Vapor Intrusion Investigation
DECORAH SHOPPING CENTER ANNEX
WEST BEND, WASHINGTON COUNTY, WISCONSIN
The Wisconsin Department of Natural Resources (WDNR) asked The Wisconsin Department ofHealth and Family Services (DHFS) to assess possible vapor migration of waste solvents from adry cleaning facility in West Bend, Wisconsin. Groundwater monitoring and soil samplingindicate that volatile organic compounds (VOCs) on this site are too concentrated to exclude thepotential for this type of vapor intrusion into basements of nearby residences. The vaporintrusion pathway is being investigated and addressed as part of the scope of work at this site. No complete exposure pathways, via either direct contact or ingestion of VOC-contaminated soiland groundwater, exist at the site. In a limited investigation, a complete vapor migration andintrusion pathway to nearby residences was not observed downgradient from the source area. However, uncertainties remain in the conceptual site model regarding the environmental fate ofVOCs from the site. PCE (tetrachloroethene) and TCE (trichloroethylene) soil gasconcentrations downgradient from the source area were much less than expected; however, theVOCs may be migrating north from the source area along a utility corridor on the eastern edge ofthe property. Pending an environmental investigation that conclusively excludes the possibilityof organic vapor migration into basements, the site poses an indeterminate public health hazard. The previous soil vapor investigation should be repeated for verification, and the utility corridormentioned above should be investigated for vapor migration. Even if vapor migration isexcluded as a health threat under current conditions, it is difficult to confidently predict futureconditions related to groundwater contaminants at this site. The groundwater should beremediated to prevent future on- and off-site vapor migration concerns.
The Decorah Shopping Center Annex (Decorah Annex) is located at 1011-1025 S. Main Street,West Bend, Washington County, Wisconsin. The property is a suburban strip mall located on astreet of similar commercial properties. The property abuts the back yards of a 1960-1970sresidential neighborhood, the nearest homes being about 75 feet away. An investigationperformed by Key Engineering Group Ltd. (Hoffart and Johnson 2002) reported the presence oftetrachloroethene (PCE) and trichloroethene (TCE) in soil and groundwater in the vicinity of adry cleaning laundry formerly operating on the annex property. A preliminary soil gasinvestigation in 2003 was directed toward ruling out the potential for vapor intrusion into nearbyhomes. At the request of WDNR, DHFS reviewed the investigation work plan and thesubsequent soil vapor results from Key Engineering Group Ltd. (King and Graham 2003) for theDecorah Annex and adjacent residential property. DHFS recommendations following theseresults have been incorporated into the ongoing investigation of this property.
The soil vapor investigation follows the results of a groundwater investigation at the DecorahAnnex property (Table 1); the initial investigation detected PCE and TCE in groundwater but notin municipal water. PCE in soil (maximum 1500 mg/kg at 3-5 feet below surface) was detectedin borings beneath the paved area behind the shopping center (Key Environmental Services1997). PCE and TCE at the Decorah Annex property do not present a public health hazard fromeither dermal contact or ingestion because people do not have direct contact with affected soilsor groundwater.
However, PCE concentrations as high as 1,800 parts per billion (ppb) in groundwater near thesource area and 1,000 ppb down-plume near residences indicated a vapor intrusion potential,based on comparison to the EPA (2002) screening values for evaluating VOC migration andvapor entry into buildings. The EPA screening values for PCE (5.0 ppb) and TCE (5.0 ppb) aremodeled groundwater concentrations that predict indoor air concentrations in nearby buildingsbelow which there is no likely health risk. The maximum concentrations of VOCs found ingroundwater at the Decorah Annex property were PCE (1,800 ppb) and TCE (12 ppb) (Table 1)(Hoffart and Johnson 2002). These concentrations were detected in a monitoring well that liesnear a sewer manhole believed to be at or near the source of the contamination.
The soil vapor investigation reported by Key Engineering consisted of one sample point at thesource area and two points placed on the contour of the estimated 1,000 ppb PCE plume area(Figure 1) (King and Graham 2003). This limited investigation revealed no vapor intrusionproblems. At the two downgradient soil vapor monitoring points (Figure 1), private consultantsreport no detection of these VOCs and, to date, no data have been collected around the DecorahAnnex property to indicate that people have been exposed to PCE or TCE in their indoor air. However, the current soil vapor investigation may be insufficient to confidently conclude thatthe vapor intrusion pathway to nearby residences is incomplete. Pending an environmentalinvestigation that conclusively excludes this possibility, the site poses an indeterminate publichealth hazard from the inhalation of vapors.
The concentration of PCE and TCE vapors measured in soil gas collected at the source area andthe two downgradient sampling points (Table 1) were much lower than predicted by modeledpartitioning from groundwater. Key Engineering reported that PCE in groundwater around thesource area was 1,800 ppb in samples collected September 27, 2002 (Hoffart and Johnson2002). A groundwater-to-soil vapor attenuation factor of 0.1 to 0.01 predicts that soil vaporaround those source area should be roughly 180 to 18 ppb, respectively. PCE and TCE reportedin soil vapor around the source area were below the limit of detection (0.5 ppb), which for PCEis at least 36 times lower than the level predicted by modeled partitioning from groundwater. More investigation is needed to resolve this inconsistency.
The investigating consultant, Key Engineering, provided several possible explanations for thesefindings, the most plausible being that VOC gases from the groundwater source are quicklydiverted northward along a utility corridor that lies midway between South Main Street andLincoln Drive West (King 2003). If this is the case, this further supports the conclusion that thedown-plume houses along Lincoln Drive West and beyond are not receptors for vapor intrusion. However, this raises the possibility of vapor migration down this utility corridor. It may also bethat PCE and TCE vapors rapidly pass through sandy soil to the atmosphere and do notappreciably accumulate in the vadose zone around the source area. It should also be consideredthat PCE and TCE vapors could be vented through utility corridors that lie both in the alleybehind the Lincoln Drive residences and within Lincoln Drive. For these reasons, additionalinvestigation along and adjacent to this utility corridor should be considered. It may not benecessary to "chase" VOCs along utility corridors if an examination of city engineering recordsprovides evidence that this is not a valid pathway to nearby buildings.
The limited soil gas sampling has raised more questions than it has answered. Confidence in thisinvestigation would be improved by either repeating the present sampling regimen, particularlyunder frozen soil conditions, or by chasing the vapors to learn if PCE and TCE are travelingalong preferential pathways. The possibility of indoor air sampling to exclude individual homesas vapor intrusion receptors has been discussed with the property owner and theirrepresentatives. However, DHFS does not see a reason at this time to expand the soil vaporinvestigation beyond the area surrounding the first tier of homes already studied and the utilitycorridor that extends north from the source area.
Aside from additional investigation into the extent of PCE and TCE soil vapor migration nearthe Decorah Annex property, remediation of the source area would directly address uncertaintiesabout unknown future effects from the source. Source removal, combined with the low riskindicated by current soil gas sampling, would provide the confidence needed to inform the publicthat there is no current risk and that the environmental situation is expected to improve afterremoval or treatment of the source of contamination.
|Contaminant||Maximum detected in groundwater (ppb)||Maximum detected in shallow soil vapor (ppb)||EPA vapor intrusion groundwater screening value2 (ppb)||EPA target indoor air concentration3 (ppb)||EPA target soil vapor concentration4 (ppb)5|
1Results reported by Key Engineering Group Ltd. Project status update, Decorah Shopping Center Annex. Letter dated December 3, 2002, from CM Hoffart and GL Johnson, to B. Amungwafor, WI DNR. File 072007.
2Screening value is the EPA maximum contaminant level (MCL) for these chemicals. The modeled target groundwater concentration for PCE and TCE is lower than 5 ppb, but EPAs policy is to screen no lower than the drinking water MCL, which is the enforceable standard.
3Value satisfies both risk = 10-6 and hazard index = 1. U.S. EPA. 2002. Draft guidance for evaluating the vapor intrusion to indoor air pathway from groundwater and soils; Table 2c.
4Represents increased lifetime cancer risk of one in one million.
5ppb: parts per billion, or micrograms per liter (µg/L).
Long-term exposure to chlorinated solvents and other VOCs in indoor air is a risk to childhealth. An investigation of the vapor intrusion pathway is needed to assess whether such a risk exists for children living in homes adjacent to the Decorah Annex property.
- At the Decorah Annex property, chlorinated solvents are sufficiently concentrated ingroundwater to potentially cause vapor migration through soils and into nearby buildings.Pending an environmental investigation that conclusively excludes this possibility, the site poses an indeterminate public health hazard.
- A limited investigation of soil vapor near the source area did not indicate a complete vapor migration and intrusion pathway downgradient from the source area.
- Uncertainties remain in the conceptual site model with regard to the environmental fate ofPCE and TCE contamination at the Decorah property. PCE and TCE vapor concentrationsdowngradient from the source area were much less than expected; whether these vapors aremigrating north from the source area along the utility corridor is not known.
- Vapor migration may continue to be an issue at the Decorah Annex property as long as the groundwater is contaminated.
- Repeat the previous soil vapor monitoring (gas probes [GP] 23-25), preferably under winter conditions.
- Investigate vapor migration along the utility corridor behind the Lincoln Drive residences.
- Consider cleanup of the source of chlorinated solvents on the property to address future on- and off-site health hazards.
- Inform area residents about the results of the soil vapor investigation and any remediation plans for the property.
- DHFS will evaluate the results of any further vapor intrusion investigations and assess the potential for a completed exposure pathway.
- The property owner has received approval from WDNR to continue the investigation andremediation of the Decorah Annex property.
- DHFS and the Washington County Health Department will assist the City of West Bend withsharing the results of the environmental investigation of the Decorah Annex property with arearesidents."
Robert Thiboldeaux, Ph.D.
Health Hazard Evaluation Unit
Bureau of Environmental Health
Division of Public Health
Wisconsin Department of Health and Family Services
Hoffart CM, and Johnson GL. Project status update, Decorah Shopping Center Annex. Letterfrom Key Engineering Group Ltd. to B. Amungwafor, WI DNR. December 3, 2002. File072007.
Key Environmental Services, Inc. 1997. Soil and groundwater sample analytical results andproposed soil boring locations. Site Investigation work plan, Decorah Annex Shopping Center.File 0702007.
King, KT. E-mail communication to R. Thiboldeaux, August 5, 2003.
King KT, and Graham AB. 2003. Soil vapor investigation results, Decorah Shopping CenterAnnex. Letter from Key Engineering Group Ltd. to B. Amungwafor, WI DNR. August 11, 2002. File 072007.
US EPA. 2002. Draft guidance for evaluating the vapor intrusion to indoor air pathway from groundwater and soils. http://www.epa.gov/correctiveaction/eis/vapor.htm
This health consultation for the former Decorah Shopping Center Annex was prepared by theWisconsin Department of Health and Family Services under a cooperative agreement with theAgency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approvedmethods and procedures existing at the time the health consultation was begun.
Technical Project Officer, Cooperative Agreement Team, SSAB, DHAC
The Division of Health Assessment and Consultation, ATSDR, has reviewed this Health Consultation and concurs with the findings.
for Team Supervisor, Cooperative Agreement Team, DHAC, ATSDR