Spatiotemporal distribution characteristics and ecological risk of emerging contaminants in a typical river on the Tibetan Plateau

Along with the continuous improvement of the ecological environment in China, control and prevention of emerging contaminants have been elevated to the top of the government's agenda. The urbanization and population growth have substantially increased the emissions of anthropogenic pollutants, which threaten the fragile ecosystem and sustainable development on the Tibetan Plateau. Similar to other river systems, the Nyang River, which flows through one of the most important agricultural areas on the Tibetan Plateau, receives pollutants from treated (and untreated) municipal sewage, wastewater from animal farms and aquacultural farms, and runoff from agricultural fields. In this study, we sampled the river water at twenty sites along the Nyingchi section of the Nyang River using passive and active sampling techniques in the wet and dry seasons, respectively. A total of thirty-three antimicrobials, four paraben preservatives, and six s-triazine and benzimidazole pesticides were identified using ultra-high pressure liquid chromatography-triple quadrupole mass spectrometry (UHPLC-MS/MS). The ecological risk of the target emerging contaminants in the Nyang River was subsequently assessed based on their detected concentrations. The concentrations of individual emerging contaminants in the Nyang River ranged from 0.01 to 250 ng/L, and their detection frequency was between 3% and 100%. Among the target emerging contaminants, erythromycin (median: 5.2 ng/L), trimethoprim (median: 1.4 ng/L), norfloxacin (median: 0.30 ng/L), and oxolinic acid (median: 0.62 ng/L) were the major detected antimicrobials, methylparaben (median: 6.3 ng/L) and propylparaben (median: 3.9 ng/L) were the major detected parabens, while carbendazim (median: 66 ng/L) and thiabendazole (median: 88 ng/L) were the major detected pesticides. In addition, caffeine, a source-specific indicator for municipal sewage, was detected at all sites, in the range of 0.15-700 ng/L (median: 15 ng/L). Overall, the concentrations of most emerging contaminants were lower than those found in other rivers in China and other parts of the world. Nonetheless, erythromycin, carbendazim, and caffeine occurred at concentrations comparable to or slightly higher than those of other rivers. The concentrations of emerging contaminants exhibited significant spatial variations with antimicrobials dominating in the wet seasons and pesticides dominating in the dry seasons, respectively, which resulted primarily from the seasonal fluctuations in source inputs and river flow, as well as the attenuation of pollutants in the aquatic environment. Most of the antimicrobials in the Nyang River originated from the wastewater of animal farms and the runoff from agricultural fields applied with manure-based fertilizers, while some, particularly those used primarily in human health care, came from municipal sewage. The parabens were mainly contributed by the runoff from agricultural fields and municipal sewage in the wet and dry seasons, respectively. While the s-triazines in Nyang River were derived from the agricultural runoff, the benzimidazoles, which resulted from the wash-off of pesticide residues from fruits and vegetables in households, were primarily associated with municipal sewage. The calculated risk quotients of the target emerging contaminants indicated that several of them (i.e., erythromycin, carbendazim, thiabendazole, and caffeine) could pose medium to medium-high risk in the Nyang River in the dry season. The emerging contaminants could also pose medium to high risk to green algae in the river in both dry and wet seasons, although simultaneous exposure to them could result in even higher actual risk. While the current pollution level and risk of the emerging contaminants in the Nyang River are low, efforts should be taken to control the discharge of these pollutants in the basin in the course of urban expansion and agricultural development. In particular, the wastewater collection and treatment infrastructure in the basin should be upgraded to ensure that all municipal sewage is adequately treated prior to discharge.