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This study introduces a novel heat tracing method for estimating lateral groundwater flow velocity induced and sustained by heavy rainfall events in lowland areas, leveraging the distinct temperature difference between rainfall and groundwater. The method is motivated by the observation that the rainfall‐induced groundwater temperature signal dissipates along the flow path. To explain the observed temperature anomaly and then estimate the lateral flow velocity, we develop a semi‐analytical model for heat transport in the aquifer, accounting for conduction losses to adjacent layers. Our findings reveal that interactions between the aquifer, vadose zone, and bedrock significantly influence the temperature signal, thereby affecting velocity estimation. Inaccuracies in measured aquifer properties, such as thickness, porosity, and thermal conductivity of surrounding layers, increase the uncertainty of velocity estimates. However, variations in aquifer thermal conductivity have a minimal effect on the method's overall accuracy. When estimating multiple parameters, velocity estimates tend to be less reliable, especially if aquifer porosity remains uncertain. This is due to the challenges of simultaneously inverting both velocity and porosity. Overall, this work underscores the potential of using heat as a tracer for assessing lateral groundwater flow following rainfall, offering a practical, low‐cost solution applicable in a wide range of settings. Plain Language Summary: This study presents a new method of estimating the velocity of groundwater flowing in lowland hillslope areas, particularly during heavy rainfall. The method uses the difference in temperature between rainwater and groundwater to track the groundwater's flow. By analyzing observed temperature changes based on a semi‐analytical approach, we can estimate how fast the groundwater moves. Our findings reveal that interactions between the shallow aquifer, unsaturated soil zone, and bedrock significantly influence the temperature signal, impacting velocity estimation. Inaccuracies in measured aquifer properties, such as thickness, porosity, and surrounding layers' thermal conductivity, can adversely impact velocity estimations. Nevertheless, thermal conductivity of the aquifer layer itself doesn't noticeably affect the accuracy of this method. It is challenging in getting accurate velocity estimations when trying to estimate multiple parameters at once, especially when the aquifer porosity remains uncertain, due to the difficulty in the simultaneous determination of both velocity and porosity. Overall, this study highlights that using heat as a tracer can be an effective, affordable, and widely useable method for studying lateral groundwater flow after rainfall. Key Points: Significant temperature disturbances were observed in riparian shallow groundwater following heavy rainfallA heat tracer method was developed to estimate lateral groundwater velocity based on the attenuation of the heat signalThe accuracy of the estimated velocity depends on prior knowledge of hydrogeologic conditions [ABSTRACT FROM AUTHOR]

Macropores created by crab burrowing are commonly found in vegetated coastal wetlands. However, their impact on surface water‐groundwater interactions and salt transport is insufficiently understood. In this study, we used numerical models to quantify the impact of crab burrows with random spatial distribution, morphology, and openings on groundwater flow and salt transport in creek‐marsh systems. Results showed that these burrows can lead to a more complex network of preferential flow paths and burrow flushing capabilities than those with a single pipeline. The velocity magnitude varied over seven orders in the burrowed mud layer. Local increases in the hydraulic gradient occurred in the burrows, leading to faster water circulation and salt transport as well as turnover of soil aeration. The salinity front in the burrowed marsh reached two‐m deeper than in the unburrowed marsh. Burrow flushing depth and intensity is significantly enhanced in burrows with multiple openings. This study emphasized the complexity of tortuous burrows regulating groundwater flow and solute transport through modeling realistic crab burrows. This work supports fiddler crabs as ubiquitous "ecoengineers" who can produce more significant impact on local hydrological cycle and ecosystem‐scale geochemical processes than previously indicated. Plain Language Summary: Fiddler crabs are commonly regarded as "ecoengineers" for salt marshes, as their burrowing activity has great impact on ecological and environmental processes. This study provides innovative approaches for numerical simulations that can quantitatively evaluate the potential impact of crab burrows on groundwater flow and salt transport by considering burrow morphology. Our results showed that crab burrows can form complex preferential flow path networks, which in turn accelerates groundwater and salt circulation and enhances porewater exchange between burrows and the sediment matrix. Further, the burrow flushing depth and intensity were primarily controlled by the morphology (burrow shape, branch, and entrance) of crab burrow. For burrows with only one entrance, local circulation developed, limiting the burrow flushing to a relatively shallow depth and delaying the downward migration of salt. For burrows with multiple entrances, a more complex network of preferential flow paths will form, enhancing the bidirectional exchange between surface water and groundwater through the burrows. This study highlights the importance of biological disturbance on hydrology, which affects behavior and function of salt marshes. Key Points: Local circulation within burrows enhances porewater exchange between crab burrows and the sediment matrixCrab burrows facilitate soil aeration and alter flow paths/rate depending on burrow morphologiesCrab burrows have the potential to enhance terrigenous groundwater discharge in the creek‐marsh system [ABSTRACT FROM AUTHOR]

• We solve low-velocity non-Darcian flow to a confined and partially penetrating well in a leaky aquifer system. • An analysis is conducted to examine the temporal and spatial changes in confined groundwater head differences between Darcian and non-Darcian flow conditions. • Non-Darcian flow in the aquitard needs to be considered under small confined aquifer hydraulic conductivity and large threshold pressure gradient of the aquitard. Previous studies on the flow dynamics of leaky aquifer systems have primarily relied on Darcy's law, assuming fully penetrating wells in confined aquifers. However, when dealing with aquitards dominated by clay, the flow behavior often deviates from Darcy's law. Moreover, in the majority of cases, this entails partially penetrating well pumping. This paper introduces a mathematical model for non-Darcian flow in a confined and partially penetrating well system within a leaky aquifer system. The model is based on the low-velocity non-Darcian flow equation, incorporating the threshold pressure gradient. In the confined aquifer, the flow exhibits two-dimensional Darcian flow behavior, while in the aquitard, the flow is characterized by one-dimensional non-Darcian flow in the vertical direction. In the shallow aquifer, the flow is one-dimensional Darcian flow in the radial direction. The mathematical model is solved using the finite difference method, and the obtained results are compared with calculations based on traditional Darcian flow theory. The findings indicate that the confined groundwater head difference, as predicted by the Darcian and non-Darcian flow theories, diminishes radially as the distance from the pumping well increases and vertically as one moves away from the top of the confined aquifer. As pumping progresses, the confined groundwater head difference initially rises and subsequently stabilizes gradually. Moreover, the magnitude of the confined groundwater head difference is more pronounced when the threshold pressure gradient, vertical hydraulic conductivity of the aquitard, pumping rate, hydraulic conductivity and specific yield of shallow aquifer are larger, or when the hydraulic conductivity and specific storage of the confined aquifer are smaller. The effect of well screen length on the confined groundwater head difference is minimal. [ABSTRACT FROM AUTHOR]