A Study On Aquifer Systems And Groundwater Flow Systems In Drainage Basins

Large basins are usually abundant in energy and mineral resources, and some basins are also important grain production bases. With the increasing industrial and agricultural demands for water resources, overexpoliation of groundwater in some basins has caused a series of geoenvironmental problems. Therefore, the sustainable development of groundwater is fundamental to harmonious development of a society. A clear understanding on the pattern of groundwater flow in drainage basins is critical for sustainable development of groundwater, and is also one of the challenges facing modern hydrogeology.During the development of regional hydrogeology, the theory of groundwater flow systems initially propoased by Tóth (1963) effectively characterizes the movement of groundwater in drainage basins. Due to the periodic undulations in water table reflecting topography, local, intermediate, and regional flow systems could develop in a drainage basin, and the dynamics and chemistry of groundwater in each flow system are relatively independent. Freeze and Witherspoon (1967) further developed this theory by discussing the effects of geology and topography on the distribution of groundwater flow systems.Unfortunately, there are still some unsolved problems in the theory of groundwater flow systems. Although Freeze and Witherspoon (1967) had discussed the effects of heterogeneities caused by stratifications and faults on the development of flow systems, knowledge on the influencing factors of flow systems is still limited. Tóth (1980) pointed out that minerals and petroleum could accumulate in stagnant zones where flow systems meet, however, his understanding on the formation of stagnant zones is based on qualitative analysis. Division of flow systems of different order is essential to apply the theory of groundwater flow systems, however, the definition given by Tóth (1963) is not practicable. Groundwater age (or tracer concentration) is useful for analyzing flow systems and calibrating groundwater flow and transport models, unfortunately, the relationship between groundwater age and dynamics of groundwater in flow systems has not been established yet. After a review of these scientific problems, the theoretical and applied studies are presented in four chapters.First of all, the permeability structure of aquifer systems is analyzed. Based on previous studies, the heterogeneity of aquifer systems are classified into belting heterogeneity, layed heterogeneity, lithofacies heterogeneity and compaction heterogeneity, among which compaction heterogeneity is the most common. Based on the relationship between lithostatic stress and permeability, two different theoretical models characterizing the depth-decaying permeability for porous and fractured media are developed. In porous media, the increase in lithostatic stress with depth results in depth-decaying porosity, which leads to depth-decaying permeability. Based on the relationship among permeability, porosity and lithostatic stress, equations relating permeability with depth are derived. The equation successfully fits the permeability data of the Pierre Shale in the US (with a maximum depth of 4500 m). In fractured media, the depth-decaying fracture aperture resulting from lithostatic stress, along with the depth-decaying fracture frequency, leads to the depth-decaying permeability. Based on the relationship among transmissivity, aperture and effective normal stress, an equation relating transmissivity with depth is derived. Combined with the depth-decaying fracture frequency, an equation describing the depth-decaying permeability in fractured media is obtained, and has been found to perform better than previous equations proposed by others to interpret the permeability data of the granite at Stripa, Sweden (with a maximum depth of 2500 m). Furthermore, based on the mathematical model for fractured media, a method for estimation of fracture normal stiffness and deformation modulus of large scale rock masses using the permeability-depth correlation is proposed. In both porous and fractured media, the widely applied exponential equation describing the depth-decaying permeability can be considered as special cases of the newly derived equations under certain simplifications.Secondly, the dynamics of groundwater and influencing factors of groundwater flow systems are studied analytically. Based on the assumption that the basin is homogeneous and isotropic, Tóth (1963) derived the analytical solution of hydraulic head in a cross-section of a basin, and presented some preliminary analysis on the dynamics of groundwater. In this study, the mathematical model in Tóth (1963) is modified, and analytical solutions of hydraulic head as well as stream function in a cross-section of an anisotropic basin with compaction heterogeneity are derived. Based on the flow directions in local and regional flow systems, local flow systems can be labled as codirectional or counterdirectional. There are also two kinds of stagnation points, local and regional stagnation points. For local stagnation points, which are located below counterdirectional local flow systems, flow systems meet and part simultaneously. Based on this characteristic, local stagnation points can be used to accurately divide the flow systems of different order and determine the penetration depth of local and intermediate flow systems. For regional stagnation points, which are located on the basin bottom below the valley and the divide, only two flow systems meet or part. The compaction heterogeneity and anisotropy can greatly influence the development of groundwater flow systems. As the decay exponent increases, the local stagnation points move toward the basin bottom, thus the penetration depths of local flow systems increase; as the anisotropy ratio increases, the local stagnation points move toward the basin surface, thus the penetration depths of local flow systems decrease. The increases in decay exponent and anisotropy ratio would not change the positions of recharge and discharge zones, but could reduce the amount of total recharge in the drainage basin.Thirdly, the distribution of groundwater age in groundwater flow systems is analyzed numerically. Calibration of groundwater flow and transport models using tracer age has attracted numerous hydrogeologists recently. There are three approaches to calculate model age. Advective age is based on the assumption of the piston flow model; concentration-based model age is calculated from the results of an isotope transport model, combined with the decay rate of the isotope; directly simulated age is obtained by solving the governing equation of age mass transport. Among them, directly simulated age is most suitable for analyzing age distribution in drainage basins. Results of directly simulated age show that groundwater age distribution is highly correlated with the development of groundwater flow systems. In each flow system, following the flowpath from the recharge zone to the discharge zone, groundwater age is increasingly older. Around the local stagnation points, groundwater age is older at positions where flow systems meet. In the lower reaches of a drainage basin, there is an abrupt change in groundwater age from the shallow to the deep, which is an excellent indicator of different flow systems. In the upper reaches of a drainage basin, if the local stagnation point is close to or reaches the basin bottom, groundwater age in the corresponding stagnant zone could be the oldest in the entire drainage basin, and this stagnant zone is a potential site for mineral deposition. Compaction heterogeneity is an important influencing factor of groundwater age distribution. Neglectance of it would lead to erroneous results of model age.Finally, the aquifer systems, and dynamics and age distribution of groundwater in the Ordos Basin are analyzed. In a representative cross-section in the Cretaceous basin, heterogeneity and anisotropy co-exist, and the former includes belting heterogeneity, layed heterogeneity, lithofacies heterogeneity and compaction heterogeneity. Based on the spatial changes in dominant anion species, the flowpaths are qualitatively determined. The 2-D numerical model, which incorporates anisotropy and the four types of heterogeneity, is built and calibrated using measurements of hydraulic head and 14C age. The Sishi Ridge, which is the divide, separates the Cretaceous basin into two independent drainage basins, the western and the eastern drainage basins. Due to differences in thickness of aquifer systems, the distributions of flow systems and groundwater age in the two drainage basins differ greatly. In the eastern drainage basin, local flow systems dominate, and groundwater age in most part of the basin is young. At the local low near the Sishi Ridge, the local stagnation point is close to the basin bottom, groundwater in the corresponding stagnant zone is the oldest in the entire basin. In the western drainage basin, local, intermediate, and regional flow systems develop. Groundwater in the shallow part is relatively younger, while groundwater in the deep part is relatively older. The local stagnation point near the Dosit River is located in the middle part of the basin, and groundwater in the stagnant zone below the local stagnation point is older than nearby zones, thus in boreholes within the stagnant zone, groundwater in the middle part can be older than groundwater in the deep part of the basin. The case study confirms the theoretical results of dynamics of groundwater and age distribution in groundwater flow systems.The results presented in this dissertation can provide guidance on the exploration of groundwater in large drainage basins and advance in knowledge on the pattern of groundwater flow in large drainage basins, thus lead to a sustainable development of groundwater resources.