Paul Gettings, Rick Allis, Robert N. Harris, David S. Chapman
High-precision gravity monitoring provides a cost-effective method for studying mass change in aquifer systems over large areas and times. We have established a network of 8 stations over the Sandy aquifer in northern Utah. The network extends 13 km from the Wasatch Mountains to the Jordan River in the middle of the Salt Lake Valley. The results of 20 months (December 1998 to September 2000) of monitoring the aquifer indicate a seasonal gravity change of up to 0.035 mGal. Refinement of data acquisition and processing techniques yields an accuracy of &plusminus;0.005 mGal.
Gravity data were collected in 18 gravity campaigns, at approximately 4 week intervals. Water levels at nearby wells were monitored concurrently with the gravity surveys. Seasonal water level variations of between approximately 1 and 15 m are observed. Three precision GPS campaigns were conducted, at approximately 6 month intervals from June 1999, for elevation change information. Station elevations dropped by as much as 7 cm during the 20 months of study. Gravity changes adjusted for elevation and water level changes agree in direction, timing, and duration. These data are consistent with the seasonal pumping of the system; the aquifer is pumped during summer to provide municipal water supply, and is recharged during fall and spring by rainfall and snow melt.