Groundwater is an important source of water for human use, especially in the arid Southwest. Changes in the amount of groundwater stored in an aquifer are typically estimated by observing groundwater-level change in a well and multiplying the observed change by an assumed value of aquifer porosity, which is the void space that contains groundwater in a subsurface rock formation. USGS scientists are leveraging microgravity measurements as a way to calculate aquifer proosity. These measurements capture subsurface changes in mass that are caused by groundwater-level changes in an aquifer. Repeated measurements of gravity at a network of sites at the land surface can be used to measure changes in the mass of groundwater at the water table. Combined with water-level measurements in nearby wells, the change in groundwater mass can be used to calculate the porosity of an aquifer.
The project has established a network of microgravity stations in southeastern Albuquerque, New Mexico, where groundwater levels declined by as much as 120 feet from 1950 to 2008, but have risen since 2008 when changes in groundwater pumping were implemented. The amount of water stored in the aquifer changes as water levels change, but without microgravity measurements the volume of groundwater storage change has to be estimated based on an assumed porosity value. Given microgravity measurements at a site where water-level measurements are also available, the porosity of the aquifer can be calculated and used to obtain a better estimate of the volume of groundwater storage change at that site. When microgravity and water-level measurements are made at enough sites, the groundwater storage change can be extrapolated to an entire well field. In addition, if the porosity of the aquifer is known, gravity measurements can be made at a site without a well to estimate how much and how quickly the groundwater level is changing.