We think of pollution as a human phenomenon, when in fact natural pollution from the break-down of minerals is also important, and can come from unexpected sources. The bedrock Wisconsin and Minnesota contains lots of sandstone beds. One of the purest is the St. Peter Sandstone, which consists almost entirely of well-rounded quartz grains. This is the rock that forms the white cliffs along the Mississippi River in St. Paul, the lower layer at Minnehaha Falls, and the steep road cuts around River Falls, Wisconsin. Where this formation is below the surface, it forms an important aquifer.
Water moves readily through the spaces between the sand grains.
Measurements in Outagamie and Winnebago Counties find some wells with arsenic concentrations up to 2,000,000 micrograms per liter. A home owner's first reaction , after going to get bottled water will be to look around to sue whoever it was who has been dumping arsenic. The problem is that the arsenic is coming from the sandstone itself. If one looks at the St. Peter sandstone, one sees scattered rusty red zones. This is the result of the weathering of pyrite.