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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. [ABSTRACT FROM AUTHOR]

• We summarize field observations of 14C age showing obvious GW aging in two basins. • Pumping causes not only rejuvenation, but also aging of GW in layered basins. • Although GW rejuvenation is larger in area, aging is greater in magnitude. • We identify two new mechanisms of pumping-induced GW aging in layered basins. • Topography-driven flow is the main cause of GW aging in the discharge area. As an indicator of the renewability of a groundwater basin, the spatial distribution of groundwater age has attracted intensive research. As reported in the literature, 3H measurements in 74 US aquifers showed pumping-induced groundwater rejuvenation, whereas 14C ages in two heavily developed aquifers in France and China indicated pumping-induced groundwater aging. Unfortunately, the mechanisms leading to the seemingly opposite behavior of pumping-induced aging and rejuvenation remain unclear. In this study, we conducted numerical simulations of groundwater age in layered aquifer-aquitard systems under different pumping scenarios and aquitard permeability to show that pumping-induced groundwater rejuvenation and aging always occur simultaneously. Groundwater rejuvenation widely occurs throughout the basin wherever the well is located, whereas groundwater aging mainly occurs to the downgradient parts of the basin. Although groundwater rejuvenation is larger in area, groundwater aging is greater in magnitude, and the magnitude of aging increases with pumping intensity and duration. Moreover, when pumping occurs in the discharge area, aging could occur in almost the entire aquifer to the downgradient of the pumping well, and the magnitude of aging is larger. The large area and large magnitude of aging in the discharge area is a direct result of upward leakage of old groundwater from underlying aquitards/aquifers, which is controlled by the topography-driven groundwater flow. By demonstrating the importance of topography-driven groundwater flow and underlying formations in groundwater age redistribution, this study has implications for determining the bottom of a groundwater flow and solute transport model and interpretation of various tracers. [ABSTRACT FROM AUTHOR]

• We separate the roles of upward discharge and aquitards in flowing artesian zone. • Basinal flow are coupled with well hydraulics to examine hydraulics of flowing wells. • A low K aquitard causes a large flowing artesian zone but low stable discharge rate. • Baseflow reduction is the main source of groundwater derived in flowing wells. Causes and water sources of flowing artesian wells attracted the interest of many hydrogeologists throughout history, however, a quantitative model that satisfactorily considers the roles of topography, groundwater recharge/discharge and aquitards on hydraulics of flowing wells is still lacking. In this study, a three-layer river-valley basin with a recharge boundary is used to obtain the basinal flow field, spatial distribution and transient discharge rates of flowing wells. Even if there is an aquitard separating the unconfined and the confined aquifers, groundwater discharge to the river still plays a critical role in occurrence of flowing wells. An aquitard with lower permeability would enlarge the zone with flowing wells, implying that flowing wells are more likely to occur in basins with continuous aquitards. By comparing flowpaths and inter-aquifer leakages before and after numerically installing a flowing well in a three-layer basin, we find decreased upward leakage in the discharge limb plays a much larger role than increased downward leakage in the recharge limb in the stable discharge rate of a flowing well. This recognition is different from previous studies where the stable discharge rate is supported only by downward leakage from the overlying aquifer. By considering well hydraulics in a basinal background flow field, this study improves understanding of the mechanisms of flowing artesian wells and the interaction of surface water and deep groundwater, indicating that flowing wells are valuable for sampling deep groundwater representative of deep and old groundwater in the discharge area of a basin. [ABSTRACT FROM AUTHOR]