The arsenic concentration in groundwater varies significantly with time and space in groundwater‐surface water exchange zones. Various processes have been identified that control arsenic concentration distribution and mobility in laboratory systems. However, it is still challenging to identify important processes controlling arsenic concentration distribution at the field scale due to the complex coupling of hydrobiogeochemical processes. In this study, a reactive flow and transport model was used to identify dominant processes controlling arsenic distribution and seasonal variations in groundwater‐surface water exchange zones using Jianghan Plain, China, as an example. The results revealed the importance of river water and groundwater interactions on seasonal changes in arsenic concentration; however, the affected region is limited to within 50‐m distance to the river. The modeling results, unexpectedly, revealed the predominant importance of groundwater extraction to the seasonal variation in arsenic concentration. The groundwater extraction changed the groundwater flow pattern and induced vertical leakage of oxygen‐containing surface water into the aquifer, which triggered a series of biogeochemical reactions that changed groundwater redox conditions and promoted arsenic sorption, resulting in a rapid decrease in arsenic concentrations in groundwater. After groundwater extraction ceased, aquifer recovered to anoxic condition, promoting arsenic release from the sorbed phase, leading to a rapid rebounding in groundwater arsenic concentrations. Overall, this study provided a tool to identify coupled hydrobiogeochemical processes on arsenic spatiotemporal distribution and migration in groundwater.

It is a challenge to describe the hydrogeological characteristics of karst aquifers due to the complex structure with extremely high heterogeneity. As the response of karst aquifers to rainfall events, spring discharge variations after precipitation can be used to identify the internal structure of karst systems. In this study, responses of spring discharge to different kinds of precipitations are investigated by continuously monitoring precipitation and karst spring flow at a single-conduit karst aquifer in western Hunan province, China. Recession curves were used to analyze hydrodynamic behaviors and separate recession stages. The results show that the shape of the recession curve was changed under different rainfall conditions. Recession processes can be divided in to three recession stages under heavy rain conditions due to water drainage mainly from conduits, fracture, and matrix at each stage, but only one recession stage representing drainage mainly from matrix in the case of light rain. With the change in amount and intensity of precipitation, the calculated recession coefficient at each stage changes in an order of magnitude. The influence of precipitation on the recharge coefficient and the discharge composition at each recession are discussed, and then the conceptual model diagram of water filling and releasing in the single-conduit karst aquifers is concluded. The findings provide more insight understand on hydraulic behaviors of karst spring under different types of rainfall events and provide support for water resource management in karst regions.

This study provides experimental evidence of Forchheimer flow and the transition between different flow regimes from the perspective of the pore size of permeable stone. We first carry out seepage experiments on four kinds of permeable stones with mesh sizes of 24, 46, 60 and 80, corresponding to mean particle sizes (50 % by weight) of 0.71, 0.36, 0.25 and 0.18 mm, respectively. The seepage experiments show that an obvious deviation from Darcy flow regime is visible. In addition, the critical specific discharge corresponding to the transition between flow regimes (from pre-Darcy to post-Darcy) increases with increasing particle size. When the “pseudo” hydraulic conductivity (K, which is computed as the ratio of the specific discharge q and the hydraulic gradient) increases with increasing q, the flow regime is denoted pre-Darcy flow. After q increases to a certain value, the pseudo hydraulic conductivity begins to decrease; this regime is called post-Darcy flow. In addition, we use the mercury injection technique to measure the pore size distributions of four permeable stones with different particle sizes. The mercury injection curve is divided into three stages. The beginning and end segments of the mercury injection curve are very gentle, with relatively small slopes, while the intermediate mercury injection curve is steep, indicating that the pore size in permeable stones is relatively uniform. The porosity decreases as the mean particle sizes increases. The mean pore faithfully reflects the influences of the particle diameter, sorting degree and arrangement mode of the porous medium on seepage parameters. This study shows that the size of pores is an essential factor for determining the flow regime. In addition, the Forchheimer coefficients are discussed. The coefficient A (which is related to the linear term of the Forchheimer equation) is linearly related to $\mathrm{1}/d^{\mathrm{2}}$ : $A=\mathrm{0.0025}\left(\mathrm{1}/d^{\mathrm{2}}\right)+\mathrm{0.003}$ . The coefficient B (which is related to the quadratic term of the Forchheimer equation) is a quadratic function of $\mathrm{1}/d$ : $B=\mathrm{1.14}\times {\mathrm{10}}^{-\mathrm{6}}{\left(\mathrm{1}/d\right)}^{\mathrm{2}}-\mathrm{1.26}\times {\mathrm{10}}^{-\mathrm{6}}\left(\mathrm{1}/d\right)$ . The porosity (n) can be used to reveal the effects of the sorting degree and arrangement on the seepage coefficients. A larger porosity leads to smaller coefficients A and B for the same particle size.