Water level fluctuation induced variation of sulfamethazine biogeochemical behavior and expression of resistance genes in the hyporheic zone: A case study of Chaobai river, China

It is well known that the intensive biogeochemical activity in hyporheic zone of groundwater and antibiotic contaminated river water induced the pollution diffusion and variation of biogeochemical transformation. However, the complex interaction process caused by water level fluctuation is difficult to accurately depict on a larger, catchment scale. Therefore, a loosely coupled HYDRUS-3D/GMS was established in Chaobai river basin to simulate the sulfamethazine (SMZ) pollution interaction process caused by river and groundwater level fluctuation. The upward fluctuation of river water level increased the migration of SMZ from surface water to the hyporheic zone soil and groundwater with the rate of 3.04 m/(y·m), which accelerated the pollution diffusion. Moreover, the rise of river water level reduced DO and ORP in hyporheic zone, which induced the biochemical transformation of SMZ from aerobic to anoxic and anaerobic, thus reduced 25% decomposition rate and increasing 7 times expression of resistance genes. The expression of ARGs sul1, sul2, sul3 were positively correlated with the SMZ concentration with coefficient of 0.9954, 0.9856, 0.9689. The biogeochemical behavior of SMZ was completely opposite when the river water level fell back in the dry season compared with that of water level rising, which contributed to the decomposition of accumulative antibiotics in the hyporheic zone. However, the upward fluctuation of groundwater table led to the secondary release of 20% SMZ accumulated in hyporheic zone soil and the reverse interaction of river water pollution, which induced increase in the abundance of resistant bacteria, thereby enhancing the expression intensity of ARGs. In addition, the accumulation and diffusion of SMZ were also closely related to soil physicochemical properties (P<0.01 to BET; P<0.001 to TOC) and microbial community structure during the interaction between groundwater and surface water in hyporheic zone. The accumulation of SMZ increased the ecological risk and induced the variation of microbial community structure and relative abundance. The enrichment of these antibiotic degrading genera, Hyphomicrobium, Thermomonas and Comamondaceae, improve the degradation of antibiotics and enhanced the expression of resistance genes sul1 and sul2, which increased the risk of drug resistance of superbacteria. This study provided a new approach to predict the variation of SMZ biogeochemical behavior and expression of resistance genes in the hyporheic zone due to water level fluctuation, which can effectively improve removal technologies and the drinking safety of groundwater.