- Widespread in the estuary, both geographically and in the food chain
- Present even though it was banned decades ago
- Persistent and bioaccumulative
- Acutely toxic, but also has sublethal effects
DDT is a persistent organochlorine pesticide that was widely used for decades to control agricultural pests and mosquitoes that carried diseases such as typhus and malaria. DDT is a probable human carcinogen, and its breakdown products—DDE and DDD—also have toxic effects.
DDT is a legacy pesticide, meaning that it is still found in the environment even though it was banned in 1972 because of its toxic effects on fish, wildlife, and people. Legacy pesticides resist dissolving in water. Instead, they adhere to soil and sediment, where they can remain for years. They bioaccumulate readily and can biomagnify up the food chain.
Aldrin, dieldrin, and chlordane are examples of other legacy pesticides. Aldrin and dieldrin are chlorinated insecticides developed in the 1940s as an alternative to DDT. They were widely used to control termites and other soil insects until they were banned in 1987. Aldrin breaks down quickly into dieldrin, which is persistent and bioaccumulates. When exposed to sunlight, dieldrin can transform into photodieldrin, a more toxic compound.
Chlordane is a mixture of related chemicals, such as heptachlor, that was used on food crops and for termite control in the United States until it was phased out between 1978 and 1988. Chlordane bioaccumulates readily, is commonly found in human body fat, and can affect the liver, nervous system, and digestive system. It is still manufactured for export.
DDT was found in the tissue and stomach contents of juvenile salmon from sites all along the estuary, although detections in water and on suspended sediment were rare. Dieldrin also was present in salmon tissue from all sites (Lower Columbia River Estuary Partnership 2007). Heptachlor epoxide has been detected in salmon tissue and stomach contents in the estuary.
DDT has both acute and long-term effects on microorganisms, invertebrates, amphibians, fish, mammals, and birds, including (notoriously) the reproduction of bald eagles. In juvenile salmon, exposure to DDT can cause immune suppression, reduced growth, hormone disruption, disrupted reproduction, and physical/developmental abnormalities.
DDT levels in some juvenile salmon in the estuary are at or above the threshold level (5,000 – 6,000 nanograms per gram lipid) for health effects (Lower Columbia River Estuary Partnership 2007).
Because DDT and other legacy pesticides have been banned, it is likely that they are being released to the environment only in small amounts, if at all. However, these pesticides are persistent. It appears that DDT that entered the environment some time ago is recirculating through the food chain, meaning that it reaches the top of the food chain, is re-released to the environment when the top predator dies and decomposes or is eaten, and enters the food chain once again.
In the Columbia River estuary, juvenile salmon are exposed to DDT through their prey and, less significantly, through hatchery feed (Lower Columbia River Estuary Partnership 2007).
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i Lower Columbia River Estuary Partnership. 2007. Lower Columbia River and Estuary Ecosystem Monitoring: Water Quality and Salmon Sampling Report.
ii Ibid.
iii Ibid.
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