2012 Jetta 2.0 heater core

So my mechanic found "a gravel like something" that is plugging my heater core. He has flushed it 4 times and has gotten a lot of it out but it is still clogged. He switched the hoses to run the coolant backwards through it instead to see if that will eventually help. We're hoping to avoid replacing the heater core for obvious reasons....

My question is: is there ANYTHING to ANYONE'S knowledge that could cause this "gravel like substance" to end up in my antifreeze resulting in a clogged heater core, OR are my paranoid thoughts of sabotage maybe not so paranoid??

I trust this mechanic fully and there is someone at work who has already gone to great lengths to make my life miserable because I'm good at what I do and she feels threatened by me and is psychotic.

Running regular tap water instead of distilled water could cause this. High mineral content.