3.1 Estimation of antibiotic emissions in the Yangtze River Estuary
Due to the lack of official statistics on the use of antibiotics, we converted them from the permanent population statistics of the Yangtze River Estuary and the breeding scale data of livestock and poultry from 2011 to 2019 (Table S6). As shown in Fig.1a, the total population in the study area remained stable from 2011 to 2019, the rural population decreased slightly, from 311million to 2.3million, and the urban population increased by 1.5million during the 10 years, with a limited increase relative to the total number. Due to the decrease of agricultural population and the decline of breeding scale in the Yangtze River estuary, the number of breeding livestock and poultry has decreased significantly. In 2011, the number of livestock was 2.67million, and in 2019, it was only 1.17million, which decreased by 60%. The number of poultry breeding has also decreased sharply, from 43million in 2011 to 8.44million in 2019, and the breeding scale has decreased to 20% of the original. Overall, the population of the Yangtze River estuary did not change significantly, but the scale of aquaculture in the region was seriously reduced.
Different types of antibiotics have different retention times after entering the body, resulting in different proportions of final excretion in vitro. Therefore, when considering the total amount of antibiotics entering the environment, antibiotics absorbed by people or animals need to be subtracted (Table S10 and Table S11). It can be seen from the rate of T02 (Fig. 1b) and T04 (Fig. 1c) of the six target antibiotics released to surface water and soil during 2011–2019 that the amount of these antibiotics released to soil and surface water has been decreasing. In surface water, the emission rate of tetracycline decreased from 0.635 mol/h to 0.272 mol/h, the emission rate of amoxicillin decreased from 0.341 mol/h to 0.154 mol/h, the emission rate of ofloxacin decreased from 0.085 mol/h to 0.038 mol/h, the emission rate of oxytetracycline decreased from 0.023 mol/h to 0.011 mol/h, the emission rate of norfloxacin decreased from 0.019 mol/h to 0.009 mol/h, and erythromycin hardly emitted into surface water. In soil, tetracycline emission rate decreased from 8.125 mol/h to 3.224 mol/h, amoxicillin emission rate decreased from 4.714 mol/h to 1.503 mol/h, ofloxacin emission rate decreased from 4.092 mol/h to 0.988 mol/h, oxytetracycline emission rate decreased from 0.512 mol/h to 0.115 mol/h, norfloxacin emission rate decreased from 0.398 mol/h to 0.121 mol/h, erythromycin emission rate decreased from 1.821 mol/h to 0.407 mol/h. The main reason is that the decrease in the scale of aquaculture has led to the decrease in the amount of veterinary antibiotics. The rate of tetracycline and amoxicillin discharged into the water environment is large, on the one hand because of their large consumption, and on the other hand because the degradation efficiency of these two substances in sewage treatment plants is not high, resulting in a large rate of discharge into the water environment. In addition to high tetracycline and amoxicillin, the rate of ofloxacin directly discharged into the soil was also relatively high because it was mainly used for bacterial and mycoplasma infections in livestock and poultry. Due to the lack of facilities, sewage and feces in rural areas are discharged directly into the outdoor environment and enter the soil, resulting in the rate of these six antibiotics entering the soil environment is much higher than the rate of pollution entering the water phase.
3.2 Model validation
In order to verify the validity of the model, the simulated value of the model was compared with the measured value. The validation data were from the literature (as show in Table S12) and the measured values of this study. Because the predicted antibiotic concentration in the air was lower than the detection limit, the simulated values of the other five environmental phases were compared with the measured values. It can be found in Fig. 2 that the difference between the simulated concentration and the measured concentration of the six antibiotics in each environmental medium was less than a logarithmic unit, so the model simulation was effective. The simulation results of OTC were also within the allowable range of the model, and the errors in each environmental phase were large compared with other substances. These may be due to the selection of antibiotic emission parameters in this study. Input of OTC in water was from sewage discharge, while the input rate of upstream incoming water was small, which might be the reason for the low simulation value in water. For underground water phase, partition coefficient and degradation rate in environmental medium were very important parameters. The degradation rate of selected antibiotics varied in different environmental media. The values of this study may be different from the actual situation, as the simulation value was higher than the measured value. This is because the hydrodynamic factors of estuary were ignored.
3.3 Interphase distribution
Overall, as the aquaculture area studied shrank (Fig. 1a), the concentrations of those antibiotics in each environmental phase showed a decreasing trend year by year. Among them, the aquaculture area in this region increased by 30% from 2017 to 2018, resulting in the increase of various antibiotic dosage. The predicted ambient concentrations of the six antibiotics (namely, oxytetracycline, tetracycline, ofloxacin, ofloxacin, ofloxacin, erythromycin and amoxicillin) remain extremely low (10− 23-10− 25 ng/L), because of their low volatility.
As shown in Fig. 3a, the concentration of oxytetracycline in freshwater was 0.14 ng/L in 2011 and decreased to 0.06 ng/L in 2019 (a 60% decline), while its concentration in seawater and groundwater was in the order of 10− 3 ng/L. The concentration of tetracycline in freshwater was higher than that of oxytetracycline, reaching 1.20 ng/L, and its lowest concentration was 0.50 ng/L during 2011–2019 (Fig. 3b). Tetracycline in the seawater phase was up to 0.02 ng/L due to the effects of migration, diffusion and advection. The degree of contamination in the groundwater phase equaled to that of oxytetracycline, because tetracycline itself has a high octanol-water partition coefficient Kow and it tends to be adsorbed by sediments or soils. The concentration of norfloxacin in freshwater, seawater and groundwater was higher than tetracycline antibiotics, and its distribution was more uniform than that of tetracycline antibiotics. The concentration range of norfloxacin in freshwater, seawater and groundwater was 0.10–0.21 ng/L, 0.03–0.06 ng/L and 0.02–0.06 ng/L, respectively (Fig. 3c). The pollution of norfloxacin in seawater and groundwater was more serious than tetracycline antibiotics, but the concentration in freshwater was much lower than tetracycline and was almost the same as that of oxytetracycline. The concentration change of ofloxacin was completely different from that of norfloxacin. From 2011 to 2019, the pollution level of ofloxacin did not change over time, that is, it did not decrease with the decrease of aquaculture. The concentration of ofloxacin in freshwater was maintained at a high level of 12.00 ng/L, and the concentration of ofloxacin in seawater was maintained at 0.28 ng/L, while that in groundwater was about 0.08 ng/L (Fig. 3d). The overall pollution degree was high. The reason is that both the hydrolysis degree and the degradation rate of ofloxacin are slow, and a certain amount of ofloxacin was imported into the upstream of the river. The concentration of erythromycin in various environmental phases continued to decrease. In 2011, the concentration of erythromycin in groundwater was as high as 16.00 ng/L, but it fell to about 3.00 ng/L in 2019, and the overall concentration decreased by 82.5%. Erythromycin was at a high concentration level in freshwater and seawater. During 2011–2019, its concentration in freshwater was 2.00–8.50 ng/L, and its concentration range in seawater was 2.00–7.00 ng/L (Fig. 3e). However, comparatively, erythromycin pollution was more serious, no matter it was in groundwater, fresh water or seawater. That is because wastewater treatment plants applied better techniques to remove tetracycline and quinolones from water, but they almost failed in addressing the erythromycin issue. (Xu et al., 2007). Amoxicillin concentration peaked at 8.00 ng/L in the freshwater phase, and decreased to 3.00 ng/L in 2019. Amoxicillin concentrations in the seawater and groundwater phases were 0.10–1.00 ng/L (Fig. 3f), and the pollution status was better than erythromycin. The result indicates that beta-lactam antibiotics had not been used on a large scale in the Yangtze River Estuary region. Similar results can be found in the concentration levels of amoxicillin antibiotics in Guangzhou, China (Golet et al., 2003).
As shown in Fig. 4, the pollution of these six antibiotics in soil and sediment was more serious, and the concentrations of tetracycline, norfloxacin, ofloxacin, erythromycin and amoxicillin were basically higher than 0.10 ng/g. The concentration of tetracycline in soil decreased from 14. 00 ng/g to 5. 00 ng/g, and the pollution status improved significantly. Nevertheless, there was still serious pollution. Because the molecules of tetracycline antibiotics contained more polar/ionic functional groups (Chopra and Roberts, 2001), the adsorption ability in soil was strong. The concentration of ofloxacin in sediments has been maintained at about 13. 00 ng/g, with no decrease in recent 10 years. It may be attributed to the higher concentration of quinolones antibiotics in sewage discharge; on the other hand, the adsorption coefficient (Kd) value of quinolones antibiotics is higher, and the quinolones is easy to transfer from the freshwater phase to the sediment phase (Tolls, 2001). Moreover, researchers (Yang et al., 2011) found that sediment adsorption was one of the most important ways to remove quinolones antibiotics from sewage. The molecular structure of quinolones contains positively charged nitrogen atoms or dimethylamino groups. Therefore, under the electrostatic attraction, quinolones antibiotics are easier to be adsorbed than other types of antibiotics in negatively charged sediments (Wang et al., 2010). Quinolones are not easy to be biodegraded, so it is assumed that adsorption removal may be a major approach to remove the quinolones in sewage. Quinolones enter the sediment through adsorption. If they have not been degraded, they will accumulate continuously, pollute the water or soil environment, and then pose threats to human health.
3.4 Sensitivity analysis
The sensitivity of the relevant parameters can determine the source of uncertainty of the model parameters and identify the importance of the parameters to the model. In this regard, a quantitative analysis of the sensitivity of many parameters was carried out and the parameters were calculated with the following formula:
$$\begin{array}{c}S=\frac{{X}_{1.1}-{X}_{0.9}}{0.2{X}_{1.0}}\left(18\right)\end{array}$$
The input of the X1.1 was increased to 110% of the original parameter value; the \({X}_{1.0}\) stands for the original value of the parameter; and the input of the X0.9 was reduced to 90% of the original parameter value; S represents the sensitivity coefficient.
After a series of analysis on sensitivity and calculation of the main model input parameters, we found that the two emissions were important parameters for the model, as they had exerted considerable influence on the concentration of six environmental phases, which might be attributable to the uncertainty of the model concentration (Fig. 5). The discharge in the soil phase affects that in the groundwater, soil and air phase. Meanwhile, the discharge of freshwater showed an impact on sediment, seawater and freshwater, which was caused by the migration tendency of antibiotics in the environmental phase. Emissions from freshwater had little effect on OFX and AMOX, and this is because that the regional emission volume were much smaller compared to the upstream advection input rate.
Different input parameters demonstrated huge influence on the concentration of each environmental phase. During the sensitivity analysis, we found that the temperature variation had little effect on the six antibiotics. In general, when there was no significant temperature change, the disturbance of the material properties turned to be weaker. The degradation rate (Km) and the soil-to-water runoff coefficient (Kl) were the main influential parameters. The migration, transformation and fate of the six antibiotics were all affected by these parameters. This also confirms from the model perspective that hydrodynamic conditions are important for the migration and redistribution of typical antibiotics (i.e., oxytetracycline, tetracycline, norfloxacin, ofloxacin, erythromycin and amoxicillin) in various environmental phases of the region.
3.5 Interphase distribution under tidal effect
The tidal action of the Estuary will bring a large number of salt-rich seawater into the area studied. Moreover, the freshwater brought by the upstream runoff changes the salinity distribution of the whole Estuary area. Therefore, when considering the model parameters, we first determined the intrusion length of salt water and the salinity at different intrusion lengths. Combined with the measured data (Table S13), the salinity values of water bodies at different distances from the Estuary are calculated (Fig. 6).
As shown in Fig. 7, after taking the tidal effect into consideration, the results showed that the measured concentrations of six antibiotics in various environmental media and the concentration difference in tidal phase were in a logarithmic unit, in addition to the fact that the logarithmic difference of oxytetracycline and tetracycline in sediment and groundwater environmental phases was slightly greater than 1. The model simulation was generally effective. All of simulated values of all antibiotics in tidal phase are closer to the measured values than in analog state, except amoxicillin in seawater and oxytetracycline in soil. In general, the simulation effect further increased after the introduction of tidal effect, especially the seawater and soil phases.
Concentration distribution of antibiotics in different phases of the Yangtze River Estuary changed greatly after tidal effect was introduced (Fig.8 and Fig.9). The concentration showed a consistently decreasing trend over the time period. Compared with the steady state stage, the occurrence of antibiotics in each phase of the tidal stage increased, and the concentration of some antibiotics rose as well (Fig.8a). As one of the main pollutant discharge phases, the overall concentration of seawater remained high in several phases. The concentration of oxytetracycline was 0.26 ng/L in 2011 and decreased to 0.13 ng/L in 2019. The concentration increased a lot compared with that in the steady state, and the concentration of oxytetracycline in groundwater increased to about 0.10 ng/L. The concentration of tetracycline in seawater phase also reached to 0.34 ng/L during 2011–2019 (Fig.8b), and the highest value reached 0.80 ng/L, which was an order of magnitude higher than that before, but less than the concentration of freshwater phase at the steady state. The pollution of groundwater environment in the tidal stage was also aggravated, and the highest pollution concentration reached to 0.03 ng/L. The concentration range of norfloxacin in seawater phase was 0.07–0.16 ng/L, and that in groundwater was 0.03–0.08 ng/L (Fig.8c). The pollution level in seawater and groundwater in tidal phase was higher compared with tetracycline antibiotics, which was different from that in the steady phase. At the same time, the concentration of seawater phase in tidal phase was also lower than that in the freshwater phase and the seawater phase in the steady phase, indicating that more norfloxacin lost its migration and transformation in the water phase in the tidal phase. Changes in ofloxacin concentrations continued to show the same trend during the period 2011–2019, in other words, not decreasing with the scale of aquaculture in the Yangtze River Estuary, and the ofloxacin concentrations in seawater remained high at 6.80 ng/L (Fig.8d). Erythromycin pollution was serious in both groundwater and seawater (Fig.8e). Amoxicillin pollution was milder than erythromycin pollution, but more serious than tetracycline antibiotics (Fig.8f). On the whole, the pollution in surface water was weakened, but the groundwater was aggravated, indicating that the tides aggravated the pollution of oxytetracycline in groundwater. The possible reason was that the tides increased the exchange between water bodies, causing more oxytetracycline entering into the groundwater.
As shown in Fig. 9, the concentrations of these six antibiotics in soil and sediment were still high under tide effect. The concentration of tetracycline in soil decreased from 14.00 ng/g to 5.00 ng/g, and the decreasing trend had not changed, which was consistent with that in the steady state. However, the concentration of ofloxacin in sediment fell to about 8.00 ng/g, a significant decrease. The concentrations of norfloxacin, ofloxacin, erythromycin and amoxicillin in soil and sediment were higher than 0.10 ng/g, and the content of oxytetracycline in sediment was lower than 10− 3 ng/g. It can be seen that tidal action exerted great influence on the occurrence of stable antibiotics, such as ofloxacin, in the environment.
3.6 Risk assessment
Ecological risks of typical antibiotics in freshwater and soil were evaluated at the Yangtze Estuary in 2019 (Fig.10a and 10b). Oxytetracycline, tetracycline and norfloxacin had low risks to the most sensitive aquatic microorganisms (algae) in the area studied, which were all lower than 0.1, indicating that these antibiotics were unlikely to cause adverse effects on algae. Erythromycin and ofloxacin extended moderate risk to algae, while amoxicillin showed high risk to algae.In soil, erythromycin and ofloxacin demonstrated high risks, and that was especially the case of erythromycin, as its RQ was close to 16, much larger than 1. The risk of norfloxacin was moderate, and tetracycline antibiotics were at a low risk level. Despite that, erythromycin and ofloxacin were two considerable threats to the environment, because the amount of the two antibiotics was large and they were difficult to be degraded. As shown in Fig.10c, the HQ of the five selected antibiotics was less than 1, a status that was not threatening to human health. That is to say, the water in the drinking water source had not affect human health. However, the fish resources in the estuary area were relatively concentrated, and the enrichment effect might be harmful to human health. Therefore, it can be concluded that more attention should be paid to the pollution caused by antibiotics.