DDT was used as a pesticide on farms.
It is insoluble in water and bonds strongly to particles.
It enters marine/fresh water food webs through land runoff, precipitation and dumping.
DDT is absorbed by plankton but cannot be metabolized.
It accumulates in the food chain until it reaches the top carnivore, where more drastic effects are seen.
For example, in birds, DDT interferes with calcium deposition in the eggshells. The eggs shells are extremely thin and therefore easily broken.