Measurement

You can view a national map of hardness in surface water at water hardness map.
Yes, it is not unusual for salt marshes to experience dieback in small patches in some years. However, these areas are usually less than an acre or so in size and generally regenerate the next year or over the next several years.
Most survey marks were set by the US Coast & Geodetic Survey (now called the National Geodetic Survey) and information for those marks is available on the web.
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.
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.
The USGS Grand Canyon Monitoring and Research Center in Flagstaff, Arizona, conducts at least eight trips each year to study endangered fish populations.
Since 1950, the U.S. Geological Survey (USGS) has collected and analyzed water-use data for the United States and Territories.
The phenomenon of brown marsh has been observed along Louisiana's coastline. Hardest hit is the Barataria-Terrebonne estuary extending from the west side of the Mississippi River to the Atchafalaya River.