Groundwater flow systems and stagnant zones in drainage basins are critical to a series of geologic processes. Unfortunately, the difficulty of mapping flow system boundaries and no field example of detected stagnant zones restrict the application of the concept of nested flow systems. By assuming the variation in bulk resistivity of an aquifer with uniform porosity is mainly caused by groundwater salinity, the magnetotelluric technique is used to obtain the apparent resistivity of a profile across a groundwater-fed river in the Ordos Plateau, China. Based on the variations in apparent resistivity of the Cretaceous sandstone aquifer, the basin-bottom hydraulic trap below the river has been detected for the first time, and its size is found to be large enough for possible deposition of large ore bodies. The boundaries between local and regional flows have also been identified, which would be useful for groundwater exploration and calibration of large-scale groundwater models.

Hydraulic connections between a river and an adjacent aquifer are controlled by the river resistance and aquifer diffusivity. In this paper, we derive a spectral solution linking the power spectrum of river stage fluctuations to that of the hydraulic head of a confined aquifer by means of a physical scaling factor. The physical scaling factor represents an algebraic expression of the river resistance and aquifer diffusivity and is included in an exact spectral solution derived herein. Statistical measures of the aquifer diffusivity and river resistance are provided by fitting the solution versus observed groundwater hydraulic head obtained at several distances and/or frequencies. At a study site in the middle reach of the Yangtze River and downstream of the Three Gorges Dam in China, we find systematic damping of the hydraulic head variations with distance from the river, which follows a fractal pattern driven by the river stage. In general, the estimated parameters are consistent with results reported in the literature, which supports the validity of the proposed spectral approach, although the paper discusses advantages and limitations due to application conditions.

Although it has been increasingly acknowledged that groundwater flow pattern is complicated in the three-dimensional (3-D) domain, two-dimensional (2-D) water table-induced flow models are still widely used to delineate basin-scale groundwater circulation. However, the validity of 2-D cross-sectional flow field induced by water table has been seldom examined. Here, we derive the analytical solution of 3-D water table-induced hydraulic head in a Tóthian basin and then examine the validity of 2-D cross-sectional models by comparing the flow fields of selected cross sections calculated by the 2-D cross-sectional model with those by the 3-D model, which represents the "true" cases. For cross sections in the recharge or discharge area of the 3-D basin, even if head difference is not significant, the 2-D cross-sectional models result in flow patterns absolutely different from the true ones. For the cross section following the principal direction of groundwater flow, although 2-D cross-sectional models would overestimate the penetrating depth of local flow systems and underestimate the recharge/discharge flux, the flow pattern from the cross-sectional model is similar to the true one and could be close enough to the true one by adjusting the decay exponent and anisotropy ratio of permeability. Consequently, to determine whether a 2-D cross-sectional model is applicable, a comparison of hydraulic head difference between 2-D and 3-D solutions is not enough. Instead, the similarity of flow pattern should be considered to determine whether a cross-sectional model is applicable. This study improves understanding of groundwater flow induced by more natural water table undulations in the 3-D domain and the limitations of 2-D models accounting for cross-sectional water table undulation only.