David Hansen writes:

Where do you think these very high levels of iron will come from?

Iron in surfaces that are in contact with the water.

If the water in my water heater stands for a long period, it looks a
bit rusty the next time I take water from it, and I presume that this
is iron oxide, since that's what it looks like.

I doubt if many houses have galvanised iron water tanks these days, or
for at least a decade. New work since the late 1960s has involved
plastic tanks.

I don't know exactly what installations may be used. I was simply
going on the principle that hot water is a much better solvent than
cold water, and if it stands under pressure for a time, it might have
quite a bit of junk in it.

Other than the mains supply, where do these metals or minerals come
from?

The supply plumbing would be sufficient to provide them.

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