A Methodological Study On The Identification Of Hierarchically Nested Groundwater Flow Systems In Drainage Basins

The theory of regional groundwater flow, which well characterizes the general behaviors of the basin-scale groundwater circulation and evolution, is a powerful tool to instruct the engineering application and research of hydrogeology. Identifying the spatial distributions of local, intermediate and regional flow systems is critical to this theory. In this doctoral dissertation, based on the scrutinizations of the artesian flow conditions, the groundwater salinity distributions and the groundwater residence time distributions (RTDs), three identification methods are proposed.Although it has been reported that flowing artesian wells could be topographically controlled, there is no quantitative research on artesian flow conditions in drainage basins. By introducing the relative amplitude factor to quantitatively establish the relationship between the topography and the water table and by defining the artesian head as the difference between the hydraulic head at a specific point and the corresponding elevation of the land surface, analytical solutions of artesian head for a unit basin and a Tothian basin are derived. The analytical solutions are then used to quantitatively analyze the artesian flow conditions, i.e., the spatial distributions of artesian head and artesian/non-artesian zones, and to sensitively examine the key-controlling factors. In the unit basin, the artesian head is positive nearby the valley and negative nearby the divide, and varies exponentially with depths. Its iso-contour with a value of zero is the dividing boundary between the artesian and non-artesian zones, which also gives the critical depth of being a flowing artesian well. Such key factors as the basin depth-to-width ratio, permeability depth-decay and anisotropy and the relative amplitude factor control the spatial distributions of artesian head and artesian/non-artesian zones. In the Tothian basin, the artesian flow conditions are influenced by the hierarchically nested groundwater flow systems a lot. Several artesian and non-artesian zones could develop alternatively. In the local divide nearby the regional valley (or in the local valley nearby the regional divide), an extreme value exists in the profile of artesian head versus depth, around which an internal stagnation point develops. Based on the theoretical findings, two field cases in the Ordos Plateau are related to and explained by artesian flow conditions. In the field case of a unit basin, the slope of the profile of artesian head versus depth is used to estimate the permeability anisotropy and the decay exponent. In the field case of a Tothian basin, the extreme value of the profile of artesian head versus depth is used to identify the hierarchical pattern of the nested groundwater flow structure.Groundwater flow systems in drainage basins are critical to a series of geologic processes. Unfortunately, the difficulty of mapping flow system boundaries restrict the application of this concept. The groundwater salinity, also named the total dissolved solids (TDS), bears the information on the basin-scale groundwater circulation and evolution. Firstly, by introducing the governing equation of the groundwater salinity, the spatial distribution of groundwater salinity in a Tothian basin is simulated using COMSOL Multiphysics. Two distinct features are found:there is an abrupt interface between the local and the regional flow systems in the lower reach of a basin; there is a high-salinity strip-shaped zone between two adjacent local flow systems. These two features serve as a theoretical basis for the interpretation of the field geo-physical exploration results. Then, by assuming that the variation in bulk resistivity of the Cretaceous sandstone aquifer with uniform porosity is mainly caused by the groundwater salinity, the magnetotelluric technique implemented by the STRATAGEM EH-4 is used to obtain the apparent resistivity of a profile across a groundwater-fed river in the Dosit River Basin, the Ordos Plateau. Using the local borehole data and the measured groundwater salinity, the credibility of the geo-physical results is verified, and the spatial distribution of groundwater salinity in the profile is obtained with the help of the relationship between the apparent resistivity and the groundwater salinity. Finally, in analogy to the simulated groundwater salinity distribution, in the geo-physical profile, the boundaries between local and regional flows are identified, and the groundwater flow pattern is characterized.Previous studies on the characterization of hierarchically nested groundwater flow systems have mainly been based on spatial distribution analyses of groundwater pathways. By considering the discrete nature of the temporal behavior induced by different hierarchical flow systems, a new method is proposed. The core of this method is to use the critical residence times defined by the late-time peaks of the RTD to divide the groundwater residence time field to identify local, intermediate and regional systems as described by Toth (1963). In order to demonstration the feasibility of this new method, Tothian basins of a 2D cross section and a 3D domain are introduced as hypothetical test cases. The results show that the spatial distributions of local, intermediate and regional groundwater flow systems identified by the late-time peaks of RTDs are in good agreements with the results given by the dividing streamlines associated with internal stagnation points (in 2D) and by linking recharge zones to discharge zones using tracked groundwater pathways (in 3D).Finally, the Dosit River Basin is introduced as a synthetic study. Based on the hydrogeologic conditions in the Dosit River Basin, a basin-scale groundwater flow model is built using MODFLOW, and the observed hydraulic heads and the investigated flowing artesian wells are used to conduct the calibration. Within the calibrated model, firstly, MODPATH is used to quantitatively study the groundwater pathways and the residence times. It is found that each two of the groundwater pathway lengths, penetration depths and residence times have a power-law relationship. Then, one distinct late-time peak is identified from the RTD to divide the Dosit River Basin into a two-order hierarchical flow structure as the local and the regional flow systems. The local flow systems are quantitatively characterized by short pathway lengths (l*<0.53), shallow penetration depths (d*<0.2), short residence times (t*<0.18), the low occupancy ratio of the basin volume (～22%) and the high contribution rate of the groundwater circulation (～78%); while the regional flow systems have opposite features. Finally, the identification result given by the late-time peaks of the RTD is validated by comparing with the results given by the hydro-geochemistry cluster analysis and by the magnetotelluric geo-physical exploration.The results presented in this doctoral dissertation, which contribute to the theory of regional groundwater flow, not only can advance in knowledge on the groundwater circulation and evolution in the Ordos Plateau, but also can provide guidance on the sustainable development of local groundwater resources.