Aquaculture is the farming of aquatic organisms by individuals or groups with the goal of enhancing their production. Rapid population growth in the 21st century requires a sharp increase in seafood production [1
]. Seafood provides a good source of high-quality protein and forms an important cash crop in many parts of the world. However, the majority of aquaculture production relies on the use of antibiotics and input of other artificial substances to treat or prevent disease and to accelerate growth. Therefore, studies on antibiotic residues in the aquatic environment as well as in fish products are numerous [2
]. These residues could increase the occurrence of antibiotic-resistant bacteria in the aquatic environment, increase antibiotic resistance in fish pathogens, lead to variations of the bacterial flora both in sediments and water systems, and transfer these resistance determinants to human pathogens and bacteria of terrestrial animals [5
Approximately 70 classes of antibiotics have been reported in the aquatic environment [6
]. Among these, sulfonamides (SA) are the most widely used in aquaculture cultivation [2
]. Furthermore, SA have been detected in the muscles of fish species through bioaccumulation according to recent research [7
]. Given sufficient time, SA can accumulate in the body and adversely impact human health. Moreover, bacteria will become resistant to SA, which tends to contribute to multidrug and cross resistance. It is difficult to predict future negative effects caused by the potential environmental and health risks. Therefore, the fate of SA in the aquatic environment, as a basis of the current problem, plays an important role in relevant research fields.
With regard to the investigation of antibiotic occurrence and transport, many scholars focus on empirical methods. The most popular method is liquid chromatography tandem mass spectrometry (LC-MS/MS) [8
]. However, most empirical methods focus on the occurrence, not the transport process, of antibiotics. With respect to the numerical method, there are many mathematical models for pollutant transport in the aquatic environment [11
], while very few attempts to simulate the transport of antibiotics have been made [13
]. Therefore, a promising numerical method, the lattice Boltzmann method (LBM), is used in this study to address this issue. The LBM was developed during the last few decades. It evolved directly from the Lattice Gas Automata (LGA) introduced by Frisch [14
]. The LBM is recommended as a mesoscopic method due to its nature. It can describe particle microscopic behaviors with a simpler form and accurately reflect flow movements at a macroscopic level compared with traditional methods. Consequently, it has been extensively used by researchers for the simulation of fluid flows [15
]. This study is the first to study the transport of antibiotics using the LBM.
In this paper, the shallow-water equations and depth-averaged advection–diffusion (AD) equation are solved using the LBM, and then are used to simulate the transport and occurrence of SA in Laizhou Bay, a highly polluted sea area of China. The results of this study can be used to aid the understanding the environmental risks of antibiotic mixture exposures to aquatic species and human health in the future.
5. Results and Discussion
shows that the SMX and SMZ concentrations change over time at the three selected points. SMX is the dominant SA compared to SMZ and poses a high risk to aquatic organisms. Out of all the rivers, Xiaoqing River and Guangli River are the two most contaminated; therefore, the SA concentrations at P1 and P2 are higher than that at P3. The transport process also indicates increasing pollution of the aquatic environment without control of antibiotic abuse.
Comparison between simulation results and experimental data [8
] with different SA and locations after 42 h are shown in Table 2
. The results illustrate that the proposed model is sufficiently accurate to obtain the occurrence of SA. The results also emphasize that the concentrations of SA are high in the near-shore area—especially for SMX at P2, which is comparatively high in the marine area at 80 ng/L.
In Table 2
, the simulation value of SMZ at P3 is available with 0.087 ng/L compared to the undetected experimental measurement. Owing to limitations of instrument precision, the results are deficient at small magnitudes. Therefore, the numerical method, the lattice Boltzmann method is recommended for this work owing to its simplicity and accuracy. Less than 5 h are required to simulate 42 h of real transport, highlighting the efficiency of the model. In this case, the flows have Re < 250, belonging to the laminar flow, a range within which the LBM has very good stability. Furthermore, complex boundary conditions can be processed well, and parallel operations can be conducted.
The occurrence of SMX and SMZ has been plotted in Figure 5
to show the spatial distribution of SA after 30 days. On the last day, the pollution covers the whole bay, and SA concentrations > 0.005 g/L are dominant.
Due to the high density of human activities and the many rivers flowing into the bay, the high SA concentrations in the model show the dominant impact of riverine inputs on the marine environment. This finding coincides with previous studies of the sources of antibiotics in marine systems [8
]. Most estuaries are located in the northwest of Laizhou Bay, leading to higher pollution levels in this area. The bay is semi-enclosed with poor water exchange, resulting in high-level environmental risk. Moreover, the results for the selected points are lower than those in most river estuaries, which may result from seawater dilution where rivers discharge into the bay [23
This paper proposes a two-dimensional model, coupling hydrodynamic and water quality models. The LBM is used to discretize the model and the bed shear stress is also considered in this study. Moreover, the coefficients for the diffusion and relaxation times are calibrated, and the coupled model is applied to simulate the transport and occurrence of SA, including SMX and SMZ in Laizhou Bay. The numerical results are in good agreement with the experimental data, showing that the proposed model is able to predict the transport of sulfonamides accurately. It is reasonable to expect that the proposed model, as an efficient tool, has the potential to be used to investigate antibiotic transport problems in complex environmental conditions.