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EPA Response to Comment Document 

April, 2016

EPA Reponses to Peer Reviewer Comments in 

the Peer Review Summary Report, July 28, 2015

Peer Review of the Draft Aquatic Life and AquaticDependent Wildlife Selenium Water Quality Criteria 

for the San Francisco Bay and Delta (2014)
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EPA RESPONSE TO COMMENT DOCUMENT

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.

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.

EPA: Thank you for your comments.

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 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. 

EPA: Thank you. Since birds are considered important eco-receptors in the San 

Francisco estuary, EPA felt it was necessary to consider them in this site-specific 

analysis. 

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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EPA: EPA agrees that uncertainty in the analysis is important. We have added 

information on uncertainty and sensitivity in the document.

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. 

EPA: Thank you for your comments. EPA agrees that uncertainty analyses would be 

useful; we have added information and discussion in the document.

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 

range of bay salinities (<1 to 33 parts per trillion, ppt)” should be parts per thousand. 

EPA: Thank you for your comments. 

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.

EPA: Thank you for your comments.
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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.

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 

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.

EPA: EPA acknowledges your comments concerning applying data from freshwater 

species to saltwater species. However, the freshwater species sensitivity distribution

for the national criterion includes several anadromous fish, particularly at the sensitive 

end of the distribution. Also, the available toxicity data indicates little difference in the 

sensitivity between freshwater, saltwater, and anadromous estuarine species of fish.

Based on the concerns presented, the use of data for freshwater species is likely 

conservative for saltwater species, given that ionic content (i.e., sulfate) of saltwater 

limits the bioavailability of selenite. Given the lack of specific data, and the 

uncertainty, using the national freshwater tissue criterion for the estuary is reasonable.

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 
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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 g/L and g/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.)

EPA: Thank you for your comments. Given the lack of specific data, with the exception 

of the available information on anadromous species, EPA believes it would not be 

sufficiently informative to solicit expert opinion on an issue that has a paucity of data.

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 

al. 2014).

EPA: Thank you for your comments. EPA does not agree that selenium may be an 

endocrine disruptor in the sense that it is receptor-based; the general scientific 

consensus is that excess selenium is involved with oxidative damage, but the evidence 

is not conclusive. Despite some uncertainty, EPA believes using the national freshwater 

tissue criterion for the estuary is reasonable. 

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.
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EPA: Medaka are freshwater species while anadromous fish are specifically adapted to 

a certain lifestyle and undergo major physiological changes when transitioning from 

freshwater to saltwater lifestyles. In the study of medaka, salinity may have been more 

of an acute stressor that affected homeostasis causing indirect effects manifested as 

developmental toxicity in offspring. If the medaka had been acclimated over time to a 

given salinity regime, as is the case for anadromous species, the effects may have a 

more significant meaning. 

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.

EPA: The salinity of the San Francisco Bay and Delta ranges from freshwater (1 ppt) to 

marine (approximately 33 ppt); the conditions in the estuary are not hypersaline (above 

the salinity of seawater, or above 35 ppt). A key uncertainty is the level of maturity of 

eggs and whether vitellogenesis is still actively occurring. If vitellogenesis is complete 

or nearly complete before entering the estuary, then only minimal amounts of 

selenium from diet in the estuary would be transferred to eggs.

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). 

EPA: A key issue is the species and their exposure to selenium in marine systems. The 

document looks at salmonid smolts, since adults are semelparous and receive most 

(almost all) of their selenium exposure from an ocean diet outside of the estuary before 

returning to spawn. Salmonid smolts are exposed through diet in a freshwater 

environment until they transition to saltwater, and are then exposed through diet to 

selenium in the estuary for three months to a year before moving out into the ocean.

We have clarified this in the document.

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). 

EPA: Both the national freshwater selenium criterion and the proposed criteria for the 

San Francisco estuary do not consider and/or incorporate the marine dietary exposure 

to selenium. Although it is uncertain as to how much selenium returning salmon take 

up as they pass through the estuary, it is likely very small in comparison to the dietary 

component taken up and delivered via maternal transfer to eggs during oogenesis and 

vitillogenesis which primarily (almost exclusively) occurs prior to entering the estuary. 

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 
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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 

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

EPA: Thank you for your comments and the proposed scientific rationale as to why the 

national freshwater criterion would be reasonable.

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.

EPA: Thank you for your comments.

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 ‘no-effect’ concentration for growth or other effects, and is similar to 

the draft national whole body criterion 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.

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 

of 8.1 ppm.

EPA: EPA agrees that the difference between 8.0 - 8.1 mg/kg dw and 6 - 7 mg/kg dw is 

not significant. The study supports a whole body EC10 value of 6 – 8 mg/kg dw for 

juvenile growth and development (non-reproductive effects) in both green and white 

sturgeon, given that effects would be expected above this range. 

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 
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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 heuristiclike “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 ) 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.

EPA: If under your analysis of less critical vital rates the proposed criteria might be 

overprotective, since green sturgeon are listed as Federally threatened and

populations in the system are dwindling, the proposed criteria could be considered 

reasonable and protective of the species. 

 EPA notes that the Linville (2006) study on white sturgeon has been added to the 

database to derive the national freshwater criterion. This significantly strengthens 

the assertion that the national value will be protective of green sturgeon, since it is 

based on a genus level surrogate of the green sturgeon. 

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.
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EPA: Thank you for your comments. (The estimated value is an EC10 value [an effect 

concentration affecting 10% of the species’ population] determined from a regression 

analysis of the four most sensitive Genus Mean Chronic Values [GMCVs] and an 

extrapolation to the 5th percentile of the distribution represented by the tested 

genera.) 

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 ± 

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.
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EPA: Thank you for your comments. EPA notes your concern of uncertainty in the De 

Riu et al. (2014) study, given that the exposure concentrations in diet were 

unrealistically high.

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 

using the micro-injection to maternal transfer ratio further increases the uncertainty in 

this analysis.

EPA: Micro-injection is an unsupportable route of exposure for toxicity tests, 

particularly with selenium which is available in several forms in organic matter, and 

which is regulated by organisms as a nutrient. Micro-injection exposure contains 

significant uncertainty as it is usually only one type of selenium (selenomethionine) and 

the injection is a bolus dose instead of an acquisition over time through diet. A bolus 

dose may overwhelm the body’s metabolic response to selenium as its function as a 

nutrient.

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. 

EPA: Thank you for your comments. The data in the Linville (2006) study supports an 

EC10 egg/ovary value of 16 mg/kg dw for white sturgeon. Including the Linville (2006) 

study in the dataset for the draft national criterion results in a whole body criterion of 8 

mg/kg dw. The De Riu et al. (2014) study shows the control green sturgeon had a 

whole body level of 6 – 7 mg/kg dw; this indicates that juvenile green sturgeon may 

have a similar sensitivity to selenium as juvenile white sturgeon at this whole body 

concentration level. This strongly supports using the national freshwater threshold for 

the San Francisco estuary.
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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 Brain 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?

Reviewer Comments

Reviewer 1 I do not know.

(Declined to respond.)

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

best for their particular data set.

EPA: Thank you for your comments. EPA agrees that if sufficient dose levels in the 

range of essentiality are unavailable, the use of the biphasic model is inappropriate 

and if used, may lead to bias. 

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.

EPA: Thank you for your comments. EPA agrees that if sufficient data are not available 

to use a biphasic model, this type of model should not be used. In those cases, EPA 

agrees that a standard log-logistic model should 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, 

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)
13

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. 

EPA: Thank you for your comments. EPA agrees that a log-log regression is more 

appropriate than a biphasic model for the data from the published studies that have 

been reviewed. EPA acknowledges the citation from Beckon et al. (2008) concerning 

cautions against using the biphasic model, and the comment that selenium appears to 

have very poor regulatory homeostasis as an essential trace element compared to 

copper or manganese, and that this confounds the use of biphasic models for selenium.

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 

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.

EPA: Thank you for your comments. EPA agrees that where sufficient and appropriate 

data exist, non-standard models like biphasic models should be considered. EPA does 

not find that the data sets in the studies discussed include sufficient and/or appropriate 

data for utilization of a biphasic model.
14

2.4 continued: Toxicity Benchmarks/Model:

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.

Reviewer Comments

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

(Declined to respond.)

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.

EPA: Thank you for your comments. EPA agrees that if studies have sufficient data 

points in the lower dose regions, the use of a biphasic model may be more appropriate. 

However, for the studies reviewed, sufficient data in the lower dose ranges does not 

exist and therefore, EPA supports the use of a standard log-logistic model to determine 

EC10 values. 

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

provide adequate statistical power.

EPA: Thank you for your comments. Please see EPA response to Reviewer 2.

Reviewer 4 See response provided under 2.4.a.

EPA: Thank you. Please see EPA response provided under 2.4.a.

Reviewer 5 See response provided under 2.4.a.

EPA: Thank you. Please see EPA response provided under 2.4.a.
15

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.

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).]

The goal of the site specific water quality criteria is to protect the system as a whole 

based on the available toxicity data for sensitive species. In this case, we have data for 

sturgeon, salmonids and birds as sensitive receptors for selenium toxicity. These species, 

although consuming a mixed diet, do have documented “prey” preferences (e.g. 

sturgeon-mollusc, and juvenile salmon-insect), for which data is available. Since there 

are preferences, conservative assumptions are applied since there is uncertainty about 

other prey items in the diet. The result will theoretically be protective, since the 

sensitivity of surrogates consuming the modeled diets containing selenium is 

representative (and maybe overly conservative) for other organisms in the system 

without data.

EPA: Thank you for your comments. EPA acknowledges that in nature, diet can vary 

greatly for a single species. For the San Francisco estuary, we have known sensitive 

species with known dietary preferences; those species have been modeled using 

reasonably conservative assumptions in order to determine protective criteria values to 

capture a range of consumption rates.

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 

each calculated TTF. The uncertainty is likely large relative to that variation in TTF values 

among species.
16

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.

EPA: Thank you for your comments. EPA agrees that an end member analysis involving 

N, C and S stable isotopes would help to determine dietary components. A study using 

stable isotope analyses has been completed for white sturgeon that live in the estuary 

(Zeug et al., 2014) and is considered in the analysis; EPA is not aware of any other 

similar studies for the critical species that live in the estuary. EPA agrees that a 

discussion of uncertainty would be useful; we have added information and discussion in 

the document. (See EPA response to question 2.1.) 

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.

EPA: EPA agrees that the lifestage of the species is important and necessary; the 

document discusses juvenile salmonids as the endpoint associated with a TTF based on 

an insectivorous diet. Adult salmonids moving back into freshwaters from a marine 

environment have very limited dietary exposure to selenium in the estuary, since it is 

used mainly as a migratory corridor and staging area for salmonids preparing to enter 

freshwater natal streams and rivers. Thus assigning a TTF to this species lifestage 

combination would not be meaningful with regard to selenium exposure related to 

maternal transfer of selenium to eggs. We have clarified this in the 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.

EPA: Thank you for your comments. 

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.
17

EPA: Thank you for your comments. 

2.5 continued: Species and Diet: 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?

Reviewer Comments

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

EPA: Thank you for your comment. 

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.

EPA: Thank you for your comments. Most of the TTFs used in the analysis are based on 

(and/or validated using) matched field data sets, reflecting what is happening in the 

environment. One reason to explain the similarity in TTF values between fish and 

birds, even though cold-blooded fish and warm-blooded birds may have significantly 

different metabolic rates, is that in this case, fish size might ameliorate some of the 

metabolic differences, e.g., a 100+ pound sturgeon will need quite a bit of food to 

maintain its bodyweight, while a small bird, even if it has a much higher metabolic 

rate, may eat substantially less.

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

EPA: Thank you for your comment. 

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 
18

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. 

EPA: Thank you for your comments. EPA has clarified in the document that even 

though splittail may eat up to 34% mollusks, a portion of which may include Corbula, it 

is less than the amount of Corbula that white sturgeon are estimated to consume.

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.

EPA: Thank you for your comments. EPA has added information and discussion on 

uncertainty and sensitivity in the document. EPA agrees that risk is a combination of 

exposure and effect. For splittail, studies indicate that diet is likely no greater than 34% 

clam-based. The available studies on splittail do not indicate whether they are more or 

less sensitive to selenium than other species whose diet is also clam-based (e.g., white 

sturgeon).
19

2.5 continued: Species and Diet: Specifically for Clam-eating Fish:

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.

Reviewer Comments

Reviewer 1 These questions are outside my expertise.

(Declined to respond.)

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 

accuracy of estimates. Perhaps a formal expert elicitation would improve future 

estimates.

EPA: Thank you for your comments. EPA agrees that including information on 

exposure duration and frequency of intake (diet) would improve our understanding of 

risk for migratory sturgeon, but these kinds of very specific data are not available. We 

have added more detail in the document to clarify our assumptions concerning white 

and green adult and juvenile sturgeon.

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.

EPA: Different fish species have different strategies for depositing vitellogenin into 

eggs prior to spawning; zebrafish may not be representative of other fish species, 

particularly sturgeon.

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 
20

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.

EPA: A key uncertainty is whether selenium is being deposited from body burden to 

eggs via maternal transfer during vitellogenesis, or whether selenium is being 

deposited more directly through current diet, or a combination of the two pathways. 

EPA noted this issue in the document.

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.

EPA: EPA has added discussion in the document concerning these issues to clarify what 

information currently exists.

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.

EPA: Thank you for your comments. EPA has added discussion in the document 

concerning these issues to clarify what information currently exists. We have also 

noted its uncertainty. However, for selenium exposure in the estuary to be 

toxicologically significant, several key conditions must occur: 1) green sturgeon must 

consume a significant amount of selenium-contaminated prey items over the course of 

their occupancy during migration, and 2) a pathway for maternal transfer of the current 

diet-based selenium must be present, i.e., immature/developing eggs are still 

predominant in the female during migratory processes in the estuary, and/or turnover 

of vitellogenin is occurring, as observed by one reviewer above in zebrafish (note: a key 

issue here is that zebrafish have a different spawning strategy than sturgeon in the 

wild, but are easily manipulated in the lab, with unknown effects on the reproductive 

cycles when compared with wild populations). It is not apparent that either of these 

conditions are occurring in the estuary, and we therefore do not consider exposure to 

adult green sturgeon in the estuary to be toxicologically significant. 
21

2.5 continued: Species and Diet: 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.

Reviewer Comments

Reviewer 1 This definitely makes sense.

EPA: Thank you for your comment. 

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

EPA: Thank you for your comment. 

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

species.

(Declined to respond.)

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.

EPA: Thank you for your comment.
22

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?

EPA: Thank you for your comments. Given the lack of Corbula’s preferred habitat 

overlapping with rail feeding areas, the likelihood of rails consuming specifically more 

Corbula (not mollusks generally) is low; and given the other mollusks have substantially 

lower TTFs, EPA does not believe substantial risk exists to the general population, and a 

highly exposed rail due to Corbula consumption would be an exception. EPA has 

included information and discussion on sensitivity in the document.

2.5 continued: d. continued: Species and Diet: Specifically for Clam-eating Birds:

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.

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.

EPA: Your concerns appear to reflect whether acute exposures to adult birds are toxic, 

whereas our question concerned whether selenium exposures shortly prior to 

migration have the potential for maternal transfer at a later time when the birds reach 
23

breeding grounds. We have no evidence that the level of acute exposures from clams 

in the estuary has led to death in these adult birds. 

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. 

EPA: Thank you for your comments. EPA agrees that additional information concerning 

exposure duration and frequency may be helpful in refining the approach in the future. 

We have added discussion in the document to reflect what is currently known about 

the migratory movements of scoter and scaup to help reduce uncertainty.

Reviewer 3 See response provided under 2.5.d.

(Declined to respond.)

Reviewer 4 See response provided under 2.5.d.

EPA: Thank you for your comments.

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).

EPA: EPA has added information and discussion in the document concerning what is 

currently know about the timing of selenium incorporation into eggs for fish species 

(sturgeon) and bird species (migratory ducks/waterfowl), as well as its depuration in 

fish species and bird species. As noted in the comment, evidence indicates that 

selenium is depurated rapidly in waterfowl, and that marine duck species deposit less 

selenium into their eggs than freshwater ducks species. 
24

2.5 continued: Species and Diet: 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.

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.

EPA: Thank you for your comments. EPA has added a discussion on uncertainty to 

account for the variability of selenium in diet due to prey species; variability of 

selenium levels from (and in) prey species is a reflection of abundance, timing and 

location of predation.

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.

EPA: Thank you for your comment. 

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.

EPA: EPA has clarified its discussion in the document concerning life stages and 

associated diet mixes. EPA does not consider inbound salmonids in the analysis due to 

their minimal exposure to selenium in the estuary.

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. 

EPA: Thank you for your comments. 

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.

EPA: Thank you for your comments. 
25

2.6 Trophic Transfer Factors (TTFs):

a. 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?

Reviewer Comments

Reviewer 1 These TTFs look good to me. 

EPA: Thank you for your comment. 

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,EACanuel 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 

EPA: Thank you for your comments. Please see EPA response to question 2.5.b. 

concerning TTF values for cold-blooded and warm-blooded species. Please see EPA 

response to question 2.5.a. concerning studies using stable isotope tracking to 

determine diet composition. 
26

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).

EPA: Thank you for your comments. EPA has clarified its discussion in the document 

concerning life stages and associated diet mixes. EPA does not consider inbound adult 

salmonids in the analysis due to their minimal exposure to selenium in the estuary.

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. 

EPA: Thank you for your comments. 

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 

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.

EPA: Thank you for your comments. The analysis for the San Francisco estuary looks at 

categories of species based on diet preferences. Although sturgeon may have an 

estimated TTF of 0.8 with respect to its primary food source (Corbula), it is not the only 

clam-eating species; the average fish TTF of 1.1 is a reasonable, conservative estimate 

to represent all fish species in the clam-eating fish category. 
27

2.6 continued: Trophic Transfer Factors (TTFs):

b. 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 Agencyshould consider for developing a site-specific TTF for 

Corbula?

Reviewer Comments

Reviewer 1 I don’t know of any involving Corbula beyond what is here.

EPA: Thank you for your comment. 

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 

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.
28

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 

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 TTF for C.

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.” 

EPA: Thank you for your comments. We have added additional detail and discussion in 

the document concerning the calculations for the Corbula TTF value. 
29

Reviewer 3 The site-specific model used to calculate TTF for Corbula is scientifically sound.

EPA: Thank you for your comment. 

Reviewer 4 See response provided under 2.6[a].

EPA: Thank you for your comments. 

Reviewer 5 See response provided under 2.6[a].

EPA: Thank you for your comments. Please see EPA response to question 2.6.a.

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.

Reviewer Comments

Reviewer 1 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 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.

EPA: EPA agrees that Kds (or CFs) can vary significantly within a system depending on 

season (i.e., water flow). Using a 75th percentile Kd value is intended to reasonably 

capture the variability, and provides a more conservative value for modeling low flows, 

which have been occurring in the estuary for the last several years. We have included a 

more detailed explanation in the document of our rationale, and have included the Kd

values in the sensitivity analysis to show how values will change using a different 

metric (e.g., mean or median, larger percentile). In response to the comment that 

phytoplankton assemblages change with season and nutrient concentrations, EPA 

notes that trophic level 2 organisms (e.g., Corbula) will consume and take up selenium 

from a variety of particulate/algal matter.
30

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.

EPA: Thank you for your comments. EPA has added an analysis and discussion in the 

document concerning the distribution of Kd values. 

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.

EPA: EPA has added an analysis and discussion in the document concerning the 

distribution of Kd values, and to clarify our rationale for using 75th percentile values. 

The 95th percentile would be more conservative but may incorporate outliers that are 

not representative of the normal distribution and seasonal variability of Kd values for 

the system. EPA believes that using 75th percentile Kd values reasonably represents a 

conservative approach to modeling probable wet and dry conditions in the estuary.

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.

EPA: Thank you for your comments. 

Reviewer 5 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 
31

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.

EPA: Thank you for your comments. We feel that using 75th percentile Kd values and 

reasonable, conservative estimates of other variables in the model, the proposed 

criteria values are both reasonable and 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.

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 

(No. 9) on South Bay being less well flushed.

EPA: Please see EPA response to question 2.7. 

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.

EPA: Thank you for your comments. 

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.

EPA: Thank you for your comments. 

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 
32

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. 

EPA: EPA will continue to review all available information on the system to identify 

where the analysis can be improved. As the dry conditions in California continue, we 

will continue to review new data and consider adjustments to the analysis as 

necessary.

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.

EPA: Thank you for your comments. EPA has and will continue to review all available 

data reflecting the current dry conditions; if the dry conditions continue, EPA will 

consider adjustments to the analysis as necessary.

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-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.

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.

(Declined to respond.) EPA: EPA has clarified its rationale in the document for 

including the South Bay in the applicable area of the water quality criteria.

Reviewer 2 This decision does seem reasonable in my opinion.

EPA: Thank you for your comment. 

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 
33

upon specific areas particularly with an element with incredible geospatial movement 

may not be fruitful. 

(Declined to respond.) EPA: By modeling a smaller area, EPA can focus on the areas of 

greatest importance and limit complexity which helps determine appropriate criteria 

values.

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. 

EPA: Thank you for your comments. EPA hopes that by focusing on important areas 

and modeling dry (conservative) conditions, we can determine appropriate and 

reasonable criteria values. EPA agrees that modeling critically dry or extremely wet 

conditions may introduce extreme results.

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.

EPA: Thank you for your comments. EPA has added information and discussion 

concerning the presence/absence of relevant species in the Central Bay near the 

Golden Gate, and clarified the rationale for using focused transects in the Suisun Bay 

area.
34

Results and Application

2.10 Results: a. 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.

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.

EPA: EPA modeled the three food chains independently, then chose a set of results 

that would reasonably protect all three food chains; we did not model one food chain 

as a surrogate for another. 

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.

EPA: Thank you for your comments. Please see EPA response to question 2.5.a. 

concerning studies that use stable isotope analysis. 

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. 

EPA: As discussed in EPA’s response to question 2.5.e. and 2.6., EPA does not consider 

inbound adult salmonids in the analysis due to their minimal dietary exposure to 

selenium in the estuary; adult salmonids use the ecosystem mainly as a migratory 

corridor. Sturgeon, although large, are mainly bottom feeders and consume large 

amounts of Corbula. Some sturgeon may also consume a small amount of small 

(forage) fish, but the selenium load from that would be minimal.

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 
35

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 

differences in exposure and Se sensitivity (not recommended) there are no other 

common approaches that can be applied across species.

EPA: Thank you for your response. 

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. 

EPA: Thank you for your response. 

2.10 continued: Results:

b. 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.

Reviewer Comments

Reviewer 1 See response provided under 2.10 [a].

EPA: Please see EPA response to question 2.10.a.

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.

EPA: Thank you for your comments. (The analysis for San Francisco estuary does not 

include fish-eating birds; it is assumed the reviewer is referring to clam-eating birds.)

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 

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. 
36

EPA: EPA does not consider inbound adult salmonids in the analysis due to their 

minimal exposure to selenium in the estuary as a migratory corridor. Almost all of the 

selenium exposure to inbound salmonids is outside the estuary. In addition, a key step 

is maternal transfer of selenium to the ova via vitellogenesis (not exposure to the 

developing ova); if maternal transfer of vitellogenin is ongoing during the migratory 

passage and staging, and adult salmon are actively feeding within the estuary during 

that process, then some dietary selenium exposure from the estuary may take place. 

However, it would be much lower than full-time residents of the estuary. 

Reviewer 4 See response provided under 2.10.[a].

EPA: Thank you for your response. 

Reviewer 5 See response provided under 2.10.[a].

EPA: Thank you for your response. 

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.

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. 

EPA: Thank you for your comments. EPA agrees that the San Francisco estuary is wellstudied, however the USGS model can be used in other places. For less studied areas, 

data gaps can filled by carefully selecting surrogate species and model variables.

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 
37

data and its use to produce predictive models is definitely defensible, and from my 

reading of the documents, is protective.

EPA: Thank you for your comments. 

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. 

EPA: Thank you for your comments. EPA focused on clam-eating fish, clam-eating 

birds, and insect-eating fish (including juvenile salmonids) because they were identified 

as the most at-risk set of species for this system (FWS, 2008; FWS, 2009; FWS, 2012), 

based on their sensitivity to selenium and their dietary exposure to selenium. EPA 

concurs with this assessment; adult salmon have minimal exposure, and other 

broadcast spawning species within the estuary have less exposure and sensitivity than 

the clam-eating fish and bird species that are discussed. 

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.

EPA: Thank you for your comments. 

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.

EPA: Thank you for your comments. 
38

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.

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.

EPA: For selenium, EPA agrees that the dissolved form does not better represent the 

bioavailable fraction, since particulate selenium is easily taken up into biota (e.g., by

filter feeders). However, for permit purposes, a total recoverable value is needed, and 

the issue is how to translate the dissolved value into a total recoverable value for this 

waterbody, the San Francisco estuary. 

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.
39

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 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. 

EPA: Thank you for your comments. 
40

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 

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. 

EPA: Organic and inorganic forms of selenium can exist as dissolved or particulate. 

Organic forms are more readily available to biota than inorganic forms, but inorganic 

selenium (selenate and selenite) can be transformed into organic forms. Thus, all forms 

of selenium have the potential to be taken up into the food chain, although some forms 

will undergo a longer transformation process. 

Both dissolved and particulate forms of selenium are bioavailable, and they transform 

back and forth in the system depending on the conditions in the system. Therefore, the 

translator should equal 1, i.e., all dissolved selenium should be considered bioavailable 

as part of the total recoverable fraction. 

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.

EPA: EPA does not agree; both dissolved and particulate selenium can be bioavailable 

and toxic; some forms are more bioavailable than other forms, e.g., organo-selenium is 

more available than selenite, but selenite can be transformed into organo-selenium. 

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.

EPA: Thank you for your comments. 
41

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.
42

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 References

Reviewer 3 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 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.
43

Reviewer 5 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.