Thank you for the response, the 4th point is really interesting and makes a lot of sense that putting it in the center of land would help. I'll definitely try to keep that in mind for the future.
I ended up using the console to clear pollution and it's spreading at a reasonable pace now, I think I had just such a massive backlog of pollution built up from when I was cleaning up the factory (I had a prod module'd EM plant running for a few minutes at one point) that it just wasn't being absorbed fast enough to make the negative pollution visible even with literally nothing running electricity for an hour.
Thank you for the breakdown and advice, much appreciated.