[mrob]

Microplastics were found in bottled water, but I see no evidence that they actually came from the bottles, and I can't think of any mechanism that could cause particles to break off from the insides of the bottle under normal storage conditions. It seems more likely that they were already present in the water when the bottles were filled.

[idDriven]

It's likely a result of the manufacturing process, at least partially. I found the journal article the WHO pulled data from. Heres the abstract (I italicized for emphasis):

Eleven globally sourced brands of bottled water, purchased in 19 locations in nine different countries, were tested for microplastic contamination using Nile Red tagging. Of the 259 total bottles processed, 93% showed some sign of microplastic contamination. After accounting for possible background (lab) contamination, an average of 10.4 microplastic particles >100 um in size per liter of bottled water processed were found. Fragments were the most common morphology (66%) followed by fibers. Half of these particles were confirmed to be polymeric in nature using FTIR spectroscopy with polypropylene being the most common polymer type (54%), which matches a common plastic used for the manufacture of bottle caps. A small fraction of particles (4%) showed the presence of industrial lubricants. While spectroscopic analysis of particles smaller than 100 um was not possible, the adsorption of the Nile Red dye indicates that these particles are most probably plastic. Including these smaller particles (6.5–100 um), an average of 325 microplastic particles per liter of bottled water was found. Microplastic contamination range of 0 to over 10,000 microplastic particles per liter with 95% of particles being between 6.5 and 100 um in size. Data suggests the contamination is at least partially coming from the packaging and/or the bottling process itself. Given the prevalence of the consumption of bottled water across the globe, the results of this study support the need for further studies on the impacts of micro- and nano- plastics on human health.

URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6141690/

[smsm42]

That should be not so hard to check - some companies bottle water both in plastic and glass, levels can be compared. Others use specific sorts of plastics, so one could check if the particles in their water match the chemical composition of bottle plastic or contain other plastics that are not part of the bottle. Also, many companies supply water coolers which use big bottles made of different kind of plastics than retail water - but likely the water itself, from the same company, comes from the same manufacturing process. The profile of these can be compared and one can see if the bottles are the likely source.