Using Water Level Responses to Atmospheric Pressure Variations to Measure and Monitor Vertical Leakage Through Confining Units, With Application to the Jurassic Shaximiao Crust, China

The upper few hundreds of meters of the crust often hosts leaky aquifers. Quantifying leakage is important if those aquifers are used as a water resource. The responses of water level to external forcing such as tides and barometric pressure changes offer the opportunity to measure aquifer hydrogeological properties and monitor possible changes in those properties. Around the Huayingshan faults adjacent to Sichuan and Chongqing provinces, China, inclined fold-and-thrust belts form the crust, and frequent earthquakes might impact aquifers in the shallow crust that are used for drinking water. We introduce a new computational approach for continuous modeling of water level changes in response to barometric pressure variations to identify when the signals are reliable and then determine values of aquifer transmissivity and aquitard hydraulic diffusivity. Computed aquifer transmissivity agrees with values from well tests. We obtain horizontal and vertical hydraulic parameters for more than 10 years (from 2008 to 2019). Of the six wells studied, five have aquitard vertical hydraulic diffusivities at least two orders of magnitude greater than aquifer horizontal transmissivity. Although several regional and teleseismic earthquakes caused changes in water levels in one of the wells with relatively low vertical permeability, we do not see clear changes in hydraulic properties in response to the earthquakes. We also identify small long-term trends and seasonal variations in hydrogeological properties.