We can see from aerial photos that the floodwater flowed down the rivers and out into the Gulf of Mexico. Then the currents took them across the north Atlantic via the Gulf Stream. Six weeks later they were off the coast of Newfoundland. The rivers, seas and oceans are not separate. They mix together.
Air too flows from one place to another.
And they stay around a long time. Many of these chemicals take a very long time to break down in the body. DDT for example has a half-life of 57 years in temperate zones. Hormones get their name from the Greek word meaning 'to urge on'. They are chemical messengers produced and released into the bloodstream by organs known as the endocrine glands.