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