Water pollution primarily comes from cities, industry and farming. Urban and industrial pollution is localized and can be partially treated (see below), whereas agricultural pollution is dispersed in fields and gradually reaches groundwater and rivers. The main types of contaminants are listed below

Suspended matter
Suspended matter* is made up of fine particles. Some are present naturally in river water, such as plankton, fine plant debris and minerals, while others stem from human activity (organic and inorganic matter). Suspended matter can make water more turbid, which has a negative impact on river and stream biology.

Organic pollution
Some discharge contains organic matter*. Organic matter is biodegradable, which means that it can be broken down into water and carbon dioxide by bacteria and algae. However, the biodegradation process uses oxygen that is therefore no longer available for fish and other aquatic fauna. For this reason, organic pollution is often measured in terms of biological oxygen demand* (BOD), i.e., the amount of oxygen used by microorganisms to decompose the waste.
Another standard measurement concerns chemical oxygen demand* (COD), i.e., the amount of oxygen needed to oxidize the organic matter.

Microbiological pollution
Water can contain pathogenic organisms such as viruses, bacteria and parasites. Since these pathogens are dangerous to human health, they limit the possible uses of water (for example for swimming pools or shellfish farms).

Fertilizers
Excessive amounts of nitrogen and phosphorus in streams leads to the proliferation of algae that reduce available light and, more important, consume oxygen in the water. This phenomenon, known as eutrophication*, is caused by pollution from urban wastewater, certain types of industrial effluent and runoff containing excess fertilizer from fields.

Metals and persistent chemical contaminants
Certain chemical contaminants, such as pesticides, are particularly persistent in that they resist biological, chemical or photochemical degradation. Along with heavy metals* like lead, mercury, cadmium, copper, zinc, nickel and chromium, these chemicals form a family of very hazardous contaminants. They are absorbed by plants or small animals and build up throughout the food chain, which ends with people.

Mineral salts
Small amounts of mineral salts, including calcium, magnesium, sodium and potassium chloride or sulfate are naturally present in water. Their concentration can rise, however, as a result of industrial discharge, with a negative impact on aquatic biology.