Simulation And Mechanism Of Antibiotics Reactive Transport In The Tidal Estuary

As a class of emerging contaminants,antibiotics have received increasing attentions due to their great impacts on the ecosystem.Antibiotics from domestic sources,animal breeding,pharmaceutical industries as well as other pollution sources can be discharged to rivers,and migrating to ocean eventually.Residues of antibiotics in the surface water posed great threats to the aqueous ecosystems.Located at the landsea interface zone,estuary is an important channel for the transport of terrestrial pollutants to the sea.Meanwhile,estuary is a highly dynamic environment with its physical,chemical,and biological conditions characterized by spatial and temporal variability.The variable hydrodynamic conditions and geochemical gradients in estuary have a direct impact on the fate and transport of antibiotics.To better understand the transport and fate of antibiotics from land to sea,there is necessary to clarify their pollution sources,spatio-temporal distribution,and migration mechanism in estuarine environment.In this paper,Xiaoqing River tidal estuary is selected as the study area because there are many potential pollution sources of antibiotics in the Xiaoqing River Basin.Further,Xiaoqing River is the main source of pollution in Laizhou Bay.The water quality of Xiaoqing River affects the aquatic ecosystem of Laizhou Bay significantly.Based on the field surveys,chemical and tracer experiments,and numerical simulation,this study reveal the distribution,environmental behaviors and migration mechanisms of antibiotics in the Xiaoqing River tidal estuary.Field investigation showed that the concentration of trimethoprim(TMP)and ofloxacin(OFL)in the surface water of Xiaoqing River are up to the level of ?g/L.According to the risk quotients(RQs),5 antibiotics [i.e.OFL,ciprofloxacin(CIP),sulfamethoxazole(SMX),erythromycin(ETM),and clarithromycin(CTM)] posed high risk(RQ>1.0)to the sensitive aquatic organisms.Principal component analysismultiple linear regression(PCA-MLR)analysis indicated that the antibiotics in the surface water of the Xiaoqing River were mainly derived from domestic sewage(37%)and animal breeding wastewater(63%).5 sampling events were conducted from May 2015 to May 2017 in the Xiaoqing River tidal estuary to investiagte antibiotics [SMX,sulfamethoxazole(SMZ),TMP,CIP,OFL,ETM and roxithromycin(RTM)] pollution.The results show that the pollution of TMP in the estuarine water decreased;the concentration of sulfonamides and macrolide antibiotics almost remained at the same level within 3 years;while contamination of fluoroquinolones was increasing.The spatial distribution of 7 antibiotics showed that the TMP pollution source is located in the lower reaches of the estuary;while the main source of the other 6 kinds of antibiotics are located in the upper reaches of the estuary.Flood discharge from Wang Dao sliuce has an important effect on the antibiotic pollution in the estuary.In addition,the spatial distribution of antibiotics in the estuary were controlled by tidal.The degradation of SMX,CIP and TMP in the estuarine water were mainly dominanted by photolysis.After entering the estuarine water environment,SMX,CIP,TMP,and the similar antibiotics decay rapidly through the adsorption process.Then,photolysis and biodegradation reactions result in further attenuation of these antibiotics with relative slower rates.Delft 3D model was used to simulate the fate and transport of SMX and TMP in the Xiaoqing River tidal estuary.There was a significant positive correlation between the measured and simulated concentrations of the two antibiotics(p<0.01).Further,the average accuracy of the simulation results was greater than 70%.This indicated that the accuracy of this model is high enough.Several scenarios were setted to study the fate and transport of antibiotics under unexpected circumstances.Simulation results showed that the transport of antibiotics in the upper reaches of the Xiaoqing River tidal estuary are mainly controlled by river runoff,where the effect of tidal is relatively small.Tidal can hinder the migration of antibiotics from river to sea,and the obstruction of tidal is stronger near to the lower reaches of estuary.Correspondingly,antibiotics will retard for a long time when they reach the lower reaches of the Xiaoqing River tidal estuary.River flow is usually small during the nonflood period,transport of antibiotics in the Xiaoqing River tidal estuary will be very slow at this period.The retention of a large number of antibiotics in the estuary will affect the aquatic ecosystem.With the increase of river runoff,the migration of antibiotics to the sea will be faster.Overall,this paper systematically studied the migration mechanism and transmission process of antibiotics in the Xiaoqing River tidal estuary through combinding the field investigation,indoor simulation experiments,and numerical simulation.The results obtained from this study provide supports for antibiotic pollution assessment and pollution control decision in estuaries.