Kun Huang

Karst conduits play an important role in the process of groundwater and material transport, in which pollutants can move quickly and pollute groundwater easily. Considering the different shapes of karst conduits, hydrodynamic conditions and other factors, eddy generation is very common. After the solute is trapped in the eddy zone, which usually results in the tailing of solutes, typically referred to as non‐Fickian migration or anomalous transport. Many solute transport models have been proposed to explain this phenomenon reasonably and prepare to capture the solute breakthrough curve (BTC), although the model parameters lack a clear physical background. In this study, a series of different types of rough conduits were designed to quantitatively evaluate the influence of eddies on solute transport. A new method for the quantitative identification of the eddy zone is proposed. The quantitative relationship between different flow velocities, roughness shape, relative roughness and the eddy area proportion is summarized. A new model of solute transport under the influence of eddies was proposed based on the traditional mobile‐immobile model, and the physical meaning of the model parameters was clarified. The ability of the new model to capture the breakthrough curve is satisfactory, which can effectively serve the engineered settings.

This study provides experimental evidence of Forchheimer flow and transition between different flow regimes from the perspective of pore size of permeable stone. We have firstly carried out the seepage experiments of permeable stones with four different mesh sizes, including 24 mesh size, 46 mesh size, 60 mesh size, and 80 mesh size, which corresponding to mean particle sizes (50 % by weight) of 0.71 mm, 0.36 mm, 0.25 mm, and 0.18 mm. The seepage experiments show that obvious deviation from Darcian flow regime is visible. In addition, the critical specific discharge corresponding to the transition of flow regimes (from pre-Darcian to post-Darcian) increases with the increase of particle sizes. When the “pseudo” hydraulic conductivity (K) (which is computed by the ratio of specific discharge and the hydraulic gradient) increases with the increase of specific discharge (q), the flow regime is denoted as the pre-Darcian flow. After the specific discharge increases to a certain value, the “pseudo” hydraulic conductivity begins to decrease, and this regime is called the post-Darcian flow. In addition, we use the mercury injection experiment to measure the pore size distribution of four permeable stones with different particle sizes, and 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, and the mean pore size can faithfully reflect the influence of particle diameter, sorting degree and arrangement mode of porous medium on seepage parameters. This study shows that the size of pores is an essential factor for determining the flow regimes. In addition, the Forchheimer coefficients are also discussed in which the coefficient A (which is related to the linear term of the Forchheimer equation) is linearly related to 1/d ² as A = 0.0025 (1/d ²) + 0.003; while the coefficient B (which is related to the quadratic term of the Forchheimer equation) is a quadratic function of 1/d as B =1.14E-06 (1/d)² − 1.26E-06 (1/d). The porosity (n) can be used to reveal the effect of sorting degree and arrangement on seepage coefficient. The larger porosity leads to smaller coefficients A and B under the condition of the same particle size.

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.

Based on the integrated ParFlow.CLM modeling in the Little Washita basin located in the southwestern Oklahoma in the U.S., the long-term effects of groundwater (GW) pumping on ground surface temperature (GST) are studied. Conclusions are obtained based on the simulation results as follows. The subsurface can be conceptualized as a buffer on variations of GST while GW pumping can weaken this buffer causing hotter summer and colder winter. In the long-term pumping, the variations of GST (ΔGST) present nonlinear warming trend in average by rapidly increasing in the beginning and gradually reaching dynamic equilibrium. This is due to the attainment of a new equilibrium of the GW flow system with a sustainable pumping rate. Whereas, for unsustainable pumping, it is mainly attributed to the fact that the water table depth finally becomes lower than the critical depth range (1–10 m). Different coupling depths between ParFlow and CLM are tested in the long-term pumping scenarios since it determines the heat capacity of the subsurface buffer which is important on regulating GST. The buffer with deeper coupling depth is more effective on damping the nonlinearity and the amplitude of ΔGST. In addition, the effects of coupling depth on GST are more prominent when pumping occurs due to the degenerated thermal properties. In other words, the time-scale for GST to response the different coupling depth is greatly shortened under pumping in contrast to natural state. This study diagnosed the subsurface buffer on variations of GST with the role of coupling depth in the integrated modeling under long-term GW pumping. The results are expected to have implications on integrated modeling in GW depletion areas worldwide.