I found a quarter that was on the beach for a long time, and it was eaten away quite a bit, surprised me that it was even there because the water rises and recedes, so I thought it should have been washed away, that was glacier water that it was in, so, I think either water can eat away at them if they are left there long enough, with moving water, I don't know what standing water would do to them, probably the same thing, but might take longer. Hope this helps you out.
