Scientists have run high-tech tests on harmful bacteria in local rivers and streams and found that many of the germs — and in the Potomac and Anacostia rivers, a majority of them— come from wildlife dung. The strange proposition that nature is apparently polluting itself has created a serious conundrum for government officials charged with cleaning up the rivers.

Part of the problem lies with the unnaturally high populations of deer, geese and raccoons living in modern suburbs and depositing their waste there. But officials say it would be nearly impossible, and wildly unpopular, to kill or relocate enough animals to make a dent in even that segment of the pollution.

That leaves scientists and environmentalists struggling with a more fundamental question: How clean should we expect nature to be? In certain cases, they say, the water standards themselves might be flawed, if they appear to forbid something as natural as wild animals leaving their dung in the woods.