Folks have been testing wastewater for legal and illegal drugs for years (https://score-cost.eu has an interesting report if you're interested).

There's been talk amongst Police Departments in whether investing in wide scale wastewater monitoring could help locate drug dealers/labs. Personally, I think this is a huge invasion of privacy, but since wastewater is a "public good" police currently can do whatever they want with it (source: I work in a wastewater testing facility).