There was a recent article in "The Week" titled Toilets that tell secrets. Researchers at Oregon State University have been drug-testing entire cities by analyzing a single teaspoon of wastewater in sewage plants. The tests reveal what drugs, quantities and what days of the week the drugs are used. While several illegal drugs were mentioned in the article, marijuana was not. Reasons were not stated, but more than likely the tests show that a large number of toilets users are also bong users.