Occurrence and risk assessment of antibiotics in river water in Hong Kong

• A wide distribution of antibiotics in the main rivers of Hong Kong.
• The high detection rate of antibiotics occurred where there was a high level of human activity.
• Ofloxacin was the most frequently detected antibiotic in the rivers.
• Ofloxacin and doxycycline present a medium to low ecological risk.

The occurrence and distribution of six typical antibiotics in the main rivers of Hong Kong were investigated using high-performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ES-MS/MS). The results revealed that the antibiotics were widely distributed in the area studied. Of the target antibiotics, ofloxacin was the most frequently detected in the rivers, with a detection rate of 69.6% and a median concentration of 0.7 ng/L. Sulfadimidine (n.d–580.4 ng/L) and doxycycline (n.d–82.2 ng/L), with detection frequencies of 65.2% and 30.4%, respectively, were found at the same level as in rivers in North America, Spain, France, Australia, and in the Yangtze and Pearl Rivers of China, while the other target antibiotics were found at lower levels. According to the ratios of the measured environmental concentration to the predicted no-effect concentration, ofloxacin and doxycycline could present a medium to low ecological risk to algae, while sulfonamides posed no obvious ecological risk to the relevant aquatic organisms (algae, Daphnia magna, and fish). A high detection rate of antibiotics occurred in densely populated areas, revealing that population activities might be greatly contributing to the increasing levels of antibiotics in the area. Thus, the residues of antibiotics present in the waters of Hong Kong need to be closely monitored.