Dry Cleaner Site: Groundwater Plume, Soil Gas, and a Push for No Further Action

The recommendation is not supportable on the vapor intrusion pathway

1. The screening is not passed. 850 µg/m³ against a 120 µg/m³ screening level is 7.1× the level. A screening exceedance does not prove a risk, but it absolutely requires the pathway to be evaluated rather than declared irrelevant.
2. 'Soil gas is not groundwater' is a category error. Vapor intrusion is its own exposure pathway with its own criteria. One pathway clearing does not clear another; here the groundwater standard is exceeded and the soil gas screening value is exceeded, which is worse than either alone.
3. Estimate what reaches the indoor air. With a default sub-slab-to-indoor attenuation factor, even the low end is uncomfortable: 850 × 0.03 ≈ 25.5 µg/m³; 850 × 0.01 ≈ 8.5 µg/m³. States typically set PCE residential indoor-air values in the low single digits µg/m³ because PCE is a probable human carcinogen. On a default attenuation factor, the predicted indoor concentration is above what any of those values would tolerate.

Next 30 days

• Sample indoor air and outdoor ambient air at the residence under normal occupied conditions (normal HVAC, winter/closed-house conditions are more conservative), with proper canister QA and duplicate/blank.
• Check the soil-gas sample quality: 5 ft below slab is deeper than a typical sub-slab probe; verify sample depth, leak testing, and whether a shallow sub-slab point was ever taken directly under the slab.
• Install a sub-slab depressurization system as an interim measure now. It is inexpensive, effective, and buys time; do not wait for the indoor air results to schedule it.
• Tell the buyer plainly: no further action is not available on this record, and closing without a VI evaluation transfers an unquantified liability.

The plume

45 µg/L at MW-1 is 9× the 5 µg/L standard, with 12 µg/L one well down and non-detect at MW-3 and MW-4.

Is DNAPL likely? Probably not at MW-1. PCE aqueous solubility is roughly 150–200 mg/L, and the effective solubility in the presence of free product is typically a few percent of that - thousands of µg/L. At 45 µg/L the groundwater is 2–3 orders of magnitude below that range, which argues against free-phase DNAPL in the sampled interval. It does not rule out DNAPL near the former dry-cleaning machine or the still residue area, which is where you would look for it (soil concentrations near the water table, high dissolved concentrations at the source, and soil boring observations).

Is MNA defensible? Not yet. Stable-looking data are not MNA. The four lines of evidence needed:

1. A statistically demonstrated trend, not two or three points eyeballed. Typically 4+ sampling rounds over 2+ years with a formal trend test.
2. A degradation mechanism. PCE dechlorinates reductively: PCE → TCE → cis-DCE → VC → ethene. If the daughter products are absent, that is evidence of no degradation, not of attenuation. The molar-fraction analysis is the diagnostic.
3. An attenuation rate that outruns transport, documented with a decay rate and a plume-length/center-of-mass comparison over time.
4. No completed exposure pathway and enforceable controls (ordinance, easement, well prohibition), plus a monitoring plan with a sentinel well.

What is missing

• The plume is not delineated laterally - MW-1 to MW-4 is one line and there is nothing cross-gradient.
• No downgradient sentinel well between the plume and the private well 300 ft away.
• The private well's construction is unknown: screened at 60–80 ft, deeper than the 22 ft water table, could be in a lower unit or could be an improperly grouted conduit. Either way, you cannot assume it is protected until you have the log and a vertical gradient measurement.
• Source-area soil and any DNAPL evidence near the former machine/still.

Frame the constraint honestly. A deed restriction does not clean up a plume, does not stop vapor intrusion, and does not by itself protect a buyer from CERCLA liability - a bona fide prospective purchaser needs all appropriate inquiries, compliance with land-use restrictions, reasonable steps to prevent releases and to stop continuing threats, and cooperation with response actions. Any path that skips the VI work is a risk transfer, not a risk reduction.

Defensible closing structure

1. Environmental covenant / institutional control that prohibits residential use of the ground floor or requires an operating SSD system with a maintenance obligation - paired with a recorded notice.
2. Interim SSD installed and operating before closing, with a service contract and a named responsible party.
3. Indoor-air verification sampling completed or scheduled with defined decision criteria written into the purchase agreement, so no one has to litigate later about what 'clean' means.
4. Escrow or holdback sized to the remaining delineation and long-term O&M, released against delivered milestones.
5. Buyer-side environmental insurance (pollution legal liability) as a backstop, priced against a real conceptual site model - insurers will not write against 'no further action' where the VI pathway is unaddressed.
6. Regulatory posture in writing. The 45 µg/L result is a release above the state standard; confirm whether the state requires release notification and whether a 'no further action with conditions' letter is even achievable. Get the case manager's position before signing anything that promises closure.

What to tell the seller: 60 days is enough to install an SSD system, run indoor air, and negotiate an escrow. It is not enough to defend a no-further-action position on 850 µg/m³ of PCE under an occupied house, and a consultant's letter that says otherwise will not survive diligence.