Groundwater

You can view a national map of hardness in surface water at water hardness map.
Regulatory limits for safe levels of elements in water and foodstuffs are established by the U.S. Environmental Protection Agency and the U. S. Food and Drug Administration.
Safe water means water that will not harm you if you come in contact with it. The most common use of this term applies to drinking water, but it could also apply to water for swimming or other uses.
There are many different kinds of treatments depending on what contaminant is in the water. Biological contamination is usually treated by heat (boiling) or by filtration.
A basic introduction to natural filtering of water can be found on-line at the USGS 'Water Science for Schools' website. The address for the site's ground-water-quality page is:
Since 1950, the U.S. Geological Survey (USGS) has collected and analyzed water-use data for the United States and Territories.
Sulfur is not regulated as a primary drinking-water contaminant, so there is no official level of sulfur that represents a threshold between healthy and unhealthy concentrations.
Yes, it is called reclaimed wastewater, though its use is limited. Before you start to feel ill, no, it is not used further down the line as drinking water. It is most often used for irrigation and for water parks and golf courses.
Yes. The main body of subsurface water is found in the saturated zone of aquifers. Aquifers can be only a few feet below the surface or more than a thousand feet deep.