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I wondered the same thing. I imagine it's because it used to pump the water out before it became toxic from absorbing the metals.

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More likely the water was pumped out when the mine was operating because digging, blasting, and other mining activities are easier when the equipment and people can breathe the atmosphere.

I don't know when the pumps were turned off but it would have been after the mine ceased operations, possibly after the owners threatened to walk away completely. Because they could do that in the USA, and many other parts of the world.

That was my first thought too. They are spending so much money on gear and employing someone full time - wouldn't it be cheaper to just drain it? There must be a reason they aren't doing it.

Also, he can't be there 24 hours - what happens if a bird lands on the water at night?

I assume the reason is that draining it just moves the toxins elsewhere. No one else wants to drink this stuff or have it in their surface water either.

Might also be an issue of rainwater collection, and then the enormous cost of filling the pit.

They are pumping water out, treating it and returning the water at a higher pH (more alkaline). But oxidized metal minerals plus rain water results in acid.

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