HEALTH CONSULTATION
FORMER MOBILE OIL VAPOR INTRUSION INVESTIGATION
MARSHFIELD, WOOD COUNTY, WISCONSIN
The Wisconsin Department of Natural Resources (WDNR), the Wisconsin Department of Commerce, and the Wood County Public Health Department asked the Wisconsin Department of Health and Family Services (DHFS) to assess the potential for vapor intrusion of petroleum fuels within a railroad corridor into the basement of a private residence and to determine if this is a pathway of concern. Based on groundwater monitoring and soil sampling conducted on Canadian National property and an adjacent private residence in Marshfield, Wisconsin, the potential for vapor intrusion of volatile organic compounds (VOCs) cannot be ruled out. Therefore, volatile organic compounds (VOCs) on this site are an indeterminate public health hazard. The remediation of this property is currently being planned. The vapor intrusion pathway should be formally investigated and addressed as part of the scope of work at this site.
BACKGROUND AND STATEMENT OF ISSUES
Petroleum from a former petroleum storage area within a railroad industrial corridor contaminates the soil of an adjacent residential property in Marshfield, Wisconsin. The site is undergoing remediation as part of a road construction project. Percent levels ("free product") of petroleum-related compounds, including benzene, xylenes, 1,2,4-trimethylbenzene, ethylbenzene, and 1,3,5-trimethylbenzene have been detected in soil 75 feet from the residence, with somewhat lower concentrations detected at other locations 10-100 feet from the building (Table 1; Envirogen 2002). Of these, benzene is addressed as the contaminant of greatest concern for affecting indoor air because of its high volatility and status as an Environmental Protection Agency (EPA) Class A carcinogen. The risk of chronic exposure of benzene to people living in an adjacent residence was investigated, at the request of WDNR, the Wisconsin Department of Commerce, and the Wood County Health Department.
Table 1. Concentration of benzene in soil and groundwater near the former Mobile Oil lease site, Marshfield, Wisconsin1.
| Location1 | Concentration in soil (µg/kg) |
Concentration in groundwater (µg/L) |
Distance to house (ft.) |
Depth (feet below surface) |
| MW-14 | 540 | 120 | 50 | 10-12 |
| SB-8 | 356 | Not avail. | 50 | 2-4 |
| SB-9 | 565 | Not avail. | 50 | 2-4 |
| MW-2 | Not avail. | Free product | 75 | 10 |
1Sample locations and results reported by Envirogen (2002).
Site visit. The residence was visited on February 27, 2003, by representatives of the WDNR, Wood County Public Health, and DHFS. Prior to the home visit, fuel odors were expected at the residence based on the close proximity of fuel-contaminated soil and groundwater, the relatively shallow local water table, and the high porosity of the century-old stone-and-mortar foundation at the residence. At the time of the February 27 visit, petroleum odors were not detected indoors, nor were there anecdotal reports of odors from the residents.
Evidence of occasional groundwater infiltration into the basement was noted. Water infiltration from a source contaminated with VOCs is a condition that could cause vapor intrusion. Lighter components of petroleum compounds such as benzene might be expected to intrude into buildings before the heavier components. It cannot be determined solely on the lack of petroleum odors in the residence whether there is an ongoing exposure to fuel contaminants in soils outside the house. Ultimately, eliminating the potential for vapor intrusion is dependent upon eliminating or reducing the source of soil vapor contaminants. Exposure to petroleum via drinking water is not a concern at this property because the residence is served by municipal water supplies unaffected by this contaminant source.
Interpretation of Environmental Investigation. Contamination in soils from the 0-3 foot depth, and at distances from 8-100 feet from the house, exceeds Wisconsin Administrative Code Ch. NR 720 and 746 standards (below screening values) for several fuel-related compounds (see Envirogen 2002, table 1). However, these concentrations are not a public health threat for either incidental ingestion, direct contact with contaminated soils, or from transfer of contaminants from surface soils to air. This conclusion is based on comparison to health-based values from the federal Agency for Toxic Substances and Disease Registry (ATSDR soil comparison values) and EPA (Region III risk-based concentrations).
In contrast, the potential for a health threat from organic vapor migration from soil or groundwater to building foundations cannot be excluded based on the Envirogen (2002) report. EPA (2002) has published model-based guidance and screening values for evaluating VOC migration and vapor entry into buildings. A strict application of these screening values to the benzene measured at this site (Appended) quantitatively predicts a strong potential for vapor entry into buildings at this site. However, EPA acknowledges that at petroleum-contaminated locations, site-specific features dominate, making modeling less predictive. DHFS guidance (2003) agrees with this position. Site-specific features would include microbial degradation, preferential pathways and other subsurface features, and the response of the source to high-water events. The lack of odors within the basement suggests that fuel contaminants are not continually migrating into the building. However, the presence of this contaminant source in soil and groundwater very near the home is cause for concern for future exposures. Under changing environmental conditions such as heavy rainfall, high water table, or excavation work on the property, contaminants could easily move the short distance to the home. Consequently, the vapor intrusion pathway should be formally investigated and addressed as part of the scope of work at this site.
Toxicology of benzene. The petroleum fuels found in soil at this site contain a mixture of chemicals, each having individual properties of toxicity, metabolic fate, and environmental fate. Of this mixture, benzene is the chemical with perhaps the most characterized toxicity to humans. It is also among the most volatile chemicals in the fuel mixture. Consequently, benzene receives focal attention in the public health evaluation of petroleum-contaminated property.
EPA classifies benzene a class A carcinogen, which means that benzene is known to cause cancer in humans. Much evidence supports benzene as a leukemogen in humans, where reactive intermediates formed during the metabolism of benzene preferentially affect the bone marrow. Over the long-term in a residential exposure situation, benzene levels at the odor threshold, roughly 5 parts per million (ppm), are an increased cancer risk. This exposure is more than one thousand times greater than health officials would permit in drinking water. Although benzene is rarely present above the odor threshold, its presence at harmful concentrations may still be suspected when the odors from other petroleum chemicals are noticed. When faint but perceptible gasoline odors are noticed, benzene levels can still be ten-to-one hundred-fold higher than levels considered safe and appropriate for residential indoor air. Other components of petroleum, such as toluene, xylenes, and naphthalene, can be smelled at very low concentrations that signal the presence of benzene. Long-term exposures to these and related VOCs pose a risk of non-cancer health effects including damage to the liver, kidneys, and nervous system. These non-cancer health effects can be addressed by addressing unnecessary cancer risks (DHFS, 2003). Although the odor of petroleum indoors is usually enough to conclude there is a problem without analyzing the air, the absence of odor may not rule out potential health effects when there is a known source of volatile compounds in soil or groundwater near a building.
The inhalation of petroleum-related chemicals, where present in indoor air, is a risk to child health. Although there are children living in the house, there is no known current exposure of health concern to petroleum from contaminated soils and groundwater.
Robert Thiboldeaux, Ph.D.
Toxicologist
Health Hazard Evaluation Unit
Bureau of Environmental Health
Division of Public Health
Wisconsin Department of Health and Family Services
American Industrial Hygiene Association. Odor Thresholds of Chemicals with Established Occupational Health Standards. Akron, OH: AIHA. 1989.
Envirogen, Inc. Site Investigation Report, Former Mobile Oil Lease Site, Marshfield, WI. PIN 5662443. September 2002.
U.S. EPA. 2002. Draft guidance for evaluating the vapor intrusion to indoor air pathway from
groundwater and soils. http://www.epa.gov/epaoswer/hazwaste/ca/eis/vapor.htm 
Wisconsin Department of Health and Family Services. 2003. Chemical vapor intrusion and
indoor air: Guidance for environmental consultants and contractors.
http://www.dhfs.state.wi.us/eh/Air/
Environmental fate of benzene. Using guidance developed by EPA (2002), a screening value (based on predictions of the Johnson and Ettinger model) of 14 micrograms/liter (µg/L) for benzene in groundwater is estimated for this site. This is based on a vapor attenuation factor of 1x10-4 (EPA, fig. 3b, loam) and a cancer risk factor of 1x10-6. The corresponding generic screening value for groundwater, where soil information is absent, is 5.0 µg/L. In addition, the EPA recommends that these screening values not be used when actual groundwater concentrations exceed 20-times the screening value (in this case, 280 µg/L). At concentrations this high, the predictive value of the Johnson and Ettinger model (U.S. EPA, 2000) diminishes, and must be supplemented with target-specific information. According to the Envirogen report, benzene exceeds 280 µg/L at several sample locations within 100 feet of the residence.
This public health consultation for the former Mobile Oil facility was prepared by the Wisconsin Department of Health and Family Services under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the Public Health Consultation was begun.
Gail Godfrey
Technical Project Officer, S.P.S., SAAB, DHAC
The Division of Health Assessment and Consultation, ATSDR, has reviewed this Public Health Consultation and concurs with the findings.
Roberta Erlwein
Chief, State Program Section, DHAC, ATSDR
