A curious case of flickering lights and a well pump

This has to be one of the most unexpected troubleshooting finales I've seen in a long while.

I'm renting a home in the Colorado Front Range, and I've been annoyed by flickering ceiling lights ever since I moved in. At seemingly random times, there would be periods from several seconds to several minutes long when lights would flicker erratically. I'd describe it as "fast and shallow flickering", as if caused by high-frequency pollution of the mains AC signal. For months, I simply assumed that it's just dirty "rural power", but curiosity always wins in the end...

All of the affected lights are controlled by sliding dimmer switches. There are multiple sets of lights on separate dimmers, and even separate circuits. Those analog dimmers can act up sometimes, but my lights don't always flicker, and when they do, they all flicker at the same time. With that, I assumed the issue must be down to power quality rather than a malfunctioning dimmer.

I started by looking at the AC waveform at a wall socket (during a flickering event), but didn't notice anything abnormal.

Here's the waveform outside of an event (lights not flickering):

Here's it is during an event (lights flickering):

The waveform oscillates ever so slightly, which you can't see on the screenshot, but it still looks close enough to a normal sine wave.

Now, here's a "clean" wave from a dimmer outside of an event:

The partial waveform is normal, that's how those dimmers work.

And here's a "dirty" wave from a dimmer while the lights are flickering:

This is the same wave zoomed in, clearly showing high frequency aberrations:

Evidently, the analog dimmers are affected by signal pollution in the mains line, and just freak out. But why?

I noticed that the issue comes and goes in contiguous events: there's non-stop flickering (and a dirty wave) for a number of seconds, then it abruptly disappears again, then it comes back. After scratching my head for a few minutes, I had a lucky breakthrough: the issue happened to coincide with me using water, such as showering or washing dishes. It couldn't be the water pump putting a high-frequency signal on the mains, could it?

I opened the tap in the kitchen, ran to the utility room and waited for the well pump to start, and there it was! With the well pump running, I'm getting high frequency pollution on the dimmer! The pump stops, and the signal is clean again!

I looked up the pump model number, and sure enough, it's using a smart controller with variable voltage and frequency (0-240V, 0-400Hz). Due to the location of the well, the controller output is buried right alongside the power line running between the transformer and the house. I've no idea if well wiring is supposed to be shielded, but it certainly should be, and it certainly isn't.

So there you have it, a pump controller causing flickering lights. That wasn't on my bingo card for this week.