Surfactant-Enhanced In Situ Remediation Beneath an Occupied Residential Building: Reduction of the BTEX Vapor-Intrusion Source Term and Field Validation in a Single Field Season

Petroleum hydrocarbon contamination beneath occupied buildings poses a chronic BTEX vapor-intrusion risk, yet field-scale evidence for in situ remediation under occupied residential structures remains scarce. We report a full-scale surfactant-enhanced remediation (SER) campaign at a former service-station site in Quebec City (Canada), where 235 m2 of contaminated soil extended directly beneath an occupied multi-unit residential building. Through 46 dual-action injection–extraction wells, 15.3 m3 of a biodegradable polysorbate-family washing solution and 31.2 m3 of rinse water were delivered by a stop-and-go protocol over 115 days. Pre-treatment soils reached Criterion C for BTEX (xylenes up to 335 mg kg−1). Validation sampling at three sectors showed compliance with Quebec Criterion B at two of them, with a single residual xylenes exceedance (9.0 mg kg−1; Criterion B = 5) at Sector 1. Sector-mean abatement reached >99% for ethylbenzene and xylenes at Sector 2, 79% and 90% at Sector 1, and 60–61% for TPH C10–C50. All validation results complied with the provincial criterion for the methylnaphthalenes, yet 1-methylnaphthalene remained 2.2–5.0 times its inhalation-driven USEPA residential screening level. Biodegradable SER can substantially reduce the BTEX vapor-intrusion source term within a single season without displacing occupants; occupant exposure itself was not measured.