PU580 Unit 10 Vapor Intrusion Investigation Report Example

A Dry Cleaner That Closed in 1998 and an Apartment Building Breathing Its Solvent: A Unit 10 Exposure Investigation Report Reading a Composite Plume Map, Sub-Slab Gas and Paired Indoor and Outdoor Air Into One Set of Recommendations

[Student Name]

Purdue University Global

PU580: Environmental Epidemiology

Unit 10 Exposure Investigation Report

[Instructor Name]

[Date]

The strip mall, the apartment building and all sample results are composite; the vapor intrusion science, the screening approach and the research cited are real.

Site and History

A six-unit strip mall built in 1972. Unit 3 operated as a dry cleaner from 1974 to 1998 using tetrachloroethylene, with a floor drain connected to the sanitary sewer and a documented spill in 1986. Behind the mall, separated by a parking lot, a 48-unit three-story apartment building constructed in 1981 on a slab with an unsealed utility chase. Groundwater is 4.2 meters below grade and flows toward the apartment building.

Why Vapor Intrusion Rather Than Drinking Water

The apartments and the mall are on municipal water drawn from a wellfield 6 kilometers upgradient, which the investigation confirmed is unaffected. The exposure pathway of concern is therefore volatilization from groundwater and soil into the buildings above, which is the pathway the conceptual model of vapor intrusion describes: a vapor source below, a driving force from pressure differences across the slab, and an opening (Johnson & Ettinger, 1991).

The Sampling Design

1. Groundwater: eleven monitoring wells, four quarters, defining a plume 180 meters long and 60 meters wide, with maximum tetrachloroethylene of 2,400 micrograms per liter beneath the former dry cleaner and 310 micrograms per liter beneath the apartment building's eastern third.

2. Sub-slab soil gas: twelve probes, five under the mall and seven under the apartment building, sampled twice, in February and July.

3. Indoor air: 22 apartments across the three floors, with a paired outdoor sample at each sampling event and a product inventory of every unit sampled.

4. Timing: winter and summer, because the pressure difference across a slab is largest in cold weather when heating draws air upward, and winter is when the pathway is strongest.

The Results

Sub-slab soil gas under the apartment building's eastern third: 1,200 to 4,100 micrograms per cubic meter in February, 400 to 1,100 in July. Indoor air, eastern ground-floor apartments, February: 11 to 34 micrograms per cubic meter. Same apartments in July: 2 to 7. Indoor air, western apartments and upper floors: 0.8 to 3.1 in February. Paired outdoor air: 0.4 to 1.2 throughout. Attenuation factor, indoor divided by sub-slab, in the eastern apartments: roughly 0.008, which sits in the middle of the empirical range for this pathway. The highest indoor concentrations and the lowest outdoor ones occur in the same week, which is what separates vapor intrusion from a neighborhood air problem.

Ruling Out the Indoor Sources

The product inventory found two apartments with solvent-containing products, and both were excluded from the eastern cluster analysis. The concentrations in those two units were no higher than their neighbors'. The pattern that establishes the pathway is geographic and seasonal rather than per-unit: concentrations fall with distance from the plume edge, fall with height above the slab, and fall by a factor of five between February and July, none of which a stored product would produce.

What the Numbers Mean for the Residents

The February concentrations in the eastern ground-floor apartments exceed the screening levels used for residential indoor air by roughly an order of magnitude for long-term exposure. Nine apartments are involved, with 21 residents including four children under six. The exposure is intermittent in the sense that summer concentrations are much lower, and continuous in the sense that these households have lived there for a mean of 6.4 years and the pathway has probably operated since the building's construction. The health literature on long-term exposure to this solvent in drinking water, where exposure can be reconstructed, reports associations with several outcomes and remains contested in its details (Aschengrau et al., 2015). The report therefore states exposure and does not estimate individual risk, which is a distinction the recommendations depend on.

Why the Winter and Summer Difference Decides the Sampling Plan

The factor of five between February and July is the most practically important number in this report, and it deserves a paragraph. In cold weather a heated building is warmer than the ground around it, air rises and escapes at the roof, and the slab becomes the inlet, so soil gas is drawn in. In summer the pressure difference is smaller and often reverses in air-conditioned buildings. An investigation that sampled only in July at this site would have found indoor concentrations of 2 to 7 micrograms per cubic meter against an outdoor background near 1, would have reported a modest exceedance, and would probably have recommended monitoring rather than mitigation. The same building in February is an order of magnitude above the screening level. This is why the report specifies winter sampling for every future round, and why any site where a single summer round produced a clean result should be regarded as unsampled rather than as cleared.

Five Recommendations, in Order of Urgency

1. Install sub-slab depressurization under the apartment building's eastern third within 60 days, which is the proven engineering control for this pathway and reduces indoor concentrations within days of commissioning.

2. Pending installation, seal the utility chase and the two slab penetrations identified, and provide interim ventilation guidance to the nine apartments. Do not rely on sealing as the solution, since sealing alone performs inconsistently.

3. Re-sample indoor air in all nine apartments 30 days after the system is commissioned, in winter, and annually thereafter in winter only, since summer sampling would produce reassuring numbers that mean nothing about the worst case.

4. Notify every current and former tenant of the eastern apartments since 1998 in writing, with the results, which the state's own guidance requires and which the property owner has resisted.

5. Investigate the sanitary sewer as a second pathway, since the former floor drain connection makes the sewer line a potential preferential route into both buildings, and sewer gas has been documented as an overlooked pathway at sites of this kind (McHugh et al., 2017).

What the Investigation Did Not Establish

It did not sample the strip mall's occupied units in winter, because two tenants declined, so the commercial exposures are characterized only from sub-slab data. It cannot reconstruct concentrations before this year, so the residents' past exposure is inferred from the plume's age rather than measured. And it cannot say whether the plume is still fed by residual solvent in the soil beneath the former cleaner, which determines whether the problem has a horizon of years or decades, and which is the single most useful thing the next phase of work could resolve.

References

Aschengrau, A., Winter, M. R., Vieira, V. M., Webster, T. F., Janulewicz, P. A., Gallagher, L. G., Weinberg, J., & Ozonoff, D. M. (2015). Long-term health effects of early life exposure to tetrachloroethylene (PCE)-contaminated drinking water: A retrospective cohort study. Environmental Health, 14, 36. https://doi.org/10.1186/s12940-015-0021-z

Johnson, P. C., & Ettinger, R. A. (1991). Heuristic model for predicting the intrusion rate of contaminant vapors into buildings. Environmental Science and Technology, 25(8), 1445-1452. https://doi.org/10.1021/es00020a013

McHugh, T., Loll, P., & Eklund, B. (2017). Recent advances in vapor intrusion site investigations. Journal of Environmental Management, 204, 783-792. https://doi.org/10.1016/j.jenvman.2017.02.015