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None of these substances are toxic to your horses. The only problem that you should have with them is palatability (taste) and this will make your horses hesitant to drink it. However, if they have been drinking it for a lifetime, they might not even notice the taste.
When you compare the human vs horse standards, you have to remember that it's done by weight. So, if your well is at 2X the recommended limit for people, it's not going to affect a 1200 lb horse.
You can read the proof that states this here in this article:
It states that manganes, sodium and iron are not toxic.
I WOULD talk with your equine vet and ask for some simple bloodwork to test for levels in the blood if you still have concerns. I would also ask if he thinks that you need to use potted water for the foals.
I hope that this helps you.
Let me know if you still have questions.