GRAVITY MONITORING OF AQUIFER STORAGE PROJECTS: RESULTS FROM THE WEBER RIVER PROJECT

Repeat precision gravity measurements provide an economical way to track aquifer storage changes through time and space. In early 2004, the Weber River Water Conservancy District in northern Utah began an aquifer storage and recovery pilot project by infiltrating river water into a depleted unconfined aquifer. We have tracked infiltrated water through two infiltration cycles (early 2004-fall 2005) over a network of 28 stations around the site and adjacent mountains. Gravity surveys were repeated at approximately two week intervals; monthly rapid-static GPS campaigns monitored ground deformation across the stations.

In Feburary 2004, we established a baseline prior to infiltration to quantify natural signals and measurment noise, resulting in a significance level of 20 μGal. Initial infiltration occured between March and July 2004, reaching a total infiltrated volume of 750 000 m³. Gravity changes at the site peaked at ~100 μGal above baseline. Gaussian integration of the 2004 peak gravity signal is consistent with the total infiltration volume.

Repeat gravity surveys through the fall of 2004 tracked the decline of the infiltration mound to a minimum of ~45 μGal. The second year of infiltration spanned March to May 2005, with concurrent gravity monitoring. As in 2004, gravity monitoring indicated the presence of a recharge mound under the infiltration site, with a peak value of 179 μGal. Monitoring during and after infiltration in 2004 and 2005 tracked horizontal migration of mass consistent with known hydraulic gradients. GPS monitoring showed negligable station motion during the study.