Quality

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.
It is not an easy task to keep a swimming pool so clear and clean. If you just set a pan of water outdoors in the middle of summer, you'll see that it ends up containing gunk very quickly.
"Animal Feeding Operations: Effects on Hydrologic Resources and the Environment" highlights many potential effects of animal feeding operations (AFOs) that were discussed at a meeting co-sponsored by the USG
Information on pesticides and herbicides and on nutrients is available from the USGS National Water-Quality Assessment Program 
The salinity of Great Salt Lake is measured by taking specific gravity and temperature measurements and comparing them to standardized values reported in a table.
Different treatment is used depending on the source of your water. Ground water taken from wells has been filtered through rocks, so it is usually quite free of particles.
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:
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.
Several of these trace elements are regulated by the EPA and are on their list of primary drinking water standards. These include arsenic, copper, and lead, as well as cadmium, chromium, mercury, and selenium.