[AHW31-01] Numerical analysis of the hyporheic flow in a gravel aquifer subjected to an extraction scheme using multiple radial collector wells
キーワード：hyporheic flow, radial collector well, water resource, FEFLOW
Clean water stored in a hyporheic zone can be directly used as a source of municipal water supply such that the cost of water treatment can be reduced. Another advantage of hyporheic water is its stable reserve even in dry seasons when the river water is depleted to a large extent. In the literature, however, no detailed numerical simulations of the response of a hyporheic zone corresponding to the extraction of water from radial collector wells have been published. In addition, the desaturation process during the extraction has not been rigorously considered. Therefore, this study aims to develop a numerical approach for investigating the physical processes in a hyporheic zone during the extraction period.
A river in northern Taiwan was selected to evaluate its feasibility of exploring additional water resources for municipal water supply. In general, the simulation domain needs to be selected from the reach that is free from contamination. For the selected river, hydrogeological characterization of its river basin is difficult but not impossible because not a single geological borehole is present in this area. Therefore, the simulation domain is characterized based on logging data form a borehole that is nearby the simulation domain.
Only the topmost unconfined aquifer is investigated in this study. Based on the existing logging data, this aquifer is conceptualized as five homogeneous but anisotropic sublayers of variably saturated gravel with only one thin layer representing the fine sand. The commercial software FEFLOW is used to simulate the three-dimensional steady-state hyporheic flow by solving the Richards equation combined with the van Genuchten model such that the desaturation process can be taken into account. Constant head boundary conditions are assumed at the western and eastern boundaries. No flux boundary condition is assumed elsewhere.
It was found that the direction of hyporheic flow is not normal to the river channel but parallel to it. Based on the presence of circulated sub-vertical exchange of groundwater in the vicinity of the fine sand layer, the hyporheic zone is defined as the unconfined aquifer between the river bed and the first layer with low hydraulic conductivity. In our model, three radial collector wells were located at the flood plain with high hydraulic conductivity. Simulation results have confirmed that water is extracted from the hyporheic zone but not from the river. Furthermore, the drawdown obtained by multiple collectors is significantly smaller than that obtained by a single point well.
In summary, a methodological approach has been developed. In particular, the detailed variation of hydraulic head and Darcy flux in the neighborhood of a radial collector well can be vividly revealed thanks to the mesh refinement and three-dimensional visualization capabilities provided by FEFLOW. Once a detailed hydrogeological conceptual model is available, this approach can be repeated to provide prediction results of higher precision to facilitate the decision making regarding the design of water resource exploitation.
A river in northern Taiwan was selected to evaluate its feasibility of exploring additional water resources for municipal water supply. In general, the simulation domain needs to be selected from the reach that is free from contamination. For the selected river, hydrogeological characterization of its river basin is difficult but not impossible because not a single geological borehole is present in this area. Therefore, the simulation domain is characterized based on logging data form a borehole that is nearby the simulation domain.
Only the topmost unconfined aquifer is investigated in this study. Based on the existing logging data, this aquifer is conceptualized as five homogeneous but anisotropic sublayers of variably saturated gravel with only one thin layer representing the fine sand. The commercial software FEFLOW is used to simulate the three-dimensional steady-state hyporheic flow by solving the Richards equation combined with the van Genuchten model such that the desaturation process can be taken into account. Constant head boundary conditions are assumed at the western and eastern boundaries. No flux boundary condition is assumed elsewhere.
It was found that the direction of hyporheic flow is not normal to the river channel but parallel to it. Based on the presence of circulated sub-vertical exchange of groundwater in the vicinity of the fine sand layer, the hyporheic zone is defined as the unconfined aquifer between the river bed and the first layer with low hydraulic conductivity. In our model, three radial collector wells were located at the flood plain with high hydraulic conductivity. Simulation results have confirmed that water is extracted from the hyporheic zone but not from the river. Furthermore, the drawdown obtained by multiple collectors is significantly smaller than that obtained by a single point well.
In summary, a methodological approach has been developed. In particular, the detailed variation of hydraulic head and Darcy flux in the neighborhood of a radial collector well can be vividly revealed thanks to the mesh refinement and three-dimensional visualization capabilities provided by FEFLOW. Once a detailed hydrogeological conceptual model is available, this approach can be repeated to provide prediction results of higher precision to facilitate the decision making regarding the design of water resource exploitation.