Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency’s regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M₂ and the O₁ tides and to the barometric pressure across a large (2.046 × 10⁶ km²) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China.

Permeability is an important parameter of shallow crust that affects underground water migration. It is important to calculate the permeability changes caused by earthquakes for underground engineering safety and pollutant dispersion. The response analysis of groundwater in wells, to Earth tides, has been widely used to calculate permeability. However, the calculation results and the judgment of leakage are controversial. In this study, the water level response of both the M₂ and O₁ tidal waves in the Zuojiazhuang well was analyzed. An optimization fitting, using the mixed flow model, was performed to calculate the hydraulic parameters of aquifers and aquitards. After the Wenchuan earthquake, the horizontal transmissivity of aquifers increased temporarily and then slowly recovered. However, after the Tohoku earthquake, the horizontal transmissivity increased permanently by one order of magnitude. The vertical hydraulic conductivity increased permanently by three orders of magnitude, meaning that vertical leakage occurred. As both the M₂ and the O₁ waves are used as constraints for the calculation with the mixed flow model, the results of this study are more precise than those of previous ones. This article also gives some suggestions for utilizing the mixed flow model.

It is well known that the storage of the valuable helium and other natural gases needs thick well-confined aquitards. Meanwhile, the assumption that deep aquifers are confined is most evident from the wide practices of targeting deep aquifers as the storage of toxic wastes and CO₂. With the negative phase shifts of the M₂ tidal wave, previously, deep Gaocun well (3500m) in the North China Platform is assumed to be confined fairly well. However, water level of the Gaocun well has been continuously decreasing for 40 years without coseismic variations and without explainable mechanism. In this study, innovatively, we find that even buried in 3500 meters deep, and covered by thick compact mudstones, the aquifer of well Gaocun is calculated to be continuously leaking for 40 years probably induced by the vertical leakage incurred by aquitard fractures, which is under the assumptions of the none-leaking bottom layer and none direct pumping or exploiting in the observation aquifer layer. Meanwhile, for the first time, we did a detailed systematic comparison between the leaky models of the tidal response and the barometric response of water level, which indicate a consistency leaky result, and the minor difference mainly induced by the frequency differences. Merits and demerits of both models are also analyzed. Last but not least, underground fluid leakages might frequently occur, which are becoming increasingly worldwide urgent since that might also be related with surface sourced contaminations, toxic waste burials, and burial and exploration of the natural gas reservoirs.