[Wuli]

I am wondering if the same process - breakdown of the metals in rocks through long erosion of the rocks via water - is true for deep groundwater that supplies wells used for human water supplies.

It at least makes me wonder how well and how often residential well water is tested, in general, on average.

[shotgun]

Yes, groundwater wells will also contain various metals, no mater what the depth. And just because a well is drilled and tested with levels lower than the Maximum Concentration Level (MCL) those levels can increase over time and even cause them to be abandoned.

I once worked for a small city in Eastern Washington which got all of its water from deep (1,000 foot +) wells and the scientist from the Hanford area would come get background water from us because of the purity to compare to the wells on the Hanford Reservation.

[BwanaNdege]

It at least makes me wonder how well and how often residential well water is tested, in general, on average.

We have two adjacent subdivision utilizing the same deep well. We get periodic reports on the lab test results of our system water.