Effectively lost mean different things on a human and a geological timescale. It could take millennia (or longer) for the water to slowly push through various

Effectively lost mean different things on a human and a geological timescale. It could take millennia (or longer) for the water to slowly push through various kinds of bedrock, to finally get above ground again. By that time whatever contaminants were in it will have been filtered away. It's an incredibly slow process, but it's not as if we could somehow drain the oceans by fracking at full speed for several millennia.