> flame retardant chemicals used to treat geofoam were shown to be accumulating in the ground, which led to their [i.e., the flame retardants’] discontinuation in Europe but not the US

It appears that people are paying attention to the ecological impact. (In some places, at least.)

Its inert, so its not biodegradable, but it also doesn't break down the way that styrofoam* does. And it can be re-used. I'm not convinced that any of this makes it A Good Thing tbh, but it is possibly less bad than some of the alternatives.

* which is an absolute menace in my opinion

Edit: s/expanded polystyrene/styrofoam

Um, this is expanded polystyrene, just with a fancy "polite" name. Per TFA:

Despite its uncanny appearance, geofoam is incredibly prosaic. Essentially, it’s polystyrene. Expanded polystyrene (EPS) geofoam — the most common type — is made from small polystyrene resin beads that contain microscopic cells filled with a blowing agent, which expand when exposed to steam under controlled pressure and are molded and fused into a brick shape.

Geofoam is expanded polystyrene.

If it were biodegradable, geofoam hills would turn into geofoam-vacant sinkholes.

Many substances are selectively biodegradable i.e. remain shelf-stable forever as long as they are not exposed to oxygen or sunlight or whatnot.

Also, concrete degrades by itself. In decades it crumbles, pulverizes and goes back into being soil.

But, importantly, it does not become toxic particulate like plastic.

[edit: corrected phrasing, thanks zdragnar]

Same for other sustainable materials like compacted soil, gravel, sand...

The production of concrete is responsible for a ton of pollution in the world and while there's significantly lower environmental impact concrete available, few projects use them because of its higher costs for primarily environmental benefits. The distribution of the lifecycle of the environmental impact matters and if things are pretty bad now I'd strongly consider anything with backloaded environmental costs over ones that have environmental costs incurred up-front.

Isn't that simply weathering?

Biodegradable would seem to imply to me that bacteria and fungi feast on it and convert it to what we consider soil.

The bit about petroleum solvents, makes me wonder about sinkholes. "causes it to dissolve and break down into a non-load-bearing sludge", so if some joker tips a bottle of that in the park then a sludgy hole in the ground will start forming?

If there is any reasonable amount of soil above it, I suspect the bacteria and fungi will eat it well before it gets down to the foam.

"Biodegradable essentially means that an item can be broken down into increasingly smaller pieces by bacteria, fungi or microbes to be reabsorbed by the surrounding environment, ideally without causing any pollution."

So it degrades, but causes pollution, therefore it is not biodegradable. That is my understanding.

> Am I missing something?

If the foam blocks are buried, it's less likely they'll make it into the water cycle. (That's not to say they won't cause other problems.)

Also, all the samples collected are not biodegraded, but rather the actual molecule of plastics that's simply gotten smaller and smaller. See "microplastics":

* https://oceanservice.noaa.gov/facts/microplastics.html

* https://en.wikipedia.org/wiki/Microplastics

Biodegrade means that living organisms have decomposed the substance. But it can still degrade in other ways.

Maybe https://duckduckgo.com/?q=worms+eating+polysterene ?