Are Deep Aquifers Really Confined? Insights From Deep Groundwater Tidal Responses in the North China Platform

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 M-2 and the O-1 tides and to the barometric pressure across a large (2.046 x 10(6) km(2)) 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.

Automated summary

By inverting groundwater responses in the North China Platform, it was found that deep confined aquifers are not depth-dependent and may become leaky after distant earthquakes. Monitoring of aquifer confinement is crucial to ensure the safety of wastewater injection into targeted deep aquifers.