TCE Vapor Intrusion at a Strip-Mall Daycare
A Phase II investigation is underway at a strip mall built in 1974 over the plume from a former dry cleaner. Groundwater contains TCE at 850 µg/L at 12 ft below grade; soil gas at 5 ft is 8,000 µg/m³. Sub-slab sampling in Unit C, occupied by a licensed daycare with 22 children under age 5, reports 410 µg/m³. Indoor air in Unit C measured 6.8 µg/m³ on an 8-hour Summa canister in January and 1.9 µg/m³ in July. Outdoor ambient air collected upwind in January was 0.4 µg/L-equivalent and showed detectable PCE as well as TCE. The dry cleaner currently operating three units away uses PCE, and the indoor air sample also contains PCE at 2.4 µg/m³.
The state's residential indoor-air screening level is 2.1 µg/m³ for TCE. The landlord wants to wait a year and re-sample in summer "when it will be lower," and has not told the daycare operator about the results.
1. Interpret the data. Does it demonstrate vapor intrusion, and does the summer result change the conclusion?
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The data support vapor intrusion, and the summer result does not rescue the conclusion.
- The sub-slab-to-indoor ratio is 6.8/410, about 0.017. That is at the high end of the 0.001–0.01 range normally observed, but plausible for an older slab with seams and penetrations. Sub-slab > indoor > outdoor at the same location points to a soil-gas source rather than an indoor source.
- The winter/summer difference (6.8 vs 1.9 µg/m³) is expected. Heating-season stack effect and closed windows increase pressure-driven flow from the sub-slab into the building. Seasonal variability is a reason to sample in the heating season, not a reason to average the January result away.
- Competing sources must be evaluated. The 0.4 µg/L-equivalent outdoor concentration is elevated for TCE and suggests an outdoor source, probably the operating dry cleaner. Indoor product inventories (cleaning solvents, stored degreasers) also need checking. Even so, the indoor concentration is roughly 17 times the outdoor ambient.
- 6.8 µg/m³ is more than three times the state residential screening level. For a daycare with children under five, use the residential target, not an occupational target, because of longer exposure and greater susceptibility.
Conclusion: an unacceptable inhalation risk cannot be excluded, and the daycare is a sensitive receptor. Act now; do not average or wait.
2. What immediate actions, notifications, and confirmation sampling do you recommend?
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Immediate (days, not months). Increase ventilation in Unit C (HVAC economizer mode, operable windows in mild weather), seal obvious slab penetrations, and restrict use of any back room showing higher readings. Portable carbon air cleaning is a stopgap, not a remedy.
Notify. Inform the daycare operator and the landlord in writing. Report to the state voluntary remediation or vapor intrusion program and the local health department. If the landlord refuses access or refuses notification, put the refusal in writing, and ask the state program about an access or response order.
Confirmation sampling. Run 24-hour (or at minimum 8-hour) canisters in occupied rooms during the heating season, with co-located sub-slab points, an upwind outdoor sample, a field duplicate, and a blank. Include the other units and the dry cleaner's unit. Analyze by TO-15 with reporting limits well below the screening level, and conduct a product inventory to rule out indoor sources.
Mitigation. Sub-slab depressurization is the presumptive remedy: pilot test with communication testing, install the full system, verify with post-installation indoor-air sampling, and pair it with a monitoring plan and, if contamination remains, institutional controls.
Documentation. Record the recommendation, the landlord's response, and the basis for any delay in the project file. If the client declines action for the daycare, say so in writing.
3. Both PCE and TCE are present. How do you apportion sources, and what does that mean for the parties involved?
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You cannot apportion from a single indoor-air sample, and you should not try.
TCE and PCE are related: TCE is a common degradation product of PCE, so the TCE plume may originate from the former dry cleaner even where PCE dominates today. The operating cleaner three units away uses PCE and is a plausible current contributor to both the outdoor 0.4 µg/L-equivalent reading and the 2.4 µg/m³ indoor PCE.
The correct approach is forensic rather than arithmetic:
- Run a groundwater and soil-gas transect between the two cleaners and the daycare, and check for degradation products (cis-1,2-DCE, vinyl chloride) that indicate a weathered PCE source.
- Compare chemical signatures and historical operating records (when each cleaner operated, solvent purchase records, wastewater disposal practices).
- Sample the dry cleaner's unit and the space between it and Unit C to determine whether that source is affecting Unit C.
- Consider compound-specific isotope analysis if the parties are litigating allocation.
For the parties: the daycare's exposure does not wait for allocation. The landlord and both cleaners may be liable under CERCLA and state law regardless of contribution share, and joint and several liability is possible. Advise your client to secure access, mitigate the daycare, and preserve the allocation fight for cost recovery, not for the decision whether to protect children.