Seasonal change of groundwater response to Earth tides

Groundwater is an important natural resource and its security depends largely on the confinement of the aquifer. Here we use ten-years of continuous groundwater data from a well in Southwest China to show that seasonal precipitation may change the aquifer's response to Earth tides from being confined-like during dry seasons to being unconfined-like during rainy seasons. Comparing the response of water level in this well with the theoretical prediction from a leaky aquifer model, we show that the observed changes of aquifer confinement reflect a seasonal increase of the vertical hydraulic conductivity from being low in the dry season when the water level is low to being high in the rainy season when the water level is high. Such seasonal change of aquifer confinement has not been reported before and may impact our understanding on the subsurface transport, the security of groundwater resources, and the safety of toxic waste repositories.