Microgravity Monitoring of Artificial Reservoir Recharge at Little Cottonwood Canyon, Wasatch Front, Utah

A series of repeated high-precision (±5 μGal) gravity surveys are being used to monitor an artificial recharge project at the mouth of Little Cottonwood Canyon, Utah. This recharge location allows direct infiltration into the main municipal supply aquifer for the Salt Lake valley which provides water to >33% of Utah's population. The pilot project under study has three infiltration sites within 100 m of each other to test differing infiltration techniques: open shallow infiltration pond, medium-depth (20 m) cased well, and near-surface- (1 m) buried drain field. To capture the expected horizontal migration (>500 m/yr) of the infiltrated water, the survey network includes 12 stations within 500 m of the three closely-spaced infiltration points, ll stations within 500 and 1000 m from the site, and 4 stations over 1500 m distant from the infiltration site for regional and environmental background control. Prior to infiltration, a set of five background campaigns have been acquired on the survey network between spring 2006 and summer 2007. Background (natural/environmental) variability is thus robustly estimated at 10 μGal. Infiltration commenced in mid- September 2007 with bimonthly gravity surveys over the entire network during the last quarter of 2007. Based on a previous artificial recharge experiment approximately 60 km north of the current project, we expect a peak signal of >40 μGal from the approximately 300 acre-ft to be infiltrated. Detailed study of the gravity changes at each infiltration site will allow a comparison of the relative recharge and migration rates from each technique, and will inform future large-scale recharge projects for the southern portion of the Salt Lake valley.