A Methodological Study On The Hydrogeochemical Characterization Of Hierarchically Nested Groundwater Flow Systems

The groundwater circulation which is a critical component of water cycle. The element of the groundwater could be modified by the interactions between groundwater and its ambient environment along the groundwater flow. Therefore,identification of the geochemical features of the groundwater could provide a detailed statement of the groundwater circulation. The theory of reginal groundwater flow characterizes the general behaviors of the basin scale groundwater circulation and evolution, could be treated as the benchmark for studying reginal groundwater flow.In this doctoral dissertation, based on the hydrogeochemical information and the electrical resistance of the groundwater, hierarchically nested groundwater flow systems and the evolution processes have been characterized in the discharge area.Meanwhile, the processes of river water and groundwater interaction have been identified in such site.Although the hierarchically nested flow system could be identified through the hydraulic aspect. However, little research has been devoted to identify the evolution of nested groundwater flow system. The lowest reaches of a large-scale basin could be the discharge area of local, intermediate and regional groundwater flow systems with different travel distances and travel times. In this study, groundwater samples were collected from wells with different depths in the discharge area of the Dosit River Watershed in the Ordos Cretaceous Basin. Using the major ions and pH as the input,hierarchical cluster analysis was conducted, which leads to five clusters(from C1 to C5) with distinct geochemical compositions. Both hydrochemistry and isotopes versus depth show a three-part structure with boundaries around depths of 200 m and 600 m,which were assumed to be associated with the circulation depths of local and intermediate flow systems. Four geochemical indicators(Cl-, SO42-,(Ca2+ + Mg2+)-(SO42-+HCO3-) versus Na+- Cl-) and CAI-1) imply that the processes of the evolution processes of groundwater flow.The evolution and circulation of groundwater could have resulted the regional changing of watertype and TDS which externalized the pattern of groundwater flow.The watertype shows an evolution trend which started from HCO3-through SO42-to Cl-, while, the TDS was gradually increased, which indicated the groundwater evolution trend. Based on the statistical analyses of the electrical resistance data,distinct characteristics of the electrical resistance could be found not only in the upstream and downstream of the Cretaceous sandstone aquifer but also between the Cretaceous and Jurassic layers, indicating different evolution stages of the groundwater in the study area.The evolution processes in hierarchically nested groundwater flow system could be highly possible related with the distinct circulation path, residence time and the recharge area for each groundwater flow system. Therefore, the groundwater flow length and residence time have been calculated using backward simulation based on the 3D numerical model of the Dosit River Basin. The apparent trend could be found that groundwater in the deep part of the aquifer has a relatively long flow length and residence time comparing to the shallow part, indicating the distinct evolution stages of the groundwater in different parts of the study area. In addition, recharge areas for the groundwater in the upstream and downstream have been identified based on this method which are helpful for understanding the groundwater evolution trend when integrating the lithofacies and paleogeography in the recharging site.Surface water is mainly recharged by the groundwater in the arid or semi-arid area. The interactions between surface water and groundwater have been identified based on the geochemical features integrating with statistical method. For the upstream area, surface water was recharged by groundwater from both of the shallow and deep parts of the aquifer, with the domination process of ion-exchange. While, the surface water was got recharged by the groundwater from deep part of aquifer in the downstream area.The results of this doctoral dissertation not only reveals the mechanisms of groundwater evolution and interaction between groundwater and surface water in the study area, but also provides a method to identify flow systems using hydrochemistry and isotopes, which enhanced the understanding of the theory of regional groundwater flow.