Using high-resolution gravity and pumping data to infer aquifer parameters

Drawdown measurements during a pumping test are often of limited spatial extent but typically provide the only available head observations for developing and evaluating a groundwater model. Previous field and modeling studies have shown gravity data to be complementary to head observations for aquifer-storage-change monitoring and estimating aquifer properties. Although spatially distributed gravity measurements have proven useful for monitoring aquifer storage change and oil and gas extraction, to date, no direct superconducting gravimeter measurements of aquifer response to high-volume pumping have been reported. Data collected from March 2010 to March 2011 with a superconducting gravimeter located 200 m from a public supply well in Tucson, AZ, show a decrease in gravity-up to 10 microgals- primarily correlated with withdrawal of water-mass from pumping, and to a lesser degree, with pumping-induced land surface elevation changes that indicate elastic aquifer compressibility. This small gravity change was scarcely detected with a co-located A10 absolute gravimeter. These gravity data provide an additional tool to evaluate and calibrate a groundwater flow model, and suggest more interesting aquifer-storage dynamics than a simple correlation between drawdown and gravity change. The superconducting gravimeter, together with the desert environment and urban setting where direct recharge is not present and non-pumping hydrologic effects are minimal, provides a high signal-to-noise ratio not possible with other instruments.