The water in the streams is as acidic as lemon juice.
When you hold a coin in the rivers on the island, the acidic content in the water will actually clean corrosion from it's surface! Just a few seconds in the stream will do it.
The water also has a high amount of minerals and metals. If you dip your finger in the water and then taste it, the iron levels make it taste like blood.
The miners who lived on the island tried to boil the water to make it potable, but because of the PH level, it would rot their teeth, turning them black over time.