Repeated high-precision gravity surveys were conducted over 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 after infiltration events indicate that a perched groundwater mound formed during infiltration events and decayed smoothly following infiltration. Data also suggest the groundwater mound migrated gradually south-southwest from the surface infiltration site. Maximum measured gravity changes associated with the infiltration were 111μGal during the first event (2004) and a net 130-μGal increase during the second event (2005). Gaussian in-tegration of the spatial gravity anomaly yields an anomalouscausative mass within 10% of the 106m3(109kg) of infiltrated water measured in 2004. The spatial gravity field is consistent with a groundwater mound at the end of the infiltration cycle approximately equivalent to a cylindrical disc of height 13.5m and a radius between 300400m. 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 for the decay of a groundwater mound through a saturated porous media. This comparison places relatively tight bounds on the hydraulic conductivity of the alluvial-fan material below the infiltration site with a preferred value of 80m/day on a length scale of a few hundred meters.

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