Occurrence Characteristics Of Typical Pollutants In The Auqtic Environment Of River-reservoir System

River system is considered as an important water collection and drainage system in the aquatic environment,and it plays a vital role in maintaining the social development and ecological environment.In contrast,the reservoir system is a modern aquatic environment that is established by humans to regulate the original river system,often receiving the water drainage from river systems in the basin water environment.However,studies on the occurrence and distribution patterns of environmental pollutants in the river system and reservoir system,as well as their environmental behavior are still scarce.Therefore,more comprehensive investigations about the characteristics of environmental pollutants in the river-reservoir system along with the environmetal gradient form river system to reservoir system will be of great significance for a better understanding of their environmental behavior in actual aquatic environment,and consequently have important implications for the watershed pollution control and risk assessment.To accomplish this goal,the Headwater Region of the Dongjiang River?HRDR?was regarded as a representative drinking water in China,and was also selected as the case study.Conventional pollutants?nitrogen,phosphorus,and chromophoric dissolved organic matter?,typical emerging pollutants?pesticides and antibiotics?and antibiotic resistance genes?ARGs?were chosen as target pollutants in the various aquatic environment including bulk waters,sediments,and soils.The levels of their occurrence were measured by the means of three-dimensional fluorescence spectrometer,gas chromatograph mass spectrometry,ultra-high performance liquid chromatography coupled with tandem mass spectrometry,and high-throughput fluorescence quantitative PCR instrument,respectively.Then,based on these,the occurrence patterns and risk levels asociated with these pollutants in the river system and reservoir system?river-reservoir system?were systematically investigated.Meanwhile,the main factor influencing the spread and dynamic of antibiotic resistance genes in aquatic environment were revealed using the structural equation models?SEMs?.The specific findings were as follows:?1?.In the bulk waters of the HRDR,the principal component of nitrogen was inorganic nitrogen during the summer?normal season?and spring?wet season?,when the nitrogen self-cleaning behavior was relatively high.However,the principal component of nitrogen was organic nitrogen,when the self-purification of nitrogen was relatively low.Chemometrics studies indictated that the limiting factors for the growth of algae in waters of source regions are mostly phosphorus;while the main limiting factor for the nitrogen cycle in the summer?normal season?and spring?wet season?was organic carbon.In general,the risk of eutrophication in the HRDR was not high,and it was at a moderate level of eutrophication.Furthermore,the concentrations of nitrogen and phosphorous in the pore water of Fengshuba Reservoir sediments and nitrogen composition were relatively stable,respectively.However,the pore water might still be an important source of NH₄⁺-N for the bottom water.The total concentration of nitrogen in the reservoir sediments was heavily polluted,and was positively correlated with the sand particle size ratio in the sediments.In contrast,the total concentration of phosphorus was slightly polluted.Additionally,the levels of nitrogen and phosphorus in the sediments were significantly higher than those in the soils,respectively.?2?.The dissolved organic matter?DOM?in the waters of the HRDR was mainly influenced by agricultural activities in the summer?normal season?,and was mainly influenced by the urban activities in the autumn?dry season?and spring?wet season?.However,the chromophoric dissolved organic matter?CDOM?was highly influenced by human activities over the three seasons.In the summer?normal season?,the reservoir system had the potential to reduce the level of DOM exported from the river system.Meanwhile,in the autumn?dry season?,the reservoir system had the potential to reduce the level of CDOM and its molecular weight exported from the river system.The exchange behavior of CDOM between the pore water in the sediment and the bottom water might occur in the reservoir,but the pore water in the sediment had little effect on DOM molecular weight and its aromatization degree of the overlying bottom water.There was no significant difference in the level of DOM between the sediments and soils of the reservoir,but the level of CDOM in the sediments was obviously higher than that in the soils.In addtion,three fluorescent components including two humic-like components?C1 and C2?and a tryptophan-like component?C3?were identified from the excitation-emission matrix coupled with parallel factor analysis?EEM-PARAFAC?for all samples.Meanwhile,the reservoir system had the potential to reduce the level of microbial humic-like and terrestrial humic-like components exported from the river system,but it did not influenced the level of tryptophan-like component in the river system.Seasonal hydrological processeses in the reservoir could influence the levels of microbial humic-like and tryptophan-like components in the pore waters,but they had little effect on the composition of the fluorescent components in the sediments.Interestingly,the intensity of each fluorescent component could be chosen to quickly evaluate the level of DOC in the river waters and the levels of DON and DTP in the reservoir water,respectively.However,microbial humic-like or terrestrial humic-like components could be suggested as the indictor for the rapid assessment of the CDOM level in the reservoir waters.?3?.The total concentrations of 8 organochlorine pesticides?OCPs?,16organophosphorus pesticides?OPPs?and 7 pyrethroid pesticides?SPs?in the bulk waters of the HRDR were 107.57³40.35 ng/L,222.25¹119.95 ng/L and 86.27²45.09 ng/L,respectively.For the bulk waters of the river system,the levels of three types of pesticides were closely related with the the use of pesticides in seasonal agricultural management activities.For the bulk waters of the reservoir,seasonal hydrological processes might be the important factor in influncing the environmental behavior of the three types of pesticides.Meanwhile,the reservoir system had the potential to reduce the total concentrations of OCPs,OPPs,and SPs exported from the river system during the summer?normal season?.The balances of pesticide exchange fluxs between the sediment and pore water in the reservoir indicated that the sediment phase had the potential to release OCPs?1.74³.81 kg?and SPs?8.99¹2.19 kg?into the water phase,respectively.However,the sediment phase still had the potential to accommodate the OPPs?267.35³47.27 kg?from the water phase.Furthermore,ecological risk assessment indicated that the total risk of the three types of pesticides in the water phase and sediment phase were all at the high risk,especially for the SPs,hence our study suggested that the SPs should be given more priority to control.?4?.The total concentration of antibiotics in the water of the HRDR was193.59⁸63.27 ng/L.During the three seasons,the reservoir system had the potential to reduce the level of antibiotic contamination in the river system.However,there still had seasonal differences in the environmental behavior of antibiotics from the river system to reservoir system.In the summer?normal season?,the primary environmental process that took place from the river to reservoir system might be the physical dilution behavior.In contrast,this process might be the biogeochemical behavior in the autumn?dry season?and spring?wet season?.Moreover,the extension of the hydraulic retention time would contribute to influencing the environmental behavior of antibiotics in the reservoir.The balance of antibiotic exchange flux between the sediment and pore water in the reservoir indicated that the sediment phase still had the potential to accommodate total antibiotics?217.90³13.24 kg?from the water phase.Furthermore,risk assessment indicated that the ecological risk of tetracycline in the water phase was at the high risk,and ciprofloxacin was at the high risk in inducing microbial resistance,hence our study suggested that the tetracycline and ciprofloxacin should be given more priority to control.In contrast,all the antibiotics in the sediment phase were at the low risk.?5?.A total of 242 species,184 species and 137 species of antibiotic resistance genes?ARGs?were detected in the bulk waters,sediments and soils the HRDR,respectively.Meanwhile,the corresponding absolute levels of ARGs were 6.57×10⁷².06×10¹¹ copies/L,5.33×10⁷⁴.34×10⁸ copies/g,and 1.45×10⁹².64×10⁹copies/g.In the autumn?dry season?,the reservoir system had the potential to reduce the absolute levels of ARGs in the river system.The correlation analysis indictated that vertical gene transmission?VGT?and horizontal gene transmission?HGT?might have together aided the dissemination of ARGs in the water phase,however only HGT had a potency to facilitate the spread of ARGs in the sediments of reservoir system.Furthermore,a clinical class I integron-integrase gene?cintI-1?could be used to quickly assess the absolute abundance of ARGs in the aquatic environment.Pathogenic bacteria in the reservoir system might be sedimented by particles and thus tended to be more concentrated in the sediments,which could influence the migration of ARGs between the water phase and sediment phase in the reservoir to some extent.Structural equation models?SEMs?indicated that the factor that have the greatest impact on the dynamic of ARGs in the water phase?river and reservoir?and sediment phase?reservoir?was the mobile original gene?MGEs?.In addition,the pathogenic bacteria and antibiotic residues in the river system water could significantly promote the spread of ARGs.However,the reservoir system had the potential to regulate the ARGs dynamics,which was mainly stimulated by antibiotics and pathogenic bacteria in the water phase of river system.Generally,in addition to the fact that the existence of reservoir system could change the original hydrological characteristics of the river system,it could also partly influnce the environmental behavior of nitrogen,phosphorus,CDOM,pesticides,antibiotics and ARGs associated with the river system via the various environmental comparments including the water phase,sediment phase and soil phase.