An impaired water is one that doesn’t meet water quality standards. It’s as simple and complex as that.
It depends, what is your waterbody used for?
In Minnesota there are five beneficial uses that have separate standards:
A waterbody can have one or more of these uses and must meet all the requirements under those uses. These can range from chloride to ammonia, dissolved oxygen to mercury. To see more information about the beneficial uses go here: https://www.pca.state.mn.us/water/water-quality-standards
Every two years the list of impaired waters is updated. Occasionally, waterbodies will be removed from the list. But usually the list gets longer. If you are interested in seeing the list go here: https://www.pca.state.mn.us/water/minnesotas-impaired-waters-list
Identifying all the impaired waters in the state is a long a lengthy process. In order to asses every waterbody in the state, they would all need to be monitored. That is largely infeasible. However, the Minnesota Pollution Control Agency created a system to find most of the impaired waters. This process in involves a process called Watershed Restoration and Protection Strategies (WRAPS). In Otter Tail County, one watershed is currently undergoing this process; the Otter Tail River Watershed. To learn more information about the Otter Tail River WRAPS go here: http://www.eotswcd.org/river-wraps/