Drug Testing an Entire Community
You won’t identity individual users, but you can test for the prevalence of drug use in a community by testing the sewage water.
Presumably, if you push the sample high enough into the pipe, you can test groups of houses or even individual houses.
EDITED TO ADD (7/13): Here’s information on drug numbers in the Rhine. They estimated that, for a population of 38,5 million feeding wastewater into the Rhine down to Düsseldorf, cocaine use amounts to 11 metric tonnes per year. Street value: 1.64 billion Euros.