A study in the journal Addiction shows that a viable system for measuring the consumption of illegal drugs in various communities is to analyze samples of untreated wastewater--which contains the leftovers.

It is not just waste water that is contaminated. The AP report says that watersheds, the natural source of most of the country's drinking water, are also affected. The AP investigators said tests were carried out in the watersheds of 35 of the 62 major water utilities they surveyed and drugs were found in 28 of them. Six of the 28, when contacted by the AP, said they did not test their drinking water, despite the watersheds being contaminated.

from the Associated Press investigation