EPA Response to Comments from

External Peer Review of the 

“Technical Review of Hydrogen Sulfide”

Office of Environmental Information

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, NW

Washington, DC 20460

October 5, 2009
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Peer-Review Comments and EPA Responses on

Technical Review of Hydrogen Sulfide

CHEMISTRY AND FATE

Comment 1-1: Little information is given for the amount of hydrogen sulfide generated at the 

sediment-water interface. This is the information necessary to correlate the environmental 

exposure and effects concentrations. (peer reviewer: #1)

EPA Response: 

Environmental exposure based upon the amount of hydrogen sulfide generated at the 

sediment-water interface is beyond the scope of the technical report. The technical report does 

not contain an exposure assessment or any evaluation of the potential for concentrations of 

concern to exist in the environment as a result of any industrial releases or other sources of 

hydrogen sulfide. The purpose of the report is limited to the chemical properties, fate, and 

potential ecological effects of hydrogen sulfide. However, the report does provide general 

information on the environmental generation of hydrogen sulfide, (some occurring at the 

sediment-water interface) stating:

“An estimated 100-324 million tons of hydrogen sulfide are naturally released to the global 

atmosphere each year (Pouliquen et al., 2005). Releases from ocean waters are believed to 

account for half of this volume, with the remainder originating from volcanoes, flooded ground, 

and hydrogeological sources such as thermal springs (Pouliquen et al., 2005).” 

Comment 1-2: There are no data regarding the residence time of hydrogen sulfide in water 

containing natural organic matter. (peer reviewer: #2)
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EPA Response: Limited data are available regarding the residence time of hydrogen 

sulfide in aquatic systems. The following has been added to section 1.6.2 concerning abiotic 

oxidation half-lifes: 

“Reported oxidation half-life values measured in air-saturated solutions at 25 °C and pH 

8 are 50 hours in fresh water and 26 hours in seawater (Millero et al., 1987). Other studies of 

oxidation rates of hydrogen sulfide in fresh and sea water have reported a variable range of halflife values which may is in part be due to differences in experimental conditions including pH, 

temperature and ionic strength of the test solution and analytical methodology employed (Millero 

et al., 1987) .”

Comment 1-3: All the important chemical properties are listed in Table 1-1. Because H2S is a 

gas, listing of color (colorless) and odor (rotten egg) on the table may be useful. (peer reviewer: 

#3)

EPA Response: These basic properties are reported in the front-end of the report and but EPA 

does not believe they need to be reiterated in the table.

Comment 1-4: A more complete account of air-water partitioning is required including the 

effect of pH. See previous comments. (peer reviewer: #4)

EPA Response: See response to comments 3-4, 3-6, and 3-9.

Comment 1-5: Under Section 1.4, it should be noted that hydrogen sulfide is released via oil 

and gas extraction activities. Natural gas and oil extraction in estuarine and marine 

environments often have hydrogen sulfide in the produced water that is discharged after 

separation of oil and gas. In the U.S., this has become an environmental issue requiring 

hydrogen sulfide permit limits for oil extraction platforms offshore of California, and to some 

extent in the Gulf of Mexico. (peer reviewer: #1)

EPA Response: The following text has been added to Section 1.4:
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“Hydrogen sulfide is released to the environment via water discharge during oil 

and gas extraction activities (EPA, 1993).”

Comment 1-6: It would also be appropriate to note in this section that H2S is typically 

measured as total sulfide and the unionized form is calculated using well established dissociation 

formulae that take into account pH, salinity, and temperature (e.g., Standard Methods). (peer 

reviewer: #1)

EPA Response: The following text has been added to Section 1.4:

“The concentration of undissociated hydrogen sulfide in an environmental water 

sample can be calculated from the dissolved sulfide concentration measured in 

the sample and the pH, salinity, and temperature of the solution (APHA 1998).”

Comment 1-7: The review concludes with virtually 100% of the hydrogen sulfide being in the 

atmosphere. The residence time in aquatic ecosystems is a necessary piece of information to 

adequately quantify risk. (peer reviewer: #2)

EPA Response: Limited data are available regarding the residence time of hydrogen sulfide in 

aquatic systems. The following information has been placed in section 1.6.2:

 “Reported oxidation half-life values measured in air-saturated solutions at 25 °C and pH 8 are

50 hours in fresh water and 26 hours in seawater (Millero et al., 1987). Other studies of 

oxidation rates of hydrogen sulfide in fresh and sea water have reported a broader range of halflives which may is in part be due to differences in experimental conditions including pH, 

temperature and ionic strength of the test solution and analytical methodology employed (Millero 

et al., 1987).”

EPA also notes that the use of experimental data in the rerunning of the EPIWIN fugacity model 

predicts an aquatic sink of some 15 to 41 percent of the hydrogen sulfide released, depending 

upon the compartmental release scenarios used for the model. The sink value initially reported 
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in the document stated 100 percent to the air compartment. This value was incorrect due to some 

of the program default values used.

Comment 1-8: Considering the high vapor pressure, hydrogen sulfide is expected to be present 

predominantly in the atmosphere. In the atmosphere, H2S does undergo oxidation and hydroxyl 

radical reactions. This has been mentioned on P6, lines 8-21. However, the degradation or 

transformation products of these reactions have not been mentioned/reported. It would be 

helpful to indicate if the degradation products are toxic or are involved in acid rain (P7, line 24) 

or other environmental changes/effects. This should be mentioned in summary. (peer reviewer: 

#3)

EPA Response: The sentence on P7, line 24 has been changed to include hydroxyl radicals in 

the list of atmospheric species that can oxidize hydrogen sulfide to form sulfur dioxide and 

sulfuric acid (acid rain). This information has not been included in the summary (Section 

1.1), due to its high level of detail.

Concerning the formation of acid rain from H2S degradation products, the following language 

can be found in section 1.6.5: 

“Atmospheric oxidation of hydrogen sulfide by oxygen, ozone, or hydroxyl radicals 

produces sulfur dioxide (Hill, 1973). Sulfur dioxide is ultimately oxidized to form 

sulfate, which can be washed out of the atmosphere as sulfuric acid when dissolved 

in rain droplets (Hill, 1973).”

Comment 1-9: P5, line 24: It is also worth indicating that in aerated (oxidized conditions) 

water, hydrogen sulfide is oxidized to sulfates and biologically oxidized to elemental sulfur. In 

anaerobic water, microbial reduction of sulfate to sulfide can occur. This will highlight the fate 

in water bodies, as there is information for the fate in atmosphere. (peer reviewer: #3)

EPA Response: Section 1.6.2 has been reorganized to clarify the abiotic oxidation process 

that converts hydrogen sulfide in water to sulfur dioxide and sulfate. Biotic oxidation and 
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reduction processes that convert between sulfide, sulfate, and sulfur (the sulfur cycle) are 

mentioned in section 1.6.3.

Comment 1-10: The fate assessment is generally satisfactory, except that rates of volatilization 

may be in error as a result of relying on the EPIWIN program that is not really designed to treat 

gases and dissociating substances. (peer reviewer: #4)

EPA Response: The following caveat has been included in Section 1.6.1:

“Hydrogen sulfide is not a typical organic molecule and therefore caution must 

be used when interpreting the results of the EPI Suite program for this substance. 

These volatilization half-lives may overestimate the rate at which volatilization 

may be occurring due to the fact that the EPI Suite software does not take into 

consideration the dissociation of hydrogen sulfide which increases with 

increasing pH above a pH of 5 (Balls and Liss, 1983).”

Comment 1-11: The formation of toxic by-products is largely irrelevant because the parent H2S 

is so toxic. By-products are probably less toxic! (peer reviewer: #4)

EPA Response: Section 1.6.5 (toxic transformation products) is a standard component of the 

technical review documents. Information is provided to give a quick indication of the relative 

toxicities of the degradation products of hydrogen sulfide. 

ENVIRONMENTAL EFFECTS

Comment 2-1: The ecological hazards of hydrogen sulfide are correctly portrayed and the data 

presented are generally accurate. It should be acknowledged that test durations under 28d are not 

considered chronic tests for fish species according to EPA. Similarly, test durations < 21d are 

not considered chronic tests for invertebrates. (peer reviewer: #1)

EPA Response: Fish toxicity tests on adults with durations less than 28 days (but greater than 

96 hours) and aquatic invertebrate tests with durations less than 21 days (but greater than 48 
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hours) will be maintained in the chronic section of the table but will receive footnotes 

indicating the period of testing is less than routine chronic testing periods. All juvenile, fry, 

larvae, and egg testing data will be placed in a new section of Table 2-1 entitled “Early-Life 

Stage Toxicity Testing.” 

Comment 2-2: This report should also acknowledge that toxicity testing of hydrogen sulfide is 

very challenging and typically is accompanied by low oxygen as well (H2S is highly unstable in 

well-aerated water and at neutral pH as this review points out in the fate section). Some of the 

tests cited in fact had low oxygen as well as hydrogen sulfide (e.g., Kang and Matsuda 1994) 

which complicates interpretations. Because pH also affects whether the total sulfide present is 

H2S, HS or both in some combination, it would be useful to report pH for the tests in Table 2-1 if 

available. (peer reviewer: #1)

EPA Response: The following sentences were added to the last paragraph of Section 2.2: 

“When interpreting results of hydrogen sulfide aquatic toxicity testing, care should 

be taken to consider the potential effects of pH, temperature, and dissolved oxygen 

concentration in combination with hydrogen sulfide exposure. As indicated in 

section 1.6.1, the proportion of hydrogen sulfide is dependent on the pH of the 

water, with more basic pH levels leading to decreased amounts of hydrogen sulfide 

and increased amounts of the bisulfide anion (HS-). Additionally, as stated in 

section 1.6.1, increasing temperature generally increases the rate of hydrogen 

sulfide volatilization from water. Regarding dissolved oxygen, some data indicate 

that hydrogen sulfide toxicity can increase when combined with hypoxic test 

conditions (Kang & Matsuda, 1994).”

Comment 2-3: The oil and gas industry has compiled some information regarding sulfide 

toxicity to marine species, which may fill in some data gaps in this review. References that I am 

aware of include:
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Ivanov,V.N., T.G.Usenko, and A.V.Parkhomenko Publication Year: 1976 

Title: Effect of Hydrogen Sulfide on the Survival Rate of Eggs and Embryonal 

Mitoses of the Black Sea Turbot Reference Source: Hydrobiol. J. 12(2):60-62 

Knezovich, J.P., D.J. Steichen, J.A. Jelinski and S.L. Anderson 1996. Sulfide 

Tolerance of Four Marine Species Used to Evaluate Sediment and Pore-Water 

toxicity. Bull. Env. Chem. Tox. 57:450-457.

Bagarinao, T., R.D. Vetter. 2005. Sulphide tolerance and adaptation in the 

California killifish, Fundulus parvipinnis, a salt marsh resident. J. Fish 

Biology 42:729-748. 

Vismann, B. Sulfide species and total sulfide toxicity in the shrimp Crangon 

crangon. J. Exp. Mar. Biol. Ecol. 204:141-154. 

Marcus, N.H., R.V. Lutz, J.P. Chanton 1997. Impact of anoxia and sulfide on 

the viability of eggs of three planktonic copepods. Mar. Ecol. Prog. Ser. 

146:291-295

Llanso, R.J. 1991. Tolerance of low dissolved oxygen and hydrogen sulfide by 

the polychaete Streblospio benedicti (Webster). Journal of experimental 

marine biology and ecology 153:165-178.

(peer reviewer: #1)

EPA Response: The above-mentioned 6 journal articles were retrieved and reviewed. The 

article by Knezovich et al. (1996) contained 48-hr EC and LC50 values for four saltwater 

invertebrate species, and these values were added to the assessment. Bagarinao & Vetter 

(1993): The study reports a 96-hour LC50 value for fish of 42 μM H2S, equivalent to 1.43 

mg/L. This value was added to the assessment. Ivanov et al. (1976): This study reports a 24-

hour LC100 value for saltwater fish eggs. This value was included in the early life stages 

toxicity testing section of the assessment.
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The remaining 3 articles were determined to be unsuitable for inclusion in the assessment for 

reasons outlined below: 

 

Vismann (1996), Llanso (1991), Marcus et al. (1997): The experimenters investigated 

exposure time, not concentration, to produce 50% mortality in saltwater invertebrates (median 

survival time or LT50); LT50 is not an endpoint that is typically used in these assessments. 

Additionally, the focus of the studies by Marcus et al. (1997) and Llanso (1991) were effects of 

hypoxia and/or anoxia in combination with H2S exposure. So in these studies, the only 

exposure group that was exposed to H2S was concurrently exposed to anoxic conditions. This 

may be a confounding factor since it would be difficult to know whether effects seen were due 

to the complete deoxygenation of the water, or the H2S, or some combination of the two. 

Furthermore, the sulfide concentration in Llanso (1991) is not precisely reported (it is 

indicated as averaging 20 μM, with a maximum concentration of 66 μM).

Comment 2-4: The literature search appears to be thorough but all of the data are with aquatic 

organisms. If all of the hydrogen sulfide will reside in the atmosphere, data on terrestrial 

organisms should be included. Hydrogen sulfide has been implicated in human mortalities; it 

makes sense that data for non-human terrestrial organisms should be included. (peer reviewer: 

#2)

EPA Response: The use of experimental data in the rerunning of the EPIWIN fugacity model 

predicts an aquatic sink of some 15 to 41 percent of the hydrogen sulfide released, depending 

upon the compartmental release scenarios used for the model. The sink value initially reported 

in the document stated 100 percent to the air compartment. This value was incorrect due to some 

of the program default values used. The objective of this technical report is to assess if a 

chemical presents an environmental hazard. Based upon the revised aquatic sink information for 

hydrogen sulfide and its high toxicity demonstrated to aquatic organisms, it can be concluded 

that hydrogen sulfide is an environmental hazard. No information was identified concerning the 

toxicity of hydrogen sulfide to terrestrial organisms, outside of humans, which will be covered in 

EPA’s assessment of the human health hazards of hydrogen sulfide. 
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Comment 2-5: The summary statement for environmental fate covers all the key elements. 

Inclusion of a statement regarding the formation of SO4 and sulfuric acid as products of

atmospheric reactions of H2S in the summary is needed. Also, indicate that H2S is a component 

of the natural sulfur cycle in the summary. (peer reviewer: #3)

EPA Response: The formation of sulfate and sulfuric acid as products of atmospheric 

reactions of hydrogen sulfide is too much detail for the summary section (1.1). The following 

statement has been included in the summary:

“Hydrogen sulfide is a component of the natural sulfur cycle”

Comment 2-6: The summary should be rewritten to address the partitioning and pH issues 

discussed above. (peer reviewer: #4)

EPA Response: The second to last paragraph of section 1.1 has been rewritten as two 

separate paragraphs to better summarize the degradation and partitioning of hydrogen 

sulfide; however, these subjects are not described in detail since this would be inappropriate 

for the summary section. The influence of pH on partitioning is only mentioned briefly. 

Greater detail is provided in section 1.6.1, “Environmental Transport.”

ADDITIONAL COMMENTS

Comment 3-1: Purpose of Document: Text says EGBE in second paragraph. This should be 

changed to H2S. (peer reviewer: #1)

EPA Response: This correction has been made.

Comment 3-2: General Comments: This review appears to be pretty thorough and should 

provide information required for section 313 of the Emergency Planning and Community RightTo-Know Act. The biggest disparity in the report is the emphasis on air being the ultimate sink 
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of hydrogen sulfide while all the toxicity data has been generated with aquatic organisms. (peer 

reviewer: #2)

EPA Response: See response to comment 2-4.

Comment 3-3: Line 16: Insert “of” after ‘consideration’ (peer reviewer: #3)

EPA Response: This suggested edit (page 5 of the assessment) does not appear necessary 

since the sentence reads appropriately as written (“These volatilization half-lives may 

overestimate the rate at which volatilization may be occurring due to the fact that the EPI 

Suite software does not take into consideration the dissociation of hydrogen sulfide which 

increases with increasing pH above a pH of 5”).

Comment 3-4: This report is generally well written, however, there are some areas in which 

there should be improved treatment of chemical properties and fate.

H2S is a gas thus it does not have a unique solubility in water as is implied on p1 line 10. The 

“solubility” of a gas in water is proportional to the partial pressure of the gas in the air space as 

described by Henry’s Law, i.e.

P = H3x

where P is partial pressure (units such as atm), H3 is a Henry’s Law Constant (units such as atm) 

and x is mol fraction in water (dimensionless). H3 appears to be about 477 atm at 20oC and 591 

atm at 30oC. (Al-Haddad et al. 1989). H3 can be converted to units of atm m3/mol by 

multiplying by the molar volume of water, i.e. 18 x 10-6 m3/mol yielding values of 8.4 x 10-3 and 

10.8 x 10-3 as correctly given in Table 1.1. If we designate this as H, then the air water partition 

coefficient KAW is H/RT where R is 82 x 10-6 atm m3/mol K and T is absolute temperature (K). 

The KAW values are thus 

0.35 at 20oC and 0.43 at 30oC

These values are consistent with the 0.4 given by Balls & Liss (1983).

H2S does not really have a vapor pressure or a solubility as is implied in Table 1.1. It is 

preferable to delete these entries or explain them. It is H or KAW that matters. (peer reviewer: #4)
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EPA Response: In response to these comments, the following changes have been made. Airwater partitioning coefficient (KAW) values have been added to Table 1-1. The vapor pressure 

value has been removed from the table as requested. Water solubility values from Carroll and 

Mather (1989) have been added to the table along with the following footnotes:

“bThe solubility of hydrogen sulfide in water is calculated from the strict Henry’s 

Law constant over a wide temperature and pressure range (Carroll and Mather 

1989): mH2S H = yH2S P, where yH2S is the mol fraction of hydrogen sulfide in the 

vapor phase, P is the total pressure, and mH2S is the mol fraction of hydrogen 

sulfide in solution. Multiplying yH2S by P yields the partial pressure of hydrogen 

sulfide above the solution. 

c

These solubilities were originally reported as mol percent in Carroll and Mather 

(1989) and were converted to mg/L for this table.”

Comment 3-5: In the EPIWIN results (which were kindly provided) there are multiple H and 

KAW values given. On p5 and p8 the H is given as 8.56 x 10-3 atm m3/mol which is ok but on P9 

it is given as 0.163 resulting in a KAW of 6.67. This is about a factor of 10 too high. This 

erroneous value is, I believe, used in the STP and Fugacity model. This is not a disaster but it is 

a glaring inconsistency. The overall coclusion is not significantly affected. (peer reviewer: #4)

EPA Response: The EPI Suite fugacity calculation was performed again to ensure that the 

user-entered measured value (8.4×10 -3atm m3/mol at 20°C) was the actual value used by the 

program. Although other values are listed in the results output, these values were not used in 

the calculation. The STP model was not used for this assessment.

Comment 3-6: Because H2S dissociates with a pKa of 7.04 (p5 line 18) the report correctly 

points out that both H2S and HS2 are present at normal pHs. The KAW for H2S is ok, but the 

ratio of gas/water total concentrations (neutral and ionic) is much lower at high pH because (as 

the report points out) at high pH the dominant species in the water is HS2. So fate is also 

influenced by pH. This issue deserves some expansion and discussion. (peer reviewer: #4)
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EPA Response: The discussion of the pH-dependent speciation of hydrogen sulfide is located 

in the section regarding environmental transport (Section 1.6.1). A speciation diagram has 

been included as Figure1-1 to help explain the nature of hydrogen sulfide speciation at 

environmental pH. 

Comment 3-7: There is a related ambiguity on p6 line 1 where a “sorption capacity” is given 

for H2S in soils. These “capacities” must depend on the partial pressure of H2S. (peer reviewer: 

#4)

EPA Response: The paragraph discussing the sorption capacities of soils has been rewritten 

to emphasize the conclusions of the study by Smith et al. (1973) that soil may be a natural sink 

for H2S and that soils can differ in their sorption capacities. The conditions of the study to 

measure soil sorption capacities were also included in the paragraph. In addition, the 

following statement related to partial pressure was added to the first paragraph of Section 

1.6.1:

“Along with sorption to soils, the natural diffusion of hydrogen sulfide from 

areas of greater concentration to areas of lower concentration will also influence 

the amount of hydrogen sulfide found in soil compartments.”

Comment 3-8: What is needed, I believe, is a re-write of the section on properties to give a more 

complete account of the air-water and air-soil partitioning of H2S and the effect of pH. 

Unfortunately the EPIWIN program is not really designed to treat gases or ionising substances. 

It is primarily designed to treat non-polar organic liquids and solids. (peer reviewer: #4)

EPA Response: The effect of pH on the partitioning of hydrogen sulfide in the environment is 

discussed in Section 1.6.1. A diagram has been included as Figure 1-1 to help clarify the 

speciation of hydrogen sulfide in the environment; however, a more detailed discussion of this 

subject is beyond the scope of this assessment. The following paragraph has been added to 

Section 1.6.4:
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“A measured Henry's Law constant of 8.4×10 3 atm-m3/mol at 20°C and pH 7.1 

was used for this fugacity calculation. The volatilization half-lives calculated 

within the fugacity model may overestimate the rate at which volatilization may 

be occurring due to the fact that the EPI Suite software does not take into 

consideration the dissociation activity of hydrogen sulfide which increases with 

increasing pH above a pH of 5 (Balls and Liss, 1983). ” 

Comment 3-9: P1 line 10 see above comments on solubility. (peer reviewer: #4)

EPA Response: See response to comment 3-4.

Comment 3-10: P1 line 17 “by chemical means”. This is rather vague. (peer reviewer: #4)

EPA Response: This phrase has been replaced with “hydrogen sulfide is also manufactured 

via reactions involving other industrial chemicals such as sulfur and hydrogen gas (Pouliquen 

et al. 2005)”. 

Comment 3-11: P1 line 24 Is this the US or the global release? (peer reviewer: #4)

EPA Response: This section now reads “...released to the global atmosphere...”.

Comment 3-12: P3 Table 1.1 delete entries for vapor pressure and solubility. (peer reviewer: 

#4)

EPA Response: See response to comment 3-4.

Comment 3-13: P5 line 12 see above comments about EPIWIN. (peer reviewer: #4)

EPA Response: See response to comment 3-8.
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Comment 3-14: P5 line 16 “dissociation activity” is a vague term. H2S certainly dissociates 

more at high pH. Is this what is intended? (peer reviewer: #4)

EPA Response: The word “activity” has been deleted.

Comment 3-15: P6 line 2 see comments above. (peer reviewer: #4)

EPA Response: See response to comment 3-7.

Comment 3-16: P7 line 19 I would prefer “H2S will mostly remain in air”. Delete 100%. (peer 

reviewer: #4)

EPA Response: As stated in the response to comment 2-4, the fugacity calculation has been 

performed again using different release scenarios. This has resulted in a change in 

partitioning results, which are reported in the revised text.

Comment 3-17: P18 line 19 This section on BCF is a repeat of p12 line 2. I suspect that the 

BCF is largely irrelevant because of toxicity and rate of oxidation. The figure of 3.162 would be 

more honestly replaced by “approximately 3". (peer reviewer: #4)

EPA Response: This change has been made as requested.

Comment 3-18: I appreciate the inclusion on the CD of the selected cited papers. That was 

helpful. Since the assessment relies heavily on the EPIWIN program it would be worth 

considering appending the output, or at least selected, output to the review. (peer reviewer: #4)

EPA Response: A pdf file of the EPI Suite output that was used for this assessment has been 

created for reference.