Undrainable pore spaces comprise half of US groundwater storage

Groundwater is vital to global freshwater access, streamflow generation, and biogeochemical cycling, but not all groundwater can be drained due to adhesive and capillary forces. Quantifying the proportion of groundwater that can be drained—and is, thus, theoretically recoverable—is critical for characterising groundwater’s role in earth system processes. Unfortunately, estimates of theoretically recoverable groundwater are poorly constrained due to a lack of three-dimensional lithologic observations. Here we analyse ~19.2 million 3D lithologic observations recorded in ~3.7 million drilling reports across the United States. We show that only half of US groundwater is theoretically recoverable by wells due to the abundance of aquitards, which retain most of their water when drained. The abundance of aquitards emphasizes that the great majority of groundwater is stored in confined aquifers, which are often more sensitive to rapid groundwater-level declines than shallower unconfined aquifers. The widespread prevalence of aquitards and confined conditions suggests that even modest groundwater pumping can lead to substantial drawdown in many aquifers, inducing land subsidence and creating potential water quality risks.