Grass (the trimmed always green lawn type) is more demanding than many other crops. If the grass is growing there, then the topsoil is good enough for some other things too. Also the topsoil is something you can develop, especially on such small scale as personal garden. Make compost, grow less demanding plants first nad your soil will get better. You can grow things on sand mixed with a bit of compost.
Single use plastic items laying on the beach is what bothers people the most, but this doesn’t mean it is the biggest problems. There is much more plastic in the oceans that we do not see.