0.15 Air, water and soil  (Page 4/7)

Processing water in treatment plants can reduce the amounts of infectious agents, oxygen-depleting wastes, inorganic chemicals, organic chemicals and plant nutrients. Bans and restrictions on the use of certain chemicals, such as those on DDT and hexavalent chromium compounds, are also very helpful in reducing the amounts of these chemicals in the environment. By limiting exposure to these harmful substances, their negative effects on humans and local ecosystems can be greatly reduced.

Soil pollutants

The persistence of pesticides in the soil is related to how quickly these chemicals degrade in the environment. There are three ways pesticides are degraded in the soil: biodegradation , chemical degradation , and photochemical degradation . Microorganism activity plays the predominant role in the biodegradation of pesticides. Water plays an important role in the chemical degradation of pesticides (e.g. some pesticides are hydrolyzed on the surfaces of minerals by water). Exposure to sunlight can also degrade some pesticides.

A variety of pesticides are used to control insects, weeds, fungi, and mildew in agricultural, garden, and household environments. There are three classes of pesticides: insecticides , which kill insects; herbicides , which kill plants; and fungicides , which kill fungi. Each of these classes includes different types of chemicals. These chemicals differ in chemical composition, chemical action, toxicity, and persistence (residence time) in the environment. Some of these pesticides can bioaccumulate (e.g. they concentrate in specific plant and animal tissues and organs). Pesticides can accumulate in the soil if their structures are not easily broken down in the environment. Besides rendering the soil toxic to other living organisms, these pesticides may leach out into the groundwater, polluting water supplies.

The five classes of insecticides are: chlorinated hydrocarbons, organophosphates, carbamates, botanicals and synthetic botanicals. Chlorinated hydrocarbons such as DDT, are highly toxic in birds and fishes, but have relatively low toxicity in mammals. They persist in the environment, lasting for many months or years. Because of their toxicity and persistence, their use as insecticides has been somewhat restricted. Organophosphates , such as Malathion , are more poisonous than other types of insecticides, but have much shorter residence times in the environment. Thus, they do not persist in the environment and cannot bioaccumulate. Carbamates , such as Sevin , are generally less toxic to mammals than are organophosphates. They also have a relatively low persistence in the environment and usually do not bioaccumulate. Botanicals , such as camphor , are derived from plant sources. Many of these compounds are toxic to mammals, birds, and aquatic life. Their persistence in the environment is relatively low, and as a result bioaccumulation is not a problem. Synthetic botanicals , such as Allethrin , generally have a low toxicity for mammals, birds, and aquatic life, but it is unclear how persistent they are and whether or not they bioaccumulate.