Measurement

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.
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.
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.
Data are offered as a file or personal geodatabase, but can also be downloaded as a shapefile.
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.
This is a very broad question, and difficult to answer without a bit more context. Pollution comes in many forms; chemical, thermal, acoustic, and physical for example.