When people take drugs, they end up in the water, either unchanged or broken down into specific metabolites. [W]ater can be tested to gauge how much drug use is going on in an area.

[R]esearchers detected cocaine in 93 percent of the untreated water samples. Based on the relative level of cocaine’s metabolites, they determined that most of the drug ended up there via human excretion, rather than direct disposal.

[M]orphine was found in 100 percent of the untreated water [probably from heroin being broken down in the human body].

Other, similar tests have uncovered related findings in other areas, reported in Environmental Health News:

• In London, cocaine and ecstasy spike on weekends while methadone is used more consistently.
• In Italy, cocaine use has declined while use of marijuana and amphetamines has increased.
• In Sweden and Finland, people use more amphetamines and methamphetamines and less cocaine than other European cities. Also, in Finland, stimulants were more common in large cities.
• In Zagreb, Croatia, marijuana and heroin were the most commonly found illicit drugs, but cocaine and ecstasy showed up more frequently on weekends.
• In Oregon, cocaine and ecstasy are more common in urban than in rural wastewater according to a 2009 study.
• Amphetamine use on his campus skyrockets during finals.

Via Popular Science.