There is a new process for treating wastewater sludge that destroys the microfibers, so that is good news at least. I think it may be expensive, of course. It is called “hydrothermal carbonization”. Basically put the sludge in a giant pressure-cooker and the heat breaks the plastics into carbon and some water-soluble residual molecules which can go back to the start of the wastewater treatment plant to be biodegraded. But like others say, the main source in general is tires - not sure if they know whether tire microplastics are the main source in agricultural land though.

From the immortal Journal of Irreproducible Results, “The Data Enrichment Method”: “. . .its principal shortcoming is that before the enrichment process can be started, some data must be collected. It is quite true that a great deal is done with very little information, but this should not blind one to the fact that the method still embodies the ‘raw-data flaw’. The ultimate objective, complete freedom from the inconvenience and embarrassment of experimental results, still lies unattained before us.”

Swimming pools are normally constructed empty. They were withstanding surrounding soil before they were filled, and concrete strength increases with age (for about 90 days, typically). On the other hand, a sunken structure like a pool that is roofed over, becomes a “confined space”. Unlike a typical structure, heavier-than-air gases cannot escape from the pool. Such gases could originate from the drain system or flow from leakage outside the pool area. For examples, leaking propane or various gases from sewer lines in the vicinity. A sunken greenhouse would almost certainly be a building code violation for that reason. If you build it, ventilate it by means both active and passive and do not enter if you can’t verify that ventilation is working.

You know, just wearing a seatbelt is not enough to keep you from being killed in a car wreck, so might as well not wear it, eh?