### 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 , 20092 

### 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 technic al 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

- wate r 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 remainde r 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. (pe er reviewer: #2)3 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 half

life val ues 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 a re listed in Table 1

- 1. Because H 2 S 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 believ e 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.

Com ment 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 dischar ged 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 Respo nse: The following text has been added to Section 1.4:4 “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 H 2 S 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 (AP HA 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 half

lives which may is in part be due to differences in experimental co nditions 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 sin k 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 reported5 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, H 2 S 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 2 4) 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 sulf ide 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 H 2 S 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 sulf uric 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

anaero bic 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 and6 reduction processes that convert between sulfide, sulfate, and sulfur (the sulfur cycle) are
mentioned in section 1.6.3.

Comment 1

- 10: The fat e 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 follow ing 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 overestima te 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 H 2 S 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. (pe er 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 487 hours) will be maintained in the chronic sectio n 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 Testi ng.”

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 (H 2 S is highly unstable in
well

- aerated water and at neutral pH as this review point s 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
H 2 S, HS or both in some c ombination, 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 toxic ity 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 sulf ide 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 informa tion regarding sulfide toxicity to marine species, which may fill in some data gaps in this review. References that I am
aware of include:8 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. Ec ol. 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: T he above

- mentioned 6 journal articles were retrieved and reviewed. The article by Knezovich et al. (1996) contained 48

- hr EC and LC 50 values for four saltwater invertebrate species, and these values were added to the assessment. Bagarinao & Vetter (1993): The study reports a 96

- hour LC 50 value for fish of 42 μ M H 2 S, equivalent to 1.43 mg/L. This value was added to the assessment. Ivanov et al. (1976): This study reports a 24

- hour LC 100 value for saltwater fish eggs. This value was included in the earl y life stages toxicity testing section of the assessment.9 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 inves tigated exposure time , not concentration, to produce 50% mortality in saltwater invertebrates (median
survival time or LT 50

); LT 50 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 H 2 S exposure. So in these studies, the only exposure group that was exposed to H 2 S 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 H 2 S, or some combination of the two. Furthermore, the sulfide concentration in Llanso (1991) is not precisely reported (it is indicated as averaging 20 μ M, with a maximum concentration of 66 μ M).

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 in cluded. 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 mo del 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. T his 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.10 Comment 2

- 5: The summary statement for environmental fate covers all the key elements. Inclusion of a statement regarding the formation of SO 4 and sulfuric acid as products of atmospheric reactions of H 2 S in the summary is needed. Also, indicate that H 2 S 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 par titioning 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

Co mment 3

- 1: Purpose of Document: Text says EGBE in second paragraph. This should be changed to H 2 S. (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 Right

To

- Know Act. The biggest disparity in the report is the emphasis on air being the ultimate sink11 of hydrogen sulfide while all the toxicity data has been generated w ith 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.

H 2 S is a gas thus it does not h ave 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 20 o C and 591
atm at 30 o C. (Al

- Haddad et al. 1989). H3 can be converted to units of atm m 3 /mol by multiplying by the mola r volume of water, i.e. 18 x 10 - 6 m 3 /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 K AW is H/RT where R is 82 x 10 - 6 atm m 3 /mol K and T is absolute tem perature (K).
The K AW values are thus
0.35 at 20 o C and 0.43 at 30 o C These values are consistent with the 0.4 given by Balls & Liss (1983).
H 2 S does not really have a vapor pressure or a solubility as is implied in Table 1.1. It is preferable to delete t hese entries or explain them. It is H or K AW that matters. (peer reviewer: #4)12 EPA Response: In response to these comments, the following changes have been made. Air

water partitioning coefficient (K AW ) values have been added to Table 1

- 1. The vapor p ressure 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:

“

b The solubility of hydrogen sulfide in water is calculated from the stric t Henry’s Law constant over a wide temperature and pressure range (Carroll and Mather 1989): m H2S H = y H2S P, where y H2S is the mol fraction of hydrogen sulfide in the vapor phase, P is the total pressure, and m H2S is the mol fraction of hydrogen sulfide i n solution. Multiplying y H2S 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
K

AW values given. On p5 and p8 the H is given as 8.56 x 10 - 3 atm m 3 /mol which is ok but on P9 it is given as 0.163 resulting in a K AW 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 - 3 atm m 3 /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 mod el was not used for this assessment.

Comment 3

- 6: Because H 2 S dissociates with a pKa of 7.04 (p5 line 18) the report correctly points out that both H 2 S and HS2 are present at normal pHs. The K AW for H 2 S is ok, but the ratio of gas/water total concentrat ions (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 H S 2. So fate is also influenced by pH. This issue deserves some expansion and discussion. (peer reviewer: #4)13 EPA Respons e: 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 speciatio n at environmental pH.

Comment 3

- 7: There is a related ambiguity on p6 line 1 where a “sorption capacity” is given
for H 2 S in soils. These “capacities” must depend on the partial pressure of H 2 S. (peer reviewer: #4)

EPA Response: The paragraph discu ssing 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 H 2 S and that soils can differ in their sorption capacities. The conditions of the study to

measu re 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 f rom 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 H 2 S 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 r eviewer: #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; howeve r, a more detailed discussion of this subject is beyond the scope of this assessment. The following paragraph has been added to
Section 1.6.4:14 “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 calculatio n. 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 sul fide 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

- 8.15 Comment 3

- 14: P5 line 16 “dissociation activity” is a vague term. H 2 S 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 “H 2 S 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 chan ge 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 wou ld 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 relie s 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 re ference.