LONDON — Researchers seeking evidence of chemical “micropollution” in five rural English rivers have found pesticides in many of the freshwater shrimp they tested. And cocaine in all of them.

The presence of the illegal drug was unexpected because the sites where the researchers gathered their samples, in the eastern coastal county of Suffolk, were miles away from any large city, said the study’s lead author, Thomas Miller, a researcher at King’s College London.

“Although we know that pharmaceuticals, pesticides and so on are in our rivers, most studies in the world don’t look at what’s inside wildlife,” Dr. Miller said in a telephone interview on Friday.

Drugs and other chemicals that are flushed into the sewage system have presented regulators with a puzzle for years. At the low concentrations in which they generally reach waterways, the substances have been found unlikely to affect human health, but their presence is hard to ignore. Recent concerns about plastic waste in rivers and oceans have helped focus attention on the species that live in those waters as victims of pollution.