Over the past few days, you might have seen some blaring headlines and hyperventilating ledes about recent British tests detecting trace amounts of cocaine metabolites—evidence that cocaine has passed through a human body—in public drinking water. Most of these articles commenced to fretting over whether Britain has a cocaine problem. (Answer: The entire western hemisphere has a cocaine problem.) As the Gloucester Citizen put it:

Cocaine is now so widely used in Britain it can be found in our drinking water, tests have shown.

Inspections of tap water at four different sites found a metabolised form of the illegal drug, which showed it had already passed through the human body.

The levels were so low that they posed no danger to health, but come as a startling indication of how widespread drug use has become.

The Guardian calmly pointed out that there are trace elements of all kinds of alarming-sounding things in our water supply, so the presence of cocaine metabolites isn't necessarily a "startling indication" of anything—except, perhaps, how much urine gets into the water supply.

How much piss has to get into a public water system to show detectable amounts of drug use? And how common is it for wastewater—even treated wastewater—to wind up back in public drinking water?

I emailed the question to Seattle Public Utilities and Lynn Kirby, a "water quality engineer" for SPU, sent this very helpful reply:

I’ll use the example of the Mississippi River. There are cities and towns along the entire stretch of that river, and many rivers in the US and Canada. Almost all wastewater treatment plant outlets (the water that is produced after the waste has been treated) end up going to the nearest river. And many towns also draw their drinking water from rivers. So the further downstream you go on a river, the more likely you are to find medications, personal care product components, and their metabolites. The source is humans who are taking the medications, and either having it pass through their bodies (ending up in the wastewater), or humans dumping their extra medications down the toilet to dispose of them. The wastewater treatment processes do not aim to remove these trace compounds. In addition, drinking water treatment processes do not aim to remove these trace compounds.

In Seattle we are extremely fortunate to have source waters that do not have human influences like most rivers. There is no industry, agriculture, recreation, habitation, or wastewater treatment plants in our watersheds. But out of curiosity, we did test our sources a few years ago for a variety of medications and personal care products. Not surprisingly we didn’t find anything. Our drinking water comes from the mountains. The wastewater effluent from the City of Seattle goes to Puget Sound.

Thanks, Lynn. And chalk that up as another benefit of living in a city wedged between the mountains and the sea.