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Groundwater sampled at the outlets of uncased flowing wells in a thick unconfined aquifer, which has undergone mixing, has been found to have hydrochemistry similar to deep groundwater in discharge areas. To identify the hydrodynamic causes, transient models of groundwater flow and age in a three-dimensional homogeneous unit basin with flowing wells are constructed to obtain flow rates in wells and groundwater mean age around wells. Inflow of groundwater to the well in the deep part leads to mixing of groundwater from different sources, and the finally mixed groundwater is found to have the same age as groundwater in the aquifer at a specific depth, termed the equivalent position (EP). EP is always found in the lower half of the flowing well, indicating that a mixed sample at the outlet could represent deep groundwater. Outflow from the well to the unconfined aquifer in the shallow part results in aging of groundwater around the well. For fully penetrating flowing wells in confined aquifers, EP is found in the upper half of the aquifer. The different relative depths of EP to the screen interval in the two types of flowing wells are mainly due to the profiles of horizontal velocity in the inflow segment, which is basically uniform in a confined aquifer but increases from zero to a maximum value in unconfined aquifers. Thus, groundwater at the outlets of topography-controlled flowing wells is a window of the deep part of a basin, and existing long-screen wells could have the potential for groundwater sampling.

The Badain Jaran Desert (BJD) in China is a desert with impressive sand dunes and a groundwater situation that has attracted numerous researchers. This paper gives an overview of the mysteries of groundwater in the BJD that are exhibited as five key problems identified in previous studies. These problems relate to the origin of the groundwater, the hydrological connection between the BJD and the Heihe River Basin (HRB), the infiltration recharge, the lake–groundwater interactions, and the features of stable isotope analyses. The existing controversial analyses and hypotheses have caused debate and have hindered effective water resources management in the region. In recent years, these problems have been partly addressed by additional surveys. It has been revealed that the Quaternary sandy sediments and Neogene-Cretaceous sandstones form a thick aquifer system in the BJD. Groundwater flow at the regional scale is dominated by a significant difference in water levels between the surrounding mountains and lowlands at the western and northern edges. Discharge of groundwater from the BJD to the downstream HRB occurs according to the regional flow. Seasonal fluctuations of the water level in lakes are less than 0.5 m due to the quasi-steady groundwater discharge. The magnitude of infiltration recharge is still highly uncertain because significant limitations existed in previous studies. The evaporation effect may be the key to interpreting the anomalous negative deuterium-excess in the BJD groundwater. Further investigations are expected to reveal the hydrogeological conditions in more detail.

The Ordos Basin is one of the most intensively studied groundwater basins in China. The Ordos Plateau, located in the north part of the Ordos Basin, is ideal to study the pattern of regional groundwater circulation induced by water-table undulations due to the wavy topography and the relatively simple aquifer systems with macroscopically homogeneous sandstone. In catchments located near the first-order divide, the water table is found to be a subdued replica of the topography, and the nonclosed water-table contours in topographic highs of a catchment are indicative of regional groundwater outflow to other catchments. In topographic lows, groundwater-fed lakes/rivers, topography-driven flowing wells, water-loving and/or salt-tolerant vegetation, and soap holes are all indicative of discharge areas. In discharge areas, although groundwater inflow from recharge areas is relatively stable, seasonal variations in groundwater recharge and evapotranspiration lead to significant seasonal fluctuations in the water table, which can be used to estimate groundwater inflow and evapotranspiration rates based on water balance at different stages of water-table change. In the lowest reaches of a complex basin, superposition of local flow systems on regional flow systems has been identified based on groundwater samples collected from wells with different depths and geophysical measurements of apparent resistivity, both of which can be used for characterizing groundwater flow systems. This study enhances understanding of the pattern of regional groundwater circulation in the Ordos Plateau, and also tests the effectiveness of methods for groundwater flow-system characterization.

The theory of regional groundwater flow is critical for explaining heat transport by moving groundwater in basins. Domenico and Palciauskas’s (1973) pioneering study on convective heat transport in a simple basin assumed that convection has a small influence on redistributing groundwater temperature. Moreover, there has been no research focused on the temperature distribution around stagnation zones among flow systems. In this paper, the temperature distribution in the simple basin is reexamined and that in a complex basin with nested flow systems is explored. In both basins, compared to the temperature distribution due to conduction, convection leads to a lower temperature in most parts of the basin except for a small part near the discharge area. There is a high-temperature anomaly around the basin-bottom stagnation point where two flow systems converge due to a low degree of convection and a long travel distance, but there is no anomaly around the basin-bottom stagnation point where two flow systems diverge. In the complex basin, there are also high-temperature anomalies around internal stagnation points. Temperature around internal stagnation points could be very high when they are close to the basin bottom, for example, due to the small permeability anisotropy ratio. The temperature distribution revealed in this study could be valuable when using heat as a tracer to identify the pattern of groundwater flow in large-scale basins. Domenico PA, Palciauskas VV (1973) Theoretical analysis of forced convective heat transfer in regional groundwater flow. Geological Society of America Bulletin 84:3803–3814

The exchange rate between seawater and groundwater in a tidal flat was investigated at Laizhou Bay, China, where there are large-scale seepage faces with horizontal extension of several hundred meters developed during low tides. Taking into account the effects of seepage face and density, a simple and efficient method for estimating seawater–groundwater exchange rate is proposed, based on field measurements of groundwater hydraulic head, temperature and salinity. First, the exchange rate at each well was obtained using the generalized Darcy’s law, then the results were interpolated and integrated along the whole transect. The total submarine groundwater discharge (SGD) and inflow were estimated to be 8.8 and 15.3 m3 d−1 m−1, respectively. The spatial distributions of SGD and inflow were different from those of sandy or gravel beaches possibly owing to the low-permeability sediment (silty sand with mud), very gentle slope, and the large-scale seepage faces. A freshwater discharge tube was identified near the low-tide line, as evidenced by significant increase in outflow and low salinity of groundwater observed there. The SGD from the seepage faces accounted for ∼21 % of the total SGD. The outflow rate that occurred from the seepage faces, and the ratio of the outflow from the seepage faces to the total outflow, decreased seaward significantly and monotonically.