Dissolved organics
Organic contaminants include pesticides, herbicides,, decayed plant and animal tissues, gasoline (centuries old decayed plant and animal tissue), and plasticizers (made from petroleum products).  Plasticizers leach out of  plumbing lines, fittings and storage tanks, so ironically, the purification system itself can contaminate the water if it is not properly constructed.

The  absence of organic substances is especially important in research that involves tissue cultures, and also in HPLC (high performance liquid chromatography), gas chromatography, electrophoresis, and fluoroscopy. 

Removing Organic contamination from water
Carbon filtration, ultraviolet oxidation, and organic scavenging are all excellent methods of removing organics from water.