A Monitor's Guide to Water Quality

Download PDF

What are they?

Pesticides are substances used to control organisms that are considered pests, such as insects, bacteria, fungi, plants and animals. Pesticides are designed to deter or harm the organisms they target, but they often have negative impacts on other organisms as well, including humans.

Many pesticides that are in widespread use are organic compounds, and these are often grouped into categories like organochlorines, organophosphates and carbamates. Organochlorines include the well-known pesticide DDT (dichlorodiphenyltrichloroethane), which was widely used until the 1960s and 1970s, when its use was phased out.

Chemical compounds can be organic or inorganic. This depends on what elements they are made of. Elements are substances that cannot be broken down into simpler substances. There are 118 known chemical elements and all matter on earth is made up of some combination of them.

Organic compounds are substances made up mostly of hydrogen and carbon (or hydrocarbon). Inorganic compounds do not contain hydrocarbon, and most do not contain carbon.

Why do they matter?

By their very nature, pesticides negatively impact organisms. Certain pesticides have toxic effects only at very high concentrations, while others are toxic at very low concentrations. The effect they have on plants, animals and humans varies depending on the type of pesticide.

Organochlorine pesticides are generally persistent in the environment and they can bioaccumulate (build up in the tissues) in organisms. Organophosphorus pesticides are less persistent.

Some organochlorine pesticides that are persistent in the environment can end up in remote rivers and lakes far away from areas where pesticides are being used. This happens when these pesticides evaporate from soils in warm weather and travel through the atmosphere on wind currents. When temperatures cool, they fall back to the ground in rain or snow.

In rivers and lakes, many organochlorine pesticides tend to accumulate primarily in the sediments and in the tissue of some aquatic organisms, rather than in the water. Because of this, national guidelines for these types of pesticides apply to how much is present in sediments and biological tissues. In contrast, other pesticides dissolve easily in water and these have guidelines that apply to how much is present in the water.

Organic compound pesticides with national guidelines set by the Canadian Council of Ministers of the Environment (CCME).

* These pesticides do not have applicable CCME water guidelines, but do have sediment and/or tissue guidelines.

Persistent organic pollutants (POPs) are transported from warmer regions to colder regions through a process called global distillation (also described as the grasshopper effect). POPs evaporate more readily in warmer environments and travel on wind currents before being deposited back to the land and water in cooler environments. This keeps happening until they arrive where temperatures are not warm enough to cause further evaporation. This explain why relatively high concentrations of these pollutants have been found in cold polar regions and on mountain tops.