Using chlorofluorocarbons (CFCs) and tritium (3H) to estimate groundwater age and flow velocity in Hohhot Basin, China
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Abstract:
The concentrations of chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and tritium (3H) content in groundwater were used to date groundwater age, delineate groundwater flow systems and estimate flow velocity in the Hohhot basin. The estimated young groundwater age is fallen in the bracket of 21 ~ 50 a and indicates the presence of two different age profiles and flow systems in the shallow groundwater system. Older age waters occur under the topographically low areas, where the aquifer is double-layer aquifer system consisting of shallow unconfined-semi-confined aquifer and deep confined aquifer. This reflects long flow paths associated with regional flow. Groundwater (range from 21 to 34 years) in the north piedmont and east hilly areas, where the aquifer is a single-layer aquifer consisting of alluvial fans, are typically younger than those in the low areas. The combination of CFCs dating with hydrogeological information indicates that both local and regional flow systems are present at the basin. The regional groundwater flow mainly flows from the north and east to the southwest, the local groundwater flow system occurs nearby the Hohhot city. The mean regional groundwater flow velocity of the shallow groundwater is estimated about 0.73 km/a. These findings can aid in refining hydrogeological conceptual model of the study area. Copyright © 2012 John Wiley & Sons, Ltd.Keywords:
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Abstract In general, groundwater flow and transport models are being applied to investigate a wide variety of hydrogeological conditions besides to calculate the rate and direction of movement of groundwater through aquifers and confining units in the subsurface. Transport models estimate the concentration of a chemical in groundwater which requires the development of a calibrated groundwater flow model or, at a minimum, an accurate determination of the velocity and direction of groundwater flow that is based on field data. All the available hydrogeological, geophysical and water quality data in Musi basin, Hyderabad, India, were fed as input to the model to obtain the groundwater flow velocities and the interaction of surface water and groundwater and thereby seepage loss was estimated. This in turn paved the way to calculate the capacity of the storage treatment plants (STP) to be established at the inlets of six major lakes of the basin. The total dissolved solid was given as the pollutant load in the mass transport model, and through model simulation, its migration at present and futuristic scenarios was brought out by groundwater flow and mass transport modeling. The average groundwater velocity estimated through the flow model was 0.26 m/day. The capacities of STP of various lakes in the study area were estimated based on the lake seepage and evaporation loss. Based on the groundwater velocity and TDS as pollutant load in the lakes, the likely contamination from lakes at present and for the next 20 years was predicted.
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Abstract Hydrogeological parameters are very uncertain to specify in the groundwater flow models. Hence, it is important to specify a range for these parameters to be further utilized by the groundwater model users. In El-Qaa Plain, previous researchers have determined the hydrogeological parameters such as transmissivity, hydraulic conductivity, and mean annual recharge rate using geophysical methods, pumping tests, hydrological models, and groundwater flow models. However, they are also limited to specific locations and do not spatially distribute. In this paper, an effort has been accomplished to specify a specific range for the aquifer hydrogeological parameters using inverse groundwater modeling taking into account the spatial heterogeneity. These ranges could be taken into consideration during modeling groundwater flow. Hence, a finite-difference groundwater flow model has been applied using the ModFLOW model. Regularized pilot points method (RPPM) has been chosen as an inverse modeling technique to specify the accurate values of the aquifer parameters. Model-independent parameter estimation and uncertainty analysis (PEST) has been utilized to estimate the parameters. The results show that the geometric mean of the hydraulic conductivity in the study area ranges from proximity 1 to 2 m/day. Additionally, the geometric mean of the transmissivity ranges from nearly 200 to 500 m 2 /day. A relationship between the mean annual recharge rate and hydrogeological parameters has been determined. Besides, maps have been generated showing the spatial heterogeneity of the hydraulic conductivity. These maps could be further utilized by groundwater flow modelers to predict the potential of groundwater flow and pollution in the future.
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A study of hydrogeological process involves movement of water beneath the ground surface. Water content in the aquifer influences the quantitative determination of aquifer hydraulic parameters. The limited opportunity to
explore and demonstrate groundwater processes is the reason why students have inappropriate understanding of groundwater concept. The visualisation of groundwater flow is quite difficult as it deals with subsurface condition which cannot be seen. In research, field experiments on groundwater are difficult to carry out because time consuming and involves uncertainty in aquifer conditions. Physical models have been used in classroom as a tool for teaching hydrogeology. Further understanding was developed by demonstration and
observation of groundwater flow using simple sand tank. Previous research implemented sand tank under controlled conditions to investigate the mechanism and flow process of groundwater. A large artificial physical aquifer
model was developed in this study as an alternative to show the students the real aquifer condition and hydrogeology processes. The model consisted of
three different layers of soils, in which water table level was controlled using water tank at both sides of the physical model structure. Hydraulic parameters
of the artificial aquifer and performance of production well were evaluated by pumping tests. The groundwater flow in the artificial aquifer model was simulated accordingly to Darcy‟s law. Analysis of pumping test was computed by an Aquifer Test software. Well performance measurement provided by a step drawdown pumping test estimated the efficiency of well as 99%. The artificial aquifer model was verified by constant rate discharge pumping test and found to be a leaky aquifer. The pumping test analyzed the aquifer with transmissivity of 78.50m2/day and hydraulic conductivity of 7.37m/day while
recovery test analyzed the transmissivity to be 8.22m2/day and hydraulic conductivity of 7.34m/day. Both test analyzed the storage coefficient as 0.5.
This artificial aquifer physical model was designed and developed to enhance student‟s understanding of groundwater theory. Through hands-on pumping test on the aquifer model, students would be able to visualize clearer the groundwater processes.
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Based on the conditions of hydrogeology, groundwater exploitation and data of the research area, the hydrogeological concept model was conceptualized as a 2-D heterogeneous, isotropic and unstable groundwater flow system. A groundwater flow model was built by using the FEFLOW software (Finite Element Subsurface Flow and Transport Simulation System). This model can forecast the groundwater dynamic variation under the different exploitation plans and provide a scientific foundation for the groundwater development, utilization and optimum allocation.
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