Gas chromatography-mass spectrometry (GC-MS) is a method that combines two standard methods, to identify carbon compounds, so-called „organic” molecules, in bodies of water. Including (among others) pesticides, most drugs and many pollutants.
Liquid chromatography-mass spectrometry (LC-MS) works in a similar way to GC-MS. It allows larger, polar compounds, like many endogenous substances to be analysed.

The international main warning centres on the Rhine report incidents downstream, so that if the pollution of Rhine water occurs, the waterworks can stop water intake in time.

Source: International Commission for the Protection of the Rhine IKSR – International Warning- and Alarm Plan Rhine (1.7.2009)