MONITORING VADOSE ZONE INFILTRATION WITH TIME-LAPSE GRAVITY DATA AT A MUNICIPAL RECHARGE AND WITHDRAWAL FACILITY (Invited Presentation)

Time-lapse gravimetry is a direct measurement of changes in terrestrial water storage, but comprehensive evaluation has been hindered by limited instrument availability and typically low signal-to-noise ratios. A recent study deploying a large gravimeter network in southern Arizona addresses these limitations. The study area comprises three artificial recharge basins, each about five acres, at which a total of about 15,000 acre-feet of water is recharged annually. Two iGrav superconducting gravimeters and three gPhone spring-based relative gravimeters collect continuous data adjacent to the basins. An A-10 absolute gravimeter and two metal-spring relative gravimeters are used for spatially distributed measurements, and an FG-5 absolute gravimeter and is used to calibrate the other gravimeters.

One eight week flooding cycle has been completed, during which 315 liters/second (5,000 gallons/minute) of water was applied continuously at each of the three basins. The total increase in gravity at each basin during this period was about 160 microgals, but the rates of change differed between the basins and reflect differences in infiltration rates. At the basin with the fastest infiltration, gravity increased rapidly at the onset of basin flooding and decreased rapidly when the basin inflow was turned off. After a 3 month drying period, maximum gravity change at this basin had nearly returned to pre-flooding levels, indicating little long-term storage within the gravimeter's support volume. During this same flooding and drying cycle, water level in a well 125 m from the edge of the basin increased continuously, from 69.2 to 61.7 meters bls. At an adjacent basin, gravity increased more slowly but eventually reached the same maximum gravity change. Three months after drying, this basin had a cumulative gravity change 40 microgals higher than the pre-flooding state, indicating lower flux and longer-term storage below the basin. Together, the gravity data are used for testing hypotheses concerning coupling geophysical measurements with hydrologic models, common-mode rejection for removal of tide and barometric pressure effects, efficient survey-network design, and measurement uncertainty.