Final answer:
DDT enters the environment through direct application as a pesticide to control pests. Its persistence and bioaccumulation lead to environmental and ecological impacts.
Step-by-step explanation:
The entry of DDT into the environment primarily occurs through direct application as a pesticide. DDT was sprayed in residential areas, agricultural lands, and forests to control pests, particularly malarial mosquitoes. This direct application led to the persistence and bioaccumulation of DDT in the environment due to its resistance to breakdown.
As a result, it biomagnified through the food chain, affecting top predators like birds. Indirectly, DDT can also enter water bodies through runoff, but its initial entry into the ecosystem is through intended use as a pest control agent.