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HEALTH CONSULTATION

Review of Hydrogen Sulfide Risk Assessment

FRANK STREET DUMP
SANTA MONICA, LOS ANGELES COUNTY, CALIFORNIA


SUMMARY

The California Department of Health Services (CDHS) Environmental Health Investigations Branch (EHIB) was requested by the regional representative of the Agency for Toxic Substances and Disease Registry (ATSDR) to review a health risk assessment for hydrogen sulfide emissions from the Frank Street Dump. According to the ATSDR regional representative, it was necessary to send a rapid response to the City of Santa Monica. Therefore, EHIB staff wrote a letter dated August 8, 2001, to the regional representative in which we reviewed the risk assessment. The content of that letter is the basis of this health consultation. EHIB has a cooperative agreement with the Agency for Toxic Substances and Disease Registry and the letter is being forwarded to ATSDR as a health consultation for their concurrence.


BACKGROUND AND STATEMENT OF ISSUES

The Frank Street Dump is an inactive landfill in Santa Monica, California, that accepted municipal waste from about 1949 through 1964 (1). Although the precise extent of the dump is not known, the approximate boundaries are Olympic Boulevard, Cloverfield Boulevard, Stewart Street, and the Santa Monica Freeway. Several light industrial facilities, a park, and portions of the Mountain View Mobile Home Park are located over the former landfill. Approximately 140 mobile homes are located on the approximately 5-acre Mountain View Mobile Home Park.

In 1999, the residents of the Mountain View Mobile Home Park asked ATSDR to assess the public health hazard associated with exposures to landfill gas at the mobile home park. ATSDR evaluated the hazards and wrote a health consultation dated December 30, 1999, in which the agency summarized its findings (1). ATSDR concluded that the landfill gas emissions and the volatile organic chemicals in the open air at the mobile home park do not pose a public health hazard. However, ATSDR recommended a study to evaluate the ambient levels of hydrogen sulfide at the mobile home park because this had not been characterized.

Hydrogen sulfide is a gas occasionally found in landfills and has a characteristic odor of "rotten eggs" or "sewer gas." Hydrogen sulfide gas might be released from a number of other sources, including certain manufacturing processes, natural hot springs, waste treatment plants, livestock framing, oil and gas drilling, and sewers (3). Some of the residents of the mobile home park have expressed concern about a rotten-egg odor near Frank Street Dump (2).

In 2000, ATSDR was asked by the City of Santa Monica to review its work plan for hydrogen sulfide emissions testing at the former Frank Street Dump. The city proposed to characterize the surface emission rates of hydrogen sulfide using the surface isolation emission flux chambers and a Jerome 631-X hydrogen sulfide analyzer. ATSDR reviewed the work plan and wrote a health consultation dated July 24, 2000, in which the agency summarized its comments (2). ATSDR concluded that "based on the results of the surface isolation flux chamber testing, an air dispersion model might be used to estimate nearby residents' exposure to hydrogen sulfide" and suggested that additional sampling might be needed to further evaluate exposure to hydrogen sulfide near the site. ATSDR was unable to evaluate the appropriateness of sample locations to characterize potential exposure to residents because the basis for the sample number and locations were not included in what ATSDR reviewed. ATSDR also requested that the analysis of the resulting flux chamber data should include a discussion of how temporal, spatial, temperature, barometric pressure, soil moisture and laboratory variability might affect the results.


DISCUSSION

On July 13, 2000, the consultants for the City of Santa Monica conducted surface flux emission testing at two locations on city property and over the historic landfill footprint, five locations on the mobile home property and over the landfill footprint, four locations on the mobile home property and not over the landfill footprint, and two over sewer covers--one within the mobile home and one not within the mobile home (4). Soil gas probes were co-located with the two flux chamber locations on the city property. For more detail about the methodology, see the attached letter.

Except for two locations, the flux chamber results were approximately 1 part per billion per volume (ppbv), or 0.052 microgram per meter squared per minute--the detection limit of the field measuring device. At one location on city property and over the historic landfill footprint, a flux of 3 ppbv (0.16 microgram per meter squared per minute) was measured. At the on-site sewer system location, a flux of 8 ppbv (0.42 microgram per meter squared per minute) was recorded.

Using the information from the hydrogen sulfide flux chamber collection, consultants for the City of Santa Monica conducted a health risk assessment for hydrogen sulfide exposure to the nearest residents (5). The focus of the risk assessment is on the landfill, so the flux chamber results from the locations over the landfill were considered for the exposure calculations. The consultant took the public health protective step of using the maximum flux measured (0.42 microgram per meter squared per minute) instead of a statistical summary of all the measurements collected over the landfill. In addition to looking at the theoretical health risk from hydrogen sulfide exposure alone, the consultant also added this risk with that from the other exposures that had been evaluated in the original health risk assessment for the site. Exposure point concentrations for outdoor and indoor exposure were derived by applying modeling to the flux chamber measurements (see Appendix A letter for more detail).

The outdoor exposure point concentration for hydrogen sulfide was calculated to be 0.0063 micrograms per meter cubed. The indoor air exposure point concentration was calculated to be 0.011 micrograms per meter cubed.

For the risk assessment, the consultant characterized potential noncarcinogenic adverse health effects by comparing the predicted outdoor and indoor air concentrations of hydrogen sulfide with the U.S. Environmental Protection Agency (EPA) reference concentration (RfC). The ratio of the predicted air concentration to the RfC is termed the hazard index (HI). If the HI is less than 1, then no noncancer adverse health effects are expected. The HI for outdoor exposure was estimated at 0.0063 and the HI for indoor exposure was estimated at 0.011. The consultant also added the hydrogen sulfide HI together with HI from the original risk assessment in which exposure to 15 other chemicals of potential concern was estimated. The cumulative HI for outdoor exposure was 0.027, and for indoor exposure was 0.017. Thus, the consultant concluded that "there are no noncarcinogenic health hazards associated with former land filling activities at or near the site."

Potential Health Effects from Exposure to Hydrogen Sulfide

The health effects of exposure to hydrogen sulfide depend mainly on the concentrations in the air and how one is exposed. In general, levels of a few parts per million (ppm) of hydrogen sulfide in the air might cause eye and respiratory irritation. Breathing in hydrogen sulfide on a long-term basis might result in fatigue, loss of appetite, headaches, irritability, poor memory and dizziness (2).

ATSDR has established noncancer health comparison values, such as minimal risk level (MRL), for hydrogen sulfide exposure (2). An MRL is "an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse noncancer health effects over a specified duration of exposure." The acute MRL exposure of 1-14 days is 0.07 ppm (100 micrograms per meter cubed) hydrogen sulfide. This finding was based on a one-time exposure study involving respiratory endpoints. The intermediate MRL, exposure of 15-364 days, is 0.03 ppm (37 micrograms per meter cubed) hydrogen sulfide. This was based on a 90-day study of effects in mice. ATSDR has not established a chronic MRL exposure greater than 364 days.

The U.S. Environmental Protection Agency (EPA) has determined that hydrogen sulfide is not carcinogenic.

CDHS's Review of the Flux Chamber Data and the Hydrogen Sulfide Health Risk Assessment

Both the indoor and outdoor exposure point concentration estimates of hydrogen sulfide arising from the landfill are several orders of magnitude smaller than the acute or intermediate MRL. Thus, using the flux chamber measurements, there does not seem to be a health hazard from the hydrogen sulfide emissions. The modeling approach and risk assessment for the outdoor health risk posed by the hydrogen sulfide emissions from the landfill seems appropriate. However, the approach the consultant took to evaluate the indoor health risk is not typical and grossly underestimates the risk posed by soil gas intrusion into a structure.

Unlike the flux of soil gas in an open area, a structure creates a pressure-driven flow of air through the soil and through penetrations in the building substructure (6). The flux of soil gas beneath a building cannot be estimated using flux chamber data. A flux chamber collects only the diffusion-based soil gas flow that occurs in the absence of a force being applied to the surface. This is further supported by the information in the hydrogen sulfide health risk assessment that barometric pressure and temperature do not affect flux chamber measurements, but have been shown to alter soil gas infiltration into a structure by two orders of magnitude.

The preferred approach for investigating a soil gas intrusion problem is to use soil gas data in the Johnson and Ettinger model (7). This model takes into account diffusive and convective forces to estimate the flux rate and indoor air concentration. If the model indicates there could be a concern, then further soil gas data should be gathered by placing probes close to the structure or placing diagonal probes underneath the structure to measure the effect the structure is having on the soil. If the problem is confirmed with the soil gas probes, then the next step involves conducting indoor air measurements, taking into account possible indoor sources of the chemical.

Thus, the approach taken in the hydrogen sulfide risk assessment would underestimate the actual soil gas intrusion problem. This leads one to ask, could this underestimation change the conclusion of risk for the indoor exposure? It is not clear. However, radon studies show the entry rate due to pressure-driven flow exceeds by an order of magnitude that of molecular diffusion flow (8). Other studies indicate the flow rates changes can be even greater. It is possible that the indoor air hydrogen sulfide estimates, which are about two orders of magnitude smaller than the health comparison values, might not exceed the health comparison value even if the convective force of the structure were taken into account. However, this should be confirmed using the suggested approach of the Air/Superfund National Technical Guidance Series (9). Namely, use the existing soil gas monitoring wells located around the landfill to collect hydrogen sulfide measurements. Use this data in the Johnson and Ettinger model to estimate indoor air concentrations and compare to health comparison values.


CONCLUSIONS AND RECOMMENDATIONS

Hydrogen sulfide soil gas emissions from the landfill do not pose a health risk to those breathing ambient air. It is not possible to conclude that hydrogen sulfide soil gas emissions would pose a health risk to indoor air. Though this was evaluated in the risk assessment, the approach taken would not adequately address the pathway of soil gas entry, and, thus, the conclusion that the pathway does not pose a risk cannot be supported. Therefore, the soil gas pathway is categorized as an indeterminate health hazard.

CDHS recommends that existing soil gas monitoring wells located around the landfill be utilized to collect hydrogen sulfide measurements. Use this data in the Johnson and Ettinger model to estimate indoor air concentrations, and compare the results to health comparison values. If a problem might exist based on the model, place near-structure soil gas probes and/or conduct indoor air monitoring.


PUBLIC HEALTH RECOMMENDATIONS AND ACTIONS

The Public Health Recommendations and Actions Plan (PHRAP) for this site contains a description of actions taken, to be taken, or under consideration by ATSDR and CDHS at and near the site. The purpose of the PHRAP is to ensure that public health activities not only identifies public health hazards, but also provides a plan of action designed to mitigate and prevent adverse human health effects resulting from exposure to hazardous substances in the environment. The CDHS and ATSDR will follow up on this plan to ensure that actions are carried out.

Actions Completed

  1. ATSDR evaluated the health hazard that landfill gases posed to the nearby residents.


  2. ATSDR reviewed and commented on the work plan for hydrogen sulfide flux chamber testing.


  3. CDHS evaluated the hydrogen sulfide flux chamber results and the health risk assessment of hydrogen sulfide.

Recommendations for Future Action

  1. Collect hydrogen sulfide measurements using the existing soil gas monitoring wells located on the landfill.


  2. Use this data in the Johnson and Ettinger model to estimate indoor air concentrations, and compare to health comparison values.


  3. If the Johnson and Ettinger model results using soil gas data from the existing wells exceed health comparison values, conduct soil gas probe sampling near the structures around the landfill.


  4. Use the Johnson and Ettinger model to estimate indoor concentrations using the near-structure soil gas probe data and compare to health comparison values.


  5. If the Johnson and Ettinger model results using soil gas data from the near-structure probes exceed health comparison values, conduct indoor air sampling in the structures.


  6. If indoor air sampling indicates there might be hydrogen sulfide coming from the landfill at a level of health concern, eliminate or reduce this exposure.

PREPARERS OF REPORT

Environmental and Health Effects Assessors

Marilyn C. Underwood, PhD
Staff Toxicologist
Environmental Health Investigation Branch
California Department of Health Services


ATSDR Technical Project Officer

Debra Gable
Environmental Health Scientist
Division of Health Assessment and Consultation
Superfund Site Assessment Branch, State Programs Section


ATSDR Regional Representatives

William Nelson
Gwendolyn Eng
Dan Strausbaugh
Regional Representatives, Region IX
Agency for Toxic Substances and Disease Registry


REFERENCES

  1. Agency for Toxic Substances and Disease Registry. Health consultation concerning Frank Street Dump--evaluation of landfill gas at the Mountain View Mobile Home Park. Atlanta: US Department of Health and Human Services; 1999 Dec 30.


  2. Agency for Toxic Substances and Disease Registry. Toxicological profile for hydrogen sulfide. Atlanta: US Department of Health and Human Services; July 1999.


  3. Agency for Toxic Substances and Disease Registry. Health consultation concerning Frank Street Dump--review of proposal to sample for hydrogen sulfide emissions. Atlanta: US Department of Health and Human Services; 2000 Jul 24.


  4. City of Santa Monica Environmental Programs Division. Draft of technical memorandum prepared for Teri Copeland, consulting toxicologist, by CE Schmidt, environmental consultant, concerning results of the surface flux chamber testing for hydrogen sulfide at the Mountain View Mobile Home Park site in Santa Monica, California. July 2000.


  5. City of Santa Monica Environmental Programs Division. Health risk assessment for the evaluation of hydrogen sulfide at the Mountain View Mobile Home Park, Santa Monica, California. Prepared by Teri L. Copeland, consulting toxicologist. Santa Monica, California: City of Santa Monica; 2000 Aug.


  6. Little JC, Daisey JM, Nazaroff WW. 1992. Transport of subsurface contaminants into buildings. Environ Sci Technol 26:2061-2066.


  7. Johnson PC, Ettinger RA. 1991. Heuristic model for predicting the intrusion rate of contaminant vapors into buildings. Environ Sci Technol 25:1445-1452.


  8. Nazaroff WW, Lewis SR, Doyle SM, Moed BA, Nero AV. 1987. Experiments on pollutant transport from soil into residential basements by pressure-driven airflow. Environ Sci Technol 21:459-466.


  9. US Environmental Protection Agency. Air/Superfund national technical guidance study series: assessing potential indoor air impacts for Superfund sites. 1992 Sep. Report No.: EPA-451/R0-92-002.

CERTIFICATION

The California Department of Health Services prepared this Frank Street Dump Health Consultation under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). The health consultation is in accordance with approved methodology and procedures existing at the time the health assessment was begun.

Debra Gable
Technical Project Officer
Superfund Site Assessment Branch (SSAB)
Division of Health Assessment and Consultation (DHAC)
ATSDR


The Division of Health Assessment and Consultation, ATSDR, has reviewed this health consultation and concurs with its findings.

Roberta Erlwein
Section Chief,
SPS, SSAB, DHAC,
ATSDR


APPENDIX A: LETTER TO THE REGIONAL REPRESENTATIVE OF THE AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY CONCERNING HYDROGEN SULFIDE EMISSIONS FROM THE FRANK STREET DUMP FROM THE CALIFORNIA DEPARTMENT OF HEALTH SERVICES DATED AUGUST 22, 2001


DEPARTMENT OF HEALTH SERVICESDEPARTMENT OF HEALTH SERVICES
1515 CLAY STREET, SUITE 1700
OAKLAND, CA 94612
(510) 622-4500

August 22, 2001

William Q. Nelson
Senior Regional Representative
Agency for Toxic Substances and Disease Registry
1500 Hawthorne Street
Suite 100, MS HHS-1
San Francisco, CA 94105

Dear Mr. Nelson:

The California Department of Health Services Environmental Health Investigations Branch (CDHS-EHIB) was asked by you to review the "Health Risk Assessment for the Evaluation of Hydrogen Sulfide, Mountain View Mobile Home Park, 1930-1932 Stewart Street, Santa Monica, California". This document is dated August 2000; it was written by Teri I. Copeland, Consulting Toxicologist for the City of Santa Monica Environmental Programs Division. You asked us to review the health assessment as part of our cooperative agreement work that we have with your agency, the Agency for Toxic Substances and Disease Registry (ATSDR). In this letter, we will comment on the health risk assessment. In addition to the health risk assessment, we reviewed three additional, related documents (two written by ATSDR and a third one prepared by C.E. Schmidt, Environmental Consultant for Teri Copeland, Consulting Toxicologist). A summary of the pertinent parts of these reports is provided below.

According to the information reviewed, the Frank Street Dump is an inactive landfill in Santa Monica, California, which accepted municipal waste from approximately 1949 through 1964. Although the precise extent of the dump is not known, the approximate boundaries are Olympic Boulevard, Cloverfield Boulevard, Stewart Street, and the Santa Monica Freeway. Several light industrial facilities, a park, and portions of the Mountain View Mobile Home Park are located over the former landfill.

Evaluation of Landfill Gas at the Mountain View Mobile Home Park

In 1999, ATSDR evaluated the potential health hazards from landfill gases at the Mountain View Mobile Home Park. After reviewing available sampling data of methane and other volatile organic compounds, ATSDR concluded that the landfill gas emissions and the volatile organic chemicals in the breathing air at the mobile home park do not pose a public health hazard. However, ATSDR recommended a study to evaluate the ambient levels of hydrogen sulfide at the mobile home park because this had not been characterized.

Review of Proposal to Sample for Hydrogen Sulfide Emissions

In 2000, ATSDR was asked by the City of Santa Monica to review their proposed hydrogen sulfide emissions testing at the former Frank Street Dump to determine if the results would be useful in a public health evaluation. The city proposed to characterize the surface emission rates of hydrogen sulfide using the surface isolation emission flux chambers and a Jerome 631-X hydrogen sulfide analyzer. ATSDR concluded that "based on the results of the surface isolation flux chamber testing, an air dispersion model may be used to estimate nearby residents' exposure to hydrogen sulfide". They suggested that additional sampling might be needed to further evaluate exposure to hydrogen sulfide near the site. ATSDR was unable to evaluate the appropriateness of sample locations to characterize potential exposure to residents since the basis for the sample number and locations were not included in what ATSDR reviewed. ATSDR also requested that the analysis of the resulting flux chamber data should include a discussion of how temporal, spatial, temperature, barometric pressure, soil moisture and laboratory variability may affect the results.

Results of the Surface Flux Chamber Testing for Hydrogen Sulfide

On July 13, 2000, the consultants for the City of Santa Monica conducted surface flux emission testing at two locations on City property and over the historic landfill footprint, five locations on the mobile home property and over the historic landfill footprint, four locations on the mobile home property and not over the landfill footprint, and two over sewer covers, one within the mobile home and one not within the mobile home. Soil gas probes were co-located with the two flux chamber locations on the City property.

Surface flux measurements were collected for a period of 30 minutes at each site with an inert gas sweeping through the chamber at a rate of 5.0 liters per minute. The flux chamber was placed on barren soil or grass, or in the case of the sewer locations, over the lid of the sewer. All the measurements were taken on the same day. The outdoor temperature ranged from 72 to 88 degrees Fahrenheit.

Except for two locations, the flux chamber results were approximately 1 ppbv (0.052 microgram per meter squared per minute), the detection limit of the field measuring device. At one location on City property and over the historic landfill footprint, a flux of 3 ppbv (0.16 microgram per meter squared per minute) was measured. At the on-site sewer system location, a flux of 8 ppbv (0.42 microgram per meter squared per minute).

Health Risk Assessment for the Evaluation of Hydrogen Sulfide

Using the information from the hydrogen sulfide flux chamber collection, consultants for the City of Santa Monica conducted a health risk assessment for hydrogen sulfide exposure to the nearest residents. The focus of the risk assessment is on the landfill so the flux chamber locations over the landfill were considered for the exposure calculations. The consultant took the public health protective step of using the maximum flux that was measured (0.42 microgram per meter squared per minute) instead of a statistical summary of all the measurements collected over the landfill. In addition to looking at the theoretical health risk from hydrogen sulfide exposure alone, the consultant also added this risk with that from the other exposures that had been evaluated in the original health risk assessment for the site. Exposure point concentrations for outdoor and indoor exposure were derived by applying modeling to the flux chamber measurements. For the outdoor air concentration, the standard "box model" was used. The model assumes the emission of the chemical from the soil surface (flux measurement) into an airborne box, with time- dependent dilution based on wind speed (135 meters per minute). For the indoor air concentration, an ASTM model was applied. This model applies measured flux rates to an indoor dispersion factor which considers volume of a residence (272 meters squared), infiltration into the building in cracks in the foundation (0.1% of surface area), and using a minimum residential air exchange rate (0.583 liters per minute).

The outdoor exposure point concentration for hydrogen sulfide was calculated to be 0.0063 micrograms per meter cubed. The indoor air exposure point concentration was calculated to be 0.011 micrograms per meter cubed.

For the risk assessment, the consultant characterized potential noncarcinogenic adverse health effects by comparing the predicted outdoor and indoor air concentrations of hydrogen sulfide with the U.S. Environmental Protection Agency (USEPA) Reference Concentration (RfC). The ratio of the predicted air concentration to the RfC is termed the hazard index (HI). If the HI is less than one than no non-cancer adverse health effects are expected. The HI for outdoor exposure was estimated at 0.0063 and the HI for indoor exposure was estimated at 0.011. The consultant also added the hydrogen sulfide HI together with HI from the original risk assessment in which exposure to fifteen other chemicals of potential concern was estimated. The cumulative HI for outdoor exposure was 0.027, and for indoor exposure was 0.017.

Thus the consultant concluded that "there are no noncarcinogenic health hazards associated with former land filling activities at or near the site".

Review of the Health Risk Assessment for the Evaluation of Hydrogen Sulfide

The following is CDHS' review of the health risk assessment and its findings:

CDHS staff evaluated the exposure point estimates using health comparison values developed by ATSDR. ATSDR has established non-cancer health comparison values (Minimal Risk Level, MRL) for hydrogen sulfide exposure. An MRL is "an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse non-cancer health effects over a specified duration of exposure". The acute MRL, exposure of 1-14 days, is 0.07 ppm (100 micrograms per meter cubed) hydrogen sulfide. The intermediate MRL, exposure of 15-364 days, is 0.03 ppm (37 micrograms per meter cubed) hydrogen sulfide. ATSDR has not established a chronic MRL, exposure greater than 364 days.

Both the indoor and outdoor exposure point concentration estimates of hydrogen sulfide arising from the landfill are several orders of magnitude smaller than the acute or intermediate MRL.

The U.S. Environmental Protection Agency (USEPA) has determined that hydrogen sulfide is not carcinogenic. Therefore CDHS did not calculate a cancer risk for exposure to hydrogen sulfide.

General comments

The consultant took a public health conservative approach by using the maximum flux rate measured, and not using a statistical summary value of the entire data set.

The modeling approach that the consultant took to evaluate the outdoor health risk posed by the hydrogen sulfide emissions from the landfill seems appropriate. It takes the flux measured on a typical summer day and the assumes these emissions are diluted within a "box" above the soil and within the breathing zone of a person. The "box" model is the one typically used for such estimations.

On the other hand, the approach that the consultant took to evaluate the indoor health risk is not typical and underestimates the risk posed by soil gas intrusion into a structure. Unlike the flux of soil gas in an open area, a structure creates a pressure-driven flow of air through the soil and through penetrations in the building substructure. The flux of soil gas beneath a building cannot be estimated using flux chamber data. A flux chamber collects only the diffusion-based soil gas flow that occurs in the absence of a force being applied to the surface. This is further supported by the information in the hydrogen sulfide health risk assessment that barometric pressure and temperature do not affect flux chamber measurements, but have been shown to alter soil gas infiltration into a structure by two orders of magnitude.

The preferred approach for investigating a soil gas intrusion problem is to use soil gas data in the Johnson and Ettinger model (URL: http://www.epa.gov/superfund/programs/risk/airmodel/johnson_ettinger.htm). The Johnson and Ettinger model takes into account diffusive and convective forces to estimate the flux rate and indoor air concentration. If the model indicates there could a be concern then further soil gas data should be gathered by placing probes close to the structure or placing diagonal probes underneath the structure to measure the affect the structure is having on the soil. If the problem is confirmed with the soil gas probes, then the next step involves conducting indoor air measurements, taking into account possible indoor sources of the chemical.

Thus, the approach taken in the hydrogen sulfide risk assessment would underestimate the actual soil gas intrusion problem. This leads one to ask, could this underestimation change the conclusion of risk for the indoor exposure? It is not clear. However, radon studies show the entry rate due to pressure-driven flow exceeds by an order of magnitude that of molecular diffusion flow. Other studies indicate the flow rates changes can be even greater. It is possible that the indoor air hydrogen sulfide estimates which are about two orders of magnitude smaller than the health comparison values might not exceed the health comparison value even if the convective force of the structure were taken into account. However this should be confirmed using the suggested approach of the Air/Superfund National Technical Guidance Series. Namely, use the existing soil gas monitoring wells located around the landfill to collect hydrogen sulfide measurements. Use this data in the Johnson and Ettinger model to estimate indoor air concentrations, and compare to health comparison values.

Conclusions and Recommendations

Hydrogen sulfide soil gas emissions from the landfill do not pose a health risk to those breathing the outdoor air. It is not possible to conclude that hydrogen sulfide soil gas emissions would pose a health risk to indoor air. Though this was evaluated in the risk assessment, the approach taken would not adequately address the pathway of soil gas entry, and thus the conclusion that the pathway does not pose a risk cannot be supported. CDHS recommends that the existing soil gas monitoring wells located around the landfill be utilized to collect hydrogen sulfide measurements. Use this data in the Johnson and Ettinger model to estimate indoor air concentrations, and compare to health comparison values. If a problem may exist based on the model, place near-structure soil gas probes and/or conduct indoor air monitoring. Alternatively, the soil gas as entry pathway could be mitigated, i.e. decouple the trailer home effect on the soil by creating a highly ventilated crawl space area.

If we can be of further assistance, please contact Marilyn C. Underwood, Ph.D. at (510) 622-4415.

Sincerely,


Marilyn C. Underwood, Ph.D.
Staff Toxicologist
Environmental Health Investigations Branch

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