It didn't appear from nowhere via by Monkey Magic, that much is certain.
The source, then, is either
* raw lithium rich brines,
* Spodumene ore,
* waste from brine processing, OR waste from spodumene processing, (to create industrial scale supplies of industry usable lithium)
* waste from using lithium in industry to create (say) batteries.
Working through that list, lithium rich brines are largely in Chile and in some parts of China that are not Beijing, spodumene ore comes largely from Australia and parts of China that are not Beijing.
I'm not aware of, but this doesn't rule it out, brine or spodumene processing in the immediate vicinity of Beijing (it's quite likely there is some spodumene processing in that area) .. there definitely is large scale industry use of end product lithium in Beijing specifically to produce batteries.
The majority of lithium usage is related to "clean energy" .. or even just common battery use for electronics, so it's easily the most probable source of lithium turning up in an urban area such as Beijing.
Addendum: As grumpy-de-sre correctly points out below, the end of use disposal of lithium batteries in Beijing also deserves attention as a source.
The exact 'how' is as yet unknown .. it'll likely be tracked down and dealt with given the papers being published.
This will be of great interest to Texas a decade or so from now, the critical materials act and other recent activities have seen ground broken on lithium and rare earth concentrate processing plants on US soil in Texas (and elsewhere), the recent relaxing of EPA guidelines and reduction in oversight will likely lead to similar health issues appearing in due course.
Do you have any suggestions for where else it might come from?
