The chemicals industry has long opposed treating the whole class of substances the same way. It argues there is no evidence, or only limited evidence, that other PFAS are toxic, and that some have essential uses, including in life-saving medical equipment.
The decline covers PFOS and PFOA, compounds long linked by researchers to tissue damage in animals and to higher rates of serious illness, including testicular cancer.
The agency said the fall does not mean the chemicals have left the environment. Because they break down so slowly, the overall amount is likely to be unchanged: they have been diluted and washed into the sea.
Officials attributed the lower concentrations to stricter restrictions imposed as evidence grew of harm to human health and the environment.
They are called forever chemicals because that resistance means they take a very long time to break down naturally. PFOS and PFOA were among the earliest types.
About two decades ago, researchers began to link those compounds to animal tissue damage and to increased rates of certain serious health conditions, including testicular cancer.
Thousands of other forever chemicals remain in use. The regulator said its findings showed that all of them need to be better monitored and regulated.
The agency is now testing regularly for 60 other PFAS as it tries to build a national picture of contamination.
Earlier this month the government said it would begin monitoring levels of the chemicals in people's bodies, part of a wider official push to tackle forever chemicals amid growing concern.
One expert who studies the impact of the chemicals called it"brilliant"to see how far the UK has come on the issue.
The report leaves a harder point in place: lower readings in rivers can reflect dilution and a shift of pollution seaward, not the disappearance of chemicals built to last.