Chlorinated Hydrocarbons (Organochlorines) insecticides, such as chlordane, DDT, toxaphene, and dieldrin were the main family of insecticides used following their introduction after World War II. Many of these chemicals originated from attempts to develop agents of chemical warfare, but were found to be lethal to insects (Carson 1962).

DDT is a manufactured chemical widely used to control insects on agricultural crops and insects that carry diseases like malaria and typhus. The value of DDT as an insecticide was first discovered in 1939 and the discoverer won the Nobel Prize (Carson 1962). In the 1952 edition of the U.S. Department of Agriculture's book "Insects: The Yearbook of Agriculture," the agency hailed it as one or our safest all-around insecticides based on its "cost, ease of handling, safety to humans, effectiveness in destroying the pest, and safety to wildlife." It was not until decades later that the true impact of DDT on wildlife was known. Problems associated with DDT, as well as many chlorinated hydrocarbons, involved their tendency to concentrate in the fat of humans, livestock, aquatic foodchains, and wildlife. This latter phenomena, called bioaccumulation, has had and continues to have severe adverse effects on many forms of wildlife. Many predatory birds were heavily impacted by DDT. Some predatory birds, most notably the bald eagle, peregrine falcon, and brown pelican were so heavily impacted by the pesticide that they required federal listing as an endangered or threatened species under the Endangered Species Act. This was because the principal metabolite (breakdown product) of DDT, DDE, prevents normal calcium deposition during eggshell formation, causing females to lay thin-shelled eggs that often break before hatching. In addition, DDT, DDE and other chlorinated hydrocarbons, can affect the parents behavior during incubation and can result in death of unhatched embryos and eagle chicks.

Because of the potential harm to human health, in 1972, the Environmental Protection Agency (EPA) banned the use of DDT in the United States barring a public health emergency (e.g., outbreak of malaria). DDT is still used in other (primarily tropical) countries. Restrictions that control the use of aldrin and dieldrin were imposed in the United States in 1974. Since implementation of these restrictions, residues of the pesticides have significantly decreased in many regions where they were formerly used. However, DDT and DDE persist in the environment for a very long time. DDT and DDE residues can still be found in most areas of the United States. Other chlorinated hydrocarbon insecticides still being used in the US include dicofol and endosulfan. Methoxychlor is another chlorinated hydrocarbon permitted for use in the United States. However, the EPA has suspended the sole manufacturer's permit to produce and sell this pesticide until the manufacturer submits results from studies required to support reregistration of the chemical.

Carson, Rachel. 1962. Silent Spring. Houghton Mifflin Company, Boston, Massachusetts

United States Department of Agriculture. 1952. Insects: The Yearbook of Agriculture. United States Government Printing Office.

Hydrocarbons

Hydrocarbons are compounds that contain only carbon and hydrogen.  They are classified as aliphatic or aromatic.  Aliphatic hydrocarbons are not linked together to form a ring.  Compounds include the alkanes, alkenes, and alkynes, and substances derived from  them by replacing one or more hydrogen atoms by atoms of other elements or groups of atoms.  Aromatic hydrocarbons have elements linked together in rings.

Polycyclic aromatic hydrocarbons (PAHs) are a family of chemical compounds made up of carbon and hydrogen atoms, with a molecular structure consisting of at least two fused aromatic rings, each with five or six carbon atoms. The PAH family includes about 100 substances, differing in the number and position of their rings.

PAHs are generally formed from the incomplete combustion of organic matter.  Today, most of the PAHs present in the environment come from human activities. PAHs can enter the aquatic environment directly, through industrial and municipal effluents, accidental crude oil spills and PAH emissions from creosote treated materials used in water (for example, on pilings). Nearly all types of organic fuel combustion can produce PAHs. The most important sources are the incomplete combustion of fossil fuels from domestic heating systems and transportation (car exhaust fumes), garbage incineration, aluminum reduction smeSeptember 22, 2008ng of crude oil and coal liquefaction and gasification. All these human activities release PAHs into the atmosphere, where they tend to adhere to particles in suspension, some of which will enter the aquatic environment through atmospheric fallout. Owing to their low solubility and high affinity for particulate matter, PAHs are not usually found in water in notable concentrations. Their presence in surface water or groundwater is an indication of a source of pollution.

Non-point Source Pollution (NPS) pollution comes from many diffuse sources. NPS pollution is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters, and even our underground sources of drinking water.

Organophosphates pesticides affect the nervous system by disrupting the enzyme that regulates acetylcholine, a neurotransmitter. Most organophosphates are insecticides. They were developed during the early 19th century, but their effects on insects, which are similar to their effects on humans, were discovered in 1932. Some are very poisonous (they were used in World War II as nerve agents). However, they usually are not persistent in the environment (1).

Polychlorinated Biphenyls (PCBs) are mixtures of synthetic organic chemicals with the same basic chemical structure and similar physical properties ranging from oily liquids to waxy solids. PCBs were widely used as a fire preventive and insulator in the manufacture of transformers capacitors due to their non-flammability, chemical stability, high boiling point and electrical insulating properties. PCBs were also used in hundreds of industrial and commercial applications including electrical, heat transfer, and hydraulic equipment; as plasticizers in paints, plastics and rubber products; in pigments, dyes and carbonless copy paper and many other applications. More than 1.5 billion pounds of PCBs were manufactured in the United States prior to cessation of production in 1977 (USEPA. Welcome to the PCB Home Page at EPA. 10/12/2005).

PCBs entered the air, water, and soil during their manufacture, use, and disposal; from accidental spills and leaks during their transport; and from leaks or fires in products containing PCBs. They can still be released to the environment from hazardous waste sites; illegal or improper disposal of industrial wastes and consumer products; leaks from old electrical transformers containing PCBs; and burning of some wastes in incinerators (Agency for Toxic Substances and Disease Registry, ToxFAQs™ for Polychlorinated Biphenyls (PCBs) (Bifenilos Policlorados (BPCs). 10/12/2005). PCBs can be transported by by various environmental media, including air and water, spreading contamination to other areas.

The very characteristic of the PCBs that made them useful in manufacturing makes them problematic in the environment. PCBs do not readily break down in the environment and thus may remain there for very long periods of time.

Organisms such as fish and birds may accumulate PCBs from the water, from food or from sediments. The degree to which PCBs accumulate in animals is dependent on a number of factors, including their trophic position within the ecosystem, feeding strategy, longevity, fat content, sex, and reproductive status.

PCBs have been shown to cause cancer in animals. PCBs have also been shown to cause a number of serious non-cancer health effects in animals, including effects on the immune system, reproductive system, nervous system, endocrine system and other health effects (USEPA. Polychlorinated Biphenyls (PCBs) - Health Effects of PCBs. 10/12/2005).

Point Source Pollution (PS) happens when a pollutant comes from a definite source such as wastewater discharged from the pipes of industrial facilities and municipal sewage treatment plants into rivers, streams, lakes, and the ocean.

Selenium is an essential trace element that occurs naturally in the environment. Although small amounts of selenium are important for both wildlife and people, elevated concentrations can cause selenium toxicosis. Fish and wildlife can be exposed to selenium in water, sediment or food. In parts of the western United States, the soils contain rather high levels of selenium. There, selenium is most commonly found in rocks and soil that originated in the ocean (marine sedimentary deposits).

Parts of the west are highly productive farming areas. However, the long summers and hot, dry climate that makes the west so attractive for farming also demands the extensive use of irrigation. Unfortunately, long-term irrigation flushes out the salt, selenium and other trace elements found in the soil into irrigation return flows. Traditionally, these selenium-tainted agricultural waters have been disposed of in evaporation ponds. However, evaporation ponds are an attractive nuisance to birds that stop to feed on plants and animals found at the ponds. These plants and animals often contain high concentrations of selenium, which the waterbirds then accumulate in their tissue, causing the kind of problems scientists began observing in the Valley in the early 1980s, scientists documented that waterfowl in marshy areas of California's San Joaquin Valley were producing offspring that were stillborn or deformed.

Links:
For additional information on selenium and health we recommend visiting the Agency for Toxic Substances and Disease Registry (ATSDR), ToxFAQs™ for Selenium, http://www.atsdr.cdc.gov/tfacts92.html.