Discussion on hydraulic characteristics and changes of aquifer based on tidal response of well water level
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Abstract:
The traditional method of calculating an aquifer′s hydraulic characteristic parameters can not deduce the real-time continuous change of the hydraulic characteristic,which makes it difficult to analyze the change of aquifer permeability under external load.The response model of the aquifer′s head to the periodic high-frequency natural force provides a new idea for research.In recent years,it has been shown that the external stress loads may cause changes of the aquifer′s hydraulic characteristic parameters and the corresponding changes in groundwater flow movement.At present,the research mainly focuses on qualitative inference and explanation,and there were relatively few conclusions from quantitative calculation. Based on the systematic analysis and summary of the theoretical relationship between the tidal response of the well water level and the aquifer′s hydraulic characteristics,taking wells Dazu and Beibei in Huayingshan fault zone as examples,the specific storativity of the observation aquifer was deduced by the amplitude of tidal response,and the movement state of groundwater flow near the well hole was analyzed based by the phase shift of tidal analysis.Using the radial flow model and leaky aquifer system model,the permeability and the coefficient of leaky in the aquifer near the well bore were calculated.Taking the Wenchuan earthquake as an example,the changes in well water level and the aquifer′s hydraulic parameters during the earthquake were analyzed,the influence of seismic stress on the hydraulic characteristics and groundwater flow movement was discussed,and its change mechanism was proposed. The phase shift of the water level in well Dazu response to the M2 wave was negative,indicating that the groundwater movement in the aquifer near well Dazu is dominated by the radial flow.It was assumed that the specific storativity in the observation section of well Dazu is 1.55×10-6 m-1,and the horizontal permeability is 0.041 9 m/d,which was consistent with the results of the previous pumping test (0.053 4 m/d),indicating that it is feasible to calculate the hydraulic parameters of the aquifer by the tidal response of the well water level.Moreover,the locally developed fractures in the aquifer near well Beibei resulted in the coexistence of vertical and horizontal flows near the well.The specific storativity of the aquifer was estimated to be 1.41×10-6 m-1and the coefficient of leakage ranged from 1.5×10-8 to 2.5×10-8s-1.After the Wenchuan earthquake,the water levels of wells Beibei and Dazu changed step by step,and the vertical flow of groundwater in the aquifer increased. Combined with the regional hydrogeological conditions,it was proposed that the reasons for the water level changes in wells Beibei and Dazu caused by the Wenchuan earthquake were as follows:The Jurassic sandstone aquifer in this region has a unified hydraulic connection over a large range,and local fractures with large dip angle are developed.The occurrence of earthquake led to expansion deformation and fracture dredging of aquifers,under the action of gravity,groundwater flowed along the fracture channel,resulting in the decline of water level in wells Beibei and Dazu,and the enhancement of vertical water exchange near the well bores in the runoff area.The research results are helpful to improve the understanding of the interaction between aquifer stress and deformation and seepage,and further deepen the research on the response mechanism of borehole aquifer systems to seismicity.
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