Benzene is a toxic contaminant and can harm many aquatic species and cause serious damages to the river eco-system, if released to rivers. In 2012, a major spill accident occurred on the Huaihe River in Eastern China with 3 tons of benzene released to the river section 70 km upstream of a natural reserve. Two emergency measures were taken to minimize the impact of the accident on the natural reserve: 1) flow control by adjusting upstream sluices to delay the arrival of the contaminant plume at the reserve and 2) in-situ treatment using activated carbons to reduce the contaminant concentration. Here we develop a process-based mathematical model to analyze the monitoring data collected shortly after the accident, and explore not only how effective the adopted measures were over the incident but more importantly the mechanisms and critical conditions underlying the effectiveness of these measures. The model can be used as a tool for designing optimal management responses to similar spill accidents in regulated river systems, combining flow control and in-situ treatment.

Seasonal lakes appear in floodplain systems due to the seasonal variation of water level of rivers or main lakes and play an important ecological function. Previous studies have focused on exchange flux between the surface water and groundwater in seasonal lakes, but the process of solute transport at the sediment‐water interface under the effect of seasonal water level fluctuations remains unclear. In this study, laboratory flume experiments and numerical simulations were conducted to investigate non‐reactive solute transport and exchange in a seasonal lake under effects of hydrological connectivity, water level amplitude, sediment permeability and solute source. The results show that the recharge pattern between seasonal lakes and rivers or main lakes is determined by the seasonal water level. The solute transports from the groundwater of seasonal lakes to the main lake during the dry season, while the reverse applies during the wet season. The perennial hydrological isolation increases the concentration in the overlying water and pore water of the seasonal lake by 20.5% and 28.7%, respectively. The solute concentration in the overlying water and pore water of the seasonal lake decreases by 55.2% and 51.5%, respectively, when solute source occurs in the main lake compared with that occurs in the seasonal lake. The amplification of water level amplitudes leads to an increase of 6.46% and 10.55% in the solute concentration drop from the overlying water, respectively. These findings contribute to improving water resource utilization and protecting aquatic ecological environment in floodplain systems.

Macrobenthic invertebrates are widely distributed on sediments, and the micro‐morphology such as burrows and sand mounds due to their bioturbation activities can cause substantial changes in hyporheic exchange (HE). This study explores the characteristics of HE induced by benthic burrows through flume experiments and numerical simulations, focusing on the influence of burrow depth, mound height, and inter‐burrow distance. Our findings reveal that burrows enhance HE by forming preferential flow conduits. With the increased interfacial pressure drop and the newly formed circulations, the pore water tends to outflow through burrow sidewalls along the preferential paths in greater velocity under the enhanced advection, and the “pumping” effects persist even below the burrow depth. The pore water flux increased by 10.76 times on the burrow sidewall at most. Eddies are induced by mounds above the streambed, changing the inflow/outflow patterns on the interface. The interfacial exchange rate below eddies is decreased while the solute can inflow within more distant ranges away from burrows. The effects of mound height carry out in the shallow streambed, and quickly decrease as penetration depth increases. Variations in inter‐burrow distance affect the transition process of HE from advection‐ to dispersion‐dominated. As the distance increases, the solute transport is promoted first due to the enhanced advection, and then decreased with the weakened advection and the larger proportion of the dispersion. This study provides important insights into the key mechanism of HE, highlighting the critical role of benthic burrows in aquatic ecosystems.