Occurrence of Antibiotics in Water in Xiaodian Sewage Irrigation Area, Northern China

The Xiaodian sewage irrigation area in northern China has a long history of sewage irrigation. The surface water and groundwater samples were collected and analyzed in this region. All the 25 kinds of antibiotics have been detected in the surface water in this study area. The order of average detection frequency is Macrolides (MLs, 98.2%) > Fluoroquinolones (FQs, 91.3%) > Tetracyclines (TCs, 87.5%) > Sulfonamides (SMs, 75.0%). The sample with the highest concentration of antibiotics (730.19 ng/L) is located at Taiyu Drainage Canal while that with the lowest concentration (245.93 ng/L) at Xiaohe River. The type and concentration of antibiotics in SW-3 and SW-4 were almost the same, indicating the measures to prevent canal banks from seepage were able to retard their transport into the soil and groundwater system via lateral recharge. Along the Beizhang Drainage Canal, the sewage from the swine farm, the cow farm, the irrigation field and domestic usage were all discharged into the canal, so the concentration kept increasing. Along the down gradient direction of Taiyu Drainage Canal, the concentration decreased. Transport of the contaminants via lateral recharge due to lack of seepage prevention measures, therefore, poses a potential threat to the soil-groundwater system. 15 of 25 kinds of antibiotics were detected in the groundwater. The detection frequency was lower than other study areas in China. The concentrations ranged from 2.75 ng/L to 114.38 ng/L with the average concentration of 27.60 ng/L.


Introduction
Because of lack of water resources, sewage irrigation has had a long history in Taiyuan city [1] and this sewage irrigation area has became one of the major sewage irrigation areas in the north of China. By 1995, the city's total irrigation area was 834,000 mu while effective irrigation area was 812,000 mu [2], the annual irrigation requirement was 178.2 million m 3 . Under the situation of water shortage, it is difficult to ensure that all of them irrigated by clean water. At present, More than 200 million m 3 of industrial sewage and domestic sewage from the urban area is mostly discharged into Fenhe River. With the increase of sewage irrigation frequency and quantity of the sewage irrigation, environment quality of the surface water and groundwater were affected in some regions. The environmental conditions in Taiyuan City sewage irrigation area were studied by some researchers. However, the report for organic pollution of sewage irrigation area was much less. Zhang et al. studied the influence of long-term sewage irrigation to the distribution of organochlorine pesticides in water and soil [3]. Li et al. reported the distribution and sources of polycyclic aromatic hydrocarbons in the topsoil of the sewage irrigation area [1]. Liao researched the adsorption behavior of nonylphenol in sewage irrigation area [4]. However, there has been no systematic study on the environmental geochemical behavior of antibiotics in sewage irrigation area [5]. The objectives of this study are systematically study about the environmental geochemistry behavior of antibiotics in the surface water and groundwater, analyse the distribution, source and migration of these organic contaminates in the environment.

Site description
Because of lack of water resources, sewage irrigation has had a history of nearly 40 years in Taiyuan city, and this sewage irrigation area has became one of the major sewage irrigation areas in the north of China. By 1995, the city's total irrigation area was 834,000 mu while effective irrigation area was 812,000 mu, the annual irrigation requirement was 178.2 million m 3 . Under the situation of water shortage, it is difficult to ensure that all of them irrigated by clean water. At the same time, low cost is another major reason promoting the development of sewage irrigation. At present, More than 200 million m 3 of industrial sewage and domestic sewage from the urban area is mostly discharged into Fenhe River. According to statistics, in the "Eighth Five Year Planning" period (1991-1995), Taiyuan sewage irrigation area was 394,500 mu, accounting for 47.3% of the total irrigation area; the annual irrigation water capacity was 95.85 million m 3 , accounting for 53.7% of total agricultural irrigation water.

Sampling
A summary of 14 surface water samples and 9 groundwater samples were collected in the study area. Fig. 1 shows locations of surface water samples and groundwater samples. Surface waters included Fenhe River water, Xiaohe River water, water from East Main Canal and water from drainage canals. Groundwaters were all collected from wells with depth between 15 m and 35 m after 10 minutes' pumping to get fresh samples, and then stored in 4 L amber glass bottles at 4℃ and treated as soon as possible. The samples were extracted using the methods of preview studies[6]- [7]. The antibiotics were analyzed by high-performance liquid chromatography -electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS), using the MRM mode. Simetone was used as the internal standard compound. The concentrations were calculated based on the internal calibration curves.  [8]. Comparing with the detected ratio and attenuation rate coefficients, it could be indicated that there are lots of TCs enter into the surface water. The highest concentration of antibiotics (730.19 ng/L) occurs at SW-6, this is because SW-6 is located at the intersection of Beizhang Drainage Canal and Taiyu Drainage Canal, the antibiotics of both the canals integrate to this site. The site SW-11 has the lowest concentration (245.93 ng/L). FQs is the main contaminant in the East Main Canal. TCs, FQs and MLs is the main contaminant in the beginning, middle and end of the Beizhang Drainage Canal, respectively. MLs is the main contaminant in the Taiyu Drainage Canal except for the beginning site SW-8, which indicates the concentration of antibiotics in Taiyu Drainage Canal has been strongly affected by the sewage of Beizhang Drainage Canal after they intersected. FQs and MLs is respectively the main contaminant in Xiaohe River and Fenhe River. Summarily, the concentration of antibiotics in surface water shows FQs and MLs are frequently used at the Xiaodian irrigation area, which corresponds to the order of the detected ratio. Fig. 2 shows the type and concentration of antibiotics in SW-3 and SW-4 were almost the same, indicating that measures to prevent canal banks from seepage were able to retard their transport into the soil and groundwater system via lateral recharge, this is probably the reason why there is a slight increased along the direction of East Main Canal. The site SW-18 has a high concentration of antibiotics, this is because of this site close to the sewage treatment plant. The irrigation water flow the irrigation field from the East Main Canal to Beizhang Drainage Canal, the water quantity has been increased in the Beizhang Drainage Canal, so the concentration was decreased from SW-18 to SW-14. Along the Beizhang Drainage Canal, the sewage which came from the swine farm, the cow farm, the irrigation field and domestic usage were all discharged to the canal, so the concentration kept increasing. Compare with E zone and B zone, we could deduce that the compound NOR is more likely to be adsorbed in the soil or groundwater in the irrigation process. After the intersection of Beizhang Drainage Canal and Taiyu Drainage Canal, the concentration reached the peak at site SW-6. Along the down gradient of Taiyu Drainage Canal, the concentration decreased, transport of contaminants via lateral recharge due to lack of the seepage prevention measures therefore pose a potential threat to the soil-groundwater system.

Antibiotics in the groundwater
As Fig. 3 shows, 15 of 25 kinds of antibiotics were detected in the groundwater. The detection frequency was lower than other study areas in China [7]. The concentrations ranged from 2.75 ng/L to 114.38 ng/L with the average concentration of 27.60 ng/L, which is much lower than the surface water. However, the concentration is higher than some other study areas [9]. MLs are detected at 22.2% of the groundwater samples with the range from below the limit of quantification (BLQ) to 44.51 ng/L. DOC and CTC have the highest detection frequency, 88.9% and 44.4%, respectively, and with the range from BLQ to 22.06 ng/L and BLQ to 15.06 ng/L, respectively. TCs is also the main contaminant which is 100% detected with the concentration of 1.90 ng/L to 35.06 ng/L. Only 3 of 9 FQs has been detected with the concentration between BLQ to 36.30 ng/L. 7 of 8 SMs were found in the groundwater, their highest concentration was not greater than 1.60 ng/L. MLs were only detected at sites GW-19 and GW-27.

Conclusions
The antibiotics are ubiquitous in the soil and water environment. All the 25 kinds of antibiotics have been detected in the surface water in this study area. The order of average detection frequency is MLs > FQs > TCs > SMs. The concentration of antibiotics in drainage canals is higher than rivers. 15 of 25 kinds of antibiotics were detected in the groundwater. The detection frequency was lower than other study areas in China. The concentrations ranged from 2.75 ng/L to 114.38 ng/L with the average concentration of 27.60 ng/L. MLs and FQs were the main antibiotics detected in the shallow vadose zone. 22 and 20 kinds of 25 antibiotics were detected respectively in the sediment core of sewage irrigation area and groundwater irrigation area. The order of concentration of 4 categories of antibiotics in both cores was: FQs > TCs > SMs > MLs. The antibiotics in the environment mainly came from the sewage which from the sewage treatment plants and livestock breeding farms which along the drainage canals. Transport of the contaminants via lateral recharge due to lack of seepage prevention measures, therefore, poses a potential threat to the soil-groundwater system. The measures to prevent canal banks from seepage were able to retard antibiotics transport into the soil and groundwater system via by lateral recharge.