Diligent and professional service from Mike and Eric at DrainWorks. They were able to think backwards and solve a problem upstream of the drain. Here's the story:
A spill of paint thinner (2L) occurred in the basement entrance way. Apparently it had flushed clean through the drain system. Nevertheless it was still generating fumes, and the house is uninhabitable without special ventilation. They inspected the drains with a video camera, and figured it out. The fumes are probably coming from the foundation wall and floor.
They found that the drains themselves were OK all the way to the street. Out back where the spill occurred, the S-trap under the inlet was full of sediment, and therefore draining slowly. So the large volumes of water that were originally used to clean up the spill must have caused the drain to back up somewhat, at that time.
A line of 'weeping tiles' runs into the same inlet, just beneath the surface. It drains the foundation. When the water backed up, it must have backed up onto the weeping tiles, carrying the paint thinner with it. The weeping tiles are semi-permeable. So the paint thinner has probably seeped into the foundation wall and adjoining floor. From there it is evaporating into the house.
Good detective work. (Now, if anyone knows how to neutralize paint thinner that's soaked into the foundation, please let me know. :)
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Mike Allan,
mike@zelea.com