I find it interesting that there were spikes of readings at certain times of day and night. Coupled with the information that the apartments have a VoC barrier, my intuition is that there is a timer somewhere that turns on a fan to clear out the VoC capture space created by the barrier and exhaust the contaminated air to the outside.
If the VoC that's bothering her is heavier than air, it could be being blown upwards from the ground level and then settling back over the building and falling back down and finding its way back into her apartment somehow, like through the range hood, dryer exhaust, plumbing stacks, open windows, or just cracks due to poor craftsmanship.
If she was still at the site, I would recommend looking for what appears to be an exhaust vent of some kind that might be tied to the VOC barrier system and putting a sensor there and seeing if the readings correlate to the timeline of unusual readings in her home
I had a condo in San Francisco. The building was on an old gas station site. The site remediation included a vapor barrier placed below the foundation, and a vent pipe from the soil to 10 feet (I don't remember the exact height, but it was prescripted in the permit) over the roof. Said soil vent had a fan that ran nightly exactly as you suggest, again prescripted by the permit to continue for at least 10 years. So I find this quite plausible. Perhaps her apartment was next to a similar vent pipe which had a leak.
Gas stations are this big elephant that nobody speaks of. Essentially every gas station site has polluted soil that makes the site either uninhabitable or uneconomical to remediate.
Huh, that's really surprising. How does it happen? I assume it has something to do with gasoline, but I was under the assumption that most of the gasoline at gas stations was stored in giant underground concrete containers.
Fuel tanks leak. Cars leak. Waste fluids (fuel, oil, brake fluid, transmission fluid, coolant, refrigerant) leak and are improperly disposed of. Fuel spills when being pumped in or out of tanks.
Until 1996, leaded gasoline was generally legal in the United States (California phased it out in 1992). It remains legal as avgas and possibly for some specialised uses. Tetraeythyl lead is a toxic heavy metal compound thought to be a factor in the rise in urban crime, a phenomenon which traces its use and phase-out closely, though lagged about 20 years.
Among the compounds used as an alternative to lead was MTBE (Methyl tert-butyl ether), an oxygenate and ground-water contaminant, itself banned as a fuel additive in 2004 (2002 in California), though again it remains legal for other uses.
Petrol (gasoline) itself is often formulated with numerous other additives, and "is a mixture of a large number of different hydrocarbons", averaging hydrocarbon length of about 5-6. ("Naptha", shorter chains make more volatile fuels, longer chains heavier, e.g., kerosene, diesel fuel, fuel oil, bunker oil. "Octane" is an 8-chain compound with lower volatility, increasing the ignition point and reducing engine knocking due to premature ignition.) Various of the naturally-occuring, added, and refining-induced compounds themselves may be harmful or toxic, and include VOCs.
Generally the tanks are fiberglass. I don't know why they leak. From reading site reports, my impression is these leaks are usually detected by the tax authorities, who can see that the reported sales of fuel don't add up to the reported deliveries of fuel. See page 40 of this report for an idea of what an underground fuel tank looks like.
There's an old illustration from Popular Mechanics that suggests dumping used motor oil into holes in the ground. At one time that was considered a reasonable way to dispose of it.
If the VoC that's bothering her is heavier than air, it could be being blown upwards from the ground level and then settling back over the building and falling back down and finding its way back into her apartment somehow, like through the range hood, dryer exhaust, plumbing stacks, open windows, or just cracks due to poor craftsmanship.
If she was still at the site, I would recommend looking for what appears to be an exhaust vent of some kind that might be tied to the VOC barrier system and putting a sensor there and seeing if the readings correlate to the timeline of unusual readings in her home