Taiwanese researchers have sampled river and waste water samples before, during, and after a 2011 pop musical festival in Taiwan called Spring Scream. According to the authors of the study, this festival is "notorious for the problems of drug abuse and addiction."
Their study shows that the festival runoff causes a spike in compounds such as acetaminophen (Tylenol), caffeine, pseudoephedrine, MDMA (ecstasy), and ketamine. Other studies mentioned in this article have also shown increases in controlled substances (e.g. MDMA, cocaine) in water samples during New Year's Eve celebrations, graduation parties, big dances, and just weekends.
The authors couldn't characterize the local ecological impact of the surge of these compounds, but note that such studies could be concerning both for the environment and for the festival attendees.