Wastewater-based epidemiology (WBE) is increasingly used as a complementary tool for monitoring drug use at the population level, providing anonymized, real-time estimates of community drug consumption. Site-specific applications of WBE can identify localized patterns that national or municipal surveys may overlook. This study presents the first comprehensive, site-specific assessment of illicit drug use in South Korea using WBE. Influent wastewater samples were collected in 2022 and 2023 from wastewater treatment plants (WWTPs) at five strategically selected locations: an international airport, a casino, U.S. military areas, a major harbor, and rural areas. A total of 22 target compounds were analyzed to characterize drug consumption patterns across distinct functional environments. Methamphetamine (MA) was the most frequently detected substance, with the highest consumption observed at the casino (mean: 18 mg/day/1000 people) and in certain rural areas (up to 32 mg/day/1000 people), particularly those selected based on a high proportion of foreign residents. These rural areas were chosen according to specific demographic criteria and should not be interpreted as representative of the general rural population. Amphetamine (AMP) was predominantly detected in U.S. military areas (mean: 16 mg/day/1000 people), consistent with therapeutic use among military personnel. The airport exhibited the greatest drug diversity, including elevated levels of 3,4-methylenedioxymethamphetamine (MDMA) (5 mg/day/1000 people), ketamine (39 mg/day/1000 people), and cocaine (4 mg/day/1000 people), reflecting transit-related consumption. Notably, the novel psychoactive substances 25D-NBOMe and 25E-NBOMe were identified for the first time in South Korean wastewater, exclusively at the casino. These findings underscore the value of WBE in capturing region-specific drug signatures and support the need for targeted public health strategies and early warning systems in high-risk and demographically diverse environments.
