When people take drugs, either for legitimate medical reasons or to get high, they eventually eliminate traces of those drug through the body’s waste products. Those traces then enter our water supplies. When wastewater is processed to make it safe to flow into the sea, this processing does not remove all traces of drugs the humans in that area were consuming.
https://www.narconon.org/blog/drugs-in-our-bodies-become-drugs-in-our-water.html
https://www.narconon.org/blog/drugs-in-our-bodies-become-drugs-in-our-water.html