Patricia E Gettings

Monitoring aquifer recharge using repeated high-precision gravity measurements: A pilot study in South Weber, Utah

Repeatedhigh-precisiongravitysurveyswereconductedover two infiltration cycles on an alluvial-fan aquifer system at the mouth of Weber Canyon in northern Utah as part of the Weber River Basin Aquifer Storage and Recovery Pilot Project WRBASR. Gravity measurements collected before, during, and afterinfiltrationeventsindicatethataperchedgroundwatermound formed during infiltration events and decayed smoothly following infiltration. Data also suggest the groundwater mound migrated gradually south-southwest from the surface infiltration site.Maximummeasuredgravitychangesassociatedwiththeinfiltration were 111 Gal during the first event 2004 and a net 130-Gal increase during the second event 2005. Gaussian integration of the spatial gravity anomaly yields an anomalouscausative mass within 10% of the 10 6 m 3 10 9 kg of infiltrated water measured in 2004. The spatial gravity field is consistent with a groundwater mound at the end of the infiltration cycle approximatelyequivalenttoacylindricaldiscofheight13.5 m and a radius between 300‐400 m. After infiltration ceased, gravity anomalies decreased to approximately 50% of their original amplitude over a characteristic time of three to four months.The reduction of the gravity signal is simulated by analytical solutions forthedecayofagroundwatermoundthroughasaturatedporous media.Thiscomparisonplacesrelativelytightboundsonthehydraulic conductivity of the alluvial-fan material below the infiltration site with a preferred value of 80 m/day on a length scale ofafewhundredmeters.