Probably all of the above. We know microplastics are in the air, we know they are in aquifers, we know any plastic material municipal water is going through co

Probably all of the above. We know microplastics are in the air, we know they are in aquifers, we know any plastic material municipal water is going through could result in micro plastic contamination just from wear, potentially from the filters they are running the water through before bottling and any plastic fittings or piping on that fill the bottles. It's wholly plausible it's a contamination of any/all of them in any given sample.