4.1. Occurrence, Seasonal Variation
Compared with previous studies (Table 1
), the concentrations of SDZ and SQX are relatively higher than those in the Yangtze Estuary [22
], Huangpu River [21
], and Urban water [29
]. The concentration of SMM (n.d–163.2 ng/L) in this study is lower than those in the Huangpu River (ranged from 2.05–623.27 ng/L) [32
] and the Pearl River (ranged from n.d–1080 ng/L) [33
CFX and NFX were significantly higher than those in Yangtze Estuary (n.d–14.2 ng/L and n.d–2.27 ng/L, respectively) [15
] and Pearl River Estuary (n.d–34.2 ng/L and n.d–33.6 ng/L, respectively) [33
]. OFX in Qingcaosha Reservoir was much lower than the South Yellow Sea (n.d–497.6 ng/L) [30
] and Chaohu (n.d–182.7 ng/L) [34
]. It is reported that FQs showed relatively low persistence in water and had strong sorption to the solid phase [35
], so in many studies, the detection frequencies of FQs were relatively low, which is different to this study. Compared with other studies, the concentration of OFX was found in municipal sewage and animal wastewater in the area of the South Yellow Sea, so the concentration was relatively high, but the pollution level of OFX in Qingcaosha Reservoir was lower while NFX and CFX were higher than other water bodies.
PENG has been reported in ranges from n.d–250 ng/L [29
] compared to a range from n.d–289.9 ng/L in this study. In many types of research, β-lactams were thought not to be a concern as environmental pollutants due to the characteristic of fast hydrolysis. However, the result showed that β-lactams account for a large proportion of the total. This phenomenon elucidates that although these antibiotics are, generally, considered to degrade easily, a pseudo-persistence may be occurring as a result of the continuous discharge.
O is used not only in livestock as treatment and food additions, but also in the treatment of humans. Because of the strong sorption to sediments and high hydrophobicity of the MLs, the concentrations of the MLs in the aquatic environment were found to be very low. In previous investigations, TC and DC have also been detected in the Huangpu River [20
] and Yangtze Estuary [21
]. The detection frequencies of TCs in the Huangpu River were high; this might be due to the large usage and discharge in the river [20
]. Normally, TCs were seldom reported in the natural water due to their strong degradation ability as well as absorption to particles or soil. However, these antibiotics were found to be ubiquitous in rivers in Shanghai; this may reflect the large usage and discharge of the TCs in this area.
It can be summarized that tetracyclines and fluoroquinolones are widely used as both human and veterinary medicines to treat diseases or to promote growth in livestock [22
]. Because both human and livestock excreta, with metabolized or un-metabolized drugs, pass into sewage systems, the detection frequencies and concentrations of these antibiotics are high. SQX and TYL are only used in veterinary applications so they are less prevalent than other antibiotics in the aquatic environment.
The varying presence of antibiotics between four seasons may be due to the usage and prosperities of antibiotics, flow conditions and water temperature. It is worth noting that the concentrations of detected antibiotics in high flow and warm conditions were lower than those in low flow and cold conditions [20
]. From May to September, in order to prevent the eutrophication of water in the reservoir, the flow condition of Qingcaosha Reservoir was high, and from November to April, to restrain the invasion of the salt tide, the flow condition was low. The great dilution by the large flux of the Yangtze River in summer (normal above 50,000 m3
/s) led to the low concentration of antibiotics. Moreover, due to the higher microbial activity and stronger sunlight in summer, the bio-degradation and photo-degradation of antibiotics might be higher in summer than other seasons [37
]. Therefore, lower concentrations were observed in summer than in the other three seasons for most antibiotics.
It is worth noting that S4 was located in the middle of the reservoir and was close to the suburb area and S3 was located at the water pump station, which transported the water to the water plant. The above data shows that the contamination level in S3 was less serious than in the other three sampling sites. The total concentration of antibiotics at S4 was very high; this result might be explained by the settled particles releasing antibiotics into the water. Doretto indicated that settled particles with low organic carbon contents had high antibiotics desorption capacity [38
]. Sedimentation of the particles was remarkable in S4 while it was not significant downstream of the reservoir (S2/S3) (Figure A1
), suggesting that there was a high potential for surface water contamination. Furthermore, the key factor affecting the sedimentation was flow rate; the rank order of the flow rate was as follows: S1 (0.2 m/s) > S4 (0.01~0.19 m/s) > S3, S2 (0.01~0.03 m/s) [39
]. Particles were fully settled in S4 because of the relatively high velocity upstream of the reservoir. The suspended particles downstream of the reservoir were phytoplankton which would not release contaminants into the water. Qingcaosha Reservoir is located at the estuary of the Yangtze River and most domestic sewage effluents are now continuously discharged downstream of the Yangtze River since the completion of the wastewater control project. Yan claimed that wastewater treatment plants located in the upper reaches of the Yangtze River would be the primary reason for the higher concentration of antibiotics downstream of the Yangtze River [40
]. Furthermore, several drain outlets were found in the Yuxi River which was located downstream of the Yangtze River; effluents from the pond and village area were discharged into the estuary without any purification treatment. Therefore, the point source of the antibiotics also exists [34
Thus, although the tetracyclines had strong absorption to sediment, they were still detected with high frequency. The concentration of DC was found to be much higher than that of TC; the input contamination from upstream of the reservoir might be the main reason. The dominant contamination at S1 was PENV while at S2 and S4 it was PENG. This result might be due to the wide range of applications of PENG and PENV in clinical applications, considering their high effectiveness and low toxicity [29
4.2. Environmental Risk Assessment of Antibiotics
In this study, the potential ecological risks of antibiotics were assessed by using the risk quotients (RQs) approach, according to the European technical guidance document (TGD) on risk assessment. The value of RQ was defined as the ratio of the measured environmental concentration (MEC) and the predicted no-effect concentration (PNEC). The value of PNEC was assessed based on the toxicity data which were obtained from the Ecological Structure Activity Relationships (ECOSAR) and shown in Table A3
. In order to better distinguish the ecological risk levels, according to the individual RQ value, three risk levels were classified (0.01–0.1: low risk; 0.1–1: medium risk; >1: high risk) [22
The risk quotients (RQs) of antibiotics in the reservoir are shown in Figure 4
. According to the RQs, seven antibiotics (SDZ, SMM, SQX, PENG, TYL, OTC and POL) posed a low risk to the relevant sensitive aquatic organisms (S. capricornutum
, S. vacuolatus
, P. subcapitata
, M. aeruginosa
) in four seasons; four antibiotics (NFX, CFX, PENV, DC) caused high risk. The RQ values of NFX, CFX, PENV, and DC suggested that these antibiotics might present a significant risk to the algae in Qingcaosha Reservoir. Normally, RQs in winter, spring and autumn are remarkably higher than in summer. For example, OFX, ETM-H2
O and LIN caused high risk in winter, while they caused medium risk in summer, and the proportions of samples classified as high risk during the entire sampling period were 16.7%, 58.3%, 33.3%, respectively. However, LEX, may cause medium risk in the aquatic environment in April and low risk in other months.
Studies have demonstrated that the residual trace antibiotics in the aquatic environment may impose selective stress on the microbe communities and accelerate the spread of antibiotic resistance genes (ARGs) [42
]. ARGs such as sul I
, sul II
, tet (C)
, tet (G)
were the most prevalent resistance genes in raw water in Yangtze River Delta, and the absolute abundances of the sul
class genes ranged from 1010
]. In another drinking water source in Shanghai, 11 ARGs were detected with high concentrations, and sul II
was present at the highest concentration (4.19 × 108
copies/L). This phenomenon might reflect the widespread use of sulfonamides in this region [19
]. Furthermore, ARGs could be transferred between bacteria through transposons, plasmids and integrons [44
]. Xu observed that the abundances of ARGs were significantly correlated to the levels of mobile genetic elements, indicating that intI-1
and transposons may contribute to the abundances of ARGs in drinking water [45
]. Hence, the prevalence of the antibiotics in Qingcaosha Reservoir exhibited not only ecological risk in the water phase but also the risk of spread of the antibiotic resistance genes, which should be researched further in the future studies.