Mechanism of abnormal water level changes in the Wang 3 well of Tianjin

Water level in the Wang 3 well of Tianjin was higher than its normal annual values during June to September 2016. The possible mechanisms of the abnormal water level changes were analyzed in this study. Firstly, the normal annual variation of water level was extracted using spectral analysis method, and the influence of precipitation was corrected according to the relationship between the well water level and the local precipitation. Secondly, the multi-year values of water level were calculated by using the third-order polynomial fitting, and the abnormal water level in the Wang 3 well was identified quantitatively. Then, the hydrochemical analyses in the well and its surrounding water sources were made. The result shows that the groundwater in the well is mainly derived from the local precipitation, which indicates that there was some water-rock interaction rather than obvious hydraulic relationship between well water and surrounding water sources. Finally, from the wave velocity ratio in Beijing area, it is deduced that there may be a short-term stress accumulation around the Wang 3 well, which is verified by the local stress relief resulted from the Tangshan M_S4.2 earthquake on September 10, 2016, and the water level of Wang 3 well gradually returned to its normal annual values.