Groundwater flow systems and their mediation on hydrogeochemical and isotopic characterization with a case study in the Badain Jaran Desert

Abstract

Nested groundwater flow systems (NGFS) are commonplace in various hydrogeological environments. This thesis addresses the control mechanisms of NGFS and their mediation on hydrogeochemical and isotopic characterizations. Three questions are mainly addressed. First, saline water often emerges in the low-lying depressions of evaporitic basins. There are many studies about the regional groundwater flow system that leads to the formation of saline water. How the formation of saline water impacts the regional groundwater flow system has not been addressed in the literature. Second, the water table is subject to fluctuations of multiple time scales, which may have significant impacts on the NGFS and surface runoff. The responses of the NGFS to rainfall and the impacts of the NGFS on surface runoff are rarely discussed before. Third, previous theoretical studies indicate that in recharge-controlled aquifers, the regional groundwater flow system is dominant, but local groundwater flow systems are usually less developed or even non-existent. The occurrence and role of local groundwater flow systems in real recharge-controlled systems are rarely addressed. To address the first question, the classic NGFS are revisited and simulated by considering various salinities of the saline water in their discharge zones. The results show that, as the salinity in the discharge zones increases, the hydraulic gradient near the discharge zone can be significantly reduced and thus all the sub-systems of NGFS are reallocated. For the second question, the influences of variable rainfall on NGFS and runoff are explored using a fully coupled variably saturated groundwater-surface water model in conjunction with spectral analysis. The fractal behaviors of the hydraulic heads present an obvious vertical distribution and this spatial pattern can be used to define the rough spatial range of the sub-systems under transient conditions. The results also explain the controversy in previous studies that the fractal behavior of river runoff can have either positive linear relation or no relation with its watershed area. The Badain Jaran Desert (BJD) is featured in hyper-dry climate, mega sand dune-lake landscapes, and highly permeable sediments, making it an ideal case study to address the third question on the occurrence and role of local groundwater flow systems in real recharge-controlled systems. Multi-depth groundwater sampling systems are established in a representative saline lake in the BJD. Seasonal investigations of radioactive radium isotope, stable hydrogen, oxygen, and boron isotopes, trace metals, and nutrients are performed in the groundwater sampling transects and lake water. The spatial distributions of boron isotopes reveal the existence of local groundwater flow systems in hyper-arid climate conditions. The lake water in the BJD is mainly supported by intermediate and local groundwater, but nutrients in lake water are mainly supplied by the intermediate flow and lakebed diffusion. The solutes loss via downward intrusion of lake water is calculated to account for a big portion of the lake solute inventories based on the mass balances of barium, radium, and chloride. The findings of this thesis will benefit the understanding of the groundwater flow systems and their mediation on hydrogeochemical and isotopic characterization.