically defensible based on the literature (sources cited in review 

document)............................................................................................................................ 19

Rails: Specifically, discuss whether it is scientifically supportable to assume that 

California Ridgeway’s rails do not consume a large percentage of Corbula, given 

that their littoral feeding habitat does not overlap with the preferred habitat of 

Corbula. ................................................................................................................................ 19

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and 

scaup live mainly in the shallower, northern parts of the Bay (Suisun Bay and San 

Pablo Bay) where Corbula are prevalent, and consume mostly Corbula. However, 
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in the latter part of the winter and early spring prior to migration, they move 

down to the deeper, Central Bay area where Corbula are not as available, and the 

scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and 

continue to feed on herring prior to breeding and nesting. It appears that 

selenium moves directly from the diet of water birds into their eggs and not from 

internal tissue stores (Chapman et al., 2010). Given that scoter and scaup switch 

to a low selenium diet for a period prior to migration as well as during migration, 

and that selenium may be depurated at a higher rate during migration, and that 

scoter and scaup are likely to continue to feed on low selenium fish sources in the 

breeding and nesting grounds, is it reasonable to decrease the percentage of 

Corbula in scoter and scaup diet by 1⁄2 (from 90% to 45%), with respect to selenium 

exposure that could impact reproductive endpoints? Please discuss................................. 20

Specifically for Insect-eating Fish: .............................................................................................. 21

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in 

the model scientifically supportable based on the literature (sources cited in 

review document)? Please explain. ..................................................................................... 21

2.6 Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the 

literature for individual species based on their food prey are scientifically supportable, 

i.e., are the TTF factors in Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 

21 – 24; Presser and Luoma, 2010, Table 3; and USEPA, 2014, pp. 71-77, each provided 

as supporting material) scientifically supportable and appropriate? Are you aware of 

any other or more applicable data?........................................................................................... 22

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would 

you recommend a different or better way to develop a site-specific TTF for Corbula, 

given the available data? Are there additional data that the Agency should consider 

for developing a site-specific TTF for Corbula? .......................................................................... 24

2.7 Partitioning Coefficients (Kds): In general, selenium is more bioavailable for 

accumulation into the food chain during dry/low flow (lentic) conditions, which is 

represented by higher Kd values. Is it scientifically supportable to use the 75th

percentile of the Kd values for each of the landward and seaward transects in the 

modeling, since data representing critical (very dry) conditions are not available? 

Would the use of median or mean Kd values be more appropriate? Please explain your 

answer. ....................................................................................................................................... 26

2.8 Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and 

Parchaso (2010) (both provided) indicate that populations and population densities of 

Corbula appear to decline in wet years, and increase in dry years. The articles suggest 

that Corbula may be sensitive to flow into the estuary and/or the salinity of the 

estuary (and other factors). The model shows that when the system has higher Kd

values (representative of dryer years), more selenium is available for bioaccumulation 

and more selenium accumulates in the food chain (which would drive the draft criteria 

to protect the use downward), and thus, we are concerned with accounting for 

critically dry conditions. Please comment on the articles and these statements...................... 28

2.9 Transects: Discuss whether it is reasonable and representative to use the focused 

seaward transects to model the clam-based food webs, in order to eliminate the end-
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member variability at the ocean (at the Golden Gate Bridge, the entrance to San 

Francisco Bay). Does the geology of the focused area (a wide Suisun Bay flowing into a 

constricted Carquinez Strait), and the importance of the area (the location of refinery 

discharges), favor the use of focused transects to best represent the clam-based food 

web for the estuary? Please discuss........................................................................................... 29

Results and Application........................................................................................................................... 30

2.10 Results: Discuss whether the model input and results scientifically support the water 

quality criteria for the three food chains represented. Are there better ways to 

represent the three food webs in the modeling? Please explain. ............................................. 30

Discuss whether it is scientifically supportable to use the modeling results for the clam-eating 

fish as a surrogate to protect clam-eating birds and insect-eating fish (all three food 

webs) in addition to the clam-eating fish themselves. Discuss whether there are other 

scientifically defensible ways to use the modeling results to protect all the categories 

of species for the SFBD............................................................................................................... 32

2.11 Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed 

and validated for SFBD, and a considerable amount of site-specific data are available 

and are used to model the estuary. EPA is using the model results to inform the 

criteria development process, and presents draft multi-media criteria in Chapter 4. 

These criteria are a whole body or muscle fish tissue value and a clam tissue value that 

directly reflect selenium levels that are considered to be protective of aquatic biota; 

and corresponding chronic and intermittent water column values, as well as a 

particulate concentration value to facilitate regulatory controls of selenium sources............. 33

Given the relevance and detail of the USGS model and the abundance of site-specific data, 

discuss whether the criteria in the different media are scientifically defensible and 

protective for this aquatic ecosystem........................................................................................ 33

2.12 Translator (Permits): EPA regulations generally call for NPDES permit limits to be 

expressed in terms of total recoverable values (40 CFR 122.45(c)). The model applied 

in this document yields calculations of water column values in the dissolved form. For 

most metals, EPA recommends dissolved values for water quality criteria, because the 

dissolved form better represents the bioavailable fraction in the water column to 

aquatic biota. However, for selenium, aquatic biota can take in the dissolved and 

particulate fractions in the water column, which are then moved into and up the food 

chain. .......................................................................................................................................... 34

For most metals, EPA allows a translator to be used on the dissolved criteria value, to 

determine a total recoverable permit limit; the translator is the fraction of the total 

recoverable metal in the downstream water (downstream of a discharge) that is 

dissolved..................................................................................................................................... 34

Given that selenium has different bioaccumulation dynamics, what is an appropriate method 

to derive a translator? Explain your answer. ............................................................................. 34

2.13 Other Comments Provided............................................................................................................ 38
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3.0 NEW INFORMATION PROVIDED BY REVIEWERS .............................................................................38

APPENDIX A CHARGE TO REVIEWERS ...................................................................................................... 1

APPENDIX B INDIVIDUAL REVIEWER COMMENTS .................................................................................... 1

Reviewer 1..............................................................................................................................................B-3

Reviewer 2............................................................................................................................................B-11

Reviewer 3............................................................................................................................................B-25

Reviewer 4............................................................................................................................................B-35

Reviewer 5............................................................................................................................................B-43

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1.0 INTRODUCTION

This report documents the results of an independent peer review of the Draft Aquatic Life and AquaticDependent Wildlife Selenium Water Quality Criteria for the San Francisco Bay and Delta (2014), developed 

by the U.S. Environmental Protection Agency (EPA). Eastern Research Group, Inc. (ERG, a contractor to EPA)

organized this review and developed this report. Section 2 of this report provides a summary of reviewer 

comments and the individual reviewer comments for each charge question. New information provided by 

the reviewers is presented in Section 3. Appendices A and B provide, respectively, the charge to reviewers 

and the complete set of comments submitted by each reviewer.

1.1 Background

EPA’s Office of Water is charged with protecting ecological integrity and human health from adverse 

anthropogenic, water-mediated effects, under the purview of the Clean Water Act (CWA) and Safe Drinking 

Water Act. The Agency has been revising the CWA section 304(a) recommended aquatic life water quality 

criterion for selenium to reflect the latest science. States may adopt EPA’s recommended criteria, use it to 

develop site-specific criteria, or develop their own scientifically defensible criteria. Once adopted and 

approved, a state’s Water Quality Standards are used to determine the level of pollution control necessary 

to attain the aquatic life designated use in a waterbody and to assess whether the waterbodies are 

impaired.

EPA Region 9, assisted by EPA Headquarters, developed the Draft Aquatic Life and Aquatic-Dependent 

Wildlife Selenium Water Quality Criteria for the San Francisco Bay and Delta (2014), and then contracted 

with ERG to organize an independent, external peer review of the draft criteria document. The draft 

selenium criterion is intended to protect the aquatic communities of the San Francisco Bay Delta generally, 

and also focuses on aquatic-dependent waterfowl, consistent with current science and the available data for 

the Bay and Delta Region. The document is not intended to address concerns about selenium toxicity to 

human health, nor selenium contamination in other areas outside the Bay and Delta. For this reason, 

comments on human health or selenium-related aquatic issues (unless clearly relatable) were beyond the 

scope of this review.

In addition, not long before this peer review, EPA released a national external peer review draft of a 

selenium chronic criterion for the protection of freshwater aquatic life. This document was provided to 

reviewers as supporting information. Comments on the national external peer review draft criterion were 

outside the scope of this review.

1.2 Peer Reviewers

ERG searched, identified, and selected five reviewers who met the technical selection criteria provided by 

EPA and who had no conflict of interest in performing the review: 

• Kevin V. Brix, Ph.D.; Post-Doctoral Fellow, Department of Zoology, University of British Columbia 

• Nicholas Fisher, Ph.D.; Distinguished Professor, School of Marine and Atmospheric Sciences, State 

University of New York

• Gregory Möller, Ph.D.; Professor of Environmental Chemistry and Toxicology, University of Idaho

• Michael C. Newman, Ph.D.; A. Marshall Acuff, Jr. Professor of Marine Science, Virginia Institute of 

Marine Science, The College of William and Mary

• Daniel Schlenk, Ph.D.; Professor of Environmental Toxicology, Department of Environmental 

Sciences, University of California, Riverside
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ERG provided reviewers with instructions, the review document, the charge to reviewers (Appendix A) 

prepared by EPA, and supporting reference materials as described in the charge. Reviewers worked 

individually to develop written comments in response to the charge questions. After receiving reviewer 

comments, ERG summarized reviewers’ responses to the charge questions, noting areas of agreement and 

disagreement, where relevant (Section 2). 

2.0 SUMMARIES ORGANIZED BY CHARGE QUESTION

This section presents overall summaries of reviewer comments organized by charge question. Each 

summary is followed by a table presenting the individual reviewer comments organized by charge question. 

Individual comments are copied directly from written comments as submitted by each reviewer and 

presented in Appendix B. 

Overall Document Organization and Structure

2.1 Please comment on the overall construction and clarity of the document, i.e., is the layout 

of the document logical and is the document clearly written? Are there better ways to 

arrange and/or present the information? If so, please explain and describe.

Five reviewers responded to this question. All five noted that the document is well written and has a logical 

layout. One reviewer pointed out that the format and conceptual flow are similar to EPA’s External Peer 

Review Draft Aquatic Life Ambient Water Quality Criterion for Selenium—Freshwater 2014, making it easier 

to read. 

Two reviewers suggested that EPA improve its treatment of uncertainty. One suggested that EPA better 

quantify uncertainty during computational steps; the other recommended that EPA add a chapter dedicated 

to uncertainty analysis to examine the strengths and weaknesses of the assumptions used in the model. 

A reviewer suggested that the outline should more clearly communicate that the topics of “fate and 

exposure” and “effects” are addressed separately (in Chapters 2 and 3, respectively). He also commented 

that Chapter 3 could be clearer if it included more figurative descriptions of thresholds. Another reviewer 

commented that the document should include more background information concerning selenium flux, 

loading, and discharge, as well as additional discussion of water column concentrations and estuarine freshsalt water dynamics that influence selenium sedimentation. Three reviewers pointed out minor editorial 

mistakes.

Reviewer Comments

Reviewer 1 I found the writing to be quite good—that is the layout of logical argument and the clarity 

of the statements made. Considering the many other documents I have seen, this 

document is much better written than many others.

Reviewer 2 The format and conceptual flow are generally similar to those of the federal WQC – 

Selenium draft, making it easily read and understood by those who have reviewed the 

federal treatment of the issue. In my opinion, the writing was clear and logical. There 

seemed to be a richer discussion and handling of uncertainties than in the federal 

document. It helped substantially to have significant components of the material already 

published in the open literature (e.g., Luoma and Presser (ES&T 43:8483, 2009) and 

Presser and Luoma (IEAM 6:685, 2010). Those publications were polished and concise 
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discussions of the central themes, especially the methodology applied for ecosystemscale modeling.

Unlike the federal document that focused on taxonomic groupings, this draft document 

categorized species first based on their ecological qualities such as trophic ecology and 

movement patterns. Species were considered based on how much they consumed high 

(e.g., bivalves) or low (e.g., insects) selenium prey. Diet composition, species movements 

and life histories were considered throughout the draft. This made it much easier to 

understand and logically move through the materials. Whereas the federal draft 

document could not develop a comprehensive treatment of aquatic-dependent wildlife 

(birds), the SFBD draft document was able to consider such important wildlife. This was a 

welcome addition that rounded-out the entire treatment.

In my opinion, the draft SFBD document (and federal document) might have quantified 

the propagation of uncertainty more during the numerous computational steps so that 

the reader had a sound understanding with which to decide whether any modest 

differences in calculated values were real or simply reflective of the variation generated 

during the uncertain computations in the process. Question 3 below can be used as an 

example. Is the difference between 6-7 and 8.1 trivial relative to the uncertainty that 

emerges during computations? The 0.1 digit in the 8.1 suggests very high certainty that, 

given the details of the derivation, is dubious. 

Reviewer 3 The document is well constructed and relatively well written. While the document 

(Chapter 2) separately addresses fate and exposure (based on modeling) and effects in 

separate sections (Chapter 3), these are not clearly delineated in the outline. For 

example, the outline should separate environmental fate and models used to assess 

particulate vs. dissolved concentrations from bioaccumulation models that estimate body 

burdens. While Figure 2-1 attempts to provide an overall conceptual a model, a more 

simplified model documenting the estimation of water criteria from tissues would be a 

better representation, and would probably be better suited in Chapter 1.

Chapter 3 is relatively well-written. However, more figurative descriptions for thresholds 

would provide better clarity. For example, copies of the species sensitivity distribution 

(SSD) which were used to obtain EC10s would be very helpful. As a template, please see 

the EPA 2014 document. 

As a minor editorial comment, the outline for Chapter 4 is missing a subheading C.

A chapter should be dedicated to Uncertainty Analyses at some point in the document.

What are the strengths and weaknesses of the assumptions used in the model? It was 

difficult to find a specific section where validations of the models were provided. 

Reviewer 4 Overall the document is well written and well organized. It could be enhanced with 

additional summary information concerning selenium flux, loading and discharge in this 

complex ecosystem, as well as additional discussion of water column concentrations and 

estuarine fresh-salt water dynamics that impact Se sedimentation. The major routes of 

fish and avian wildlife dietary exposure are very well treated. USGS Professional Paper 

1646, USGS 2010 and the CA Water Board TMDL documents for the North Bay can be the 

source of some of this water column and loading information. Note: p13 “across the full 
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range of bay salinities (<1 to 33 parts per trillion, ppt)” should be parts per thousand. 

Reviewer 5 I found the overall construction and clarity of the document to be quite good. There was 

a nice logical progression beginning with background and then describing and assembling 

all the various components needed to develop a Se WQC for SFBD. One very minor 

comment, paginate the whole document rather than just within a section.

Questions on Use of Ecosystem-Scale Selenium Model 

2.2 Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco 

Bay and Delta (SFBD) varies from fresh to brackish to marine, and many of the species (or 

close relatives of the species) evaluated for the draft national freshwater criterion 

(USEPA, 2014 pp 57-62; 78-79 provided as supporting material), live in the estuary. Given 

these considerations, discuss whether it is reasonable to use the draft national freshwater 

selenium criterion (tissue elements) for aquatic life in the SFBD. Are there better ways to 

estimate toxicity benchmark values for key aquatic life species, many of which are 

anadromous, in the estuary? Please explain.

All five reviewers responded to this question. Three reviewers said that it is reasonable to use the draft 

national criterion. One reviewer said that the national freshwater selenium criterion was used and adjusted 

in a reasonable manner for the SFBD. He wondered, however, whether an exposure adjustment for duration 

of time within the estuary or freshwater regions could have included more detail. Another reviewer noted 

that it is reasonable to use the draft national criterion (tissue elements) because sediment selenium 

exposure is a prime pathway in the SFBD ecosystem. A third reviewer thought it was reasonable to use the 

freshwater criterion for SFPB; he was not aware of any evidence suggesting that anadromous species are 

more or less sensitive to selenium than freshwater species and, given the mechanism of action for selenium, 

saw no reason to hypothesis this might be the case. 

Two reviewers presented reasons why the draft national freshwater selenium criterion (tissue elements) 

would not be reasonable for aquatic life in the SFBD. One reviewer noted that sulfate is more abundant in 

saline waters than freshwater, resulting in higher selenate uptake by phytoplankton cells at the base of the 

food chain in saline waters. He also noted that estuarine organisms may respond to potentially toxic levels 

of selenium differently than freshwater organisms. Another reviewer agreed, noting that there may be 

differences in developmental toxicity between saltwater-adapted fish and freshwater fish. He also pointed

out that acclimation to hyper- and hypo-saline conditions has a significant impact on endocrine and 

physiological responses in anadromous fish, including altered development of offspring and diminished 

reproduction. He suggested that it may be more appropriate to differentiate criteria based on returning 

adults (ocean to stream) or outgoing younger life stages (i.e. smolts; stream to ocean), and noted that the 

proposed SFBD criteria fails to recognize in ovo exposure within returning salmonid females.

Reviewer Comments

Reviewer 1 It is not appropriate to apply Se bioaccumulation data from freshwater studies to 

estuarine or saline waters. This is because sulfate is abundant in saline waters but not in 

fresh water environments. Thus, selenate, which comes into phytoplankton cells, at the 

base of the food chain, is not appreciably taken up by cells in seawater where the sulfate 
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concentration exceeds that of selenate by 10 million times (approx.). Selenite is the main 

species of inorganic selenium that is acquired by marine phytoplankton, which can also 

take up organic selenides. Thus, selenate uptake in freshwater regions may be quite 

appreciable (due to the lack of competition from sulfate) but not in saline waters.

However, once the Se is in organisms, it is not clear whether freshwater organisms would 

respond to potentially toxic levels in the same way as would estuarine organisms. The 

abundance of potentially competing ions in saline waters may result in different 

sensitivities for the estuarine organisms, but I am unaware of a rigorous comparison of 

animal sensitivities to a given tissue Se concentration among marine vs. freshwater 

organisms. The ideal comparison would be for a euryhaline species in which Se toxicity is 

compared for the same species at freshwater and estuarine salinities. I am unaware if 

such a comparison has been made.

Reviewer 2 Although the national freshwater selenium criterion was used, its use and adjustments 

for the SFBD were reasonable and effectively incorporated. The challenge with some 

species of concern being anadromous is a difficult one and I wondered while reading the 

document whether an exposure adjustment for duration of time within the estuary or 

freshwater regions might have included more detail. Try as I might, I could not think of an 

alternative that might have been materially more accurate. As an example, the EPA 

540/1-89/002 for human risk assessment can incorporate more exposure duration and 

frequency details in its calculation of intake. But, given the lack of knowledge about 

bioaccumulation kinetics of the species of concern, this approach would likely not 

improve accuracy of estimates for the SFBD species. Perhaps a formal expert elicitation 

would further refine estimates.

As a very minor point related to the use of freshwater concepts and computations for the 

estuarine or marine regions of the SFBD, equating μg/L and μg/kg is fine for freshwater as 

done on page 11 for partition coefficients but water density differences in saline waters 

makes the equating of liters and kilograms slightly biased. (An average liter of Pacific 

seawater weighs roughly 1025 g, not 1000 g.)

Reviewer 3 It is clear that salinity values have significant variance throughout SF Bay and the Delta.

Acclimation to hyper and hypo saline conditions has significant impacts on endocrine and 

physiological responses in anadromous fish. Two of the most sensitive toxicity 

benchmarks for selenium in freshwater are altered development of offspring and 

diminished reproduction, both under endocrine control. The National Se criteria was 

estimated based on reproductive and developmental endpoints using a species 

distribution of 9 freshwater fish genera with an N=14 Genus Mean Chronic Values 

(GMCVs) (EPA 2014). Most of the exposures included a dietary component, which would 

allow all forms of Se (organic species) to be present and transported to the gonad. The 

5th percentile projection yielded criterion lower than the most sensitive fish species 

tested, brown trout (Salmo trutta) and Oncorhynchus, which has two anadromous 

species in SFBD (Steelhead and Chinook Salmon). Since at least two andromous species 

were evaluated with reproduction and developmental endpoints utilized in the 

evaluation, the National value is likely conservative. However, some uncertainty exists 

with the value as some have suggested many of the studies used dietary 

selenomethionine as the primary source of Se, and this may underestimate Se toxicity as 

other organic forms may also contribute to the toxic responses observed in fish (Ridby et 
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al. 2014).

In addition, there may be some underestimation in Saltwater acclimated animals. When 

embryos of Japanese medaka (Oryzia latipes) were acclimated to hypersaline conditions 

and treated with concentrations of 0.05 mM SeMe (derived from concentrations 

observed in mosquito fish (Gambuis affinis) from the San Joaquin River) for 12 hours 

(Kupsco and Schlenk 2014) and 24 hours (Lavado et al. 2012), significant enhancement of 

developmental toxicity was observed. While this species is not endemic to SFBD, it 

demonstrates that there may be significant differences in developmental toxicity 

between saltwater adapted fish and those of freshwater fish, particularly at early life 

stages (Larvae, alevin, smolt). And since many anadromous fish at young ages must 

acclimate to hypersaline conditions in areas where exposure to Se may still occur either 

through prior maternal transfer in ovo, or via diet, combined exposure to both stressors 

may occur, particularly in the SFBD.

Alternatively, since anadromous adult female fish are likely gravid prior to returning to 

freshwater to spawn, it is unknown whether acclimation to freshwater impairs endocrine 

signaling, which could affect maternal transfer of Se to offspring or have any reproductive 

impact on Se toxicity in the returning adults. The endocrine responses would not be due 

to accumulation or feeding preferences on the adult females but due to the effects of 

hypersaline conditions on the physiology and susceptibility of anadromous species.

Thus, it may be more appropriate to differentiate criteria based on returning adults 

(ocean to stream) or outgoing younger life stages (i.e. smolts; stream to ocean). 

In addition, the assumption of the SFBD criteria assumes that for salmonids juvenile (fry, 

alevin, smolt) life stages are the more sensitive to Se toxicity and assumes dietary 

exposures via insects. The proposed SFBD criteria fails to recognize in ovo exposure 

within returning females, which is likely more critical (Thomas and Janz 2014). If in ovo 

exposure occurs, then models must incorporate a more complex food web (inclusion of 

fish to returning adults, rather than exclusively insects as in outgoing fish). 

Reviewer 4 In the SF Bay estuarine system, the interface of marine and freshwater from the Delta 

causes sedimentation of much of the freshwater suspended particulate load and many 

dissolved ions. The major concern of Se toxicological impact modulation would come 

from the presence of dissolved sulfate at the lowest trophic levels. With sedimentation 

however, sulfate reducing bacteria come into play in the sediment forming reduced sulfur 

compounds reacting with co-precipitating dissolved metal salts and mineral adsorbents 

(see for example: Chambers, et al. Sulfate reduction and sediment metabolism in 

Tomales Bay, California. Biogeochemistry 1994, Volume 25, Issue 1, pp 1-18). Thus, it is 

reasonable to suggest that marine-freshwater interface sedimentation caused by 

common ion effect and other solution processes, would increase the sedimentation of 

dissolved and suspended Se, forming the well-recognized sediment Se exposure pathway 

of the SF Bay, while providing for a sulfate reduction pathway (and Se as well) that would 

decrease sulfate competition in selenium exposure as well as modifying Se oxidation 

state and chemical forms (see for example: Meseck & Cutter. 2012. Selenium Behavior in 

San Francisco Bay Sediments. Estuaries and Coasts (2012) 35:646–657). With sediment Se 

exposure as a prime pathway in this ecosystem, it is reasonable to use the draft national 
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freshwater selenium criterion (tissue elements) for aquatic life in the SFBD.

Reviewer 5 Yes, I think it is reasonable to use the draft national freshwater selenium tissue criterion 

for SFBD. I am not aware of any evidence that suggests anadromous species are more or 

less sensitive to Se than freshwater species. Further, considering the mechanisms of 

action for Se, I can think of no reason to hypothesize that this might be the case.

2.3 Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is 

most likely the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), 

using green sturgeon1 observed selenium concentrations of 6 – 7 mg/kg dw (whole body) 

in the control treatment, which appears to reflect essentially a ‘no-effect’ concentration 

for growth or other effects, and is similar to the draft national whole body criteria of 8.1 

mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this study (and any other relevant 

data on green sturgeon), is the draft national benchmark value of 8.1 mg/kg dw protective 

of the juvenile growth endpoint for green sturgeon? Please explain your answer.

All five reviewers responded to this question. Three reviewers agreed that the national benchmark value of 

8.1 mg/kg dw should be protective of the juvenile growth endpoint for green sturgeon. One of these 

reviewers noted that the draft national benchmark was based on reproduction and development, which are 

more sensitive than juvenile growth. Another of these reviewers pointed out that De Riu et al. (2014) has a 

very high range of exposure diets that lack clear environmental dietary relevance. He commented that De 

Riu et al. (2014) is not definitive as a single study for lower diet concentration growth effects in clam-eating 

fish, but that the no-observable-effects level of 6-7 mg/kg dw (whole body) is protected by the draft national 

benchmark value. The third reviewer disagreed that juvenile growth is likely the most sensitive endpoint for 

green sturgeon, noting that larval abnormality is generally the most sensitive endpoint for fish. However, he 

thought it clear that the draft national benchmark value would be protective of juvenile green sturgeon 

growth. The real concern, he noted, is that this value is likely statistically indistinguishable from background 

selenium in juvenile green sturgeon.

The comments of the two other reviewers suggest that they were less certain that the draft national 

benchmark value would be protective. One of these reviewers noted that the difference between 8.1 and 6-

7 mg/kg is probably not significant, considering the data variability and the fact that the 5th percentile 

projection of 8.1 is a projection, so sturgeon appear to be barely protected by the draft national benchmark 

value. Another reviewer noted that the methods in the draft national benchmark value and De Riu et al. 

(2014) focus on individuals, not populations, and that individual-based metrics are often protective of 

populations (although this may not be true for many species). 

Reviewer Comments

Reviewer 1 The difference between 8.1 and 6-7 mg/kg is probably not significant, especially given the

variability in these data and the fact that the 5th percentile projection of 8.1 is a 

projection, not a hard number. Thus, the green sturgeon have Se concentrations are

about equal to the 5th percentile projection of 8.1 whole body criterion and these green

sturgeon would appear to be just barely protected by the draft national benchmark value

 

1 Green sturgeon larvae were obtained from the offspring of three males and one female originating from the Klamath 

River along the Pacific Coast. 
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of 8.1 ppm.

Reviewer 2 The methods used in the federal draft criterion document and De Riu et al. (2014) focus 

on effects to individuals, not populations. Indeed, these documents often obfuscate this 

distinction by using laboratory study results that allow inferences about the statistical 

population of tested individuals to suggest protection of the ecological population. It is 

often the case that individual-based metrics will be protective of populations but that 

could easily not be true for many species. This point was made very clearly by Kammenga 

et al. (1996) and others almost twenty years ago. Most laboratory-derived testing is done 

with species (“r-strategists”) that are quite different from the green sturgeon. The 

sturgeon has a life history in which individuals take several years to mature, develop eggs 

over several years, do not spawn annually, live for a long time, but produce many eggs 

upon spawning. It is not a species for which an expedient heuristic like “if you protect the 

individual’s most sensitive life stage, you protect the species population viability” can be 

confidently applied.

Fortunately, because of concerns of conservationists about the endangered green 

sturgeon, information has been gathered about the vital rates of most concern for this 

species, e.g., Beanesderfer et al (2007) and Heppell (2007). Beanesderfer et al. (2007) 

used demographic models to conclude, “The model highlights the extreme sensitivity of a 

green sturgeon population to even small incremental increases in mortality where 

multiple ages are affected.” Heppell (2007) used a technique called elasticity analysis to 

identify vital rates that most influence the growth rate (log λ) of green sturgeon 

populations, concluding that “Elasticity analysis [demonstrated ] a high sensitivity of 

population growth rate to changes in the survival rate of subadult and adult fish.” The 

federal and SFBD proposed criteria focus on the less critical vital rate (reproduction and 

YOY) so they are likely protective and might even be overprotective.

Beamesderfer, RCP, ML Simpson, and GJ Kopp. 2007. Use of life history information in a 

population model of Sacramento green sturgeon. Environ Biol Fish 79:315-337.

Heppell, SS. 2007. Elasticity analysis of green sturgeon life history. Environ Biol Fish 

79:357-368.

Kammenga, JE, M Busschers, NM Van Straalen, PC Jepson, and J Baker. 1996. Stress 

induced fitness is not determined by the most sensitive life-cycle trait. Funct. Ecol. 

10:106-111.

Reviewer 3 The estimated value (EC5) should be protective of growth for juvenile green sturgeon. 

This is because the National benchmark was based on the more sensitive response of 

reproduction/development. Since growth is likely a less sensitive endpoint, the predicted 

value should be conservative and protective. In addition, the National criteria is similar to 

the criteria obtained from Linville (2006), which also used a feeding study and offspring 

development to estimate tissue values in white sturgeon.

Reviewer 4 De Riu et al. (2014) is an solid Se aquatic toxicity study (lacking genetic diversity due to 

green sturgeon listing and single egg donor), however the exposure diets of 0 (common 

1-2 mg/kg Se diet formulation background), 50, 100 and 200 (2.2 ± 0.2, 19.7 ± 0.6, 40.1 ± 
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1.5, and 77.7 ± 3.6 mg Se/kg dw tested) are a very high exposure range that lack clear 

environmental dietary relevance. Indeed Kleckner, et al. 2010 (Selenium Concentrations 

and Stable Isotopic Compositions of Carbon and Nitrogen in the Benthic Clam Corbula 

amurensis from Northern San Francisco Bay, California: May 1995–February 2010. USGS 

Open-File Report 2010–1252) observe a max Corbula Se concentration of 22 mg/kg dw. 

Thus there is really only a two point dose response curve (2.2 and 19.7 mg/kg) for the 

major dose-response range dynamic in the SFBD area of interest for clam feeding fish. 

The data of Table 1 for fixed ration Growth Performance in De Riu et al. (2014) for the 0 

and 50 exposure groups through 8 weeks appear to have very high variability and a very 

large 95% confidence interval that includes zero. The ANOVA used assumes normality but 

the data appeared skewed and may require transformation to explore effects estimation. 

The effect of time in this study is also not adequately explored. Thus the Control and 50 

dose group data for %BWI/d as presented in Table 1 are far less than definitive and not 

justified to warrant the summary conclusion of the paper characterized as “the low 

SeMet diet (19.7 ± 0.6 mg Se/kg dw), which caused severe adverse effects in green 

sturgeon”. Hence De Riu et al. (2014) is not definitive as a single study for lower (<20 

mg/kg Se) diet concentration growth effects in clam eating fish.

%BWI/d 2wk 4wk 6wk 8wk

0 control 4.5 ±1.8

(± 40%)

11.9 ± 6.1

(± 51%)

6.3 ± 15.9

(± 252%)

6.6 ± 14.9

(± 226%)

50 3.8 ± 3.9

(± 103%)

6.8 ± 8.4

(± 123%)

3.1 ± 14.8

(± 477%) 

2.6 ± 16.0 

(± 615%)

The control (2.2 mg/kg Se dw diet) green sturgeon total body Se at 4 weeks (6.5 ± 0.9) 

and 8 weeks (7.1 ± 0.9) have upper confidence bounds of 7.4 mg/kg and 8.0 mg/kg 

respectively and thus in close proximity to the draft national benchmark value of 8.1 

mg/kg dw. Given the noted genetic variability limitations of De Riu et al. (2014) as well as 

their observation of potential fixed ration study impacts to the rapid growth phase of 

juvenile green sturgeon, as well as expected routine laboratory sampling variability and 

method bias, the protection of juvenile green sturgeon is qualitatively and quantitatively 

afforded by the draft national benchmark value.

In light of the lack of precision of the De Riu et al. (2014) study to benthic clam Se 

concentration range in this ecosystem, and the variability of the Table 1 data, the NOEL of 

6-7 mg/kg dw (whole body) is protected by the draft national benchmark value of 8.1 

mg/kg dw.

Reviewer 5 I disagree with the assertion that juvenile growth is likely the most sensitive endpoint for 

green sturgeon. In general, larval abnormality is the most sensitive endpoint for fish. 

Using the micro-injection to maternal transfer ratio for white sturgeon and applying it to 

green sturgeon suggests larval abnormality may be a more sensitive endpoint. I agree 

with EPA’s current assessment that while the FWS analysis suggests this may be the case, 

the uncertainty in this extrapolation precludes its use in setting a Se WQC. The fact that 

background Se in juvenile sturgeon is higher than the estimated whole body threshold 
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using the micro-injection to maternal transfer ratio further increases the uncertainty in 

this analysis.

I think it is clear that the draft national benchmark value of 8.1 mg/kg dw is protective of 

juvenile green sturgeon growth. The real concern is that this value is likely statistically 

indistinguishable from background Se in juvenile green sturgeon. Based on the error 

estimates for week 8 whole body measurements in the De Rui et al. study (Table 2), it is 

likely that at least one control sample exceeded the 8.1 mg/kg dw threshold. This raises 

concerns that some relatively large fraction (e.g., 30% based on n=3 from De Rui et al.) of 

background tissue measurements exceed the proposed Se WQC. 

2.4 Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values 

from environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 

and Brian and Cousens, 1989, cited in review document) may be a better model to 

represent effects of selenium, since a biphasic model shows a more representative 

progression of effect to living species (selenium is a micronutrient, but is toxic in both 

deficit concentrations and excessive concentrations). Studies on selenium effects in the 

literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review 

document) do not appear to consider this issue, and study designs do not appear to take 

the issue into account. 

a. Given that many of the studies in the literature often use a small (or zero) control 

concentration of selenium, and a low level of effect is observed (reflective of a 

deficient concentration), is the response to increasing levels of selenium (into the 

excessive toxic amounts) better represented using a biphasic model or log-logistic 

model? Would the slope of the regression be skewed in either model, due to 

incomplete modeling of deficit effects?

Four reviewers responded to this question. Two of them noted that a biphasic model would likely represent 

the response to increasing levels of selenium better than a log-logistic model, but the data are insufficient to 

warrant a biphasic model. One of these reviewers clarified that exposure concentrations were not 

established to produce a sufficient number of observations. He suggested codifying more formal methods to 

guide users to decide which model is best for their particular data set. The second reviewer noted that data 

are lacking to identify EC10 values. These two reviewers preferred a log-logistic model until data are 

sufficient to warrant biphasic modeling.

A third reviewer noted several issues with the use of biphasic modeling for selenium. For example, selenium 

appears to have very poor regulatory homeostasis as an essential trace element, which confounds the use of 

biphasic models for selenium. He cited Beckon et al. (2008)2, who note that the complexity of dietary 

selenium exposure makes it a poor choice for biphasic modeling. He also pointed out that the narrow toxic 

reproductive endpoints found in selenium exposure limit the usefulness of biphasic modeling. The fourth 

reviewer thought that EPA should consider using “non- standard” models if the models better describe the 

data and there is a viable mechanism justifying the use of such a model. 

Reviewer Comments

 

2 Beckon et al. 2008. Environ. Sci. Technol. 42: 1308–1314. 
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Reviewer 1 I do not know.

Reviewer 2 This concern crossed my mind several times during the review of the EPA draft federal 

WQC document. However, many of the data sets presented in that draft document were 

not designed to generate or sufficient for generating a reliable biphasic model. Exposures 

concentrations were not established to produce the number of observations needed in 

the region where the deviation from a conventional log-logistic model would be 

observed. So, the log-logistic is generally the best model if the data are inadequate for 

fitting a more complicated one.

The USFWS document (Beckon and Maurer, 2008) does contain relevant data sets (e.g., 

Figure 23 for weights of juvenile rainbow trout) that would allow a biphasic model to be 

fit. In such cases, the biphasic model does seem best. The supplied FWS report by Beckon 

(2009, Selenium effect levels for selected representative/surrogate species) has some 

examples illustrating my point. Figure 9 (salmon survival) and perhaps Figure 1 (mallard 

hatchability) show data for which a biphasic model would be warranted. If additional 

supportive information were available, a biphasic model might be fit to the salmon 

survival data shown in Figure 10. A biphasic model for Figure 2 (mallard hatchability) 

would be an over parameterization given the nature of the data set.

Given the above discussion, it seems that professional judgment might be used to decide 

which model is warranted. I suggest that more formal methods be codified to guide a 

user in deciding which model is best for their particular data set(s).

a. If one possessed a data set with which a biphasic model could be successfully fit, the 

biphasic model would be best. Obviously the slope derived from an inadequate data 

set for fitting the deficit response dynamics would be biased. But, if one forced a 

biphasic model onto an inadequate data set, the uncertainty in parameter estimates 

would increase. Many of the species response data sets illustrated in the federal 

draft document would be inadequate for fitting a biphasic model. As stated above, I 

suggest that formal methods be established to aid users in deciding which model is 

best for their particular data set.

Reviewer 3 Se is clearly an essential nutrient and likely fits a “bi-phasic” model showing nonmontonicity or a U-shape (Eaton and Gilbert 2013). While this is likely the most accurate 

description of the relationship of Se to development, the data are significantly lacking to 

identify EC10 values. The Beckon et al. (2008) manuscript compares 3 log-logistic models 

with varied EC10 estimates. For each comparison there are limited data points showing 

the classic U-shape that essential nutrients should have with only one point showing 

reduced hatch at low concentrations. Consequently, until studies can be conducted to 

provide statistically adequate data to generate a biphasic curve, standard log-logistic 

modeling should still be utilized.

Reviewer 4 Biphasic models have good potential to address toxicological impacts from essential trace 

elements (ETEs). However the broad range of existing and published aquatic toxicology 

studies were not experimentally set up with the broad range of dosage required to 

explore effects with this model. Log-logistic models are acceptable common practice, 
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although all models have limitations. Selenium appears to have very poor regulatory 

homeostasis as an essential trace element, compared to for example copper or 

manganese, and this confounds the use of biphasic models for selenium. The 2008 work 

of Beckon notes that the complexity of dietary selenium exposure makes it a poor choice 

for biphasic modeling:

“Although biphasic relationships are common, it should not be assumed that 

dose-response relationships are biphasic as a general rule, even for substances 

that are well known to cause biphasic responses. For example, dietary exposure 

to selenium is widely understood to have biphasic effects on the health and 

survival of animals ... However, although embryonic malformations are one of 

the effects of high levels of selenium, such defects are not known to be among 

the effects of selenium deficiency. Therefore, data relating selenium exposure to 

developmental defects ... may be best represented by a monotonic dose 

response model. Affecters of organisms are more likely to exhibit biphasic doseresponse relationships if the effect or “endpoint” assessed is a general 

measurement of health, growth, or fecundity rather than a narrower effect such 

as embryonic deformities, liver lesions, or thrombosis. This is because broad 

measures of effect are more likely to represent a combination of individual 

beneficial low-dose and deleterious high-dose mechanisms or responses. A 

narrow end point is more likely to capture the consequence of a particular 

mechanism or response that may be only beneficial or only deleterious to the 

organism, and would be represented better by a monotonic dose-response 

model.” (Beckon, et al. Environ. Sci. Technol. 2008, 42, 1308–1314)

As noted, biphasic models work best when the response tracked in the study appears in 

both deficiency and toxicity realms of exposure. The somewhat narrow toxic reproductive 

endpoints found in Se exposure limit the usefulness of the biphasic approach. Thus a) the 

Se dose response analysis is better represented log-logistic model. Skewing of the slope 

of the regression due to incomplete deficit dosing maybe unlikely since all experimental 

diets will have a baseline Se concentration from the protein and other constituents in the 

diet formulation prior to test agent fortification (i.e. addition of selenate, Se-methionine 

or other quantified Se substance). Dietary formulations for deficiency studies are very 

difficult with ETEs since they are ubiquitous at low, often required, levels in many healthy 

plants and animals. The cautions of Beckon, et al. 2008 concerning the application of 

biphasic modeling to selenium suggest b) it is not reasonable and appropriate to use a 

biphasic model to determine EC10 values for studies in the literature on representative or 

key species for the SFBD. A standard log-logistic model is appropriate and acceptable in 

the exploration of high dose Se reproductive toxicity. 

Reviewer 5 For some data sets, a biphasic model may better describe the data and there is a 

potential mechanistic underpinning (i.e., Se essentiality) for why such a model would be 

appropriate. It is possible for model slopes to differ depending on the data set and the 

interpretation of what constitutes a 10% effect can be quite different. The data from 

Hilton et al. (1980) (Figure 23; USFWS 2008) is the clearest example of such a data set 

although this particular study is problematic because fish were fed an inorganic Se diet. In 

contrast, it is not clear that use of a biphasic model on data for Chinook salmon (Figure 

11; USFWS 2008) and mallards (Figure 1; USFWS 2008) is more appropriate than a 
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standard log-logistic model. In general, I think EPA should consider use of “non-standard” 

models if they better describe the data and there is a viable mechanism for why such a 

model should be used. In my opinion, only the Hilton et al. study meets both criteria.

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to 

determine EC10 values for studies in the literature on representative or key species 

for the SFBD (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in 

review document), or whether the results of using a standard log-logistic model 

would produce more representative results.

Four reviewers responded to this question, with varying levels of caveat or concern about the use of 

biphasic models for selenium. One reviewer described numerous concerns that would make it neither

reasonable nor appropriate to use a biphasic model for this purpose. A standard log-logistic model would, in 

his opinion, be appropriate and acceptable in the exploration of high-dose selenium reproductive toxicity. A 

second reviewer recommended that EPA consider use of “non-standard” models if they better describe the 

data and there is a viable mechanism for why such a model should be used; however, few studies meet both 

criteria. Another reviewer commented that a biphasic model would more accurately predict selenium 

influences on biological systems if data sets contained sufficient points in lower-dose regions. The fourth

reviewer noted that biphasic modeling would be preferable if sufficient data were available to provide 

adequate statistical power. 

Reviewer Comments

Reviewer 1 I do not know and have no opinions on this.

Reviewer 2 Yes. Fitting a biphasic model to data sets with enough points in the lower dose regions 

would result in a model that more accurately made predictions and conformed to our 

current understanding of the mechanisms of selenium influences on biological systems.

Reviewer 3 As mentioned above, a biphasic model would be preferred if the data were available to 

provide adequate statistical power.

Reviewer 4 See response provided under 2.4.a.

Reviewer 5 See response provided under 2.4.a.
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2.5 Species and Diet: Discuss whether the appropriate key species and food chains (webs) 

have been identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food 

webs are reasonable based on the literature (sources cited in review document) and 

whether the calculations of the Trophic Transfer Factors (TTFs) are scientifically 

supportable based on those diets (see specific questions below). Are there better 

ways to calculate TTFs for mixed diets for the key species? Please explain. 

All five reviewers responded to this charge question. Three reviewers noted that the diet information used 

in the document was reasonable and the TTF values were appropriate. One of these reviewers noted that 

the seminal works of Luoma and Presser have characterized selenium behavior, impacts, and risks in the 

SFBD, and subsequent studies have explored pathways and endpoints for selenium in the SFBD. TTFs are 

therefore derived from the best available information and, given this body of work, are generally 

scientifically supportable. A second reviewer thought the identified food webs were reasonable, and did not 

think there was a better way to calculate TTFs for mixed diets for the key species. The third, who clarified 

that he was not an expert in trophic ecology, thought that the diet information appeared reasonable and the 

TTF values well chosen, but suggested it might be useful to have some indication of the uncertainty 

associated with each estimate.

Two reviewers were not convinced that the TTF values were appropriate. One of these reviewers 

commented that it is possible to calculate TTFs for mixed diets, but was not sure that it would be worth 

doing, because the considerable variability in single predator/prey combinations is compounded when 

multiple combinations are considered, confounding TFF derivation. The fifth reviewer stated that 

specification of the targeted life stage is necessary to refine the food chains proposed in the document (e.g., 

because larval or Young of the Year fish moving from freshwater into saltwater are likely to have a different 

diet than adults moving back into freshwater to spawn). 

Reviewer Comments

Reviewer 1 It is possible to mathematically calculate TTFs for mixed diets for a given species, but I am

not sure that it is worth doing so, even though many animals consume more than one

prey item....that is, nature is more complex than models typically depict. The reason I

have concerns in doing so for mixed diets is that there is considerable variability for any

one predator/prey combination. If one were to consider two or more prey items, the 

variability compounds itself and does not help pinpoint or quantify a more “real” TTF. An

example of this sort of variability can be seen for 6 different metals in fish consuming

diverse prey, where about an order of magnitude differences are seen, largely based on

the variability in the metal concentrations in the different forms of prey. [see Mathews &

Fisher, Sci. Total Environ. 407: 5156-5161 (2009).]

Reviewer 2 I am not an expert on the trophic ecology of these species so the answer here is based on

the information sent with the supplemental materials and that shown in tables of the

draft SFBD document. Based on those data, the diet information appears to be

reasonable. The TTF values seem well chosen; however, it might be useful to have some 

indication of the uncertainty associated with each estimate. In future reports, the

uncertainty of each component going into the various calculations could be incorporated

into a Monte Carlo resampling routine to provide estimates of the uncertainty around
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each calculated TTF. The uncertainty is likely large relative to that variation in TTF values 

among species. 

One conventional addition would be to use end member analysis involving N, C and 

perhaps S stable isotopes to estimate dietary components for each key species. The UCDavis campus has an excellent facility to do the analyses and others have the required 

software such as the EPA’s IsoSource package. This being said, my judgment is that such 

an approach would not provide much better information than the one taken in the draft 

document.

Reviewer 3 Overall, the criteria focuses on ocean-going early life stage organisms for the salmonid 

anadromous fish species. As discussed above, larval or Young of the Year fish moving 

from freshwater into saltwater (smolts) are likely to have a different diet (only one TTF) 

than adults moving back into freshwater to spawn (Mixture 4-trophic level-see Fig 7 

USEPA, 2014). Specification of the targeted life stage is necessary to refine the food 

chains proposed in this document.

Reviewer 4 The seminal works of Luoma and Presser have wonderfully characterized Se behavior, 

impacts and risks in the SFBD. Building on these works, the SFBD is exceptionally well 

characterized through several decades of modeling, samplings and scientific analyses 

exploring pathways and endpoints for selenium in this complex and important 

ecosystem. The body of SFBD work has been thorough in its analysis of key species and 

the related food webs that contribute to our understanding of Se impacts. The dietary 

information for the key species analyzed are well cited and well explored. More recent 

work with sturgeon (Zeug et al, 2014) fills data gaps with this key species. The resulting 

TTFs thus represent the use of best available data. Opportunistic feeding and food web 

dynamics related to the hydrograph and climate variability add variability to dietary 

understanding, however it is unlikely that precise quantitative analysis that addresses the 

full range of variables is possible. Thus a) TTFs derived from best available information, 

such as done in the current analysis is generally scientifically supportable.

Reviewer 5 Yes, based on the literature provided the identified food webs are reasonable. No, I don’t 

think there are better ways to calculate TTFs for mixed diets for the key species. Also see 

specific responses below.

Specifically for Clam-eating Fish:

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile 

white and green sturgeon, and splittail scientifically supportable based on the 

available literature (Zeug et al., 2014, provided; USFWS, 2008 pp. 51-59, provided as 

supporting material; and Feyrer et al., 2003, cited in review document)? Is it 

scientifically supportable to postulate that splittail do not consume as much Corbula, 

and are therefore not as much at-risk as sturgeon?

All five reviewers commented that the diet mixes for these species are scientifically supportable based on 

the available literature. One reviewer clarified that the amount of selenium-containing food that a cold-
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blooded fish consumes would be substantially less than that consumed by warm-blooded birds, so it was 

confusing that the TTF values for fish were close to those of birds. Another reviewer specified that the 

proposed diet mixes are scientifically supportable, but the available data do not necessarily preclude 

consideration of other dietary mixes. He suggested it would be useful for EPA to perform a sensitivity 

analysis. 

All five reviewers agreed that it is scientifically supportable to assume that splittail do not consume as much

Corbula. One of these reviewers, however, disagreed that this necessarily equates to splittail being at a 

lower risk than sturgeon. The species could have different sensitivities to selenium. 

Reviewer Comments

Reviewer 1 This seems reasonable to me, but see my comment below (under “c”).

Reviewer 2 A brief clarification of one point would help relative to the general TTF values for clameating fish and birds. The fish are cold-blooded and the birds are warm-blooded. The

amount of selenium-containing food that a fish consumes to maintain itself would be

substantially less than that consumed by the warm-blooded birds. Since selenium is

accumulated mostly from food, it was confusing to me that the TTF values for fish were

so close to those of birds. For fish and birds eating Corbula as 40-45% of their diet, Table

4-8 gives TTFfish = 1.1 and TTFduck egg = 1.8. Based on a physiological rule-of-thumb, my

informal guess would have been a 5- to 10-fold difference, not the low 2-fold difference

in the tables. Clearly, there is an unexplained feature of the calculated TTF values that

results in this apparent incongruity.

b. Yes. Based on my review of the supplied materials and the discussions in the draft

document, the diet mixes seem reasonable, including the differences between

sturgeon and splittail consumptions of Corbula.

Reviewer 3 Based on the literature, these assumptions seem appropriate.

Reviewer 4 The diet mixes appear to be representative of our current understanding of clam-eating

fish in the SFBD. Newer work such as Zeug et al., 2014 are valuable assets in modeling

dietary exposure and thus TTFs. Without clear and definitive data addressing system

variability, such as the case for many organism’s diet, best informed estimates that

balance what is known against what is unknown or unquantified, is a reasonable

approach. This appears to be the approach taken when the ; USFWS, 2008 and Feyrer et

al. work is used:

Diet: Splittail are benthic (bottom) feeders. Stomach contents from 70 splittail

collected in Suisun Marsh between March 1998 and January 1999 indicate that

their diet consisted mainly (43%) of unidentified material, probably detritus. The

overbite clam (Asian clam), Potamocorbula amurensis, and other mollusks

constituted 34% of the diet (Feyrer and Matern 2000, Feyrer et al. 2003).

Thus b) it is scientifically supportable to postulate that splittail do not consume as much

Corbula, and are therefore not as much at-risk as sturgeon.
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Reviewer 5 The diet mixes proposed by USEPA are scientifically supportable, although I would not say

the available data precludes consideration of other dietary mixes. It would be useful to

perform and provide the results of a sensitivity analysis on this issue. For example, if the 

sturgeon diet was 50% Corbula, how much lower would the proposed WQC need to be? I

also agree that splittail do not consume as much Corbula as sturgeon, but do not agree

this necessarily equates to them being at lower risk. Risk is an integration of exposure 

and effect (i.e., sensitivity). Given the limited studies on sturgeon and splittail in general,

it is premature to conclude they are less or equally sensitive to Se compared to sturgeon.

Therefore, the level of risk for splittail is unknown.

c. Recent literature indicates that green sturgeon are more sensitive to selenium than 

white sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in 

review document). Studies have also shown that adult green sturgeon primarily use 

the SFBD as a migratory corridor to freshwaters for spawning 

(http://exxexpartnership.com/wp-content/uploads/2010/10/GreenSturgeon-_1_.pdf, 

cited in review document). If it is known that egg maturation in adult green sturgeon 

is completed prior to entering the estuary, discuss whether it is scientifically 

supportable to conclude that adult white sturgeon and juvenile white and green 

sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and 

discuss the potential for selenium exposure of adult green sturgeon during migration 

to play a role in reproductive toxicity, given the current state of knowledge on the 

reproductive biology and life history of green sturgeon.

Four reviewers responded to this question. Two reviewers commented that adult white sturgeon and 

juvenile white and green sturgeon will likely be more affected by elevated levels of selenium in Corbula in 

the SFBD than adult green sturgeon, though both reviewers qualified their comments. For example, one of 

these reviewers commented that EPA cannot rule out the possibility that eggs will have significant selenium 

exposure during migration of adults. 

Another reviewer noted that it is not clear whether diet-derived selenium continues to accumulate in 

animals even after maturation of gonads. The fourth reviewer referred to his response in Section 2.3 (De Riu 

et al. [2014] has a very high range of exposure diets that lack clear environmental dietary relevance) to 

describe why De Riu et al. (2014) is neither definitive nor quantitatively precise in assessing green sturgeon 

selenium exposure risk in the SFBD. 

Reviewer Comments

Reviewer 1 These questions are outside my expertise.

Reviewer 2 Yes. The judgment is sound that “adult white sturgeon and juvenile white and green 

sturgeon will be more affected than adult green sturgeon.” As discussed above, one way

of handling the migratory characteristics of each species would be to quantitatively

include more details for exposure duration and frequency. The EPA document dealing

with human risk assessment (EPA 540/1-89/002) includes exposure duration and

frequency in its calculation of intake. Given the lack of knowledge about selenium

bioaccumulation kinetics in the sturgeon, this approach would likely not improve the
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accuracy of estimates. Perhaps a formal expert elicitation would improve future 

estimates.

Reviewer 3 Since Se is likely transported to gonads via the egg-yolk precursor vitellogenin (Linville

2006: Janz et al. 2010), one could assume that animals that do not require vitellogenin 

any longer would not continue to increase gonadal concentrations of Se. However, it is 

unclear whether vitellogenin is continually deposited into “mature” gonads of green 

sturgeon. Studies in zebrafish indicate vitellogenin is constantly turned over throughout 

embryogenesis (Han et al. 2010) which would argue diet-derived Se would continue to 

accumulate in animals even after maturation of gonads.

In the recent Linares-Casenave et al. 2015 publication, of 26 female white sturgeon 

collected between San Pablo Bay and Suisun Bay, 18 were pre-vitellogenic indicating 

most fish did not have sexually mature gonadal development. It was not documented 

whether the animals were coming from saltwater to freshwater for spawning or viceversa. If animals are returning to spawn, then it can presumed that gonadal development 

and selenium burdens would be higher than in animals exciting the estuary. Animals with 

vitellogenic oocytes did have higher concentrations of Se.

Reviewer 4 While of value, De Riu et al., 2014 for reasons stated above, is neither definitive nor 

quantitatively precise in assessing green sturgeon Se exposure risk in the SFBD. If there is 

high confidence in the observation that sturgeon use the “SFBD as a migratory corridor to 

freshwaters for spawning” and that egg maturation is primarily complete before entering 

the estuary, then there is cause to c) scientifically support a conclusion that adult white 

sturgeon and juvenile white and green sturgeon will be more affected than adult green 

sturgeon from elevated levels of selenium in Corbula in the SFBD. The 0 and 50 group 

dose-response data in De Riu et al., 2014 compared to the observed Corbula Se 

concentrations in Kleckner, et al. 2010, suggest lower level risks for juvenile growth toxic 

endpoints.

Reviewer 5 I think given our understanding of adult green and white sturgeon use of SFBD, it is likely 

that juvenile white and green sturgeon will be more affected than adults of either 

species. I understand that it is likely green sturgeon eggs mature while adults are in the 

ocean and that the adults appear to primarily use SFBD as a migration corridor. However, 

I think we cannot rule out the potential for toxicologically significant Se exposure (with 

respect to the eggs) during this migration. We simply don’t have any information on this 

issue and so it would be prudent not to rule out this potential exposure pathway. I don’t 

think there is enough information to address this issue in a quantitative manner, but it 

should be highlighted that this is a potentially significant uncertainty that needs further 

study rather than eliminating it as an issue for concern as the document currently does.
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Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are

scientifically defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that 

California Ridgeway’s rails do not consume a large percentage of Corbula, given that their 

littoral feeding habitat does not overlap with the preferred habitat of Corbula.

Four reviewers responded to this question. Three supported the assumption that California Ridgeway’s rails 

do not consume a large percentage of Corbula. One of these reviewers noted that the review document 

presents a defensible understanding of diet mix and dietary exposure for the three avian species. He also 

pointed out that it appears reasonable to decrease the percentage of Corbula in scoter and scaup diet by a 

half for selenium exposure and the potential for reproductive endpoints. The fourth reviewer noted that the 

20 percent Corbula diet is speculative and the actual percentage could be higher or lower. He suggested 

conducting a sensitivity analysis to understand the implications of this assumption.

Reviewer Comments

Reviewer 1 This definitely makes sense.

Reviewer 2 Yes. This assumption seems defensible for the rail.

Reviewer 3 I cannot comment on this section as I am unfamiliar with the life histories of these avian 

species.

Reviewer 4 The review document presents a solid and defensible understanding of diet mix and this 

dietary exposure for the California Ridgway’s rail, scoter and scaup. While habitat choice 

and thus exposure potential from Corbula are beyond my area of expertise, d) it seems 

plausible and reasonable to assess exposure potential in this manner. My work with 

aquatic birds in this ecosystem (Möller, G. 1996. Biogeochemical Interactions Affecting 

Hepatic Trace Element Levels in Aquatic Birds. Environmental Toxicology and Chemistry 

15:1025-1033) suggests a complex dietary and environmental correlation of Cd–Mn–Se–

Hg–As in vivo that may confound simple monotonic dose response models. Indeed, the 

tissue data for several of the sediment feeding birds demonstrated very high Se levels 

compared to other species. While complex, d) it does appear reasonable to decrease the 

percentage of Corbula in scoter and scaup diet by one half for Se selenium exposure and 

the potential for reproductive endpoints.

Reviewer 5 While the 20% Corbula diet used in the model is not unreasonable given the available 

data, it is entirely speculative and could be either lower or higher. A sensitivity analysis 

would be useful to understand the implications of this assumption. If all of the mollusk 

component of the rail diet (60%) were Corbula, would they become the driver of the 

proposed WQC? If so, what percentage Corbula in the diet causes this to be the case?
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Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup

live mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) 

where Corbula are prevalent, and consume mostly Corbula. However, in the latter part of 

the winter and early spring prior to migration, they move down to the deeper, Central 

Bay area where Corbula are not as available, and the scoter and scaup are thought to 

consume mostly herring that do not bioaccumulate selenium as effectively. They then 

follow the coast north and continue to feed on herring prior to breeding and nesting. It 

appears that selenium moves directly from the diet of water birds into their eggs and not 

from internal tissue stores (Chapman et al., 2010). Given that scoter and scaup switch to 

a low selenium diet for a period prior to migration as well as during migration, and that 

selenium may be depurated at a higher rate during migration, and that scoter and scaup 

are likely to continue to feed on low selenium fish sources in the breeding and nesting 

grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet 

by 1⁄2 (from 90% to 45%), with respect to selenium exposure that could impact 

reproductive endpoints? Please discuss.

Four reviewers responded to this question. Two thought that this reduction seemed reasonable. One

referenced his earlier comment that it appears reasonable to reduce the percentage of Corbula in scoter and 

scaup diet by one half for selenium exposure and the potential for reproductive endpoints. The second

reviewer noted that decreasing the percentage of Corbula seems reasonable, but that exposure frequency 

and duration could be considered more fully in the future, as in human risk assessments. He also suggested 

that additional bioaccumulation/elimination pharmacokinetics models combined with knowledge of 

migratory movement of the species might enhance the certainty of reducing the percentage of Corbula in 

scoter and scaup diet. 

Two other reviewers expressed concerns about this reduction. One of these reviewers commented that it 

would be speculative and possibly not protective of the species to decrease the percentage of Corbula in 

scoter and scaup diet by half. He noted that species respond to whatever concentration of a toxic substance 

they have in their tissue at a given time, not to the average concentration over time. The other reviewer was 

not convinced that the reduction in Corbula is accurate, and suggested that EPA should consider when egg 

proteins are deposited relative to the timing of ducks feeding on Corbula. Ducks may also depurate selenium 

relatively rapidly. 

Reviewer Comments

Reviewer 1 It strikes me as rather speculative to do so, and possibly not protective of these species. 

Bear in mind that animals do not respond to average concentrations of toxic substances 

in their tissues, but rather to whatever concentration they have in their tissue at any 

particular time. So, when they are feeding on Corbula, their Se concentrations is high and 

at that time they are vulnerable. It is not appropriate to average the Se that they see, and 

respond to, by including herring for part of the time. Imagine if you drank one glass of 

water each day for 100 consecutive days, but only one of those glasses had a toxic level 

(concentration X) of arsenic. You would die after drinking that one glass of arsenic. You 

should not conclude that you would respond to X/100 arsenic (the average arsenic 

concentration of the ten glasses) and be safe. 99 of those glasses would be harmless, and 

one would be lethal. The same may occur with the Se in these birds.
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Reviewer 2 Yes. It seems reasonable and is consistent with my earlier comments about adjusting 

intake estimates based on knowledge of exposure duration and frequency. But exposure 

frequency and duration might be considered more fully in the future as done with human 

risk assessment (EPA 540/1-89/002). The treatment in “D. Draft Criteria. A. Applicable 

Waters; Magnitude, Duration and Frequency” reflects the approach in the federal draft 

document. However, a more granular treatment of the duration/frequency of exposure 

issues could improve the accuracy of judgments in my opinion. In the future, additional 

bioaccumulation/elimination pharmacokinetics models combined with knowledge of the 

migratory movement of the species between habitats differing in selenium levels might 

enhance the certainty in this judgment. 

Reviewer 3 See response provided under 2.5.d.

Reviewer 4 See response provided under 2.5.d.

Reviewer 5 While I follow the thinking used by EPA in setting the Corbula diet at 45% for scoter and 

scaup, I’m not convinced it is particularly accurate. In assessing potential exposure of 

green sturgeon, EPA asserted that sturgeon eggs mature while adults are in the open 

ocean where Se exposure is low. It seems a similar line of thinking should be used in 

assessing risks to scoters and scaups. When are egg proteins deposited relative to the 

timing of duck feeding on Corbula. If Corbula feeding occurs during this period, then 

exposure should be high, if not, then there is no relevant exposure with respect to the 

SFB WQC. It should also be pointed out that even if exposure is relatively high, there is 

evidence that ducks depurate Se relatively rapidly (19 d half life for mallards; (Heinz et al. 

1990) ) and marine ducks deposit a low percentage of Se in eggs relative to freshwater 

species (Henny et al. 1995).

Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the 

model scientifically supportable based on the literature (sources cited in review 

document)? Please explain.

All five reviewers responded to this question. Three generally agreed with this statement. Two reviewers 

commented that the insect diet mixes are well studied and scientifically supportable. One of these reviewers 

thought that EPA could have high confidence that these species were not consuming mollusks. The other 

thought the diet mixes seemed sound, but noted that he was not an expert in fish trophic ecology and 

habits. A third reviewer “supposed” that the diet mixes were scientifically supportable but noted that diet 

depends on the abundance of prey insects at the particular place and time, which probably accounts for 

much of the variability in selenium concentrations observed in predator fish. 

The fourth reviewer did not respond directly to the statement but expressed the concern that, at least for 

salmonids, the diet mixes of forage fish and insects depend on the life-stage of the animal. He suggested 

revising the models to separate adult and early life stages. 
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Reviewer Comments

Reviewer 1 I suppose so, but again, how much of any given diet these fish happen to consume in any 

one location and any one time depends on the abundance of those prey insects at that 

particular time and place. These things vary enormously and probably account for much 

of the variability that one sees in Se concentrations in predator fish.

Reviewer 2 Based on a review of the supplied documents and tabulations in the draft SFBD 

document, the diet mixes do seem sound. However, I am not an expert on fish trophic 

ecology and habits.

Reviewer 3 As discussed above, at least for salmonids, the diet mixes will differ based on the lifestage of the animal. Returning adults will have a much larger forage fish component than 

insects and likely continual deposition of Se into ova. In contrast, early life-stage 

salmonids will have a greater insect component. The models should be revised to 

separate adult and early life stages.

Reviewer 4 With several decades of monitoring, the insect diet mixes for SFBD insect eating fish are 

e) scientifically supportable and well discussed and referenced in FWS, 2008. 

Reviewer 5 The diets of SFBD steelhead, Chinook salmon and delta smelt are relatively well studied. 

These are probably the most reliable diet mixes of any of the species evaluated by EPA. 

Most importantly, there is high confidence that these species are not consuming 

mollusks.

2.6 Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the 

literature for individual species based on their food prey are scientifically supportable, 

i.e., are the TTF factors in Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 –

24; Presser and Luoma, 2010, Table 3; and USEPA, 2014, pp. 71-77, each provided as 

supporting material) scientifically supportable and appropriate? Are you aware of any 

other or more applicable data?

All five reviewers commented that the TTF values are scientifically supportable. One reviewer commented 

that the extraction and derivation of the TTF values were well founded, but suggested including stable 

isotope information for the trophic transfer for the species in this report. He said that such information is 

readily available and provided several examples. Another reviewer noted that the TTFs are scientifically 

supportable, but that a four-trophic-level mixture should be used for adult returning salmonids. A third 

reviewer suggested that EPA not use the same TTF for fish in all cases, because species-specific TTFs are 

available in some circumstances. 

Reviewer Comments

Reviewer 1 These TTFs look good to me. 
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Reviewer 2 The extraction and derivation of the TTF values seem well founded although the reason 

for such a small difference in TTF values for warm-blooded birds and cold-bolded fish 

mentioned earlier is not apparent. The TTF value computations are straightforward and 

consistent with our understanding of trophic transfer of selenium. I do wonder if there is 

useful stable isotope information available for the trophic transfer for the key species in 

this report. Several such studies exist including those by authors of several supporting 

documents for this review. Perhaps their discussion was not prominent enough for me to 

recognize the inclusion of stable isotope information in the draft SFBD document. (A 

search of the 96 page draft pdf did not find the word “isotope.”) In the future, it might be 

useful to explore and discuss more the insights available from published stable isotope 

studies of these or similar species. These kinds of data might provide reinforcing detail to 

the estimated TTF values.

Numerous publications of stable isotope studies of the subject area were found with a 

quick Google search. Some examples include the following:

Cloern, JE,EA Canuel and D Harris. 2002. Stable carbon and nitrogen isotope composition 

of aquatic and terrestrial plants of the San Francisco Bay estuarine system. Limnol. 

Oceanogr., 47(3): 713–729

Croteau, MN, SN Luoma, and AR Stewart.2005. Trophic transfer of metals along 

freshwater food webs: Evidence of cadmium biomagnification in nature. Limnol. 

Oceanogr., 50(5): 1511–1519

Davis, JA et al. 2012. Reducing methylmercury accumulation in the food webs of San 

Francisco Bay and its local watersheds. Environmental Research 119: 3–26

Gegrke, GE. 2011. Sources of mercury to San Francisco Bay surface sediment as revealed 

by mercury stable isotopes. Geochimica et Cosmochimica Acta, 75:691-705

Stewart ,AR et al. 2004. Food Web Pathway Determines How Selenium Affects Aquatic 

Ecosystems: A San Francisco Bay Case Study. Environ. Sci. Technol., 38 (17): 4519–

4526 

Reviewer 3 The TTFs appear valid and are scientifically supportable. However, as discussed above, a 4 

trophic level mixture should be used for adult returning salmonids (Figure 7; USEPA 

2014).

Reviewer 4 The TTFs taken from the work of Presser and Luoma, USGS and the 2014 Draft Criteria are 

to my knowledge the best available TTF values that could be applied in a SFBD model; I 

am not aware of any additional TTF values that would supplant this data. The additional 

Corbula TTF values developed in USGS 2010 pp21-23 is coherently presented and 

contains well supported assumptions (e.g. dissolved Se exposure is trivial). The USGS 

2010 Corbula TTF analysis is explicitly developed and cited in this reference and I am not 

aware of any additional analyses of this caliber and quality. 

Reviewer 5 In general, the TTFs taken from the literature and used in the WQC derivation are 

scientifically supportable. Specific to Corbula, yes I agree with the novel approach taken 
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by USGS and adopted by EPA in this analysis.

One issue of concern is that EPA has opted to use a TTFfish of 1.1 in all cases despite 

species-specific TTFs being available in some circumstances. For example, USGS derived a 

TTF for white sturgeon of 0.8. It is unclear why EPA selected to use a mean value from 

across species, as generally in WQC derivation, specific-specific data is given preference.

Also, see comments below on Kds and the need to consider concentration-dependent 

TTFs.

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the 

available data? Are there additional data that the Agency should consider for developing a sitespecific TTF for Corbula?

All five reviewers noted that the TTF for Corbula is scientifically supportable. One reviewer pointed out that 

the TTF values are consistent with the general approach in the federal draft criterion document and are 

well-tailored for the SFBD. He suggested supplementing relevant materials with stable isotope end member 

analyses, considering use of an expert elicitation exercise to improve the estimates. 

Reviewer Comments

Reviewer 1 I don’t know of any involving Corbula beyond what is here.

Reviewer 2 The TTF values are consistent with the federal draft document’s general approach. They 

are tailored very logically for the SFBD. Reading the relevant materials such as Luoma and 

Presser (2009) and Presser and Luoma (2010) for more details reinforces this conclusion 

of consistency. I can think of no better way to estimate TTF values. Supplementing the 

current information with stable isotope end member analyses might enhance insights. An 

expert elicitation exercise might improve TTF estimates if done carefully. Perhaps future 

reports could calculate and present the quantitative uncertainties in TTF estimates.

More specific comments can be made below about the components (IR,AE, ke) going into 

the calculations as discussed in the USGS(2010) document.

IR Estimation

“IR is determined by filtration rate (125 L/g clam/d, Cole et al., 1992) multiplied by C 

(median = 0.4 mg C/L) to achieve the units (g C/g clam/d) in the suspended particulate 

material at each sampling. In the average condition in the estuary, clams ingest 5% of 

their body weight per day in C across all days for which data is available. At an 

average of 2% C in suspended particulate material (again, the average across all data) 

they ingest 2.5 times their body weight per day in total suspended particulate 

material. If IR is calculated at each of three low river discharge months where data is 

available, the average is 1.7 g suspended particulate material/g clam/d. Experience 

has indicated that the ingestion model is more accurate when actual outcomes are 

used (or averaged) for the generic situation (i.e., 1.7 g suspended particulate 
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material/g clam/d) as compared to taking the average of each component of the 

outcome and calculating a generic average. Therefore, we recommend using 1.7 g 

suspended particulate material/g clam/d for modeling.”

Again I wonder about the variation in features such as filtration rates as a function of 

factors like temperature or salinity. Until provided with contrary information, it would be 

reasonable for a reader to suppose that filtration rate varies substantially. Also variation 

associated with the general estimate of 0.4 mg C/L might be important to describe. Given 

the wide variation in kd values throughout the region of interest, this value might have 

substantial variation associated with it. Finally, according to classic Holling’s predator 

functional responses, the efficiency of removing particles during filtration can vary with 

the density of those particles in the waters. Some additional scrutiny of the uncertainty in 

the chosen values seems warranted.

AE Estimation

“The derivation of a refined site-specific AE based on individualized bioavailabilities of 

Selenium in suspended particulate material uses observed fractions of particulate 

organo-Selenium, adsorbed Se, or elemental Se found in the estuary (Doblin et al., 

2006) combined with individual AEs for those particulate Se species from the literature 

(living phytoplankton, AE = 60%; adsorbed on seston, AE = 40%; elemental, AE = 0%; 

Schlekat et al., 2004; Wang et al., 1996). The equation is: AE = (fraction organic 

particulate Se) (AE organic particulate Se) + (fraction adsorbed particulate Se) 

(AEadsorbed particulate Se) + (fraction elemental particulate Se)(AEelemental 

particulate Se). For example, if a site-specific sample of suspended particulate 

material collected in the estuary contains 45% Se in phytoplankton at an assumed AE 

of 60%; 30% Se adsorbed on seston at an assumed AE of 40%; and 25% elemental Se 

in sediment at an assumed AE of 0%, then the composite AE = (0.45 x 0.6) + (0.30 x 

0.40) + (0.25 x 0) = 0.39 or 39% AE.”

This manner of estimating AE seems reasonable although the variation in the distribution 

of selenium among the three phases will likely change temporally and spatially. Future 

documents might spend more time discussing the variation expected in this and the 

resulting variation in AE.

Ke Estimation

“We apply the efflux rate constant derived experimentally (Lee et al., 2006): ke = 

0.03/d.” 

Seems like a reasonable value chosen from a peer-reviewed publication.

Calculations

“When we model for times when all data are available from the estuary, we use all 

data from that sampling date. When we model generically we employ mean 

parameters.” 

This is fine; however, insight might be enriched by more detail about the variation as well 

as the mean parameters. During the calculations, general decisions involving variation 
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were made such as calculating values for specific regions or “excluding April, 1999 

transects data as out of the norm (i.e., El Niño condition in the estuary).” Relative to 

the last decision, one could argue that El Niño years are normal occurrences and 

clams will be there during those times. 

Nonetheless, the handling of the calculations is fine and the report does provide some 

useful information about the expected variation, that is, “We assume a TTFC.amurensis of 

17 in modeling scenarios here. The range of TTFs across all estuarine conditions was 14-

26. These values are higher than laboratory-derived values primarily because ingestion 

rates are higher in these field systems than in experiments. This is the first calculation of a 

field-derived TTF for a marine bivalve species.” 

Reviewer 3 The site-specific model used to calculate TTF for Corbula is scientifically sound.

Reviewer 4 See response provided under 2.6

Reviewer 5 See response provided under 2.6.

2.7 Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation 

into the food chain during dry/low flow (lentic) conditions, which is represented by 

higher Kd values. Is it scientifically supportable to use the 75th percentile of the Kd values 

for each of the landward and seaward transects in the modeling, since data representing 

critical (very dry) conditions are not available? Would the use of median or mean Kd

values be more appropriate? Please explain your answer.

All five reviewers responded to this question. Two reviewers noted that the 75th percentile seems 

appropriately protective. One commented that this allows for precaution in dry/low-flow higher-known-risk 

conditions, which is important considering the exceptionally dry conditions that are currently impacting the 

SFBD hydrograph. The other suggested that EPA evaluate concentration dependency in the assessment and 

consider using a quantile regression approach for determining the 75th percentile Kd.

Two reviewers commented that it is inappropriate to use the 75th percentile of the Kd values. One of them 

noted that Kds assumes that the element in question is in equilibrium between dissolved and particulate

phases, which is not the case for suspended particles such as phytoplankton, to which the authors are 

apparently referring. He recommended that the authors refer to these values as concentration factors and 

that they use a range of values (rather than median of mean concentration factors), because the mean 

varies with phytoplankton assemblage and these natural communities typically change with the seasons. 

The other reviewer pointed out that the rationale for using the 75th percentile is not clearly stated and that, 

given the uncertainties, use of a 95th percentile would be more conservative.

The fifth reviewer pointed out that the natural variation in Kd values is high, and that means, if used, would 

not represent the populations of Kd values. He suggests using a probabilistic or conservative approach (e.g., 

use of the 75th percentile value would be better than using the mean or median). 

Reviewer Comments

Reviewer 1 I think it is inappropriate to use Kds, or at least to call them Kds. This is because Kds 
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assume that the element in question—in this case, Se—is in equilibrium between the 

dissolved and particulate phases. That may be the case for sediments, but it is not the 

case for suspended particles, which in many case would probably be phytoplankton, and I 

believe that this is the case to which the authors are referring. Se uptake by 

phytoplankton (and also other microorganisms at the base of the food chain). It is more 

appropriate to call them concentration factors. However, Se CFs can vary enormously 

(several orders of magnitude) among different types of phytoplankton, with chlorophytes 

being the lowest and diatoms and dinoflagellates among the highest (for selenite). As for 

median or mean CFs, I would instead use a range of values, since a mean for some types 

of phytoplankton would be wholly inappropriate for another phytoplankton assemblage, 

and these natural communities typically change with season (seasonal succession 

patterns among the phytoplankton communities) and nutrient concentrations.

Reviewer 2 The natural variation in the Kd values is quite high. The constrained transects have kd

values from 712 to 7,725 and the landward transect models use kd values from 554 to 

12,650. Table 8 in USGS (2010) indicates means that if used, would be unrepresentative 

of the populations of Kd values. In such a case, I would have suggested that a probabilistic 

approach be taken or that a conservative approach be applied. The conservative 

approach of using the 75% percentile value seems better than using the mean or median.

Reviewer 3 The rationale for selection of the 75th percentile is not clearly stated. Given the significant 

uncertainties in Kd as well as other components within these systems (i.e. percentage of 

microalgae, peryphyton etc), one would think a 95th percentile would be more 

conservative.

Reviewer 4 Without data, median or mean values build on absent data estimate that may 

underestimate risk without cause. Using the 75th percentile of the Kd values seems to be 

an appropriately protective estimate, allowing for precaution in dry/low flow higher 

know risk conditions. 

Point of Clarification

Without data, median or mean values build on absent data estimates that may 

underestimate risk without cause. There is currently a significant absence of data for dry 

and critically dry seasonal impacts on sediment transects that are important in calculating 

Kd values. Table 4-7 has some limited data [C. Insect-Eating Fish (see USGS, 2010 Table 

10)] for below average water-year types and the increase in observed Kd is consistent 

with our knowledge base of lentic/lotic flow dynamics on aquatic food web Se dynamics. 

While additional Se flux or mass balance analysis for the impact of less flow into the SFBD 

during dry years that would be helpful, it is reasonable to assume that system hysteresis 

from existing sediment load and potential point discharges will maintain or more likely 

increase Kd in below average water-years. Thus, using the 75th percentile of the Kd 

values seems to be an appropriately protective estimate, allowing for precaution in 

dry/low flow higher known risk conditions, especially in light of the exceptionally dry 

conditions that are currently impacting the SFBD hydrograph.

Reviewer 5 Yes, I think it is appropriate to use the 75th percentile Kd in the modeling given the 
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variability in Kds both seasonally and annually, and the uncertainty regarding Kds during 

very dry conditions. One uncertainty with Kds that EPA has not considered is that they 

tend to be inversely related to exposure concentration (see Poster by DeForest et al. 

provided with comments). EPA should evaluate concentration dependency in their 

assessment and consider using a quantile regression approach for determining the 75th

percentile Kd. It is also worth noting that if EPA were to use this type of approach at each 

step in the model (i.e., 75th percentile at each step), this would equate to a 98th percentile 

site/condition (assuming each step is independent), which I believe most would consider 

adequately protective.

2.8 Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and 

Parchaso (2010) (both provided) indicate that populations and population densities of 

Corbula appear to decline in wet years, and increase in dry years. The articles suggest that 

Corbula may be sensitive to flow into the estuary and/or the salinity of the estuary (and 

other factors). The model shows that when the system has higher Kd values 

(representative of dryer years), more selenium is available for bioaccumulation and more 

selenium accumulates in the food chain (which would drive the draft criteria to protect 

the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

All five reviewers responded to this question. The comments of four reviewers generally supported this 

concern. One reviewer commented that, given the information about population dynamics and Kd values, 

the focus on dry periods seemed appropriate. A second reviewer noted that the descriptions provided 

regarding wet and dry years and Kd values appeared to be scientifically sound. A third reviewer agreed with 

EPA’s assessment that the available literature indicates that Corbula density and distribution is constrained 

by low salinity and that the system exhibits higher Kds during drier years, resulting in more selenium 

bioaccumulation higher in the food chain. He suggested that EPA monitor the system during exceptionally 

dry conditions and revise the site-specific water quality criteria based on these new data, when available. He 

did not think it reasonable to speculate on how much Kds would change under such conditions and a priori 

increase the Kds in the models. The fourth reviewer noted that the Fuller et al. (2012) and Thompson and 

Parchaso (2010) studies were conducted before current critically dry conditions in California, and that 

incorporating recent data may help validate population relationships with the hydrograph. He also pointed 

out that lentic and lotic selenium dynamics for freshwater may not translate well to the marine sediment 

exposure pathways in the SFBD. 

The fifth reviewer questioned the use of Kd values. He reiterated his point in Section 2.7 that Kds are 

inappropriate in this scenario, and added that higher phytoplankton concentration factors in dry seasons 

would translate into higher selenium levels in Corbula predators. He also referenced his response to the next 

question (Section 2.9) that the South Bay is likely less well flushed than waters to the north, meaning that 

applying models from other areas to the South Bay may be risky. 

Reviewer Comments

Reviewer 1 Again, Kds are inappropriate here, but if phytoplankton CFs of Se are higher in dry 

seasons, then Corbula concentrations should be correspondingly higher then, and this 

would be reflected in higher Se levels in Corbula predators. Also, see comment just below 
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(No. 9) on South Bay being less well flushed.

Reviewer 2 I do not have any helpful insights to provide about Corbula population changes relative to 

flow regime. Given the information about population dynamics and kd values, the focus 

on dry periods seems appropriate.

Reviewer 3 I cannot comment on the life history of Corbula. The descriptions provided regarding wet 

and dry years and Kd values appear scientifically sound.

Reviewer 4 The work of Fuller et al. (2012) and Thompson and Parchaso (2010) predate the current 

CA critically dry conditions. Since the Fuller work appears based on ongoing BDMA 

assessments, incorporating the most recent data may help validate population 

relationships with the hydrograph which has experienced some extreme changes in 

recent years. With flow reductions in critically dry years, Se flux into the SFBD may also 

be reduced although existing sediment Se loading may provide Se exposure hysteresis 

and landward shift of the sedimentation zone at the salt – fresh water interface of the 

estuary. Increased salinity into the landward reaches of the SFBD may shift Corbula 

populations with shifting salinity. Lentic/lotic Se dynamics that are understood for 

freshwater systems may not translate well to marine sediment exposure pathways 

observed in the SFBD, as stated in “more selenium is available for bioaccumulation and 

more selenium accumulates in the food chain.” A quick trend analysis of the most recent 

BDMA data may be sufficient to explore this concern. 

Reviewer 5 I agree with EPA’s assessment that the available literature indicates the density and 

distribution of Corbula is constrained by low salinity (i.e., freshwater) and that the system 

exhibits higher Kds during drier years resulting in more Se bioaccumulation higher in the 

food chain. To some extent, EPA has accounted for the inter-annual variability by 

selecting the 75th percentile Kd for modeling. However, the possibility of higher Kds 

during exceptionally dry years or seasons cannot be ruled out. In the absence of any data 

in such conditions, I think all EPA can do at this point is commit to monitoring the system 

next time such conditions occur and ensuring the option of revising the site-specific WQC 

based on new data. I do not think it reasonable to speculate on how much Kds will 

change under such conditions and a priori increase the Kds in the models.

2.9 Transects: Discuss whether it is reasonable and representative to use the focused 

seaward transects to model the clam-based food webs, in order to eliminate the endmember variability at the ocean (at the Golden Gate Bridge, the entrance to San Francisco 

Bay). Does the geology of the focused area (a wide Suisun Bay flowing into a constricted 

Carquinez Strait), and the importance of the area (the location of refinery discharges), 

favor the use of focused transects to best represent the clam-based food web for the 

estuary? Please discuss.

All five reviewers provided responses, though two did not respond directly to the question. Three reviewers 

agreed with EPA’s approach of using focused seaward transects. One of them commented that using 

transects is a valuable way to explore sediment variability concerning flow and sedimentation dynamics in 

the SFBD; however, yearly hydrograph changes and critically dry periods may induce unacceptable variability 
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in any model input. Another pointed out that EPA eliminated the Kds from near Golden Gate to reduce 

variability; a better rationale for eliminating data would be the presence or absence of relevant species of 

fish and birds. 

The other two reviewers did not respond directly to the question. One of them noted that the South Bay is 

likely less well flushed than waters to the north, meaning that applying models from other areas to the 

South Bay may be risky. The other pointed out that selenium has incredible geospatial movement, so 

focusing on specific areas may not be fruitful given SFBD’s complexity.

Reviewer Comments

Reviewer 1 I don’t have an opinion on this. It might be worth noting here that the South Bay is 

probably less well flushed than waters to the north, so applying modeling from other 

areas to the South Bay may be risky.

Reviewer 2 This decision does seem reasonable in my opinion.

Reviewer 3 The life history and ecology of Corbula is outside of my expertise. Given the complexity of 

the SFBD and the movement of organisms throughout its confines indicates that focus 

upon specific areas particularly with an element with incredible geospatial movement 

may not be fruitful. 

Reviewer 4 Transects are a valuable way to explore sediment variability with regards to flow and 

sedimentation dynamics in the SFBD. Focused transects can help define food web 

variability that would result from geology and fresh-salt water estuarine sedimentation 

processes. Yearly hydrograph changes and critically dry periods may induce unacceptable 

variability in any model input, however. 

Reviewer 5 Yes, I generally agree with EPA’s decision to use the focused seaward transects to model 

the clam-based food webs. The end-member Kds clearly represent a set of conditions that 

differ from the focused area and the focused area is most relevant for exposure to both 

fish and bird species of interest. Having said that, as the document currently reads, it 

appears that EPA eliminated the Kds from near Golden Gate simply to reduce variability. I 

think a better rationale for eliminated these data would be related to the 

presence/absence of relevant receptors (i.e., fish and birds). Are there species we should 

be concerned about in the outer Bay where Kds appear to be significantly higher? Some 

discussion of this issue in the document would be useful.

Results and Application

2.10 Results: Discuss whether the model input and results scientifically support the water 

quality criteria for the three food chains represented. Are there better ways to represent 

the three food webs in the modeling? Please explain.

All five reviewers responded to this question. Three reviewers noted that the model input and results are 

scientifically sound and support the water quality criteria for the three food chains represented. One of 
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these reviewers suggested it would have been useful to find stable isotope-based studies from which end 

member analyses could have been done to provide additional insights and, potentially, validation. Another 

reiterated that the SFBD is exceptionally well-studied in regards to water flows, selenium, fish, and wildlife; 

the TTF model, dietary estimates, and selection of key species represent the current best available 

knowledge. The third reviewer pointed out that one of the three food chains will ultimately drive the water 

quality criteria. 

Two reviewers had reservations. One doubted whether it was appropriate to apply toxicological results for 

clam-eating fish as a surrogate for clam-eating birds, because birds and fish have different sensitivities and 

possibly different toxic endpoints. The other reiterated that, by neglecting the movement of spawning 

females to freshwater habitats, the model underestimates exposure to selenium due to consumption of 

mixed trophic levels during vitellogenesis.

Reviewer Comments

Reviewer 1 I have doubts whether it is appropriate to apply toxicological results for clam-eating fish 

as a surrogate for clam-eating birds. Birds and fish do not have the same sensitivity or 

necessarily the same toxic endpoints to particular toxicants. It may be more defensible to 

apply modeling results from clam-eating fish to insect-eating fish since the toxic end 

points in the fish would presumably be the same, or approximately the same. Probably 

the most relevant information that would be needed would be to have Se concentration 

in specific tissues following various dietary routes and a toxic response to Se 

concentrations in that tissue. Such data would be quite difficult to obtain, even in 

controlled experiments, however.

Reviewer 2 The model input and use of the three food webs do seem scientifically sound. The studies 

drawn upon relative to the trophic ecology of the key species are solid. I do wonder if 

there were any stable isotope-based studies from which end member analyses might 

have been done. Such studies might have provided additional insights and perhaps a level 

of validation.

Reviewer 3 This has been addressed above. In summary, neglecting the movement of spawning 

females to freshwater habitats may under-estimate exposure to Se due to consumption 

of mixed trophic levels during vitellogenesis. Examples would include forage fish species 

as dietary items; thus, adding another TTF. 

Reviewer 4 The review document model input and results support the water quality criteria for the 

three food chains represented. The models and model inputs used appear to be the best 

available approach using peer-reviewed data and approaches. The SFBD is an 

exceptionally well-studied aquatic ecosystem with regards to water flows, Se, fish and 

wildlife. The TTF model, the dietary estimates, and the selection of key species all 

represent our best available knowledge. Clam eating fish represent a worse case 

exposure scenario due to the Se sediment dynamics in the SFBD and the migratory habits 

and thus lessened exposure scenario of aquatic birds in this ecosystem. In this regard, it is 

scientifically supportable to use the modeling results for the clam-eating fish as a 

surrogate to protect clam-eating birds and insect-eating fish, in addition to the clameating fish themselves. Other than applying safety factors to account for species 
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differences in exposure and Se sensitivity (not recommended) there are no other 

common approaches that can be applied across species.

Reviewer 5 Yes, the proposed approach, model input and results all appear scientifically defensible 

for establishing water quality criteria protective of all three food chains. It was 

appropriate for EPA to evaluate all three chains, but ultimately one of these food chains 

will drive the WQC. It is not surprising that clam-eating fish are the driver and it is 

scientifically defensible to use WQC derived from this model to protect all species in the 

system. 

Discuss whether it is scientifically supportable to use the modeling results for the clameating fish as a surrogate to protect clam-eating birds and insect-eating fish (all three 

food webs) in addition to the clam-eating fish themselves. Discuss whether there are 

other scientifically defensible ways to use the modeling results to protect all the 

categories of species for the SFBD.

All five reviewers responded to this question. Two reviewers reiterated their earlier comments (in response 

to 2.10) that the models inputs and results are scientifically valid. 

The other three reviewers had reservations about using the modeling results. One reviewer also referenced 

his response to 2.10, in which he doubted whether it is appropriate to apply toxicological results for clameating fish as a surrogate for clam-eating birds, because birds and fish have different sensitivities and 

potentially different toxic endpoints. 

Another reviewer commented that the general approach of using clam-eating fish as a surrogate to protect 

clam-eating birds and insect-eating fish is not appropriate, but that in this particular instance involving the 

SFBD area, the protective value derived for clam-eating fish would also be protective of fish-eating birds and 

insect-eating fish. 

The fifth reviewer emphasized that the model should be revised to include “inbound” adult salmonids as a 

food-wed category due to the continued exposure to developing ova. He also suggested that EPA consider 

adding uncertainty factors for salinity and selenium interactions in species in which maternally exposed 

embryos are discharged in salt or brackish water. 

Reviewer Comments

Reviewer 1 See response provided under 2.10.

Reviewer 2 Using clam-eating fish as a surrogate to protect clam-eating birds and insect-eating fish 

does not seem the best general approach. However, in this particular instance involving 

the SFBD area, the calculations demonstrated (see quoted materials in the answer to 

Question 12 below) that the protective value derived for clam-eating fish would also be 

protective of the fish-eating birds and insect-eating fish.

Reviewer 3 As mentioned above, revision or modification of the model to include “inbound” adult 

salmonids as a food-web category is essential. Salmonids are clearly insect-eating going 

out to sea, but are fish-eating when returning. The rationale for this is the continued 

exposure to developing ova, which upon fertilization will be embryonic. In addition, the 
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addition of uncertainty factors for salinity and Se interactions may also be warranted in 

species were maternally exposed embryos are discharged in salt or brackish water 

environments. 

Reviewer 4 See response provided under 2.10.

Reviewer 5 See response provided under 2.10.

2.11 Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and 

validated for SFBD, and a considerable amount of site-specific data are available and are 

used to model the estuary. EPA is using the model results to inform the criteria 

development process, and presents draft multi-media criteria in Chapter 4. These criteria 

are a whole body or muscle fish tissue value and a clam tissue value that directly reflect 

selenium levels that are considered to be protective of aquatic biota; and corresponding 

chronic and intermittent water column values, as well as a particulate concentration 

value to facilitate regulatory controls of selenium sources. 

Given the relevance and detail of the USGS model and the abundance of site-specific data, 

discuss whether the criteria in the different media are scientifically defensible and 

protective for this aquatic ecosystem. 

All five reviewers responded to this question. Four reviewers said that the USGS model provides a defensible 

approach for generating protective criteria. Three reviewers commended the abundance of data available 

for the SFBD. One reviewer noted that the USGS Ecosystem-Scale Selenium model is the gold standard for 

modeling, balancing interpolation and extrapolation with required knowledge of species-specific and 

ecosystem-specific dynamics and values. 

The fifth reviewer held a different view. He described the criteria as “an excellent first step in setting sitespecific values,” but pointed out that the criteria are focused on Corbula-consuming predators, assuming 

these are the most sensitive species to selenium toxicity. Consequently, in his opinion, these criteria are 

more appropriately site- and species-specific criteria for sturgeon, and may not be protective of “in-bound” 

salmonid offspring or broadcast spawning species in brackish or saltwater environments.

Reviewer Comments

Reviewer 1 I think that the USGS model provides a defensible approach for generating protective 

criteria for wildlife exposed to Se in SF Bay. It seems to me that the same processes that 

operate in SF Bay would occur in other regions, so the approach taken here should, in 

principle, be applicable to other water bodies as well. However, the SF Bay system is 

unusually well studied with respect to Se, and it is doubtful that these kinds of detailed 

data for Se in other waters would be available. 

Reviewer 2 This seems to be an unusual situation in which a remarkable amount of site-specific 

information exists and appropriate synthesis of that information has already been done. 

That greatly enhances the development of the WQC draft document. The area-specific 

data and its use to produce predictive models is definitely defensible, and from my 

reading of the documents, is protective.
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Reviewer 3 The criteria are certainly an excellent first step in setting site-specific values. However, 

the criteria seem to be focused exclusively on Corbula consuming predators with the 

assumption that they are the most sensitive species with regard to Se toxicity.

Consequently, this criteria is more appropriately a site and species-specific criteria 

primarily toward sturgeon. While numerically the values may protect ocean-bound 

salmonids, it is unclear whether “in-bound” offspring would experience the same 

protection. Likewise, it is unclear the value would protect broadcast spawning species in 

brackish or saltwater environments. 

Reviewer 4 In our current understanding of Se impacts and effects, the USGS Ecosystem-Scale 

Selenium model is the gold standard for modeling, balancing interpolation/extrapolation 

with required knowledge of species specific and ecosystem specific dynamics and values. 

As presented in this review document, the criteria in the different media are scientifically 

defensible and protective for this aquatic ecosystem.

Reviewer 5 Yes, given the abundance of both site-specific and detail of the USGS model, the 

proposed criteria in different media are both scientifically defensible and protective of 

the SFB aquatic ecosystem. I have been involved with several dozen site-specific water 

quality criteria/standard projects and the amount of data and model development 

involved in deriving these site-specific criteria far exceed (by an order of magnitude in 

most cases) the level of information typically available in these efforts.

2.12 Translator (Permits): EPA regulations generally call for NPDES permit limits to be 

expressed in terms of total recoverable values (40 CFR 122.45(c)). The model applied in 

this document yields calculations of water column values in the dissolved form. For most 

metals, EPA recommends dissolved values for water quality criteria, because the 

dissolved form better represents the bioavailable fraction in the water column to aquatic 

biota. However, for selenium, aquatic biota can take in the dissolved and particulate 

fractions in the water column, which are then moved into and up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to 

determine a total recoverable permit limit; the translator is the fraction of the total 

recoverable metal in the downstream water (downstream of a discharge) that is 

dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate 

method to derive a translator? Explain your answer.

All five reviewers responded to this question. Two reviewers agreed with EPA’s approach of setting the 

translator at 1.0. One reviewer endorsed this approach because, he noted, it is clear that both dissolved and 

particulate selenium are bioavailable and toxic to aquatic organisms. The other reviewer supported EPA’s 

selection of a translator of 1 because it equates total recoverable concentrations with notionally 

bioavialable selenium concentrations; this is appropriate for selenium because total recoverable selenium is 

likely the most reflective of the selenium available to the filter feeders such as Corbula.
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The other three reviewers expressed different opinions. One of them noted that not all of the total 

recoverable selenium can be considered bioavailable and suggested that Luoma et al. (1992)3 has the most 

direct answer to the selenium translator challenge. Another disagreed with the assertion that “the dissolved 

form better represents the bioavailable fraction in the water column to aquatic biota.” For most metals, he 

noted, it is the dietary source that tends to predominate as a source of metals for aquatic animals—this is 

particularly the case in saline situations such as San Francisco Bay.

The fifth reviewer pointed out that selenium can exist in many forms, with the inorganic forms (selenite and 

selenate) predominating in most discharges; it is difficult to determine a translator from particulate to 

dissolved forms when the most toxic form is likely organic. Without the ability to measure the most toxic 

selenium species and understand the kinetics of transformation to organic forms, tremendous uncertainty 

exists in setting a translator value. Consequently, additional modeling is necessary to estimate the 

transformation rates of inorganic to organic (particulate) in a site-specific manner in which nutrient input 

and hydrology will likely play significant roles.

Reviewer Comments

Reviewer 1 I would disagree with the assertion that “the dissolved form better represents the 

bioavailable fraction in the water column to aquatic biota.” For most metals, it is in fact, 

the dietary source that tends to predominate as a source of metals for aquatic animals—

this is particularly the case in saline situations such as SF Bay.

Reviewer 2 If I understand the question, it refers to the derivation of a protective value in terms of 

dissolved selenium. What seems to be the pertinent materials from two parts of the draft 

report are pasted below. 

“Dissolved Selenium: To protect clam-eating fish at an EC10 level, the predicted 

protective dissolved selenium estuarine water column concentration is between 0.137 and 

0.685 μg/L, or between approximately 0.1 and 0.7 μg/L, based on available data. Using 

the mean value of the 75th percentile Kds (3,966), the predicted protective dissolved 

selenium estuarine water column concentration is 0.244 μg/L, or approximately 0.2 μg/L. 

Based on these analyses, the protective dissolved selenium water column concentration 

selected is 0.2 μg/L. This value should protect existing species in the estuary most of the 

time, under the environmental conditions used in the modeling.

The modeling for clam-eating birds and insect-eating fish results in predicted protective 

dissolved selenium water column concentrations between 0.126 and 2.046 μg/L, or 

between approximately 0.1 and 2 μg/L. The lower end of this range (0.1 μg/L) is the same 

as the lower end of the range of protective values for clam-eating fish (0.1 μg/L). Using 

the mean value of the 75th percentile Kds, (3,966), the predicted protective dissolved 

selenium water column concentration is 0.224 μg/L for scoter and scaup, or 

approximately 0.2 μg/L, the same predicted value for clam-eating fish. These analyses 

indicate that if clam-eating fish are protected, clam-eating birds and insect-eating fish 

should also be protected.

Particulate Selenium: To protect clam-eating fish at an EC10 level, the predicted 

 3 Luoma et al. 1992. Determination of selenium bioavailability to a benthic bivalve from particulate and solute 

pathways. Environ. Sci. Technol. 26 (3) pp. 485–491.
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protective particulate selenium concentration is 0.969 μg/g dw, or approximately 1 μg/g 

dw, based on available data. The modeling for clam-eating birds and insect-eating fish 

results in predicted protective particulate selenium concentrations between 0.890 and 

2.894 μg/g dw, or between approximately 1 and 3 μg/g dw. The lower end of this range is 

approximately equal to the protective value for clam-eating fish; if clam-eating fish are 

protected, clam-eating birds and insect-eating fish should also be protected.

Clam Tissue: To protect clam-eating fish at an EC10 level, the predicted protective 

selenium invertebrate (prey) tissue concentration is 7.4 μg/g dw; the predicted protective 

selenium Corbula tissue concentration is 6.6 μg/g dw, or approximately 7 μg/g dw, based 

on available data. The modeling for clam-eating birds results in predicted protective 

Corbula selenium concentrations between 6.8 and 9.9 μg/g dw, or between 

approximately 7 and 10 μg/g dw. The lower end of this range is approximately equal to 

the predicted protective value for clam-eating fish (6.8 vs 6.6 μg/g dw); if clam-eating fish 

are protected, clam-eating birds should also be protected.

All predicted values have been rounded to one significant digit. The rounded values 

provide a reasonable prediction of the appropriate protective values of allowable 

dissolved water column selenium, particulate selenium, and prey tissue selenium 

concentrations that will protect clam-eating fish and birds, and insect-eating fish in the 

estuary. 

And the sentences below are most relevant to this question.

Form of Permit Limits: Although draft water column criteria values are expressed in 

dissolved selenium concentrations, water quality-based permit limits, in most instances, 

are required to be in total recoverable values (40 CFR §122.45(c)). For most metals, 

criteria are in dissolved values because the dissolved fraction provides a better 

representation of the biologically active portion of the metal than does the total or total 

recoverable fraction (USEPA, 1996). When water quality criteria are in the dissolved form, 

a translator can be used to determine a total recoverable permit limit. The translator is 

the fraction of the total recoverable metal in the downstream water that is dissolved 

(ibid). However, for selenium, both the dissolved and particulate fractions are bioavailable 

to the aquatic biota. Thus, we used a translator of 1 in developing the draft criteria, an 

appropriate assumption for the Bay and Delta.

Answer

The dissolved selenium is much less relevant than bioavailable selenium in particulates so 

urging for a focus on dissolved Se would not be as helpful as for other elements and back 

calculation from tissue concentrations to total recoverable metals in lentic and lotic 

waters seems preferable. Of all of the conventional water concentrations measured, total 

recoverable selenium is likely the most reflective of the selenium available to the filter 

feeders such as Corbula. The selection of a translator of 1 equates total recoverable 

concentrations with notionally bioavailable selenium concentrations. In the case of 

selenium, that is a sound and appropriate conservative approach. The variability in total, 

recoverable, and dissolved selenium remains problematic regardless of the approach. 

Reviewer 3 Se can exist in dissolved states in many forms with the inorganic forms (selenite and 

selenite) predominating most discharges. Thus, it is difficult to determine a translator 
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from particulate to dissolved forms when the most toxic form of Se is likely organic.

Without the ability to measure the most toxic species of Se (SeMe or SeCys, or other 

organic forms) and understand the kinetics of transformation to organic forms, 

tremendous uncertainty exists in setting a translator value. Consequently, additional 

modeling is necessary to estimate the transformation rates of inorganic to organic 

(particulate) in a site-specific manner which nutrient input and hydrology will likely play 

significant roles. 

Reviewer 4 As with lead (Pb) exposure in birds and fish, not all of the total recoverable Se can be 

considered bioavailable (e.g. Zia et al., In vitro and in vivo approaches for the 

measurement of oral bioavailability of lead (Pb) in contaminated soils: A review. 

Environmental Pollution. Volume 159, Issue 10, October 2011, Pages 2320–2327). Luoma 

et al 1992 (Determination of selenium bioavailability to a benthic bivalve from particulate 

and solute pathways. Environ. Sci. Technol., 1992, 26 (3), pp 485–491) has perhaps the 

most direct answer to the Se translator challenge.

Reviewer 5 I agree with EPA’s approach of setting the translator at 1.0 for Se. For divalent metals, the 

dissolved fraction is clearly the dominant bioavailable metal fraction with respect to 

toxicity. While there is considerable evidence that particulate divalent metals are can be 

bioaccumulated, their toxic potency appears to be considerably less than the dissolved 

form in most cases. In contrast, for Se, it is clear that both dissolved and particulate Se 

are bioavailable and toxic to aquatic organisms. Consequently, it is appropriate to use a 

translator of 1.0 for Se.
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2.13 Other Comments Provided

Reviewer Comments

Reviewer 5 Section 1, Page 11, Last Paragraph: There are numerous papers and reports indicating Se 

is the most dose-sensitive nutrient, but I have yet to see a quantitative analysis 

supporting this statement. Several other metals also have very narrow ranges between 

essentiality and toxicity that are comparable to Se. For example, Cu (for snails) and Zn 

(for daphnids) both have very narrow ranges between essential requirements and 

toxicity. Unless EPA is aware of a quantitative analysis supporting this statement I suggest 

deleting it.

Section 1, Page 13, First Paragraph: “ppt” stands for parts per thousand, not trillion in this 

instance.

Section 2, Page 8, First Paragraph: Risk is an integration of exposure and effect (i.e., 

sensitivity). Just because clam-eating species have the highest exposure doesn’t

necessarily mean they are most at risk. It depends on their sensitivity as well.

Section 4, Page 11, First Paragraph: It is unclear why bioaccumulative based effects are of 

greater concern that other effects. Why would this type of effect justify a lower effect 

threshold? Regardless of the mechanism, the EC10 represents a 10% effect.

3.0 NEW INFORMATION PROVIDED BY REVIEWERS

This section compiles all new information that reviewers provided in their individual comments that were 

separate from responses to the charge questions in Section 2.0.

Reviewer Comments

Reviewer 3 References

Beckon, WN. Cary Parkins, C., Maximovich, A. and Beckon, AV . (2008). A General 

Approach to Modeling Biphasic Relationships. Environ. Sci. Technol. 42, 1308-1314.

Eaton, DL and Gilbert SG (2013) Principles of Toxicology IN: Casarett and Doull’s 

Toxicology : The Basic Science of Poisons. (C. Klaassen Editor) McGraw Hill Publishers, 

New York, NY. pp13-48.

Fan, XA, Klein, M, Flanagan-Steet, HR, Steet, R. (2010). Selective Yolk Deposition and 

Mannose Phosphorylation of Lysosomal Glycosidases in Zebrafish Journal of Biological 

Chemistry 285:32946-32953. 

Janz, D.M., D.K. DeForest, M.L. Brooks, P.M. Chapman, G. Gilron, D. Hoff, W.A. Hopkins, 

D.O. McIntyre, C.A. Mebane, V.P. Palace, J.P. Skorupa and M.Wayland. (2010). 

Selenium Toxicity to Aquatic Organisms in: Chapman PM, Adams WJ, Brooks ML, 

Delos CG, Luoma SN, Maher WA, Ohlendorf HM, Presser TS, Shaw DP (eds). Ecological 
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Assessment of Selenium in the Aquatic Environment. SETAC Press, Pensacola, FL, USA.

Linares-Casenave, J. Linville, R. Van Eenennaam, JP, Muguet,JB and Doroshovy, SI. (2015). 

Selenium tissue burden compartmentalization in resident white sturgeon (Acipenser 

transmontanus) of the San Francisco bay delta estuary Environmental Toxicology and 

Chemistry, Vol. 34, No. 1, pp. 152–160, 2015

Lavado, R., Shi, D. and Schlenk, D. (2012). Effects of salinity on the toxicity and 

biotransformation of l-selenomethionine in Japanese medaka (Oryzias latipes) 

embryos: Mechanisms of oxidative stress. Aquatic Toxicology108:18-22.

Linville RG. 2006. Effects of selenium on health and reproduction of white sturgeon 

Acipenser transmontanus): Implications for San Francisco Bay-Delta. PhD thesis. 

University of California, Davis, CA, USA.

Kupsco, A. and Schlenk D. (2014). Mechanisms of Selenomethionine Developmental 

Toxicity and the Impacts of Combined Hypersaline Conditions on Japanese Medaka 

(Oryzias latipes). Environ. Sci. Technol. 2014, 48, 7062−7068.

Rigby, MC., Lemly, AD, Gerads, R.(2014). Fish toxicity testing with selenomethionine 

spiked feed – what's the real question being asked? Environ. Sci. Processes Impacts, 

2014, 16, 511–517

Thomas, JK and Janz DM (2014) In ovo exposure to selenomethionine via maternal 

transfer increases developmental toxicities and impairs swim performance in 

F1generation zebrafish (Danio rerio) Aquatic Toxicology 152:20-29.

USEPA, 2014: U.S. Environmental Protection Agency. 2014. External Review Draft, Aquatic 

Life Ambient Water Quality Criteria for Selenium – Freshwater 2014. Office of Water, 

EPA 822-P-14-001. Washington, D.C.

Reviewer 5 References 

Heinz, G. H., G. W. Pendelton, A. J. Krynitsky and L. G. Gold (1990). "Selenium 

accumulation and elimination in mallards." Arch. Environ. Contam. Toxicol. 19: 374-

379.

Henny, C. J., D. D. Rudis, T. J. Roffe and E. Robinson-Wilson (1995). "Contaminants and 

sea ducks in Alaska and the circumpolar region." Environ. Hlth. Per. 103(4): 41-49.

Hilton, J. W., P. V. Hodson and S. J. Slinger (1980). "The requirement and toxicity of 

selenium in rainbow trout (Salmo gairdneri)." J. Nutr. 110(12): 2527-2535.

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APPENDIX A

CHARGE TO REVIEWERS

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Charge to External Peer Reviewers

Contract No. EP-C-13-009

Task Order 2014-19

January 2015

External Peer Review of Draft Aquatic Life and Aquatic-Dependent Wildlife Selenium Water 

Quality Criteria for the San Francisco Bay and Delta (2014)

BACKGROUND

The United States Environmental Protection Agency (EPA), Office of Water is charged with protecting 

ecological integrity and human health from adverse anthropogenic, water-mediated effects, under the 

purview of the Clean Water Act and Safe Drinking Water Act. At this time, the Agency is revising its Clean 

Water Act (CWA) section 304(a) recommended aquatic life water quality criterion for selenium to reflect the 

latest science. States may adopt EPA’s recommended criteria, use it to develop site specific criteria, or 

develop their own scientifically defensible criteria. Once adopted and approved, a State’s Water Quality 

Standards (WQS) are then used to determine the level of pollution control necessary to attain the aquatic 

life designated use in a waterbody and to assess whether the waterbodies are impaired.

EPA Region 9, assisted by EPA HQ, has developed the Draft Aquatic Life and Aquatic-Dependent Wildlife 

Selenium Water Quality Criteria for the San Francisco Bay and Delta (2014), and is seeking independent, 

external peer review on the draft criteria. The external peer review draft selenium criterion is intended to 

protect the aquatic communities of the San Francisco Bay Delta generally, but also focuses on aquaticdependent waterfowl, consistent with current science and the available data for the Bay and Delta Region. 

The document is not intended to address concerns about selenium toxicity to human health, nor to 

selenium contamination in other areas outside the Bay and Delta; therefore, comments on human health or 

selenium-related aquatic issues (unless clearly relatable) are beyond the scope of this peer review.

In addition, recently, the USEPA released a national external peer review draft of a selenium chronic 

criterion for the protection of freshwater aquatic life. This document will be provided as supporting 

information, but is undergoing its own peer review process, so the national external peer review draft 

criterion is also beyond the scope of this review.

Although selenium may cause acute toxicity, the most deleterious effect on aquatic organisms and aquatic 

dependent wildlife is due to its bioaccumulative properties. Organisms in aquatic environments exposed to 

selenium accumulate it primarily through their diet, and not directly through exposure to water (Chapman 

et al. 2010). Selenium toxicity occurs primarily through transfer to the eggs and subsequent reproductive 

effects. Consequently, in harmony with the recommendations of expert panels (U.S. EPA 1998, Chapman et 

al. 2010) and with peer review and public comments on the U.S. EPA (2004) draft criterion, the Agency 

recently developed a national chronic criterion reflective of the effects of selenium concentrations in the 

reproductive tissues of fish species.

EPA developed draft selenium freshwater aquatic life fish tissue guidance values based on the empirically 

measured relationship between reproductive effects and fish tissue values from dietary exposure from a 

range of field and laboratory studies (USEPA, 2014). Using a simplified set of equations derived from the U.S. 
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Geological Survey (USGS) Ecosystem-Scale Selenium Model1 (Presser and Luoma, 2010), EPA calculated 

water column values associated with the fish tissue values, for lentic and lotic systems. Since the most 

significant exposure pathway of selenium to species of concern in the San Francisco Bay and Delta is through 

diet, the current applicable freshwater criteria (5 ug/L) may not adequately protect species in the estuary. In 

particular, in the San Francisco Bay and Delta, the clam-based food webs dominant in the Bay and brackish 

Delta accumulate selenium at a much higher rate than insect-based food webs. Potamocorbula amurensis 

(P. amurensis)2 or Corbula, an introduced clam species, can now be found throughout the San Francisco 

estuary. Corbula are abundant in San Pablo Bay, and Suisun Bay and Marsh, and accumulate selenium at 

roughly three times the rate of supplanted native clam species (Cohen, 2011). The amount of seleniumladen food in an organism’s diet will affect its level of exposure. In San Francisco Bay, predator species that 

prefer to feed on Corbula, such as diving birds and sturgeon, are exposed to higher levels of bioaccumulated 

and bioconcentrated3 selenium than species that feed mainly on insects or higher order species within an 

insect-based food web.

Where national criteria guidance values may not be sufficiently protective, other scientifically defensible 

methods may be used to develop water quality criteria (40 CFR §131.11). In this document, site-specific 

selenium water quality criteria protective of aquatic life and aquatic-dependent wildlife for the San 

Francisco Bay and Delta are developed to address bioaccumulation through dietary exposure to protect the 

estuary’s diverse living resources. The criteria are intended to apply to all waters of the San Francisco Bay 

and Delta. These waters are located within the jurisdictions of the San Francisco Bay Regional Water Quality 

Control Board (SFRWQCB) and the Central Valley Regional Water Quality Control Board (CVRWQCB) of the 

State of California. 

MATERIAL PROVIDED BY EPA FOR THE PEER REVIEW

The following materials are available on a secure ERG FTP project site and include: 

• Peer Review Document: External Peer Review Draft Aquatic Life and Aquatic-Dependent Wildlife 

Selenium Water Quality Criteria for the San Francisco Bay and Delta (2014) and Appendices 

• Supporting Materials (provided for reference during the review)

(a) External Peer Review Draft Aquatic Life Ambient Water Quality Criterion for Selenium –

Freshwater 2014: Main text. The external peer review draft selenium criterion document 

includes toxicity test results and analyses used to derive an egg-ovary criterion element, a 

whole-body and muscle tissue criterion element, and two water-column elements; and 

 1 The model is a predictive tool to delineate the fate and biological uptake of selenium in an aquatic ecosystem, and is 

discussed in detail in the document to be reviewed. 

2 Also commonly known has Corbula amurensis (C. amurensis). The species correct taxonomic name is Potamocorbula 

amurensis (P. amurensis). See: Huber, M. (2014). Potamocorbula amurensis (Schrenck, 1861). Accessed through: World 

Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=397175 on 2014-09-17. 

3 Bioconcentration is the process by which an organism accumulates a chemical in its tissue from direct exposure to its 

surroundings (non-dietary exposure). 
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(b) External Peer Review Draft Aquatic Life Ambient Water Quality Criterion for Selenium –

Freshwater 2014: Appendices A-K. The appendices contain supporting information, including the 

results of several selenium toxicity tests used to derive the tissue-based criterion element and 

EPA’s interpretation of the test results, and methods, data and analyses used to derive watercolumn elements.

Note: Sections that are specifically relevant to the development of the SSC for the San Francisco 

Bay Delta are outlined in the charge questions.

(c) U.S. Geological Survey, by Theresa Presser and Sam Luoma. 2010. Ecosystem-Scale Selenium 

Modeling in Support of Fish and Wildlife Criteria Development for the San Francisco Bay-Delta 

Estuary, California Administrative Report. Menlo Park, California

Note: This is included as Appendix A of the peer review document.

(d) Theresa S. Presser and Samual N. Luoma. 2010. A Methodology of Ecosystem-Scale Modeling of 

Selenium. Integrated Environmental Assessment and Management, v.6, no. 4. p. 685-710 (plus 

Supporting Information).

Note: This is included as Appendix B in Appendix A of the peer review document.

• Supporting Reference Material: EPA is also providing supplemental information related to Corbula 

amerensis, and a key fish species of concern in the Bay Delta, the white and green sturgeon 

(Acipencer transmontanous and A. medirostrus). This information was influential in development of 

the water column, particulate and clam tissue values by informing the analysis concerning the 

percentage of Corbula in diet for sturgeon (and thus the TTF value used in the model).

(a) Nicola De Riu, Jang-Won Lee, Susie S.Y. Huang, Giuseppe Moniello, Silas S.O. Hung. 2014. Effect 

of dietary selenomethionine on growth performance, tissue burden, and histopathology in 

green and white sturgeon. Aquatic Toxicology 148, 65–73

(b) Steven C. Zeug, Annie Brodsky, Nina Kogut, A. Robin Stewart, Joseph E. Merz. 2014 Ancient fish 

and recent invaders: white sturgeon (Acipenser transmontanus) diet response to invasive 

species-mediated changes in a benthic prey assemblage. Marine Ecology Progress Series. DOI: 

110.3354/meps11002. Accepted for publication.

(c) Janet K. Thompson and Francis Parchaso, Corbula amurensis Conceptual Model. US Geological 

Survey, 2010

(d) Heather Fuller, Karen Gehrts, Dan Riordan, and Jan Thompson. 2012. The biomass of Corbula 

and Corbicula in the low salinity zone, August 2011 Poster presented at Interagency Ecological 

Program 2012 Annual Workshop (April 18-20) 

CHARGE QUESTIONS

Overall Document Organization and Structure

1. Please comment on the overall construction and clarity of the document, i.e., is the layout of the 

document logical and is the document clearly written? Are there better ways to arrange and/or 

present the information? If so, please explain and describe.

Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco Bay and 

Delta (SFBD) varies from fresh to brackish to marine, and many of the species (or close relatives of 
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the species) evaluated for the draft national freshwater criterion (USEPA, 2014 pp 57-62; 78-79 

provided as supporting material), live in the estuary. Given these considerations, discuss whether it 

is reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic 

life in the SFBD. Are there better ways to estimate toxicity benchmark values for key aquatic life 

species, many of which are anadromous, in the estuary? Please explain.

3. Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is most likely 

the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), using green sturgeon4

observed selenium concentrations of 6 – 7 mg/kg dw (whole body) in the control treatment, which 

appears to reflect essentially a ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of 

this study (and any other relevant data on green sturgeon), is the draft national benchmark value of 

8.1 mg/kg dw protective of the juvenile growth endpoint for green sturgeon? Please explain your 

answer.

4. Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values from 

environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 and Brian and 

Cousens, 1989, cited in review document) may be a better model to represent effects of selenium, 

since a biphasic model shows a more representative progression of effect to living species (selenium 

is a micronutrient, but is toxic in both deficit concentrations and excessive concentrations). Studies 

on selenium effects in the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, 

cited in review document) do not appear to consider this issue, and study designs do not appear to 

take the issue into account. 

a. Given that many of the studies in the literature often use a small (or zero) control concentration 

of selenium, and a low level of effect is observed (reflective of a deficient concentration), is the 

response to increasing levels of selenium (into the excessive toxic amounts) better represented 

using a biphasic model or log-logistic model? Would the slope of the regression be skewed in 

either model, due to incomplete modeling of deficit effects?

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to determine 

EC10 values for studies in the literature on representative or key species for the SFBD (Heinz et 

al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review document), or whether 

the results of using a standard log-logistic model would produce more representative results. 

5. Species and Diet: Discuss whether the appropriate key species and food chains (webs) have been 

identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food webs are 

reasonable based on the literature (sources cited in review document) and whether the 

calculations of the Trophic Transfer Factors (TTFs) are scientifically supportable based on those 

diets (see specific questions below). Are there better ways to calculate TTFs for mixed diets for 

the key species? Please explain. 

 4 Green sturgeon larvae were obtained from the offspring of three males and one female originating from the Klamath 

River along the Pacific Coast. 
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Specifically for Clam-eating Fish:

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile white and 

green sturgeon, and splittail scientifically supportable based on the available literature (Zeug et 

al., 2014, provided; USFWS, 2008 pp. 51-59, provided as supporting material; and Feyrer et al., 

2003, cited in review document)? Is it scientifically supportable to postulate that splittail do not 

consume as much Corbula, and are therefore not as much at-risk as sturgeon?

c. Recent literature indicates that green sturgeon are more sensitive to selenium than white

sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in review document). 

Studies have also shown that adult green sturgeon primarily use the SFBD as a migratory 

corridor to freshwaters for spawning (http://exxexpartnership.com/wpcontent/uploads/2010/10/GreenSturgeon-_1_.pdf, cited in review document). If it is known that 

egg maturation in adult green sturgeon is completed prior to entering the estuary, discuss 

whether it is scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and discuss the 

potential for selenium exposure of adult green sturgeon during migration to play a role in 

reproductive toxicity, given the current state of knowledge on the reproductive biology and life 

history of green sturgeon.

Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are 

scientifically defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that California 

Ridgeway’s rails do not consume a large percentage of Corbula, given that their littoral feeding 

habitat does not overlap with the preferred habitat of Corbula. 

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup live 

mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) where Corbula

are prevalent, and consume mostly Corbula. However, in the latter part of the winter and early 

spring prior to migration, they move down to the deeper, Central Bay area where Corbula are 

not as available, and the scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and continue to feed on 

herring prior to breeding and nesting. It appears that selenium moves directly from the diet of 

water birds into their eggs and not from internal tissue stores (Chapman et al., 2010). Given that 

scoter and scaup switch to a low selenium diet for a period prior to migration as well as during 

migration, and that selenium may be depurated at a higher rate during migration, and that 

scoter and scaup are likely to continue to feed on low selenium fish sources in the breeding and 

nesting grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet 

by 1⁄2 (from 90% to 45%), with respect to selenium exposure that could impact reproductive 

endpoints? Please discuss.
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Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the model 

scientifically supportable based on the literature (sources cited in review document)? Please 

explain.

6. Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the literature for 

individual species based on their food prey are scientifically supportable, i.e., are the TTF factors in 

Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 – 24; Presser and Luoma, 2010, Table 

3; and USEPA, 2014, pp. 71-77, each provided as supporting material) scientifically supportable and 

appropriate? Are you aware of any other or more applicable data?

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the available 

data? Are there additional data that the Agency should consider for developing a site-specific TTF 

for Corbula?

7. Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation into the 

food chain during dry/low flow (lentic) conditions, which is represented by higher Kd values. Is it 

scientifically supportable to use the 75th percentile of the Kd values for each of the landward and 

seaward transects in the modeling, since data representing critical (very dry) conditions are not 

available? Would the use of median or mean Kd values be more appropriate? Please explain your 

answer.

8. Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and Parchaso (2010) 

(both provided) indicate that populations and population densities of Corbula appear to decline in 

wet years, and increase in dry years. The articles suggest that Corbula may be sensitive to flow into 

the estuary and/or the salinity of the estuary (and other factors). The model shows that when the 

system has higher Kd values (representative of dryer years), more selenium is available for 

bioaccumulation and more selenium accumulates in the food chain (which would drive the draft 

criteria to protect the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

9. Transects: Discuss whether it is reasonable and representative to use the focused seaward transects 

to model the clam-based food webs, in order to eliminate the end-member variability at the ocean 

(at the Golden Gate Bridge, the entrance to San Francisco Bay). Does the geology of the focused 

area (a wide Suisun Bay flowing into a constricted Carquinez Strait), and the importance of the area 

(the location of refinery discharges), favor the use of focused transects to best represent the clambased food web for the estuary? Please discuss.

Results and Application

10. Results: Discuss whether the model input and results scientifically support the water quality criteria 

for the three food chains represented. Are there better ways to represent the three food webs in 

the modeling? Please explain.

Discuss whether it is scientifically supportable to use the modeling results for the clam-eating fish as 

a surrogate to protect clam-eating birds and insect-eating fish (all three food webs) in addition to 
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the clam-eating fish themselves. Discuss whether there are other scientifically defensible ways to 

use the modeling results to protect all the categories of species for the SFBD.

11. Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and validated 

for SFBD, and a considerable amount of site-specific data are available and are used to model the 

estuary. EPA is using the model results to inform the criteria development process, and presents 

draft multi-media criteria in Chapter 4. These criteria are a whole body or muscle fish tissue value 

and a clam tissue value that directly reflect selenium levels that are considered to be protective of 

aquatic biota; and corresponding chronic and intermittent water column values, as well as a 

particulate concentration value to facilitate regulatory controls of selenium sources. 

Given the relevance and detail of the USGS model and the abundance of site-specific data, discuss 

whether the criteria in the different media are scientifically defensible and protective for this 

aquatic ecosystem. 

12. Translator (Permits): EPA regulations generally call for NPDES permit limits to be expressed in terms 

of total recoverable values (40 CFR 122.45(c)). The model applied in this document yields 

calculations of water column values in the dissolved form. For most metals, EPA recommends 

dissolved values for water quality criteria, because the dissolved form better represents the 

bioavailable fraction in the water column to aquatic biota. However, for selenium, aquatic biota can 

take in the dissolved and particulate fractions in the water column, which are then moved into and 

up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to determine a 

total recoverable permit limit; the translator is the fraction of the total recoverable metal in the 

downstream water (downstream of a discharge) that is dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate method to 

derive a translator? Explain your answer.

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APPENDIX B

INDIVIDUAL REVIEWER COMMENTS

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COMMENTS SUBMITTED BY

Reviewer 1

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Overall Document Organization and Structure

1. Please comment on the overall construction and clarity of the document, i.e., is the layout of the 

document logical and is the document clearly written? Are there better ways to arrange and/or 

present the information? If so, please explain and describe.

I found the writing to be quite good—that is the layout of logical argument and the clarity of the statements 

made. Considering the many other documents I have seen, this document is much better written than many 

others.

Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco Bay and Delta 

(SFBD) varies from fresh to brackish to marine, and many of the species (or close relatives of the 

species) evaluated for the draft national freshwater criterion (USEPA, 2014 pp 57-62; 78-79 provided 

as supporting material), live in the estuary. Given these considerations, discuss whether it is 

reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic life in 

the SFBD. Are there better ways to estimate toxicity benchmark values for key aquatic life species, 

many of which are anadromous, in the estuary? Please explain.

It is not appropriate to apply Se bioaccumulation data from freshwater studies to estuarine or saline waters. 

This is because sulfate is abundant in saline waters but not in fresh water environments. Thus, selenate, 

which comes into phytoplankton cells, at the base of the food chain, is not appreciably taken up by cells in 

seawater where the sulfate concentration exceeds that of selenate by 10 million times (approx.). Selenite is 

the main species of inorganic selenium that is acquired by marine phytoplankton, which can also take up 

organic selenides. Thus, selenate uptake in freshwater regions may be quite appreciable (due to the lack of 

competition from sulfate) but not in saline waters. However, once the Se is in organisms, it is not clear 

whether freshwater organisms would respond to potentially toxic levels in the same way as would estuarine 

organisms. The abundance of potentially competing ions in saline waters may result in different sensitivities 

for the estuarine organisms, but I am unaware of a rigorous comparison of animal sensitivities to a given 

tissue Se concentration among marine vs. freshwater organisms. The ideal comparison would be for a 

euryhaline species in which Se toxicity is compared for the same species at freshwater and estuarine 

salinities. I am unaware if such a comparison has been made.

3. Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is most likely 

the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), using green sturgeon1

observed selenium concentrations of 6 – 7 mg/kg dw (whole body) in the control treatment, which 

appears to reflect essentially a ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this 

study (and any other relevant data on green sturgeon), is the draft national benchmark value of 8.1 

mg/kg dw protective of the juvenile growth endpoint for green sturgeon? Please explain your answer.

The difference between 8.1 and 6-7 mg/kg is probably not significant, especially given the variability in these 

data and the fact that the 5th percentile projection of 8.1 is a projection, not a hard number. Thus, the green 

sturgeon have Se concentrations are about equal to the 5th percentile projection of 8.1 whole body criterion 

 1 Green sturgeon larvae were obtained from the offspring of three males and one female originating from the Klamath 

River along the Pacific Coast. 
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and these green sturgeon would appear to be just barely protected by the draft national benchmark value of 

8.1 ppm.

4. Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values from 

environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 and Brian and 

Cousens, 1989, cited in review document) may be a better model to represent effects of selenium, 

since a biphasic model shows a more representative progression of effect to living species (selenium is 

a micronutrient, but is toxic in both deficit concentrations and excessive concentrations). Studies on 

selenium effects in the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, 

cited in review document) do not appear to consider this issue, and study designs do not appear to 

take the issue into account. 

a. Given that many of the studies in the literature often use a small (or zero) control concentration 

of selenium, and a low level of effect is observed (reflective of a deficient concentration), is the 

response to increasing levels of selenium (into the excessive toxic amounts) better represented 

using a biphasic model or log-logistic model? Would the slope of the regression be skewed in 

either model, due to incomplete modeling of deficit effects?

I do not know.

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to determine 

EC10 values for studies in the literature on representative or key species for the SFBD (Heinz et al., 

1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review document), or whether the 

results of using a standard log-logistic model would produce more representative results. 

I do not know and have no opinions on this.

5. Species and Diet: Discuss whether the appropriate key species and food chains (webs) have been 

identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food webs are 

reasonable based on the literature (sources cited in review document) and whether the 

calculations of the Trophic Transfer Factors (TTFs) are scientifically supportable based on those 

diets (see specific questions below). Are there better ways to calculate TTFs for mixed diets for the 

key species? Please explain. 

It is possible to mathematically calculate TTFs for mixed diets for a given species, but I am not sure that it is 

worth doing so, even though many animals consume more than one prey item....that is, nature is more 

complex than models typically depict. The reason I have concerns in doing so for mixed diets is that there is 

considerable variability for any one predator/prey combination. If one were to consider two or more prey 

items, the variability compounds itself and does not help pinpoint or quantify a more “real” TTF. An example 

of this sort of variability can be seen for 6 different metals in fish consuming diverse prey, where about an 

order of magnitude differences are seen, largely based on the variability in the metal concentrations in the 

different forms of prey. [see Mathews & Fisher, Sci. Total Environ. 407: 5156-5161 (2009).]

Specifically for Clam-eating Fish:

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile white and green 

sturgeon, and splittail scientifically supportable based on the available literature (Zeug et al., 

2014, provided; USFWS, 2008 pp. 51-59, provided as supporting material; and Feyrer et al., 2003, 
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cited in review document)? Is it scientifically supportable to postulate that splittail do not 

consume as much Corbula, and are therefore not as much at-risk as sturgeon?

This seems reasonable to me, but see my comment below (under “c”).

c. Recent literature indicates that green sturgeon are more sensitive to selenium than white 

sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in review document). 

Studies have also shown that adult green sturgeon primarily use the SFBD as a migratory corridor 

to freshwaters for spawning (http://exxexpartnership.com/wpcontent/uploads/2010/10/GreenSturgeon-_1_.pdf, cited in review document). If it is known that 

egg maturation in adult green sturgeon is completed prior to entering the estuary, discuss 

whether it is scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and discuss the 

potential for selenium exposure of adult green sturgeon during migration to play a role in 

reproductive toxicity, given the current state of knowledge on the reproductive biology and life 

history of green sturgeon.

These questions are outside my expertise.

Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are scientifically 

defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that California 

Ridgeway’s rails do not consume a large percentage of Corbula, given that their littoral feeding 

habitat does not overlap with the preferred habitat of Corbula. 

This definitely makes sense.

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup live 

mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) where Corbula

are prevalent, and consume mostly Corbula. However, in the latter part of the winter and early 

spring prior to migration, they move down to the deeper, Central Bay area where Corbula are not 

as available, and the scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and continue to feed on 

herring prior to breeding and nesting. It appears that selenium moves directly from the diet of 

water birds into their eggs and not from internal tissue stores (Chapman et al., 2010). Given that 

scoter and scaup switch to a low selenium diet for a period prior to migration as well as during 

migration, and that selenium may be depurated at a higher rate during migration, and that scoter 

and scaup are likely to continue to feed on low selenium fish sources in the breeding and nesting 

grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet by 1⁄2 

(from 90% to 45%), with respect to selenium exposure that could impact reproductive endpoints? 

Please discuss.

It strikes me as rather speculative to do so, and possibly not protective of these species. Bear in mind that 

animals do not respond to average concentrations of toxic substances in their tissues, but rather to 

whatever concentration they have in their tissue at any particular time. So, when they are feeding on 

Corbula, their Se concentrations is high and at that time they are vulnerable. It is not appropriate to average 
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the Se that they see, and respond to, by including herring for part of the time. Imagine if you drank one glass 

of water each day for 100 consecutive days, but only one of those glasses had a toxic level (concentration X) 

of arsenic. You would die after drinking that one glass of arsenic. You should not conclude that you would 

respond to X/100 arsenic (the average arsenic concentration of the ten glasses) and be safe. 99 of those 

glasses would be harmless, and one would be lethal. The same may occur with the Se in these birds.

Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the model 

scientifically supportable based on the literature (sources cited in review document)? Please 

explain.

I suppose so, but again, how much of any given diet these fish happen to consume in any one location and 

any one time depends on the abundance of those prey insects at that particular time and place. These things 

vary enormously and probably account for much of the variability that one sees in Se concentrations in 

predator fish.

6. Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the literature for 

individual species based on their food prey are scientifically supportable, i.e., are the TTF factors in 

Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 – 24; Presser and Luoma, 2010, Table 3; 

and USEPA, 2014, pp. 71-77, each provided as supporting material) scientifically supportable and 

appropriate? Are you aware of any other or more applicable data?

These TTFs look good to me. 

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the available 

data? Are there additional data that the Agency should consider for developing a site-specific TTF for 

Corbula?

I don’t know of any involving Corbula beyond what is here.

7. Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation into the 

food chain during dry/low flow (lentic) conditions, which is represented by higher Kd values. Is it 

scientifically supportable to use the 75th percentile of the Kd values for each of the landward and 

seaward transects in the modeling, since data representing critical (very dry) conditions are not 

available? Would the use of median or mean Kd values be more appropriate? Please explain your 

answer.

I think it is inappropriate to use Kds, or at least to call them Kds. This is because Kds assume that the 

element in question—in this case, Se—is in equilibrium between the dissolved and particulate phases. That 

may be the case for sediments, but it is not the case for suspended particles, which in many case would 

probably be phytoplankton, and I believe that this is the case to which the authors are referring. Se uptake 

by phytoplankton (and also other microorganisms at the base of the food chain). It is more appropriate to 

call them concentration factors. However, Se CFs can vary enormously (several orders of magnitude) among 

different types of phytoplankton, with chlorophytes being the lowest and diatoms and dinoflagellates 

among the highest (for selenite). As for median or mean CFs, I would instead use a range of values, since a 
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mean for some types of phytoplankton would be wholly inappropriate for another phytoplankton 

assemblage, and these natural communities typically change with season (seasonal succession patterns 

among the phytoplankton communities) and nutrient concentrations.

8. Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and Parchaso (2010) 

(both provided) indicate that populations and population densities of Corbula appear to decline in 

wet years, and increase in dry years. The articles suggest that Corbula may be sensitive to flow into 

the estuary and/or the salinity of the estuary (and other factors). The model shows that when the 

system has higher Kd values (representative of dryer years), more selenium is available for 

bioaccumulation and more selenium accumulates in the food chain (which would drive the draft 

criteria to protect the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

Again, Kds are inappropriate here, but if phytoplankton CFs of Se are higher in dry seasons, then Corbula 

concentrations should be correspondingly higher then, and this would be reflected in higher Se levels in 

Corbula predators. Also, see comment just below (No. 9) on South Bay being less well flushed.

9. Transects: Discuss whether it is reasonable and representative to use the focused seaward transects 

to model the clam-based food webs, in order to eliminate the end-member variability at the ocean (at 

the Golden Gate Bridge, the entrance to San Francisco Bay). Does the geology of the focused area (a 

wide Suisun Bay flowing into a constricted Carquinez Strait), and the importance of the area (the 

location of refinery discharges), favor the use of focused transects to best represent the clam-based 

food web for the estuary? Please discuss.

I don’t have an opinion on this. It might be worth noting here that the South Bay is probably less well 

flushed than waters to the north, so applying modeling from other areas to the South Bay may be risky.

Results and Application

10. Results: Discuss whether the model input and results scientifically support the water quality criteria 

for the three food chains represented. Are there better ways to represent the three food webs in the 

modeling? Please explain.

Discuss whether it is scientifically supportable to use the modeling results for the clam-eating fish as a 

surrogate to protect clam-eating birds and insect-eating fish (all three food webs) in addition to the 

clam-eating fish themselves. Discuss whether there are other scientifically defensible ways to use the 

modeling results to protect all the categories of species for the SFBD.

I have doubts whether it is appropriate to apply toxicological results for clam-eating fish as a surrogate for 

clam-eating birds. Birds and fish do not have the same sensitivity or necessarily the same toxic endpoints to 

particular toxicants. It may be more defensible to apply modeling results from clam-eating fish to insecteating fish since the toxic end points in the fish would presumably be the same, or approximately the same. 

Probably the most relevant information that would be needed would be to have Se concentration in specific 

tissues following various dietary routes and a toxic response to Se concentrations in that tissue. Such data 

would be quite difficult to obtain, even in controlled experiments, however.
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11. Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and validated for 

SFBD, and a considerable amount of site-specific data are available and are used to model the estuary. 

EPA is using the model results to inform the criteria development process, and presents draft multimedia criteria in Chapter 4. These criteria are a whole body or muscle fish tissue value and a clam 

tissue value that directly reflect selenium levels that are considered to be protective of aquatic biota; 

and corresponding chronic and intermittent water column values, as well as a particulate 

concentration value to facilitate regulatory controls of selenium sources. 

Given the relevance and detail of the USGS model and the abundance of site-specific data, discuss 

whether the criteria in the different media are scientifically defensible and protective for this aquatic 

ecosystem. 

I think that the USGS model provides a defensible approach for generating protective criteria for wildlife 

exposed to Se in SF Bay. It seems to me that the same processes that operate in SF Bay would occur in other 

regions, so the approach taken here should, in principle, be applicable to other water bodies as well. 

However, the SF Bay system is unusually well studied with respect to Se, and it is doubtful that these kinds 

of detailed data for Se in other waters would be available. 

12. Translator (Permits): EPA regulations generally call for NPDES permit limits to be expressed in terms of 

total recoverable values (40 CFR 122.45(c)). The model applied in this document yields calculations of 

water column values in the dissolved form. For most metals, EPA recommends dissolved values for 

water quality criteria, because the dissolved form better represents the bioavailable fraction in the 

water column to aquatic biota. However, for selenium, aquatic biota can take in the dissolved and 

particulate fractions in the water column, which are then moved into and up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to determine a 

total recoverable permit limit; the translator is the fraction of the total recoverable metal in the 

downstream water (downstream of a discharge) that is dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate method to 

derive a translator? Explain your answer.

I would disagree with the assertion that “the dissolved form better represents the bioavailable fraction in 

the water column to aquatic biota.” For most metals, it is in fact, the dietary source that tends to 

predominate as a source of metals for aquatic animals—this is particularly the case in saline situations such 

as SF Bay.
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COMMENTS SUBMITTED BY

Reviewer 2

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CHARGE QUESTIONS

Overall Document Organization and Structure

1. Please comment on the overall construction and clarity of the document, i.e., is the layout of the 

document logical and is the document clearly written? Are there better ways to arrange and/or 

present the information? If so, please explain and describe.

ANSWER

The format and conceptual flow are generally similar to those of the federal WQC – Selenium draft, making 

it easily read and understood by those who have reviewed the federal treatment of the issue. In my opinion, 

the writing was clear and logical. There seemed to be a richer discussion and handling of uncertainties than 

in the federal document. It helped substantially to have significant components of the material already 

published in the open literature (e.g., Luoma and Presser (ES&T 43:8483, 2009) and Presser and Luoma 

(IEAM 6:685, 2010). Those publications were polished and concise discussions of the central themes, 

especially the methodology applied for ecosystem-scale modeling.

Unlike the federal document that focused on taxonomic groupings, this draft document categorized species 

first based on their ecological qualities such as trophic ecology and movement patterns. Species were

considered based on how much they consumed high (e.g., bivalves) or low (e.g., insects) selenium prey. Diet 

composition, species movements and life histories were considered throughout the draft. This made it much 

easier to understand and logically move through the materials. Whereas the federal draft document could 

not develop a comprehensive treatment of aquatic-dependent wildlife (birds), the SFBD draft document was 

able to consider such important wildlife. This was a welcome addition that rounded-out the entire 

treatment.

In my opinion, the draft SFBD document (and federal document) might have quantified the propagation of 

uncertainty more during the numerous computational steps so that the reader had a sound understanding 

with which to decide whether any modest differences in calculated values were real or simply reflective of 

the variation generated during the uncertain computations in the process. Question 3 below can be used as 

an example. Is the difference between 6-7 and 8.1 trivial relative to the uncertainty that emerges during 

computations? The 0.1 digit in the 8.1 suggests very high certainty that, given the details of the derivation, is 

dubious. 
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Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco Bay and Delta 

(SFBD) varies from fresh to brackish to marine, and many of the species (or close relatives of the 

species) evaluated for the draft national freshwater criterion (USEPA, 2014 pp 57-62; 78-79 provided 

as supporting material), live in the estuary. Given these considerations, discuss whether it is 

reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic life in 

the SFBD. Are there better ways to estimate toxicity benchmark values for key aquatic life species, 

many of which are anadromous, in the estuary? Please explain.

ANSWER

Although the national freshwater selenium criterion was used, its use and adjustments for the SFBD were 

reasonable and effectively incorporated. The challenge with some species of concern being anadromous is a 

difficult one and I wondered while reading the document whether an exposure adjustment for duration of 

time within the estuary or freshwater regions might have included more detail. Try as I might, I could not 

think of an alternative that might have been materially more accurate. As an example, the EPA 540/1-

89/002 for human risk assessment can incorporate more exposure duration and frequency details in its 

calculation of intake. But, given the lack of knowledge about bioaccumulation kinetics of the species of 

concern, this approach would likely not improve accuracy of estimates for the SFBD species. Perhaps a 

formal expert elicitation would further refine estimates.

As a very minor point related to the use of freshwater concepts and computations for the estuarine or 

marine regions of the SFBD, equating μg/L and μg/kg is fine for freshwater as done on page 11 for partition 

coefficients but water density differences in saline waters makes the equating of liters and kilograms slightly 

biased. (An average liter of Pacific seawater weighs roughly 1025 g, not 1000 g.)

3. Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is most likely 

the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), using green sturgeon

observed selenium concentrations of 6 – 7 mg/kg dw (whole body) in the control treatment, which 

appears to reflect essentially a ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this 

study (and any other relevant data on green sturgeon), is the draft national benchmark value of 8.1 

mg/kg dw protective of the juvenile growth endpoint for green sturgeon? Please explain your answer.

ANSWER

The methods used in the federal draft criterion document and De Riu et al. (2014) focus on effects to 

individuals, not populations. Indeed, these documents often obfuscate this distinction by using laboratory 

study results that allow inferences about the statistical population of tested individuals to suggest protection 

of the ecological population. It is often the case that individual-based metrics will be protective of 

populations but that could easily not be true for many species. This point was made very clearly by 

Kammenga et al. (1996) and others almost twenty years ago. Most laboratory-derived testing is done with 

species (“r-strategists”) that are quite different from the green sturgeon. The sturgeon has a life history in 

which individuals take several years to mature, develop eggs over several years, do not spawn annually, live 

for a long time, but produce many eggs upon spawning. It is not a species for which an expedient heuristic
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like “if you protect the individual’s most sensitive life stage, you protect the species population viability” can 

be confidently applied.

Fortunately, because of concerns of conservationists about the endangered green sturgeon, information has 

been gathered about the vital rates of most concern for this species, e.g., Beanesderfer et al (2007) and 

Heppell (2007). Beanesderfer et al. (2007) used demographic models to conclude, “The model highlights the 

extreme sensitivity of a green sturgeon population to even small incremental increases in mortality where 

multiple ages are affected.” Heppell (2007) used a technique called elasticity analysis to identify vital rates 

that most influence the growth rate (log λ) of green sturgeon populations, concluding that “Elasticity 

analysis [demonstrated ] a high sensitivity of population growth rate to changes in the survival rate of 

subadult and adult fish.” The federal and SFBD proposed criteria focus on the less critical vital rate 

(reproduction and YOY) so they are likely protective and might even be overprotective.

Beamesderfer, RCP, ML Simpson, and GJ Kopp. 2007. Use of life history information in a population model of 

Sacramento green sturgeon. Environ Biol Fish 79:315-337.

Heppell, SS. 2007. Elasticity analysis of green sturgeon life history. Environ Biol Fish 79:357-368.

Kammenga, JE, M Busschers, NM Van Straalen, PC Jepson, and J Baker. 1996. Stress induced fitness is not 

determined by the most sensitive life-cycle trait. Funct. Ecol. 10:106-111.

4. Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values from 

environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 and Brian and 

Cousens, 1989, cited in review document) may be a better model to represent effects of selenium, 

since a biphasic model shows a more representative progression of effect to living species (selenium is 

a micronutrient, but is toxic in both deficit concentrations and excessive concentrations). Studies on 

selenium effects in the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, 

cited in review document) do not appear to consider this issue, and study designs do not appear to 

take the issue into account. 

ANSWER

This concern crossed my mind several times during the review of the EPA draft federal WQC document. 

However, many of the data sets presented in that draft document were not designed to generate or 

sufficient for generating a reliable biphasic model. Exposures concentrations were not established to 

produce the number of observations needed in the region where the deviation from a conventional loglogistic model would be observed. So, the log-logistic is generally the best model if the data are inadequate 

for fitting a more complicated one.

The USFWS document (Beckon and Maurer, 2008) does contain relevant data sets (e.g., Figure 23 for 

weights of juvenile rainbow trout) that would allow a biphasic model to be fit. In such cases, the biphasic 

model does seem best. The supplied FWS report by Beckon (2009, Selenium effect levels for selected 

representative/surrogate species) has some examples illustrating my point. Figure 9 (salmon survival) and 

perhaps Figure 1 (mallard hatchability) show data for which a biphasic model would be warranted. If 

additional supportive information were available, a biphasic model might be fit to the salmon survival data 
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shown in Figure 10. A biphasic model for Figure 2 (mallard hatchability) would be an over parameterization 

given the nature of the data set.

Given the above discussion, it seems that professional judgment might be used to decide which model is 

warranted. I suggest that more formal methods be codified to guide a user in deciding which model is best 

for their particular data set(s).

a. Given that many of the studies in the literature often use a small (or zero) control concentration 

of selenium, and a low level of effect is observed (reflective of a deficient concentration), is the 

response to increasing levels of selenium (into the excessive toxic amounts) better represented 

using a biphasic model or log-logistic model? Would the slope of the regression be skewed in 

either model, due to incomplete modeling of deficit effects?

ANSWER

If one possessed a data set with which a biphasic model could be successfully fit, the biphasic model would 

be best. Obviously the slope derived from an inadequate data set for fitting the deficit response dynamics 

would be biased. But, if one forced a biphasic model onto an inadequate data set, the uncertainty in 

parameter estimates would increase. Many of the species response data sets illustrated in the federal draft 

document would be inadequate for fitting a biphasic model. As stated above, I suggest that formal methods 

be established to aid users in deciding which model is best for their particular data set.

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to determine 

EC10 values for studies in the literature on representative or key species for the SFBD (Heinz et al., 

1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review document), or whether the 

results of using a standard log-logistic model would produce more representative results. 

ANSWER

Yes. Fitting a biphasic model to data sets with enough points in the lower dose regions would result in a 

model that more accurately made predictions and conformed to our current understanding of the 

mechanisms of selenium influences on biological systems.

5. Species and Diet: Discuss whether the appropriate key species and food chains (webs) have been 

identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food webs are 

reasonable based on the literature (sources cited in review document) and whether the 

calculations of the Trophic Transfer Factors (TTFs) are scientifically supportable based on those 

diets (see specific questions below). Are there better ways to calculate TTFs for mixed diets for the 

key species? Please explain. 

ANSWER

I am not an expert on the trophic ecology of these species so the answer here is based on the information 

sent with the supplemental materials and that shown in tables of the draft SFBD document. Based on those 

data, the diet information appears to be reasonable. The TTF values seem well chosen; however, it might be 

useful to have some indication of the uncertainty associated with each estimate. In future reports, the 
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uncertainty of each component going into the various calculations could be incorporated into a Monte Carlo 

resampling routine to provide estimates of the uncertainty around each calculated TTF. The uncertainty is 

likely large relative to that variation in TTF values among species. 

One conventional addition would be to use end member analysis involving N, C and perhaps S stable 

isotopes to estimate dietary components for each key species. The UC-Davis campus has an excellent facility 

to do the analyses and others have the required software such as the EPA’s IsoSource package. This being 

said, my judgment is that such an approach would not provide much better information than the one taken 

in the draft document.

Specifically for Clam-eating Fish:

ANSWER

A brief clarification of one point would help relative to the general TTF values for clam-eating fish and birds. 

The fish are cold-blooded and the birds are warm-blooded. The amount of selenium-containing food that a 

fish consumes to maintain itself would be substantially less than that consumed by the warm-blooded birds.

Since selenium is accumulated mostly from food, it was confusing to me that the TTF values for fish were so 

close to those of birds. For fish and birds eating Corbula as 40-45% of their diet, Table 4-8 gives TTFfish = 1.1 

and TTFduck egg = 1.8. Based on a physiological rule-of-thumb, my informal guess would have been a 5- to 10-

fold difference, not the low 2-fold difference in the tables. Clearly, there is an unexplained feature of the 

calculated TTF values that results in this apparent incongruity.

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile white and green 

sturgeon, and splittail scientifically supportable based on the available literature (Zeug et al., 

2014, provided; USFWS, 2008 pp. 51-59, provided as supporting material; and Feyrer et al., 2003, 

cited in review document)? Is it scientifically supportable to postulate that splittail do not 

consume as much Corbula, and are therefore not as much at-risk as sturgeon?

ANSWER

Yes. Based on my review of the supplied materials and the discussions in the draft document, the diet mixes 

seem reasonable, including the differences between sturgeon and splittail consumptions of Corbula. 

c. Recent literature indicates that green sturgeon are more sensitive to selenium than white 

sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in review document). 

Studies have also shown that adult green sturgeon primarily use the SFBD as a migratory corridor 

to freshwaters for spawning (http://exxexpartnership.com/wpcontent/uploads/2010/10/GreenSturgeon-_1_.pdf , cited in review document). If it is known that 

egg maturation in adult green sturgeon is completed prior to entering the estuary, discuss 

whether it is scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and discuss the 

potential for selenium exposure of adult green sturgeon during migration to play a role in 

reproductive toxicity, given the current state of knowledge on the reproductive biology and life 

history of green sturgeon.
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ANSWER

Yes. The judgment is sound that “adult white sturgeon and juvenile white and green sturgeon will be more 

affected than adult green sturgeon.” As discussed above, one way of handling the migratory characteristics 

of each species would be to quantitatively include more details for exposure duration and frequency. The 

EPA document dealing with human risk assessment (EPA 540/1-89/002) includes exposure duration and 

frequency in its calculation of intake. Given the lack of knowledge about selenium bioaccumulation kinetics 

in the sturgeon, this approach would likely not improve the accuracy of estimates. Perhaps a formal expert 

elicitation would improve future estimates.

Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are scientifically 

defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that California 

Ridgeway’s rails do not consume a large percentage of Corbula, given that their littoral feeding 

habitat does not overlap with the preferred habitat of Corbula.

ANSWER

Yes. This assumption seems defensible for the rail.

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup live 

mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) where Corbula

are prevalent, and consume mostly Corbula. However, in the latter part of the winter and early 

spring prior to migration, they move down to the deeper, Central Bay area where Corbula are not 

as available, and the scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and continue to feed on 

herring prior to breeding and nesting. It appears that selenium moves directly from the diet of 

water birds into their eggs and not from internal tissue stores (Chapman et al., 2010). Given that 

scoter and scaup switch to a low selenium diet for a period prior to migration as well as during 

migration, and that selenium may be depurated at a higher rate during migration, and that scoter 

and scaup are likely to continue to feed on low selenium fish sources in the breeding and nesting 

grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet by 1⁄2 

(from 90% to 45%), with respect to selenium exposure that could impact reproductive endpoints? 

Please discuss.

ANSWER

Yes. It seems reasonable and is consistent with my earlier comments about adjusting intake estimates based 

on knowledge of exposure duration and frequency. But exposure frequency and duration might be 

considered more fully in the future as done with human risk assessment (EPA 540/1-89/002). The treatment 

in “D. Draft Criteria. A. Applicable Waters; Magnitude, Duration and Frequency” reflects the approach in the 

federal draft document. However, a more granular treatment of the duration/frequency of exposure issues 

could improve the accuracy of judgments in my opinion. In the future, additional bioaccumulation/

elimination pharmacokinetics models combined with knowledge of the migratory movement of the species 

between habitats differing in selenium levels might enhance the certainty in this judgment. 
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Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the model 

scientifically supportable based on the literature (sources cited in review document)? Please 

explain.

ANSWER

Based on a review of the supplied documents and tabulations in the draft SFBD document, the diet mixes do 

seem sound. However, I am not an expert on fish trophic ecology and habits.

6. Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the literature for 

individual species based on their food prey are scientifically supportable, i.e., are the TTF factors in 

Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 – 24; Presser and Luoma, 2010, Table 3; 

and USEPA, 2014, pp. 71-77, each provided as supporting material) scientifically supportable and 

appropriate? Are you aware of any other or more applicable data?

ANSWER

The extraction and derivation of the TTF values seem well founded although the reason for such a small 

difference in TTF values for warm-blooded birds and cold-bolded fish mentioned earlier is not apparent. The 

TTF value computations are straightforward and consistent with our understanding of trophic transfer of 

selenium. I do wonder if there is useful stable isotope information available for the trophic transfer for the 

key species in this report. Several such studies exist including those by authors of several supporting 

documents for this review. Perhaps their discussion was not prominent enough for me to recognize the 

inclusion of stable isotope information in the draft SFBD document. (A search of the 96 page draft pdf did 

not find the word “isotope.”) In the future, it might be useful to explore and discuss more the insights 

available from published stable isotope studies of these or similar species. These kinds of data might provide 

reinforcing detail to the estimated TTF values.

Numerous publications of stable isotope studies of the subject area were found with a quick Google search. 

Some examples include the following:

Cloern, JE,EA Canuel and D Harris. 2002. Stable carbon and nitrogen isotope composition of aquatic and 

terrestrial plants of the San Francisco Bay estuarine system. Limnol. Oceanogr., 47(3): 713–729

Croteau, MN, SN Luoma, and AR Stewart.2005. Trophic transfer of metals along freshwater food webs: 

Evidence of cadmium biomagnification in nature. Limnol. Oceanogr., 50(5): 1511–1519

Davis, JA et al. 2012. Reducing methylmercury accumulation in the food webs of San Francisco Bay and its 

local watersheds. Environmental Research 119: 3–26

Gegrke, GE. 2011. Sources of mercury to San Francisco Bay surface sediment as revealed by mercury stable 

isotopes. Geochimica et Cosmochimica Acta, 75:691-705

Stewart, AR et al. 2004. Food Web Pathway Determines How Selenium Affects Aquatic Ecosystems: A San 

Francisco Bay Case Study. Environ. Sci. Technol., 38 (17): 4519–4526 
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The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the available 

data? Are there additional data that the Agency should consider for developing a site-specific TTF for 

Corbula?

ANSWER

The TTF values are consistent with the federal draft document’s general approach. They are tailored very 

logically for the SFBD. Reading the relevant materials such as Luoma and Presser (2009) and Presser and 

Luoma (2010) for more details reinforces this conclusion of consistency. I can think of no better way to 

estimate TTF values. Supplementing the current information with stable isotope end member analyses 

might enhance insights. An expert elicitation exercise might improve TTF estimates if done carefully.

Perhaps future reports could calculate and present the quantitative uncertainties in TTF estimates.

More specific comments can be made below about the components (IR,AE, ke) going into the calculations as 

discussed in the USGS(2010) document.

IR Estimation

“IR is determined by filtration rate (125 L/g clam/d, Cole et al., 1992) multiplied by C (median = 0.4 mg C/L) 

to achieve the units (g C/g clam/d) in the suspended particulate material at each sampling. In the average 

condition in the estuary, clams ingest 5% of their body weight per day in C across all days for which data is 

available. At an average of 2% C in suspended particulate material (again, the average across all data) they 

ingest 2.5 times their body weight per day in total suspended particulate material. If IR is calculated at each 

of three low river discharge months where data is available, the average is 1.7 g suspended particulate 

material/g clam/d. Experience has indicated that the ingestion model is more accurate when actual 

outcomes are used (or averaged) for the generic situation (i.e., 1.7 g suspended particulate material/g 

clam/d) as compared to taking the average of each component of the outcome and calculating a generic 

average. Therefore, we recommend using 1.7 g suspended particulate material/g clam/d for modeling.”

Again I wonder about the variation in features such as filtration rates as a function of factors like 

temperature or salinity. Until provided with contrary information, it would be reasonable for a reader to 

suppose that filtration rate varies substantially. Also variation associated with the general estimate of 0.4 

mg C/L might be important to describe. Given the wide variation in kd values throughout the region of 

interest, this value might have substantial variation associated with it. Finally, according to classic Holling’s 

predator functional responses, the efficiency of removing particles during filtration can vary with the density 

of those particles in the waters. Some additional scrutiny of the uncertainty in the chosen values seems 

warranted.

AE Estimation

“The derivation of a refined site-specific AE based on individualized bioavailabilities of Selenium in suspended 

particulate material uses observed fractions of particulate organo-Selenium, adsorbed Se, or elemental Se 

found in the estuary (Doblin et al., 2006) combined with individual AEs for those particulate Se species from 

the literature (living phytoplankton, AE = 60%; adsorbed on seston, AE = 40%; elemental, AE = 0%; Schlekat et 
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al., 2004; Wang et al., 1996). The equation is: AE = (fraction organic particulate Se) (AE organic particulate 

Se) + (fraction adsorbed particulate Se) (AEadsorbed particulate Se) + (fraction elemental particulate 

Se)(AEelemental particulate Se). For example, if a site-specific sample of suspended particulate material 

collected in the estuary contains 45% Se in phytoplankton at an assumed AE of 60%; 30% Se adsorbed on 

seston at an assumed AE of 40%; and 25% elemental Se in sediment at an assumed AE of 0%, then the

composite AE = (0.45 x 0.6) + (0.30 x 0.40) + (0.25 x 0) = 0.39 or 39% AE.”

This manner of estimating AE seems reasonable although the variation in the distribution of selenium 

among the three phases will likely change temporally and spatially. Future documents might spend more 

time discussing the variation expected in this and the resulting variation in AE.

Ke Estimation

“We apply the efflux rate constant derived experimentally (Lee et al., 2006): ke = 0.03/d.” 

Seems like a reasonable value chosen from a peer-reviewed publication.

Calculations

“When we model for times when all data are available from the estuary, we use all data from that sampling 

date. When we model generically we employ mean parameters.” 

This is fine; however, insight might be enriched by more detail about the variation as well as the mean 

parameters. During the calculations, general decisions involving variation were made such as calculating 

values for specific regions or “excluding April, 1999 transects data as out of the norm (i.e., El Niño condition 

in the estuary).” Relative to the last decision, one could argue that El Niño years are normal occurrences and 

clams will be there during those times. 

Nonetheless, the handling of the calculations is fine and the report does provide some useful information 

about the expected variation, that is, “We assume a TTFC.amurensis of 17 in modeling scenarios here. The 

range of TTFs across all estuarine conditions was 14-26. These values are higher than laboratory-derived 

values primarily because ingestion rates are higher in these field systems than in experiments. This is the first 

calculation of a field-derived TTF for a marine bivalve species.” 

7. Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation into the 

food chain during dry/low flow (lentic) conditions, which is represented by higher Kd values. Is it 

scientifically supportable to use the 75th percentile of the Kd values for each of the landward and 

seaward transects in the modeling, since data representing critical (very dry) conditions are not 

available? Would the use of median or mean Kd values be more appropriate? Please explain your 

answer.

ANSWER

The natural variation in the Kd values is quite high. The constrained transects have kd values from 712 to 

7,725 and the landward transect models use kd values from 554 to 12,650. Table 8 in USGS (2010) indicates 

means that if used, would be unrepresentative of the populations of Kd values. In such a case, I would have 

suggested that a probabilistic approach be taken or that a conservative approach be applied. The 

conservative approach of using the 75% percentile value seems better than using the mean or median.
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8. Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and Parchaso (2010) 

(both provided) indicate that populations and population densities of Corbula appear to decline in 

wet years, and increase in dry years. The articles suggest that Corbula may be sensitive to flow into 

the estuary and/or the salinity of the estuary (and other factors). The model shows that when the 

system has higher Kd values (representative of dryer years), more selenium is available for 

bioaccumulation and more selenium accumulates in the food chain (which would drive the draft 

criteria to protect the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

ANSWER

I do not have any helpful insights to provide about Corbula population changes relative to flow regime. 

Given the information about population dynamics and kd values, the focus on dry periods seems 

appropriate.

9. Transects: Discuss whether it is reasonable and representative to use the focused seaward transects 

to model the clam-based food webs, in order to eliminate the end-member variability at the ocean (at 

the Golden Gate Bridge, the entrance to San Francisco Bay). Does the geology of the focused area (a 

wide Suisun Bay flowing into a constricted Carquinez Strait), and the importance of the area (the 

location of refinery discharges), favor the use of focused transects to best represent the clam-based 

food web for the estuary? Please discuss.

ANSWER

This decision does seem reasonable in my opinion.

Results and Application

10. Results: Discuss whether the model input and results scientifically support the water quality criteria 

for the three food chains represented. Are there better ways to represent the three food webs in the 

modeling? Please explain.

ANSWER

The model input and use of the three food webs do seem scientifically sound. The studies drawn upon 

relative to the trophic ecology of the key species are solid. I do wonder if there were any stable isotopebased studies from which end member analyses might have been done. Such studies might have provided 

additional insights and perhaps a level of validation.

Discuss whether it is scientifically supportable to use the modeling results for the clam-eating fish as a 

surrogate to protect clam-eating birds and insect-eating fish (all three food webs) in addition to the 

clam-eating fish themselves. Discuss whether there are other scientifically defensible ways to use the 

modeling results to protect all the categories of species for the SFBD.

ANSWER

Using clam-eating fish as a surrogate to protect clam-eating birds and insect-eating fish does not seem the 

best general approach. However, in this particular instance involving the SFBD area, the calculations 

demonstrated (see quoted materials in the answer to Question 12 below) that the protective value derived 

for clam-eating fish would also be protective of the fish-eating birds and insect-eating fish.
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11. Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and validated for 

SFBD, and a considerable amount of site-specific data are available and are used to model the estuary. 

EPA is using the model results to inform the criteria development process, and presents draft multimedia criteria in Chapter 4. These criteria are a whole body or muscle fish tissue value and a clam 

tissue value that directly reflect selenium levels that are considered to be protective of aquatic biota; 

and corresponding chronic and intermittent water column values, as well as a particulate 

concentration value to facilitate regulatory controls of selenium sources. 

Given the relevance and detail of the USGS model and the abundance of site-specific data, discuss 

whether the criteria in the different media are scientifically defensible and protective for this aquatic 

ecosystem. 

ANSWER

This seems to be an unusual situation in which a remarkable amount of site-specific information exists and 

appropriate synthesis of that information has already been done. That greatly enhances the development of 

the WQC draft document. The area-specific data and its use to produce predictive models is definitely 

defensible, and from my reading of the documents, is protective.

12. Translator (Permits): EPA regulations generally call for NPDES permit limits to be expressed in terms of 

total recoverable values (40 CFR 122.45(c)). The model applied in this document yields calculations of 

water column values in the dissolved form. For most metals, EPA recommends dissolved values for 

water quality criteria, because the dissolved form better represents the bioavailable fraction in the 

water column to aquatic biota. However, for selenium, aquatic biota can take in the dissolved and 

particulate fractions in the water column, which are then moved into and up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to determine a 

total recoverable permit limit; the translator is the fraction of the total recoverable metal in the 

downstream water (downstream of a discharge) that is dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate method to 

derive a translator? Explain your answer.

ANSWER

If I understand the question, it refers to the derivation of a protective value in terms of dissolved selenium.

What seems to be the pertinent materials from two parts of the draft report are pasted below. 

“Dissolved Selenium: To protect clam-eating fish at an EC10 level, the predicted protective dissolved 

selenium estuarine water column concentration is between 0.137 and 0.685 μg/L, or between approximately 

0.1 and 0.7 μg/L, based on available data. Using the mean value of the 75th percentile Kds (3,966), the 

predicted protective dissolved selenium estuarine water column concentration is 0.244 μg/L, or 

approximately 0.2 μg/L. Based on these analyses, the protective dissolved selenium water column 

concentration selected is 0.2 μg/L. This value should protect existing species in the estuary most of the time, 

under the environmental conditions used in the modeling.

The modeling for clam-eating birds and insect-eating fish results in predicted protective dissolved selenium 

water column concentrations between 0.126 and 2.046 μg/L, or between approximately 0.1 and 2 μg/L. The 

lower end of this range (0.1 μg/L) is the same as the lower end of the range of protective values for clam-
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eating fish (0.1 μg/L). Using the mean value of the 75th percentile Kds, (3,966), the predicted protective 

dissolved selenium water column concentration is 0.224 μg/L for scoter and scaup, or approximately 0.2 

μg/L, the same predicted value for clam-eating fish. These analyses indicate that if clam-eating fish are 

protected, clam-eating birds and insect-eating fish should also be protected.

Particulate Selenium: To protect clam-eating fish at an EC10 level, the predicted protective particulate 

selenium concentration is 0.969 μg/g dw, or approximately 1 μg/g dw, based on available data. The 

modeling for clam-eating birds and insect-eating fish results in predicted protective particulate selenium 

concentrations between 0.890 and 2.894 μg/g dw, or between approximately 1 and 3 μg/g dw. The lower 

end of this range is approximately equal to the protective value for clam-eating fish; if clam-eating fish are 

protected, clam-eating birds and insect-eating fish should also be protected.

Clam Tissue: To protect clam-eating fish at an EC10 level, the predicted protective selenium invertebrate 

(prey) tissue concentration is 7.4 μg/g dw; the predicted protective selenium Corbula tissue concentration is 

6.6 μg/g dw, or approximately 7 μg/g dw, based on available data. The modeling for clam-eating birds 

results in predicted protective Corbula selenium concentrations between 6.8 and 9.9 μg/g dw, or between 

approximately 7 and 10 μg/g dw. The lower end of this range is approximately equal to the predicted 

protective value for clam-eating fish (6.8 vs 6.6 μg/g dw); if clam-eating fish are protected, clam-eating birds

should also be protected.

All predicted values have been rounded to one significant digit. The rounded values provide a reasonable 

prediction of the appropriate protective values of allowable dissolved water column selenium, particulate 

selenium, and prey tissue selenium concentrations that will protect clam-eating fish and birds, and insecteating fish in the estuary. 

And the sentences below are most relevant to this question.

Form of Permit Limits: Although draft water column criteria values are expressed in dissolved selenium 

concentrations, water quality-based permit limits, in most instances, are required to be in total recoverable 

values (40 CFR §122.45(c)). For most metals, criteria are in dissolved values because the dissolved fraction 

provides a better representation of the biologically active portion of the metal than does the total or total 

recoverable fraction (USEPA, 1996). When water quality criteria are in the dissolved form, a translator can 

be used to determine a total recoverable permit limit. The translator is the fraction of the total recoverable

metal in the downstream water that is dissolved (ibid). However, for selenium, both the dissolved and 

particulate fractions are bioavailable to the aquatic biota. Thus, we used a translator of 1 in developing the 

draft criteria, an appropriate assumption for the Bay and Delta.

Answer

The dissolved selenium is much less relevant than bioavailable selenium in particulates so urging for a focus 

on dissolved Se would not be as helpful as for other elements and back calculation from tissue 

concentrations to total recoverable metals in lentic and lotic waters seems preferable. Of all of the 

conventional water concentrations measured, total recoverable selenium is likely the most reflective of the 

selenium available to the filter feeders such as Corbula. The selection of a translator of 1 equates total 

recoverable concentrations with notionally bioavailable selenium concentrations. In the case of selenium, 

that is a sound and appropriate conservative approach. The variability in total, recoverable, and dissolved 

selenium remains problematic regardless of the approach. 
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COMMENTS SUBMITTED BY

Reviewer 3

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CHARGE QUESTIONS

Overall Document Organization and Structure

1. Please comment on the overall construction and clarity of the document, i.e., is the layout of the 

document logical and is the document clearly written? Are there better ways to arrange and/or 

present the information? If so, please explain and describe.

The document is well constructed and relatively well written. While the document (Chapter 2) separately 

addresses fate and exposure (based on modeling) and effects in separate sections (Chapter 3), these are not 

clearly delineated in the outline. For example, the outline should separate environmental fate and models 

used to assess particulate vs. dissolved concentrations from bioaccumulation models that estimate body 

burdens. While Figure 2-1 attempts to provide an overall conceptual a model, a more simplified model 

documenting the estimation of water criteria from tissues would be a better representation, and would 

probably be better suited in Chapter 1.

Chapter 3 is relatively well-written. However, more figurative descriptions for thresholds would provide 

better clarity. For example, copies of the species sensitivity distribution (SSD) which were used to obtain 

EC10s would be very helpful. As a template, please see the EPA 2014 document. 

As a minor editorial comment, the outline for Chapter 4 is missing a subheading C.

A chapter should be dedicated to Uncertainty Analyses at some point in the document. What are the 

strengths and weaknesses of the assumptions used in the model? It was difficult to find a specific section 

where validations of the models were provided. 

Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco Bay and Delta 

(SFBD) varies from fresh to brackish to marine, and many of the species (or close relatives of the 

species) evaluated for the draft national freshwater criterion (USEPA, 2014 pp 57-62; 78-79 provided 

as supporting material), live in the estuary. Given these considerations, discuss whether it is 

reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic life in 

the SFBD. Are there better ways to estimate toxicity benchmark values for key aquatic life species, 

many of which are anadromous, in the estuary? Please explain.

It is clear that salinity values have significant variance throughout SF Bay and the Delta. Acclimation to hyper 

and hypo saline conditions has significant impacts on endocrine and physiological responses in anadromous 

fish. Two of the most sensitive toxicity benchmarks for selenium in freshwater are altered development of 

offspring and diminished reproduction, both under endocrine control. The National Se criteria was 

estimated based on reproductive and developmental endpoints using a species distribution of 9 freshwater 

fish genera with an N=14 Genus Mean Chronic Values (GMCVs) (EPA 2014). Most of the exposures included 

a dietary component, which would allow all forms of Se (organic species) to be present and transported to 

the gonad. The 5th percentile projection yielded criterion lower than the most sensitive fish species tested, 

brown trout (Salmo trutta) and Oncorhynchus, which has two anadromous species in SFBD (Steelhead and 

Chinook Salmon). Since at least two andromous species were evaluated with reproduction and 

developmental endpoints utilized in the evaluation, the National value is likely conservative. However, some 
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uncertainty exists with the value as some have suggested many of the studies used dietary 

selenomethionine as the primary source of Se, and this may underestimate Se toxicity as other organic 

forms may also contribute to the toxic responses observed in fish (Ridby et al. 2014).

In addition, there may be some underestimation in Saltwater acclimated animals. When embryos of 

Japanese medaka (Oryzia latipes) were acclimated to hypersaline conditions and treated with 

concentrations of 0.05 mM SeMe (derived from concentrations observed in mosquito fish (Gambuis affinis) 

from the San Joaquin River) for 12 hours (Kupsco and Schlenk 2014) and 24 hours (Lavado et al. 2012), 

significant enhancement of developmental toxicity was observed. While this species is not endemic to SFBD, 

it demonstrates that there may be significant differences in developmental toxicity between saltwater 

adapted fish and those of freshwater fish, particularly at early life stages (Larvae, alevin, smolt). And since 

many anadromous fish at young ages must acclimate to hypersaline conditions in areas where exposure to 

Se may still occur either through prior maternal transfer in ovo, or via diet, combined exposure to both 

stressors may occur, particularly in the SFBD.

Alternatively, since anadromous adult female fish are likely gravid prior to returning to freshwater to spawn, 

it is unknown whether acclimation to freshwater impairs endocrine signaling, which could affect maternal 

transfer of Se to offspring or have any reproductive impact on Se toxicity in the returning adults. The 

endocrine responses would not be due to accumulation or feeding preferences on the adult females but due 

to the effects of hypersaline conditions on the physiology and susceptibility of anadromous species.

Thus, it may be more appropriate to differentiate criteria based on returning adults (ocean to stream) or 

outgoing younger life stages (i.e. smolts; stream to ocean). 

In addition, the assumption of the SFBD criteria assumes that for salmonids juvenile (fry, alevin, smolt) life 

stages are the more sensitive to Se toxicity and assumes dietary exposures via insects. The proposed SFBD 

criteria fails to recognize in ovo exposure within returning females, which is likely more critical (Thomas and 

Janz 2014). If in ovo exposure occurs, then models must incorporate a more complex food web (inclusion of 

fish to returning adults, rather than exclusively insects as in outgoing fish). 

3. Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is most likely 

the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), using green sturgeon1

observed selenium concentrations of 6 – 7 mg/kg dw (whole body) in the control treatment, which 

appears to reflect essentially a ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this 

study (and any other relevant data on green sturgeon), is the draft national benchmark value of 8.1 

mg/kg dw protective of the juvenile growth endpoint for green sturgeon? Please explain your answer.

The estimated value (EC5) should be protective of growth for juvenile green sturgeon. This is because the 

National benchmark was based on the more sensitive response of reproduction/development. Since growth 

is likely a less sensitive endpoint, the predicted value should be conservative and protective. In addition, the 

National criteria is similar to the criteria obtained from Linville (2006), which also used a feeding study and 

offspring development to estimate tissue values in white sturgeon.

 1 Green sturgeon larvae were obtained from the offspring of three males and one female originating from the Klamath 

River along the Pacific Coast. 
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4. Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values from 

environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 and Brian and 

Cousens, 1989, cited in review document) may be a better model to represent effects of selenium, 

since a biphasic model shows a more representative progression of effect to living species (selenium is 

a micronutrient, but is toxic in both deficit concentrations and excessive concentrations). Studies on 

selenium effects in the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, 

cited in review document) do not appear to consider this issue, and study designs do not appear to 

take the issue into account. 

a. Given that many of the studies in the literature often use a small (or zero) control concentration 

of selenium, and a low level of effect is observed (reflective of a deficient concentration), is the 

response to increasing levels of selenium (into the excessive toxic amounts) better represented 

using a biphasic model or log-logistic model? Would the slope of the regression be skewed in 

either model, due to incomplete modeling of deficit effects?

Se is clearly an essential nutrient and likely fits a “bi-phasic” model showing non-montonicity or a U-shape 

(Eaton and Gilbert 2013). While this is likely the most accurate description of the relationship of Se to 

development, the data are significantly lacking to identify EC10 values. The Beckon et al. (2008) manuscript 

compares 3 log-logistic models with varied EC10 estimates. For each comparison there are limited data 

points showing the classic U-shape that essential nutrients should have with only one point showing 

reduced hatch at low concentrations. Consequently, until studies can be conducted to provide statistically 

adequate data to generate a biphasic curve, standard log-logistic modeling should still be utilized.

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to determine 

EC10 values for studies in the literature on representative or key species for the SFBD (Heinz et al., 

1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review document), or whether the 

results of using a standard log-logistic model would produce more representative results. 

As mentioned above, a biphasic model would be preferred if the data were available to provide adequate 

statistical power.

5. Species and Diet: Discuss whether the appropriate key species and food chains (webs) have been 

identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food webs are 

reasonable based on the literature (sources cited in review document) and whether the 

calculations of the Trophic Transfer Factors (TTFs) are scientifically supportable based on those 

diets (see specific questions below). Are there better ways to calculate TTFs for mixed diets for the 

key species? Please explain. 

Overall, the criteria focuses on ocean-going early life stage organisms for the salmonid anadromous fish 

species. As discussed above, larval or Young of the Year fish moving from freshwater into saltwater (smolts) 

are likely to have a different diet (only one TTF) than adults moving back into freshwater to spawn (Mixture 

4-trophic level-see Fig 7 USEPA, 2014). Specification of the targeted life stage is necessary to refine the food 

chains proposed in this document.
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Specifically for Clam-eating Fish:

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile white and green 

sturgeon, and splittail scientifically supportable based on the available literature (Zeug et al., 

2014, provided; USFWS, 2008 pp. 51-59, provided as supporting material; and Feyrer et al., 2003, 

cited in review document)? Is it scientifically supportable to postulate that splittail do not 

consume as much Corbula, and are therefore not as much at-risk as sturgeon?

Based on the literature, these assumptions seem appropriate.

c. Recent literature indicates that green sturgeon are more sensitive to selenium than white 

sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in review document). 

Studies have also shown that adult green sturgeon primarily use the SFBD as a migratory corridor 

to freshwaters for spawning (http://exxexpartnership.com/wpcontent/uploads/2010/10/GreenSturgeon-_1_.pdf, cited in review document). If it is known that 

egg maturation in adult green sturgeon is completed prior to entering the estuary, discuss 

whether it is scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and discuss the 

potential for selenium exposure of adult green sturgeon during migration to play a role in 

reproductive toxicity, given the current state of knowledge on the reproductive biology and life 

history of green sturgeon.

Since Se is likely transported to gonads via the egg-yolk precursor vitellogenin (Linville 2006: Janz et al. 

2010), one could assume that animals that do not require vitellogenin any longer would not continue to 

increase gonadal concentrations of Se. However, it is unclear whether vitellogenin is continually deposited 

into “mature” gonads of green sturgeon. Studies in zebrafish indicate vitellogenin is constantly turned over 

throughout embryogenesis (Han et al. 2010) which would argue diet-derived Se would continue to 

accumulate in animals even after maturation of gonads. 

In the recent Linares-Casenave et al. 2015 publication, of 26 female white sturgeon collected between San 

Pablo Bay and Suisun Bay, 18 were pre-vitellogenic indicating most fish did not have sexually mature 

gonadal development. It was not documented whether the animals were coming from saltwater to 

freshwater for spawning or vice-versa. If animals are returning to spawn, then it can presumed that gonadal 

development and selenium burdens would be higher than in animals exciting the estuary. Animals with 

vitellogenic oocytes did have higher concentrations of Se.

Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are scientifically 

defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that California 

Ridgeway’s rails do not consume a large percentage of Corbula, given that their littoral feeding 

habitat does not overlap with the preferred habitat of Corbula. 

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup live 

mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) where Corbula

are prevalent, and consume mostly Corbula. However, in the latter part of the winter and early 
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spring prior to migration, they move down to the deeper, Central Bay area where Corbula are not 

as available, and the scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and continue to feed on 

herring prior to breeding and nesting. It appears that selenium moves directly from the diet of 

water birds into their eggs and not from internal tissue stores (Chapman et al., 2010). Given that 

scoter and scaup switch to a low selenium diet for a period prior to migration as well as during 

migration, and that selenium may be depurated at a higher rate during migration, and that scoter 

and scaup are likely to continue to feed on low selenium fish sources in the breeding and nesting 

grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet by 1⁄2 

(from 90% to 45%), with respect to selenium exposure that could impact reproductive endpoints? 

Please discuss.

I cannot comment on this section as I am unfamiliar with the life histories of these avian species.

Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the model 

scientifically supportable based on the literature (sources cited in review document)? Please 

explain.

As discussed above, at least for salmonids, the diet mixes will differ based on the life-stage of the animal.

Returning adults will have a much larger forage fish component than insects and likely continual deposition 

of Se into ova. In contrast, early life-stage salmonids will have a greater insect component. The models 

should be revised to separate adult and early life stages.

6. Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the literature for 

individual species based on their food prey are scientifically supportable, i.e., are the TTF factors in 

Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 – 24; Presser and Luoma, 2010, Table 3; 

and USEPA, 2014, pp. 71-77, each provided as supporting material) scientifically supportable and 

appropriate? Are you aware of any other or more applicable data?

The TTFs appear valid and are scientifically supportable. However, as discussed above, a 4 trophic level 

mixture should be used for adult returning salmonids (Figure 7; USEPA 2014).

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the available 

data? Are there additional data that the Agency should consider for developing a site-specific TTF for 

Corbula?

The site-specific model used to calculate TTF for Corbula is scientifically sound.
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7. Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation into the 

food chain during dry/low flow (lentic) conditions, which is represented by higher Kd values. Is it 

scientifically supportable to use the 75th percentile of the Kd values for each of the landward and 

seaward transects in the modeling, since data representing critical (very dry) conditions are not 

available? Would the use of median or mean Kd values be more appropriate? Please explain your 

answer.

The rationale for selection of the 75th percentile is not clearly stated. Given the significant uncertainties in Kd

as well as other components within these systems (i.e. percentage of microalgae, peryphyton etc), one 

would think a 95th percentile would be more conservative.

8. Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and Parchaso (2010) 

(both provided) indicate that populations and population densities of Corbula appear to decline in 

wet years, and increase in dry years. The articles suggest that Corbula may be sensitive to flow into 

the estuary and/or the salinity of the estuary (and other factors). The model shows that when the 

system has higher Kd values (representative of dryer years), more selenium is available for 

bioaccumulation and more selenium accumulates in the food chain (which would drive the draft 

criteria to protect the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

I cannot comment on the life history of Corbula. The descriptions provided regarding wet and dry years and 

Kd values appear scientifically sound.

9. Transects: Discuss whether it is reasonable and representative to use the focused seaward transects 

to model the clam-based food webs, in order to eliminate the end-member variability at the ocean (at 

the Golden Gate Bridge, the entrance to San Francisco Bay). Does the geology of the focused area (a 

wide Suisun Bay flowing into a constricted Carquinez Strait), and the importance of the area (the 

location of refinery discharges), favor the use of focused transects to best represent the clam-based 

food web for the estuary? Please discuss.

The life history and ecology of Corbula is outside of my expertise. Given the complexity of the SFBD and the 

movement of organisms throughout its confines indicates that focus upon specific areas particularly with an 

element with incredible geospatial movement may not be fruitful. 

Results and Application

10. Results: Discuss whether the model input and results scientifically support the water quality criteria 

for the three food chains represented. Are there better ways to represent the three food webs in the 

modeling? Please explain.

This has been addressed above. In summary, neglecting the movement of spawning females to freshwater 

habitats may under-estimate exposure to Se due to consumption of mixed trophic levels during 

vitellogenesis. Examples would include forage fish species as dietary items; thus, adding another TTF. 
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Discuss whether it is scientifically supportable to use the modeling results for the clam-eating fish as a 

surrogate to protect clam-eating birds and insect-eating fish (all three food webs) in addition to the 

clam-eating fish themselves. Discuss whether there are other scientifically defensible ways to use the 

modeling results to protect all the categories of species for the SFBD.

As mentioned above, revision or modification of the model to include “inbound” adult salmonids as a foodweb category is essential. Salmonids are clearly insect-eating going out to sea, but are fish-eating when 

returning. The rationale for this is the continued exposure to developing ova, which upon fertilization will be 

embryonic. In addition, the addition of uncertainty factors for salinity and Se interactions may also be 

warranted in species were maternally exposed embryos are discharged in salt or brackish water 

environments. 

11. Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and validated for 

SFBD, and a considerable amount of site-specific data are available and are used to model the estuary. 

EPA is using the model results to inform the criteria development process, and presents draft multimedia criteria in Chapter 4. These criteria are a whole body or muscle fish tissue value and a clam 

tissue value that directly reflect selenium levels that are considered to be protective of aquatic biota; 

and corresponding chronic and intermittent water column values, as well as a particulate 

concentration value to facilitate regulatory controls of selenium sources. 

Given the relevance and detail of the USGS model and the abundance of site-specific data, discuss 

whether the criteria in the different media are scientifically defensible and protective for this aquatic 

ecosystem. 

The criteria are certainly an excellent first step in setting site-specific values. However, the criteria seem to 

be focused exclusively on Corbula consuming predators with the assumption that they are the most 

sensitive species with regard to Se toxicity. Consequently, this criteria is more appropriately a site and 

species-specific criteria primarily toward sturgeon. While numerically the values may protect ocean-bound 

salmonids, it is unclear whether “in-bound” offspring would experience the same protection. Likewise, it is 

unclear the value would protect broadcast spawning species in brackish or saltwater environments. 

12. Translator (Permits): EPA regulations generally call for NPDES permit limits to be expressed in terms of 

total recoverable values (40 CFR 122.45(c)). The model applied in this document yields calculations of 

water column values in the dissolved form. For most metals, EPA recommends dissolved values for 

water quality criteria, because the dissolved form better represents the bioavailable fraction in the 

water column to aquatic biota. However, for selenium, aquatic biota can take in the dissolved and 

particulate fractions in the water column, which are then moved into and up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to determine a 

total recoverable permit limit; the translator is the fraction of the total recoverable metal in the 

downstream water (downstream of a discharge) that is dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate method to 

derive a translator? Explain your answer.

Se can exist in dissolved states in many forms with the inorganic forms (selenite and selenite) predominating 

most discharges. Thus, it is difficult to determine a translator from particulate to dissolved forms when the 

most toxic form of Se is likely organic. Without the ability to measure the most toxic species of Se (SeMe or 
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SeCys, or other organic forms) and understand the kinetics of transformation to organic forms, tremendous 

uncertainty exists in setting a translator value. Consequently, additional modeling is necessary to estimate 

the transformation rates of inorganic to organic (particulate) in a site-specific manner which nutrient input 

and hydrology will likely play significant roles. 

References

Beckon, WN. Cary Parkins, C., Maximovich, A. and Beckon, AV . (2008). A General Approach to Modeling 

Biphasic Relationships. Environ. Sci. Technol. 42, 1308-1314.

Eaton, DL and Gilbert SG (2013) Principles of Toxicology IN: Casarett and Doull’s Toxicology : The Basic 

Science of Poisons. (C. Klaassen Editor) McGraw Hill Publishers, New York, NY. pp13-48.

Fan, XA, Klein, M, Flanagan-Steet, HR, Steet, R. (2010). Selective Yolk Deposition and Mannose 

Phosphorylation of Lysosomal Glycosidases in Zebrafish Journal of Biological Chemistry 285:32946-32953. 

Janz, D.M., D.K. DeForest, M.L. Brooks, P.M. Chapman, G. Gilron, D. Hoff, W.A. Hopkins, D.O. McIntyre, C.A. 

Mebane, V.P. Palace, J.P. Skorupa and M.Wayland. (2010). Selenium Toxicity to Aquatic Organisms in: 

Chapman PM, Adams WJ, Brooks ML, Delos CG, Luoma SN, Maher WA, Ohlendorf HM, Presser TS, Shaw DP 

(eds). Ecological Assessment of Selenium in the Aquatic Environment. SETAC Press, Pensacola, FL, USA.

Linares-Casenave, J. Linville, R. Van Eenennaam, JP, Muguet,JB and Doroshovy, SI. (2015). Selenium tissue 

burden compartmentalization in resident white sturgeon (Acipenser transmontanus) of the San Francisco 

bay delta estuary Environmental Toxicology and Chemistry, Vol. 34, No. 1, pp. 152–160, 2015

Lavado, R., Shi, D. and Schlenk, D. (2012). Effects of salinity on the toxicity and biotransformation of lselenomethionine in Japanese medaka (Oryzias latipes) embryos: Mechanisms of oxidative stress. Aquatic 

Toxicology108:18-22.

Linville RG. 2006. Effects of selenium on health and reproduction of white sturgeon Acipenser 

transmontanus): Implications for San Francisco Bay-Delta. PhD thesis. University of California, Davis, CA, 

USA.

Kupsco, A. and Schlenk D. (2014). Mechanisms of Selenomethionine Developmental Toxicity and the Impacts 

of Combined Hypersaline Conditions on Japanese Medaka (Oryzias latipes). Environ. Sci. Technol. 2014, 48, 

7062−7068.

Rigby, MC., Lemly, AD, Gerads, R.(2014). Fish toxicity testing with selenomethionine spiked feed – what's the 

real question being asked? Environ. Sci. Processes Impacts, 2014, 16, 511–517

Thomas, JK and Janz DM (2014) In ovo exposure to selenomethionine via maternal transfer increases 

developmental toxicities and impairs swim performance in F1generation zebrafish (Danio rerio) Aquatic 

Toxicology 152:20-29.

USEPA, 2014: U.S. Environmental Protection Agency. 2014. External Review Draft, Aquatic Life Ambient 

Water Quality Criteria for Selenium – Freshwater 2014. Office of Water, EPA 822-P-14-001. Washington, D.C.
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COMMENTS SUBMITTED BY

Reviewer 4

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External Peer Review of Draft Aquatic Life and Aquatic-Dependent Wildlife Selenium Water Quality 

Criteria for the San Francisco Bay and Delta (2014)

1. Overall Document Organization and Structure

Overall the document is well written and well organized. It could be enhanced with additional summary 

information concerning selenium flux, loading and discharge in this complex ecosystem, as well as additional 

discussion of water column concentrations and estuarine fresh-salt water dynamics that impact Se 

sedimentation. The major routes of fish and avian wildlife dietary exposure are very well treated. USGS 

Professional Paper 1646, USGS 2010 and the CA Water Board TMDL documents for the North Bay can be the 

source of some of this water column and loading information. Note: p13 “across the full range of bay 

salinities (<1 to 33 parts per trillion, ppt)” should be parts per thousand. 

Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development:

In the SF Bay estuarine system, the interface of marine and freshwater from the Delta causes sedimentation 

of much of the freshwater suspended particulate load and many dissolved ions. The major concern of Se 

toxicological impact modulation would come from the presence of dissolved sulfate at the lowest trophic 

levels. With sedimentation however, sulfate reducing bacteria come into play in the sediment forming 

reduced sulfur compounds reacting with co-precipitating dissolved metal salts and mineral adsorbents (see 

for example: Chambers, et al. Sulfate reduction and sediment metabolism in Tomales Bay, California. 

Biogeochemistry 1994, Volume 25, Issue 1, pp 1-18). Thus, it is reasonable to suggest that marinefreshwater interface sedimentation caused by common ion effect and other solution processes, would 

increase the sedimentation of dissolved and suspended Se, forming the well-recognized sediment Se 

exposure pathway of the SF Bay, while providing for a sulfate reduction pathway (and Se as well) that would 

decrease sulfate competition in selenium exposure as well as modifying Se oxidation state and chemical 

forms (see for example: Meseck & Cutter. 2012. Selenium Behavior in San Francisco Bay Sediments. 

Estuaries and Coasts (2012) 35:646–657). With sediment Se exposure as a prime pathway in this ecosystem, 

it is reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic life in 

the SFBD.

3. Toxicity Benchmarks/Green Sturgeon:

De Riu et al. (2014) is an solid Se aquatic toxicity study (lacking genetic diversity due to green sturgeon listing 

and single egg donor), however the exposure diets of 0 (common 1-2 mg/kg Se diet formulation 

background), 50, 100 and 200 (2.2 ± 0.2, 19.7 ± 0.6, 40.1 ± 1.5, and 77.7 ± 3.6 mg Se/kg dw tested) are a very 

high exposure range that lack clear environmental dietary relevance. Indeed Kleckner, et al. 2010 (Selenium 

Concentrations and Stable Isotopic Compositions of Carbon and Nitrogen in the Benthic Clam Corbula 

amurensis from Northern San Francisco Bay, California: May 1995–February 2010. USGS Open-File Report 

2010–1252) observe a max Corbula Se concentration of 22 mg/kg dw. Thus there is really only a two point 

dose response curve (2.2 and 19.7 mg/kg) for the major dose-response range dynamic in the SFBD area of 

interest for clam feeding fish. The data of Table 1 for fixed ration Growth Performance in De Riu et al. (2014) 

for the 0 and 50 exposure groups through 8 weeks appear to have very high variability and a very large 95% 
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confidence interval that includes zero. The ANOVA used assumes normality but the data appeared skewed 

and may require transformation to explore effects estimation. The effect of time in this study is also not 

adequately explored. Thus the Control and 50 dose group data for %BWI/d as presented in Table 1 are far 

less than definitive and not justified to warrant the summary conclusion of the paper characterized as “the 

low SeMet diet (19.7 ± 0.6 mg Se/kg dw), which caused severe adverse effects in green sturgeon”. Hence De 

Riu et al. (2014) is not definitive as a single study for lower (<20 mg/kg Se) diet concentration growth effects 

in clam eating fish.

%BWI/d 2wk 4wk 6wk 8wk

0 control 4.5 ±1.8

(± 40%)

11.9 ± 6.1

(± 51%)

6.3 ± 15.9

(± 252%)

6.6 ± 14.9

(± 226%)

50 3.8 ± 3.9

(± 103%)

6.8 ± 8.4

(± 123%)

3.1 ± 14.8

(± 477%) 

2.6 ± 16.0 

(± 615%)

The control (2.2 mg/kg Se dw diet) green sturgeon total body Se at 4 weeks (6.5 ± 0.9) and 8 weeks (7.1 ± 

0.9) have upper confidence bounds of 7.4 mg/kg and 8.0 mg/kg respectively and thus in close proximity to 

the draft national benchmark value of 8.1 mg/kg dw. Given the noted genetic variability limitations of De Riu 

et al. (2014) as well as their observation of potential fixed ration study impacts to the rapid growth phase of 

juvenile green sturgeon, as well as expected routine laboratory sampling variability and method bias, the 

protection of juvenile green sturgeon is qualitatively and quantitatively afforded by the draft national 

benchmark value.

In light of the lack of precision of the De Riu et al. (2014) study to benthic clam Se concentration range in this 

ecosystem, and the variability of the Table 1 data, the NOEL of 6-7 mg/kg dw (whole body) is protected by 

the draft national benchmark value of 8.1 mg/kg dw.

4. Toxicity Benchmarks/Model:

Biphasic models have good potential to address toxicological impacts from essential trace elements (ETEs). 

However the broad range of existing and published aquatic toxicology studies were not experimentally set 

up with the broad range of dosage required to explore effects with this model. Log-logistic models are 

acceptable common practice, although all models have limitations. Selenium appears to have very poor 

regulatory homeostasis as an essential trace element, compared to for example copper or manganese, and 

this confounds the use of biphasic models for selenium. The 2008 work of Beckon notes that the complexity 

of dietary selenium exposure makes it a poor choice for biphasic modeling:

“Although biphasic relationships are common, it should not be assumed that dose-response 

relationships are biphasic as a general rule, even for substances that are well known to cause 

biphasic responses. For example, dietary exposure to selenium is widely understood to have biphasic 

effects on the health and survival of animals ... However, although embryonic malformations are one 

of the effects of high levels of selenium, such defects are not known to be among the effects of 

selenium deficiency. Therefore, data relating selenium exposure to developmental defects ... may be 
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best represented by a monotonic dose response model. Affecters of organisms are more likely to 

exhibit biphasic dose-response relationships if the effect or “endpoint” assessed is a general 

measurement of health, growth, or fecundity rather than a narrower effect such as embryonic 

deformities, liver lesions, or thrombosis. This is because broad measures of effect are more likely to 

represent a combination of individual beneficial low-dose and deleterious high-dose mechanisms or 

responses. A narrow end point is more likely to capture the consequence of a particular mechanism 

or response that may be only beneficial or only deleterious to the organism, and would be 

represented better by a monotonic dose-response model.” (Beckon, et al. Environ. Sci. Technol. 2008, 

42, 1308–1314)

As noted, biphasic models work best when the response tracked in the study appears in both deficiency and 

toxicity realms of exposure. The somewhat narrow toxic reproductive endpoints found in Se exposure limit 

the usefulness of the biphasic approach. Thus a) the Se dose response analysis is better represented loglogistic model. Skewing of the slope of the regression due to incomplete deficit dosing maybe unlikely since 

all experimental diets will have a baseline Se concentration from the protein and other constituents in the 

diet formulation prior to test agent fortification (i.e. addition of selenate, Se-methionine or other quantified 

Se substance). Dietary formulations for deficiency studies are very difficult with ETEs since they are 

ubiquitous at low, often required, levels in many healthy plants and animals. The cautions of Beckon, et al. 

2008 concerning the application of biphasic modeling to selenium suggest b) it is not reasonable and 

appropriate to use a biphasic model to determine EC10 values for studies in the literature on representative 

or key species for the SFBD. A standard log-logistic model is appropriate and acceptable in the exploration of 

high dose Se reproductive toxicity. 

5. Species and Diet:

The seminal works of Luoma and Presser have wonderfully characterized Se behavior, impacts and risks in 

the SFBD. Building on these works, the SFBD is exceptionally well characterized through several decades of 

modeling, samplings and scientific analyses exploring pathways and endpoints for selenium in this complex 

and important ecosystem. The body of SFBD work has been thorough in its analysis of key species and the 

related food webs that contribute to our understanding of Se impacts. The dietary information for the key 

species analyzed are well cited and well explored. More recent work with sturgeon (Zeug et al, 2014) fills 

data gaps with this key species. The resulting TTFs thus represent the use of best available data. 

Opportunistic feeding and food web dynamics related to the hydrograph and climate variability add 

variability to dietary understanding, however it is unlikely that precise quantitative analysis that addresses 

the full range of variables is possible. Thus a) TTFs derived from best available information, such as done in 

the current analysis is generally scientifically supportable.

Specifically for Clam-eating Fish:

The diet mixes appear to be representative of our current understanding of clam-eating fish in the SFBD. 

Newer work such as Zeug et al., 2014 are valuable assets in modeling dietary exposure and thus TTFs. 

Without clear and definitive data addressing system variability, such as the case for many organism’s diet, 

best informed estimates that balance what is known against what is unknown or unquantified, is a 

reasonable approach. This appears to be the approach taken when the ; USFWS, 2008 and Feyrer et al. work 

is used:
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Diet: Splittail are benthic (bottom) feeders. Stomach contents from 70 splittail collected in Suisun 

Marsh between March 1998 and January 1999 indicate that their diet consisted mainly (43%) of 

unidentified material, probably detritus. The overbite clam (Asian clam), Potamocorbula amurensis, 

and other mollusks constituted 34% of the diet (Feyrer and Matern 2000, Feyrer et al. 2003). 

Thus b) it is scientifically supportable to postulate that splittail do not consume as much Corbula, and are 

therefore not as much at-risk as sturgeon. 

While of value, De Riu et al., 2014 for reasons stated above, is neither definitive nor quantitatively precise in 

assessing green sturgeon Se exposure risk in the SFBD. If there is high confidence in the observation that 

sturgeon use the “SFBD as a migratory corridor to freshwaters for spawning” and that egg maturation is 

primarily complete before entering the estuary, then there is cause to c) scientifically support a conclusion 

that adult white sturgeon and juvenile white and green sturgeon will be more affected than adult green 

sturgeon from elevated levels of selenium in Corbula in the SFBD. The 0 and 50 group dose-response data in 

De Riu et al., 2014 compared to the observed Corbula Se concentrations in Kleckner, et al. 2010, suggest 

lower level risks for juvenile growth toxic endpoints.

Specifically for Clam-eating Birds:

The review document presents a solid and defensible understanding of diet mix and this dietary exposure 

for the California Ridgway’s rail, scoter and scaup. While habitat choice and thus exposure potential from 

Corbula are beyond my area of expertise, d) it seems plausible and reasonable to assess exposure potential 

in this manner. My work with aquatic birds in this ecosystem (Möller, G. 1996. Biogeochemical Interactions 

Affecting Hepatic Trace Element Levels in Aquatic Birds. Environmental Toxicology and Chemistry 15:1025-

1033) suggests a complex dietary and environmental correlation of Cd–Mn–Se–Hg–As in vivo that may 

confound simple monotonic dose response models. Indeed, the tissue data for several of the sediment 

feeding birds demonstrated very high Se levels compared to other species. While complex, d) it does appear 

reasonable to decrease the percentage of Corbula in scoter and scaup diet by one half for Se selenium 

exposure and the potential for reproductive endpoints.

Specifically for Insect-eating Fish:

With several decades of monitoring, the insect diet mixes for SFBD insect eating fish are e) scientifically 

supportable and well discussed and referenced in FWS, 2008. 

6. Trophic Transfer Factors (TTFs):

The TTFs taken from the work of Presser and Luoma, USGS and the 2014 Draft Criteria are to my knowledge 

the best available TTF values that could be applied in a SFBD model; I am not aware of any additional TTF 

values that would supplant this data. The additional Corbula TTF values developed in USGS 2010 pp21-23 is 

coherently presented and contains well supported assumptions (e.g. dissolved Se exposure is trivial). The 

USGS 2010 Corbula TTF analysis is explicitly developed and cited in this reference and I am not aware of any 

additional analyses of this caliber and quality. 
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7. Partitioning Coefficients (Kds):

Without data, median or mean values build on absent data estimate that may underestimate risk without 

cause. Using the 75th percentile of the Kd values seems to be an appropriately protective estimate, allowing 

for precaution in dry/low flow higher know risk conditions. .

8. Corbula Concentrations:

The work of Fuller et al. (2012) and Thompson and Parchaso (2010) predate the current CA critically dry 

conditions. Since the Fuller work appears based on ongoing BDMA assessments, incorporating the most 

recent data may help validate population relationships with the hydrograph which has experienced some 

extreme changes in recent years. With flow reductions in critically dry years, Se flux into the SFBD may also 

be reduced although existing sediment Se loading may provide Se exposure hysteresis and landward shift of 

the sedimentation zone at the salt – fresh water interface of the estuary. Increased salinity into the 

landward reaches of the SFBD may shift Corbula populations with shifting salinity. Lentic/lotic Se dynamics 

that are understood for freshwater systems may not translate well to marine sediment exposure pathways 

observed in the SFBD, as stated in “more selenium is available for bioaccumulation and more selenium 

accumulates in the food chain.” A quick trend analysis of the most recent BDMA data may be sufficient to 

explore this concern. 

9. Transects:

Transects are a valuable way to explore sediment variability with regards to flow and sedimentation 

dynamics in the SFBD. Focused transects can help define food web variability that would result from geology 

and fresh-salt water estuarine sedimentation processes. Yearly hydrograph changes and critically dry 

periods may induce unacceptable variability in any model input, however. 

Results and Application

10. Results:

The review document model input and results support the water quality criteria for the three food chains 

represented. The models and model inputs used appear to be the best available approach using peerreviewed data and approaches. The SFBD is an exceptionally well-studied aquatic ecosystem with regards to 

water flows, Se, fish and wildlife. The TTF model, the dietary estimates, and the selection of key species all 

represent our best available knowledge. Clam eating fish represent a worse case exposure scenario due to 

the Se sediment dynamics in the SFBD and the migratory habits and thus lessened exposure scenario of 

aquatic birds in this ecosystem. In this regard, it is scientifically supportable to use the modeling results for 

the clam-eating fish as a surrogate to protect clam-eating birds and insect-eating fish, in addition to the 

clam-eating fish themselves. Other than applying safety factors to account for species differences in 

exposure and Se sensitivity (not recommended) there are no other common approaches that can be applied 

across species.
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11. Draft Criteria: 

In our current understanding of Se impacts and effects, the USGS Ecosystem-Scale Selenium model is the 

gold standard for modeling, balancing interpolation/extrapolation with required knowledge of species 

specific and ecosystem specific dynamics and values. As presented in this review document, the criteria in 

the different media are scientifically defensible and protective for this aquatic ecosystem.

12. Translator (Permits):

As with lead (Pb) exposure in birds and fish, not all of the total recoverable Se can be considered 

bioavailable (e.g. Zia et al., In vitro and in vivo approaches for the measurement of oral bioavailability of lead 

(Pb) in contaminated soils: A review. Environmental Pollution. Volume 159, Issue 10, October 2011, Pages 

2320–2327). Luoma et al 1992 (Determination of selenium bioavailability to a benthic bivalve from 

particulate and solute pathways. Environ. Sci. Technol., 1992, 26 (3), pp 485–491) has perhaps the most 

direct answer to the Se translator challenge.
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COMMENTS SUBMITTED BY

Reviewer 5

Peer Review Report Task Order 2014-19, Contract EP-C-13-009

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Peer Review of Draft Aquatic Life and Aquatic-Dependent Wildlife Selenium Water Quality Criteria for the 

San Francisco Bay and Delta (2014)

Overall Document Organization and Structure

1. Please comment on the overall construction and clarity of the document, i.e., is the layout of the 

document logical and is the document clearly written? Are there better ways to arrange and/or 

present the information? If so, please explain and describe.

Response: I found the overall construction and clarity of the document to be quite good. There was a nice 

logical progression beginning with background and then describing and assembling all the various 

components needed to develop a Se WQC for SFBD. One very minor comment, paginate the whole 

document rather than just within a section.

Questions on Use of Ecosystem-Scale Selenium Model 

2. Overall Toxicity Benchmarks/Development: The salinity throughout the San Francisco Bay and Delta 

(SFBD) varies from fresh to brackish to marine, and many of the species (or close relatives of the 

species) evaluated for the draft national freshwater criterion (USEPA, 2014 pp 57-62; 78-79 provided 

as supporting material), live in the estuary. Given these considerations, discuss whether it is 

reasonable to use the draft national freshwater selenium criterion (tissue elements) for aquatic life in 

the SFBD. Are there better ways to estimate toxicity benchmark values for key aquatic life species, 

many of which are anadromous, in the estuary? Please explain.

Response: Yes, I think it is reasonable to use the draft national freshwater selenium tissue criterion for SFBD. 

I am not aware of any evidence that suggests anadromous species are more or less sensitive to Se than 

freshwater species. Further, considering the mechanisms of action for Se, I can think of no reason to 

hypothesize that this might be the case. 

3. Toxicity Benchmarks/Green Sturgeon: For green sturgeon in the SFBD, juvenile growth is most likely 

the most sensitive endpoint. A recent study, De Riu et al. (2014) (provided), using green sturgeon1

observed selenium concentrations of 6 – 7 mg/kg dw (whole body) in the control treatment, which 

appears to reflect essentially a ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criteria of 8.1 mg/kg dw (USEPA, 2014 pp. 57-62; 78-79). In light of this 

study (and any other relevant data on green sturgeon), is the draft national benchmark value of 8.1 

mg/kg dw protective of the juvenile growth endpoint for green sturgeon? Please explain your answer.

Response: I disagree with the assertion that juvenile growth is likely the most sensitive endpoint for green 

sturgeon. In general, larval abnormality is the most sensitive endpoint for fish. Using the micro-injection to 

maternal transfer ratio for white sturgeon and applying it to green sturgeon suggests larval abnormality may 

be a more sensitive endpoint. I agree with EPA’s current assessment that while the FWS analysis suggests 

this may be the case, the uncertainty in this extrapolation precludes its use in setting a Se WQC. The fact 

that background Se in juvenile sturgeon is higher than the estimated whole body threshold using the microinjection to maternal transfer ratio further increases the uncertainty in this analysis.

 1 Green sturgeon larvae were obtained from the offspring of three males and one female originating from the Klamath 

River along the Pacific Coast. 
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I think it is clear that the draft national benchmark value of 8.1 mg/kg dw is protective of juvenile green 

sturgeon growth. The real concern is that this value is likely statistically indistinguishable from background 

Se in juvenile green sturgeon. Based on the error estimates for week 8 whole body measurements in the De 

Rui et al. study (Table 2), it is likely that at least one control sample exceeded the 8.1 mg/kg dw threshold. 

This raises concerns that some relatively large fraction (e.g., 30% based on n=3 from De Rui et al.) of 

background tissue measurements exceed the proposed Se WQC. 

4. Toxicity Benchmarks/Model: EPA uses log-logistic modeling to determine EC10 values from 

environmental data. FWS has suggested that a biphasic model (Beckon et al., 2008 and Brian and 

Cousens, 1989, cited in review document) may be a better model to represent effects of selenium, 

since a biphasic model shows a more representative progression of effect to living species (selenium is 

a micronutrient, but is toxic in both deficit concentrations and excessive concentrations). Studies on 

selenium effects in the literature (Heinz et al., 1989, Hilton et al., 1980, and Hamilton et al., 1990, 

cited in review document) do not appear to consider this issue, and study designs do not appear to 

take the issue into account. 

a. Given that many of the studies in the literature often use a small (or zero) control concentration 

of selenium, and a low level of effect is observed (reflective of a deficient concentration), is the 

response to increasing levels of selenium (into the excessive toxic amounts) better represented 

using a biphasic model or log-logistic model? Would the slope of the regression be skewed in 

either model, due to incomplete modeling of deficit effects?

b. Discuss whether it would be reasonable and appropriate to use a biphasic model to determine 

EC10 values for studies in the literature on representative or key species for the SFBD (Heinz et al., 

1989, Hilton et al., 1980, and Hamilton et al., 1990, cited in review document), or whether the 

results of using a standard log-logistic model would produce more representative results. 

Response: For some data sets, a biphasic model may better describe the data and there is a potential 

mechanistic underpinning (i.e., Se essentiality) for why such a model would be appropriate. It is possible for 

model slopes to differ depending on the data set and the interpretation of what constitutes a 10% effect can 

be quite different. The data from Hilton et al. (1980) (Figure 23; USFWS 2008) is the clearest example of such 

a data set although this particular study is problematic because fish were fed an inorganic Se diet. In 

contrast, it is not clear that use of a biphasic model on data for Chinook salmon (Figure 11; USFWS 2008) 

and mallards (Figure 1; USFWS 2008) is more appropriate than a standard log-logistic model. In general, I 

think EPA should consider use of “non-standard” models if they better describe the data and there is a 

viable mechanism for why such a model should be used. In my opinion, only the Hilton et al. study meets 

both criteria.

5. Species and Diet: Discuss whether the appropriate key species and food chains (webs) have been 

identified and appropriately applied for the SFBD. 

a. Discuss whether the diets of the key species chosen in each of the identified food webs are 

reasonable based on the literature (sources cited in review document) and whether the 

calculations of the Trophic Transfer Factors (TTFs) are scientifically supportable based on those 

diets (see specific questions below). Are there better ways to calculate TTFs for mixed diets for the 

key species? Please explain. 
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Response: Yes, based on the literature provided the identified food webs are reasonable. No, I don’t think 

there are better ways to calculate TTFs for mixed diets for the key species. Also see specific responses 

below.

Specifically for Clam-eating Fish:

b. Are the diet mixes (i.e., percentage of Corbula) for adult white sturgeon, juvenile white and green 

sturgeon, and splittail scientifically supportable based on the available literature (Zeug et al., 

2014, provided; USFWS, 2008 pp. 51-59, provided as supporting material; and Feyrer et al., 2003, 

cited in review document)? Is it scientifically supportable to postulate that splittail do not 

consume as much Corbula, and are therefore not as much at-risk as sturgeon?

Response: The diet mixes proposed by USEPA are scientifically supportable, although I would not say the 

available data precludes consideration of other dietary mixes. It would be useful to perform and provide the 

results of a sensitivity analysis on this issue. For example, if the sturgeon diet was 50% Corbula, how much 

lower would the proposed WQC need to be? I also agree that splittail do not consume as much Corbula as 

sturgeon, but do not agree this necessarily equates to them being at lower risk. Risk is an integration of 

exposure and effect (i.e., sensitivity). Given the limited studies on sturgeon and splittail in general, it is 

premature to conclude they are less or equally sensitive to Se compared to sturgeon. Therefore, the level of 

risk for splittail is unknown.

c. Recent literature indicates that green sturgeon are more sensitive to selenium than white 

sturgeon (De Riu et al., 2014, provided; and Silvestre et al., 2010, cited in review document). 

Studies have also shown that adult green sturgeon primarily use the SFBD as a migratory corridor 

to freshwaters for spawning (http://exxexpartnership.com/wpcontent/uploads/2010/10/GreenSturgeon-_1_.pdf , cited in review document). If it is known that 

egg maturation in adult green sturgeon is completed prior to entering the estuary, discuss 

whether it is scientifically supportable to conclude that adult white sturgeon and juvenile white 

and green sturgeon will be more affected than adult green sturgeon from elevated levels of 

selenium in Corbula in the SFBD (please provide references). Please comment on and discuss the 

potential for selenium exposure of adult green sturgeon during migration to play a role in 

reproductive toxicity, given the current state of knowledge on the reproductive biology and life 

history of green sturgeon.

Response: I think given our understanding of adult green and white sturgeon use of SFBD, it is likely that 

juvenile white and green sturgeon will be more affected than adults of either species. I understand that it is 

likely green sturgeon eggs mature while adults are in the ocean and that the adults appear to primarily use 

SFBD as a migration corridor. However, I think we cannot rule out the potential for toxicologically significant 

Se exposure (with respect to the eggs) during this migration. We simply don’t have any information on this 

issue and so it would be prudent not to rule out this potential exposure pathway. I don’t think there is 

enough information to address this issue in a quantitative manner, but it should be highlighted that this is a 

potentially significant uncertainty that needs further study rather than eliminating it as an issue for concern 

as the document currently does.
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Specifically for Clam-eating Birds:

d. Discuss whether the diet mixes for the California Ridgway’s rail, scoter and scaup are scientifically 

defensible based on the literature (sources cited in review document). 

Rails: Specifically, discuss whether it is scientifically supportable to assume that California 

Ridgeway’s rails do not consume a large percentage of Corbula, given that their littoral feeding 

habitat does not overlap with the preferred habitat of Corbula. 

Response: While the 20% Corbula diet used in the model is not unreasonable given the available data, it is 

entirely speculative and could be either lower or higher. A sensitivity analysis would be useful to understand 

the implications of this assumption. If all of the mollusk component of the rail diet (60%) were Corbula, 

would they become the driver of the proposed WQC? If so, what percentage Corbula in the diet causes this 

to be the case?

Scoter/Scaup: It has been observed that in the early part of the winter, scoter and scaup live 

mainly in the shallower, northern parts of the Bay (Suisun Bay and San Pablo Bay) where Corbula

are prevalent, and consume mostly Corbula. However, in the latter part of the winter and early 

spring prior to migration, they move down to the deeper, Central Bay area where Corbula are not 

as available, and the scoter and scaup are thought to consume mostly herring that do not 

bioaccumulate selenium as effectively. They then follow the coast north and continue to feed on 

herring prior to breeding and nesting. It appears that selenium moves directly from the diet of 

water birds into their eggs and not from internal tissue stores (Chapman et al., 2010). Given that 

scoter and scaup switch to a low selenium diet for a period prior to migration as well as during 

migration, and that selenium may be depurated at a higher rate during migration, and that scoter 

and scaup are likely to continue to feed on low selenium fish sources in the breeding and nesting 

grounds, is it reasonable to decrease the percentage of Corbula in scoter and scaup diet by 1⁄2 

(from 90% to 45%), with respect to selenium exposure that could impact reproductive endpoints? 

Please discuss.

Response: While I follow the thinking used by EPA in setting the Corbula diet at 45% for scoter and scaup, 

I’m not convinced it is particularly accurate. In assessing potential exposure of green sturgeon, EPA asserted 

that sturgeon eggs mature while adults are in the open ocean where Se exposure is low. It seems a similar 

line of thinking should be used in assessing risks to scoters and scaups. When are egg proteins deposited 

relative to the timing of duck feeding on Corbula. If Corbula feeding occurs during this period, then exposure 

should be high, if not, then there is no relevant exposure with respect to the SFB WQC. It should also be 

pointed out that even if exposure is relatively high, there is evidence that ducks depurate Se relatively 

rapidly (19 d half life for mallards; (Heinz et al. 1990) ) and marine ducks deposit a low percentage of Se in 

eggs relative to freshwater species (Henny et al. 1995).

Specifically for Insect-eating Fish:

e. Are the diet mixes for SFBD steelhead, Chinook salmon and delta smelt used in the model 

scientifically supportable based on the literature (sources cited in review document)? Please 

explain.
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Response: The diets of SFBD steelhead, Chinook salmon and delta smelt are relatively well studied. These 

are probably the most reliable diet mixes of any of the species evaluated by EPA. Most importantly, there is 

high confidence that these species are not consuming mollusks.

6. Trophic Transfer Factors (TTFs): Discuss whether the TTF values taken from the literature for 

individual species based on their food prey are scientifically supportable, i.e., are the TTF factors in 

Table 4-3 (values taken from USGS, 2010, Figure 11 and pp. 21 – 24; Presser and Luoma, 2010, Table 3; 

and USEPA, 2014, pp. 71-77, each provided as supporting material) scientifically supportable and 

appropriate? Are you aware of any other or more applicable data?

The TTF for Corbula (17) was developed and discussed in USGS (2010) pp. 21 -23 (provided as 

supporting material). Discuss whether this analysis is scientifically supportable. Would you 

recommend a different or better way to develop a site-specific TTF for Corbula, given the available 

data? Are there additional data that the Agency should consider for developing a site-specific TTF for 

Corbula?

Response: In general, the TTFs taken from the literature and used in the WQC derivation are scientifically 

supportable. Specific to Corbula, yes I agree with the novel approach taken by USGS and adopted by EPA in 

this analysis.

One issue of concern is that EPA has opted to use a TTFfish of 1.1 in all cases despite species-specific TTFs 

being available in some circumstances. For example, USGS derived a TTF for white sturgeon of 0.8. It is 

unclear why EPA selected to use a mean value from across species, as generally in WQC derivation, specificspecific data is given preference.

Also, see comments below on Kds and the need to consider concentration-dependent TTFs.

7. Partitioning Coefficients (Kds): In general, selenium is more bioavailable for accumulation into the 

food chain during dry/low flow (lentic) conditions, which is represented by higher Kd values. Is it 

scientifically supportable to use the 75th percentile of the Kd values for each of the landward and 

seaward transects in the modeling, since data representing critical (very dry) conditions are not 

available? Would the use of median or mean Kd values be more appropriate? Please explain your 

answer.

Response: Yes, I think it is appropriate to use the 75th percentile Kd in the modeling given the variability in 

Kds both seasonally and annually, and the uncertainty regarding Kds during very dry conditions. One 

uncertainty with Kds that EPA has not considered is that they tend to be inversely related to exposure 

concentration (see Poster by DeForest et al. provided with comments). EPA should evaluate concentration 

dependency in their assessment and consider using a quantile regression approach for determining the 75th

percentile Kd. It is also worth noting that if EPA were to use this type of approach at each step in the model 

(i.e., 75th percentile at each step), this would equate to a 98th percentile site/condition (assuming each step 

is independent), which I believe most would consider adequately protective.

8. Corbula Concentrations: Information in Fuller et al. (2012) and in Thompson and Parchaso (2010) 

(both provided) indicate that populations and population densities of Corbula appear to decline in 

wet years, and increase in dry years. The articles suggest that Corbula may be sensitive to flow into 

the estuary and/or the salinity of the estuary (and other factors). The model shows that when the 

system has higher Kd values (representative of dryer years), more selenium is available for 
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bioaccumulation and more selenium accumulates in the food chain (which would drive the draft 

criteria to protect the use downward), and thus, we are concerned with accounting for critically dry 

conditions. Please comment on the articles and these statements.

Response: I agree with EPA’s assessment that the available literature indicates the density and distribution 

of Corbula is constrained by low salinity (i.e., freshwater) and that the system exhibits higher Kds during drier 

years resulting in more Se bioaccumulation higher in the food chain. To some extent, EPA has accounted for 

the inter-annual variability by selecting the 75th percentile Kd for modeling. However, the possibility of 

higher Kds during exceptionally dry years or seasons cannot be ruled out. In the absence of any data in such 

conditions, I think all EPA can do at this point is commit to monitoring the system next time such conditions 

occur and ensuring the option of revising the site-specific WQC based on new data. I do not think it 

reasonable to speculate on how much Kds will change under such conditions and a priori increase the Kds in 

the models.

9. Transects: Discuss whether it is reasonable and representative to use the focused seaward transects 

to model the clam-based food webs, in order to eliminate the end-member variability at the ocean (at 

the Golden Gate Bridge, the entrance to San Francisco Bay). Does the geology of the focused area (a 

wide Suisun Bay flowing into a constricted Carquinez Strait), and the importance of the area (the 

location of refinery discharges), favor the use of focused transects to best represent the clam-based 

food web for the estuary? Please discuss.

Response: Yes, I generally agree with EPA’s decision to use the focused seaward transects to model the 

clam-based food webs. The end-member Kds clearly represent a set of conditions that differ from the 

focused area and the focused area is most relevant for exposure to both fish and bird species of interest. 

Having said that, as the document currently reads, it appears that EPA eliminated the Kds from near Golden 

Gate simply to reduce variability. I think a better rationale for eliminated these data would be related to the 

presence/absence of relevant receptors (i.e., fish and birds). Are there species we should be concerned 

about in the outer Bay where Kds appear to be significantly higher? Some discussion of this issue in the 

document would be useful.

Results and Application

10. Results: Discuss whether the model input and results scientifically support the water quality criteria 

for the three food chains represented. Are there better ways to represent the three food webs in the 

modeling? Please explain.

Discuss whether it is scientifically supportable to use the modeling results for the clam-eating fish as a 

surrogate to protect clam-eating birds and insect-eating fish (all three food webs) in addition to the 

clam-eating fish themselves. Discuss whether there are other scientifically defensible ways to use the 

modeling results to protect all the categories of species for the SFBD.

Response: Yes, the proposed approach, model input and results all appear scientifically defensible for 

establishing water quality criteria protective of all three food chains. It was appropriate for EPA to evaluate 

all three chains, but ultimately one of these food chains will drive the WQC. It is not surprising that clameating fish are the driver and it is scientifically defensible to use WQC derived from this model to protect all 

species in the system. 
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11. Draft Criteria: The USGS Ecosystem-Scale Selenium model was originally developed and validated for 

SFBD, and a considerable amount of site-specific data are available and are used to model the estuary. 

EPA is using the model results to inform the criteria development process, and presents draft multimedia criteria in Chapter 4. These criteria are a whole body or muscle fish tissue value and a clam 

tissue value that directly reflect selenium levels that are considered to be protective of aquatic biota; 

and corresponding chronic and intermittent water column values, as well as a particulate 

concentration value to facilitate regulatory controls of selenium sources.

Given the relevance and detail of the USGS model and the abundance of site-specific data, discuss 

whether the criteria in the different media are scientifically defensible and protective for this aquatic 

ecosystem.

Response: Yes, given the abundance of both site-specific and detail of the USGS model, the proposed 

criteria in different media are both scientifically defensible and protective of the SFB aquatic ecosystem. I 

have been involved with several dozen site-specific water quality criteria/standard projects and the amount 

of data and model development involved in deriving these site-specific criteria far exceed (by an order of 

magnitude in most cases) the level of information typically available in these efforts.

12. Translator (Permits): EPA regulations generally call for NPDES permit limits to be expressed in terms of 

total recoverable values (40 CFR 122.45(c)). The model applied in this document yields calculations of 

water column values in the dissolved form. For most metals, EPA recommends dissolved values for 

water quality criteria, because the dissolved form better represents the bioavailable fraction in the 

water column to aquatic biota. However, for selenium, aquatic biota can take in the dissolved and 

particulate fractions in the water column, which are then moved into and up the food chain.

For most metals, EPA allows a translator to be used on the dissolved criteria value, to determine a 

total recoverable permit limit; the translator is the fraction of the total recoverable metal in the 

downstream water (downstream of a discharge) that is dissolved.

Given that selenium has different bioaccumulation dynamics, what is an appropriate method to 

derive a translator? Explain your answer.

Response: I agree with EPA’s approach of setting the translator at 1.0 for Se. For divalent metals, the 

dissolved fraction is clearly the dominant bioavailable metal fraction with respect to toxicity. While there is 

considerable evidence that particulate divalent metals are can be bioaccumulated, their toxic potency 

appears to be considerably less than the dissolved form in most cases. In contrast, for Se, it is clear that both 

dissolved and particulate Se are bioavailable and toxic to aquatic organisms. Consequently, it is appropriate 

to use a translator of 1.0 for Se.

Other Comments

Section 1, Page 11, Last Paragraph: There are numerous papers and reports indicating Se is the most dosesensitive nutrient, but I have yet to see a quantitative analysis supporting this statement. Several other 

metals also have very narrow ranges between essentiality and toxicity that are comparable to Se. For 

example, Cu (for snails) and Zn (for daphnids) both have very narrow ranges between essential 

requirements and toxicity. Unless EPA is aware of a quantitative analysis supporting this statement I suggest 

deleting it.

Section 1, Page 13, First Paragraph: “ppt” stands for parts per thousand, not trillion in this instance.
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Section 2, Page 8, First Paragraph: Risk is an integration of exposure and effect (i.e., sensitivity). Just because 

clam-eating species have the highest exposure doesn’t necessarily mean they are most at risk. It depends on 

their sensitivity as well.

Section 4, Page 11, First Paragraph: It is unclear why bioaccumulative based effects are of greater concern 

that other effects. Why would this type of effect justify a lower effect threshold? Regardless of the 

mechanism, the EC10 represents a 10% effect.

References

Heinz, G. H., G. W. Pendelton, A. J. Krynitsky and L. G. Gold (1990). "Selenium accumulation and elimination 

in mallards." Arch. Environ. Contam. Toxicol. 19: 374-379.

Henny, C. J., D. D. Rudis, T. J. Roffe and E. Robinson-Wilson (1995). "Contaminants and sea ducks in Alaska 

and the circumpolar region." Environ. Hlth. Per. 103(4): 41-49.

Hilton, J. W., P. V. Hodson and S. J. Slinger (1980). "The requirement and toxicity of selenium in rainbow 

trout (Salmo gairdneri)." J. Nutr. 110(12): 2527-2535.
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Attachment Provided by Reviewer 5