Popular Science has an article on 'community urine-testing', i.e. examining the drug habits (illegal or otherwise) of communities by testing the waste water they excrete.
Sewage is more than just filth. It’s evidence of our worst habits, everything from caffeine to cocaine, all ingested and flushed down the toilet. Now scientists are using wastewater to drug-test entire cities, and the results are sobering.
What is interesting are the concerns over what might be called 'collective privacy'. Communities don't want their drug habits brought out into the open - even if the habits of any one constituent are impossible to identify through the aggregated tests.
No city wants to be known as the coke capital of the world, and some critics fear Big Brother tapping their plumbing as well as their phone line.
I expect that eventually cities will start dumping masking agents down the drains. Or claim they had a bad cold and were just taking decongestants.