Taste and odor problems in water are often considered together. Many taste/odor issues are related to the disinfectants used in water treatment.
Other causes are mineral contaminants, for example:
Chlorides in excess of 500 ppm produce a "salty" taste.
Blue green algae, depending on its quantity, can cause "grassy" and "musty" tastes and odors, as well as a "spicy" odor. Algae is also blamed for "fishy, rotten, septic, and medicinal" odors.
Hydrogen sulfide gas causes what is commonly called a "rotten egg smell."
Rotting vegetation is probably the most common cause of taste/odor problems, however.
Odor is classed by the EPA as a secondary contaminant. The effects are strictly aesthetic, and no health issues have been identified. There is a classification standard called TON (Threshold Odor Number). EPA suggests a limit of 3 TON for odors.
Treatment: Activated carbon adsorption is the most standard approach to taste/odor improvement. Oxidation/reduction is also used. Chlorine is only partially effect for odor improvement, and chloramine is not effective at all. Chlorine dioxide and ozone are excellent oxidizers for odor improvement. Hydrogen peroxide is very effective against hydrogen sulfide.
"Activated carbon has an excellent history of success in treating taste and odor problems. The life of the carbon depends on the presence of organics competing for sites and the concentration of the odor causing compound." -- Enting Engineering Handbook.